Evaluating the impact of mine tailings wastes on the development of sustainable Ultra High Performance Fiber Reinforced concrete

Abstract

The increasing demand for sustainable construction materials has led to exploring alternative resources, such as mine tailings in ultra-high-performance fiber-reinforced concrete (UHPFRC). Conventional concrete production is associated with significant environmental impact and resource depletion, necessitating the search for sustainable alternatives. This study investigates the effects of incorporating mine tailings powder (MTP) and mine tailings sand (MTS) as substitutions for cement and quartz sand on the mechanical and durability properties of UHPFRC. The current study focuses on evaluating the workability, compressive strength, indirect tensile strength, modulus of rupture, sulfate resistance, autogenous shrinkage, thermal performance, porosity, hydration kinetics, and phase composition of UHPFRC with varying MTP and MTS contents. Results indicate that the optimal mix of 15% MTP and 60% MTS enhances compressive strength by 12.49% (165.2 MPa at 90 days), reduces autogenous shrinkage by 49.28% (1023 microns at 180 days), and significantly improves sulfate resistance, with a residual compressive strength of 115.2 MPa and a mass loss of 25.4%. The workability tests show a decrease in flow spread from 274 mm in the control mix to 221 mm in the mix with 25% MTP and 100% MTS. Indirect tensile strength increased by 23.53% (23.1 MPa at 90 days), and the modulus of rupture improved by 23.04% (23.5 MPa at 90 days) with the optimal mix. Mercury intrusion Porosimetry reveals increased total pore volume, especially in larger pore sizes, with the highest intrusion observed in the mix containing 5% MTP and 20% MTS. The heat of hydration analysis shows a delayed T_max from 30 h in the control mix to 36 h with 15% MTP and 60% MTS, indicating a slower and more controlled hydration process. X-ray diffraction analysis demonstrates a reduction in peak intensities, marking a transition from crystalline to more amorphous structures with increasing mine tailings content. The improved microstructural characteristics, such as reduced porosity and enhanced hydration stability, contribute to the material’s long-term durability and mechanical performance, making it a viable option for sustainable construction practices.

Introduction

Ultra-high-performance fiber-reinforced Concrete (UHPFRC) is a novel material known for improved mechanical properties, including high compressive strength, durability, and tensile strength^1,2. These characteristics are achieved through a densely packed microstructure and a very low water-to-cement ratio. This composition makes UHPC suitable for structural applications requiring reduced cross-sections, such as bridge decks, high-rise buildings, and other infrastructure projects where performance and longevity are critical³. The low water-to-cement ratio in UHPC enhances its density and reduces its permeability, improving durability and resistance to environmental factors. This dense packing is obtained by incorporating a high volume of fine materials like silica fume, fine sand, and especially Ordinary Portland Cement (OPC)⁴. The OPC content in UHPC is significantly higher than in conventional concrete, contributing to its enhanced strength and durability. However, the environmental implications of UHPC’s high OPC content are substantial. The production of OPC is highly energy-intensive, requiring approximately 1,400-1,600 kWh of energy per ton⁵. Cement manufacturing is a significant source of carbon dioxide (CO₂) emissions, contributing to about 8% of the global total. For every ton of OPC produced, approximately 0.8 to 1 ton of CO₂ is released into the atmosphere⁶. This high level of CO₂ emissions is a significant concern, considering the current emphasis on reducing carbon footprints to combat climate change. The environmental impact of UHPFRC is also considerable, originating from its extensive use of OPC, which leads to significant CO₂ emissions during production. These emissions are a substantial environmental concern, highlighting the urgency for more sustainable construction practices⁷. The low water-to-cement ratio in UHPFRC leads to a cement hydration degree of only about 30–40%, rendering the unhydrated portion of the cement as only a filler. This reduces the efficiency of cement usage and increases the cost due to the high volume of OPC required.

Additionally, the development of UHPFRC necessitates using fine materials such as river sand or quartz sand, raising the cost and intensifying the risk of depleting these natural resources⁸. The increasing global focus on sustainability in construction highlights the urgency of such substitutions, as addressing these issues could reduce environmental impacts without compromising UHPFRC’s mechanical performance. Given these factors, there is a compelling need to explore alternative substitutions for unhydrated cement and fine aggregates in UHPFRC mixtures to reduce reliance on scarce resources^9,10. It is essential to incorporate alternative binders and supplementary cementitious materials (SCMs)⁸ or adopt more efficient manufacturing processes to address these issues. Such initiatives could help reduce the environmental footprint of UHPFRC while maintaining its enhanced mechanical properties¹¹.

Mine tailings, the residual mineral waste from mining operations, present a significant global environmental challenge and offer promising opportunities for sustainable construction practices, especially in the context of UHPFRC^12,13. These tailings are finely ground rocks that remain after the desired minerals have been extracted. They can vary widely in composition based on the type of mineral extracted and the geological source¹⁴. Globally, the production of mine tailings is substantial, with significant mining countries contributing large quantities of this waste. For instance, China, the largest producer of gold and one of the top miners of other precious metals and coal, generates an estimated 3 billion tons of tailings annually¹⁵. The United States and countries in Europe also contribute significantly to the global tailings output, each producing hundreds of millions to over a billion tons of tailings per year¹⁶. These numbers highlight the potential for reusing mine tailings in construction to mitigate the environmental impacts of mining and cement manufacturing. Mine tailings are suitable for use in UHPFRC primarily due to their physical and chemical properties. Many tailings possess pozzolanic properties, which can react with calcium hydroxide released during the hydration of cement to form additional cementitious materials, thereby enhancing the durability and strength of concrete. Recent studies have shown that incorporating up to 15% mine tailings powder (MTP) results in an 8–12% compressive strength increase, as the pozzolanic reactions improve the bond between the cement matrix and the aggregates.

Furthermore, MTP has been found to significantly reduce the permeability of concrete, with chloride ion penetration decreasing by approximately 20–30%, enhancing the durability of the material in chloride-exposed environments¹⁷. On the other hand, mine tailings sand, due to its fine particle size and angular shape, plays a crucial role in improving the granular structure of UHPFRC. Studies indicate that replacing up to 40% of quartz sand with MTS can increase flexural strength by as much as 15% and reduce drying shrinkage by up to 18%. This fine particle distribution improves the packing density and inter-particle friction, which reduces the material’s shrinkage potential¹⁸. Moreover, when both MTP and MTS are combined, they complement each other by enhancing the mechanical properties and the microstructure of UHPFRC. This synergistic effect results in a more durable and sustainable concrete mix. For example, a blend of 10% MTP and 30% MTS has been shown to reduce porosity and increase the material’s resistance to sulfate attack while improving the compressive strength by approximately 10–12% compared to the control mix. These findings emphasize the importance of distinguishing between the effects of MTP and MTS. While MTP contributes mainly to improving the cementitious nature of the mix, enhancing strength, and improving the hydration process, MTS significantly enhances the granular packing and helps reduce permeability and shrinkage. This clear differentiation between the two types of mine tailings is essential for understanding their individual contributions to the final concrete properties.

Similarly, studies by Zhang et al.¹⁸ and Zhong et al.¹⁹ demonstrated that UHPC exhibited improved mechanical properties with a 15% cement replacement by mine tailings powder. Further grinding of mine tailings powder, as researched by Li et al.²⁰, showed that replacing up to 20% of the cement could maintain or increase the compressive strength compared to conventional mixes. Li et al.²⁰ even observed benefits with a 30% substitution rate. However, Shui et al.²¹ noted that increasing the amount of mine tailings powder reduces heat flow and compressive strength, though durability remains unaffected. Mine tailings sand has been effectively used as a replacement for fine aggregates in UHPC. Zhou et al.²² demonstrated that mine tailings sand could contribute to the production of UHPC, with Qiu et al.²³ finding that compressive strength and permeability peaked when mine tailings replaced 40% of the sand. Sun et al.²⁴ and Xiaowei et al.²⁵ identified a decrease in compressive strength with higher substitution rates, though the flexural strength could increase, showing mixed results at various replacement ratios. Mine tailings are viable for either aggregates or cementitious materials in UHPC formulations. A critical research gap exists in optimizing substitution rates to balance mechanical properties and sustainability.

While previous research^{13,26,27,28,29} has explored using mine tailings powder or sand as substitutes for cement or sand in UHPFRC, this study investigates a novel approach by combining both materials at higher substitution levels. Often, these studies have shown a low overall recycling rate of tailings due to the significant variation in particle size distribution. Recognizing the potential for enhanced resource utilization, Xiaowei et al.³⁰ integrated gold tailings powder and sand in UHPC mixtures, achieving a design with a significantly reduced carbon footprint and a 25% decrease in global warming potential. This highlights the importance of further research into the combined use of mine tailings powder and sand in UHPC to develop eco-friendly designs with high recycling rates of tailings. Moreover, the unique physical and mineral characteristics of mine tailings sand, differing significantly from river or quartz sand, alter its water retention properties, thereby impacting concrete’s drying shrinkage, a crucial aspect of durability in construction. Although only a few studies^15,31 have addressed it, the shrinkage performance of UHPC that incorporates both types of tailings needs a thorough evaluation. Additionally, the potential leaching of heavy metals from tailings-added concrete is a concern that must be addressed despite studies suggesting that UHPC’s dense structure may help immobilize heavy metal ions^30,32.

This study makes a significant contribution to the field of ultra-high-performance fiber-reinforced concrete by innovatively integrating both mine tailings powder (MTP) and mine tailings sand (MTS) in varying proportions to replace traditional materials like ordinary Portland cement (OPC) and fine aggregates. Unlike most existing research that typically focuses on the isolated use of either MTP or MTS in small quantities, this study pushes the boundaries by using these mine tailings as dual substitutes at much higher levels, with MTP replacing 5–25% of OPC and MTS replacing 20–100% of quartz sand. This dual-material approach improves the understanding of mine tailings’ potential in UHPFRC and explores their synergistic effects on the material’s mechanical and durability properties.

The novelty of this work lies in its comprehensive evaluation of critical factors such as chloride resistance, drying shrinkage, and the leaching potential of heavy metals, areas that are often underexplored in conventional studies. Moreover, by considering the substantial quantities of mine tailings, the study also addresses the urgent need for sustainable construction materials, contributing to a circular economy by reducing waste and enhancing the environmental performance of concrete. This research leverages advanced analytical techniques such as X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Mercury Intrusion Porosimetry (MIP) to examine the microstructural mechanisms at play, providing novel insights into the behavior of UHPFRC with mine tailings. These methods offer a deeper understanding of the hydration and pore structure development and strengthen the case for mine tailings as a viable and sustainable material for high-performance concrete applications.

Materials

In a current study, materials such as 53-grade Type I cement as per ASTM C150³³, silica fume, and quartz sand were selected based on specific criteria to ensure optimal performance in concrete applications. The 53-grade Type I cement with a specific gravity of 3.1, known for its high early strength, was sourced from local manufacturers in Lahore, renowned for their rigorous adherence to ASTM standards and quality control³⁴. This type of cement was chosen mainly for its rapid hardening properties and ability to achieve high strength within a short period, making it suitable for infrastructure projects demanding firm structural strength. Silica fume, a byproduct of silicon and ferrosilicon alloy production, was obtained from industrial operations in Karachi. The specific surface area of the silica fume is an essential factor in its selection. The specific surface area and specific gravity were 29,503 cm²/g and 2.13, which significantly enhances the durability and mechanical properties of the concrete by filling voids and increasing particle packing density. The high pozzolanic activity of the silica fume makes it an invaluable component in reducing concrete permeability and enhancing resistance to chemical attack. Quartz sand, integral for its role as a fine aggregate, was sourced from the river beds near the Sindh region. This quartz sand’s purity, grain size, and non-reactive nature were the critical deciding factors for its selection. The uniformity in size and shape ensures a consistent filling effect in the concrete matrix, which contributes to the overall strength and workability of the mixture. The careful acquisition and selection of these materials reflect a comprehensive approach to developing high-quality concrete formulations. Each material was chosen to contribute specific properties to the concrete, aiming to optimize the final product’s processing and performance in structural applications.

In the current study, mine tailings were sourced from the rich deposits near Quetta, Pakistan, a region known for its abundant mining activities. The tailings collected were initially characterized by their varied granular size. It was refined to ensure all materials were less than 75 micrometers in diameter to facilitate their use in concrete applications. Developing mine tailings powder from these tailings involved several steps, primarily focusing on mechanical grinding to achieve the desired fineness suitable for enhancing the pozzolanic properties required in concrete formulations. The first step in the preparation involved drying the raw mine tailings to remove any moisture content, which could affect the grinding efficiency and the quality of the powder. Following drying, the tailings underwent a preliminary crushing to break down large aggregates into smaller particles. Subsequently, these crushed tailings were fed into a mechanical ball mill and ground into finer particles. Throughout the grinding process, continuous monitoring was necessary to ensure the uniformity and size specification of the particles. After reaching the targeted size, the material was passed through a series of sieves to separate the fine powder from larger particles. The finest fraction, which passed through the smallest sieve (75-micrometer mesh), was collected as MTP, while the coarser particles retained on the sieve were designated as MTS. The specific gravity of MTP and MTS was 2.86 and 2.74. The chemical composition values for the materials listed in Table 1 were determined through standard laboratory analysis using X-ray fluorescence (XRF) spectroscopy. XRF is a widely used technique for accurately determining the elemental composition of materials, including cement, silica fume, and MTP. The XRF method involves irradiating the sample with X-rays, which causes the elements in the material to emit characteristic secondary (fluorescent) X-rays. These emitted X-rays are then detected and analyzed, allowing for the precise identification and quantification of the chemical elements present in the sample. OPC, SF, and MTP samples were prepared according to standard procedures to determine the chemical composition. The materials were ground to a fine powder and compacted for analysis. The XRF analysis was performed in a controlled laboratory environment, and the results were cross-validated with established compositional standards to ensure the accuracy of the values presented in Table 1.

Double hooked-end steel fibers were sourced from Karachi, Pakistan. These steel fibers, known for their enhanced mechanical anchorage, measured 35 mm in length and 0.5 mm in diameter, providing a high aspect ratio essential for improving the tensile strength and ductility of UHPFRC. The specific geometry of these fibers allows for better stress distribution and crack bridging, significantly contributing to the overall performance of the composite material. Additionally, a particular type of superplasticizer was procured from Lahore, Pakistan. This high-range water-reducing admixture, identified as a polycarboxylate ether-based superplasticizer, exhibited a water-reducing efficiency of up to 30%. It contained a solid content of approximately 40%, making it highly effective in enhancing the workability of UHPFRC while maintaining low water-to-cement ratios. Incorporating this superplasticizer ensured adequate flowability and dispersion of the cement particles, which is crucial for achieving the desired dense microstructure and superior mechanical properties in the concrete mix.

Figure 1 illustrates the particle size distribution curves for various materials used in UHPC, including Mine Tailings Powder, Mine Tailings Sand, Cement, Silica Fume, and Quartz Sand, alongside the optimal grading curve based on the Andreasen and Andersen model. The dark blue curve represents Mine Tailings Powder, showing a broad size distribution suited for filler applications. The green curve for Mine Tailings Sand indicates a coarser distribution, similar to the red Quartz Sand curve, which is essential for the aggregate phase. The purple Cement curve exhibits a steep distribution, highlighting its fineness, which is critical for hydration. The dark orange curve for Silica Fume demonstrates an excellent distribution, which is essential for enhancing the packing density and pozzolanic reactions in UHPFRC. The black dashed line represents the optimal particle size distribution according to the Andreasen and Andersen model^35,36, aiming to achieve maximum packing density and minimal voids. The alignment of these materials’ distributions with the optimal curve emphasizes their suitability in creating a densely packed, high-performance UHPFRC matrix.

To control the particle size distribution of the mine tailings during grinding, the tailings were first dried to remove any moisture content, which could negatively impact the grinding efficiency. The dried tailings were then subjected to preliminary crushing to reduce the particle size before being ground in a mechanical ball mill. The grinding process was closely monitored to achieve the desired particle size (less than 75 micrometers). A laser diffraction method was employed to measure the particle size distribution and to ensure uniformity in the final product. This allowed for the control of particle size distribution and ensured that the MTP had a fine and uniform particle size suitable for enhancing pozzolanic activity. The influence of particle size on the pozzolanic activity of MTP is significant. Finer particles have a larger specific surface area, which enhances their reactivity with calcium hydroxide, a byproduct of cement hydration. The increased surface area facilitates a higher reaction rate, forming additional cementitious compounds that improve the mechanical properties and durability of the concrete. The finer the particle size, the more effective the pozzolanic reaction, contributing to a denser and stronger UHPFRC matrix.

Table 1 Chemical composition of OPC, SF, and MTP.

Fig. 1

Gradation of Raw Materials.

Figure 2 (a) presents Mine Tailings Sand’s Energy Dispersive Spectroscopy (EDS) analysis, illustrating the sample’s elemental composition. The prominent peak at approximately 0.5 keV is attributed to oxygen (O), the most abundant element in the sample, suggesting a significant presence of oxides. Iron (Fe) is observed with a notable peak around 0.7 keV, indicative of the iron-bearing minerals within the tailings. Magnesium (Mg) and aluminum (Al) show smaller peaks near 1.0 and 1.5 keV, respectively, reflecting their lower concentrations. Silicon (Si) presents a distinct peak at around 1.8 keV, highlighting the presence of silicate minerals, which are common in mine tailings. This EDS spectrum provides a comprehensive overview of the elemental distribution within the Mine Tailings Sand, confirming the presence of critical constituents like oxygen, iron, magnesium, aluminum, and silicon, which are crucial in understanding the material’s chemical composition and potential reactivity.

Figure 2 (b) presents the XRD analysis of Mine Tailings Sand, highlighting its crystalline composition. Distinct peaks at specific 2-theta values correspond to mineral phases in the mine tailings. A peak at approximately 14.0° is attributed to calcite, signifying the presence of calcium carbonate. Prominent peaks observed at 21.0° and 27.0° are linked to quartz, indicating a notable presence of silica. The peak around 32.0° is identified as muscovite, a phyllosilicate mineral commonly found in such materials. The peak at 37.0° corresponds to magnetite, indicating iron oxide. A minor peak at 45.0° is associated with feldspar, suggesting the presence of aluminum silicate minerals. This analysis confirms the presence of essential minerals such as calcite, quartz, muscovite, magnetite, and feldspar within the mine tailings sand, providing valuable insights into its mineralogical composition and potential applications or reactivity.

Fig. 2

(a) EDS analysis of Mine Tailings Sand, (b) XRD Analysis of Mine Tailings Sand.

Sample Design and Mixing

This study formulated six concrete mixes (see Table 2) to investigate the effects of incorporating varying proportions of mine tailings as partial replacements for traditional cementitious materials and aggregates. The control mix, M1-Control, was composed entirely of conventional materials with no mention of mine tailings. Subsequent mixes incorporated increasing percentages of mine tailings powder and sand to assess their influence on the mechanical properties and durability of the concrete. Specifically, Mix M2-P5-S20 contained 5% mine tailings powder and 20% mine tailings sand, Mix M3-P10-S40 included 10% mine tailings powder and 40% mine tailings sand, Mix M4-P15-S60 comprised 15% mine tailings powder and 60% mine tailings sand, Mix M5-P20-S80 had 20% mine tailings powder and 80% mine tailings sand, and finally, Mix M6-P25-S100 was formulated with 25% mine tailings powder and 100% mine tailings sand. All mixes were uniformly reinforced with the same level (1.5% by binder wt.) of double hooked-end steel fibers to maintain consistency in reinforcement across the samples. This systematic variation in the composition allows for a comprehensive analysis of the impact of mine tailings on the structural behavior and performance characteristics of the concrete mixes.

In the present research, the preparation of UHPFRC involved a thorough and systematic mixing procedure to ensure optimal material integration and performance. The process utilized a planetary mixer, which is particularly suited for UHPFRC due to its ability to provide high-shear mixing, which is essential for achieving a homogeneous and dense mixture. The mixing sequence began with the dry mixing of the base cementitious materials. OPC and silica fume were first introduced into the mixer. This initial combination was dry-mixed for three minutes to achieve a uniform distribution of the silica particles throughout the OPC, ensuring that each cement particle was well-coated and the potential for water reduction was maximized. Following this, MTP, substituting OPC at percentages of 0%, 5%, 10%, 15%, 20%, and 25%, was added to the mix. Simultaneously, MTS was incorporated, replacing quartz sand as the fine aggregate at 0%, 20%, 40%, 60%, 80%, and 100%. The dry components, including the MTP and MTS, were then blended for five minutes to ensure thorough integration of the powders with the cementitious base. Water, measured to maintain a precise water-to-cement ratio critical for UHPFRC, was gradually added to the mixture. This was done alongside adding a polycarboxylate ether-based superplasticizer to enhance the workability without compromising the strength. The superplasticizer, key to achieving the desired slump and fluidity, was introduced into the mix over one minute while mixing continued, ensuring the superplasticizer fully interacted with the cementitious materials to reduce water demand. After adding the water and superplasticizer, mixing was carried out for another three minutes. During this stage, the matrix exhibited the high workability required for UHPFRC. Subsequently, double-hooked end steel fibers, integral for providing ductility and tensile strength to the composite, were slowly added to the mix. These fibers were dispersed under continuous mixing for two additional minutes to prevent clumping and to ensure even distribution throughout the concrete matrix. The total mixing time from the initial dry mixing to the final incorporation of fibers spanned approximately fourteen minutes. This extended mixing procedure was critical to ensure that all components, especially the supplementary cementitious materials and fibers, were well integrated to exploit their synergistic properties effectively.

A vertical lift method assessed the flow property of freshly mixed UHPFRC. Initially, the mix was poured into a steel cone set on a level surface. Upon filling, the cone was elevated directly upwards, allowing the mix to flow freely. Following this, the spread of the UHPFRC paste was measured to determine its flow diameter once the mixture ceased moving. This procedure was repeated three times to ensure precision, and the average spread was recorded as the test outcome. Subsequently, the formed specimens were covered with a plastic film and left to rest for a day before being transferred to a steam chamber maintained at approximately 75 °C for three days of curing. After this period, the specimens were removed to undergo further testing.

In formulating the six variants of UHPFRC, it was crucial to ensure repeatability and uniformity across all experimental outcomes. Consequently, four individual samples were prepared and tested for each specific mix, ranging from the control sample to those with increasing proportions of MTP and MTS. This procedure was planned to minimize variability and enhance the cohesiveness of the data. The preparation of four samples per mix allowed for reliable analysis, ensuring the results were replicable and significant in illustrating the effects of substituting traditional materials with mine tailings in UHPFRC. Each sample was cast using the same batch to avoid any discrepancies in material properties or external conditions that might influence the results. They were then cured under identical conditions, and each was tested under the same parameters for mechanical strength, durability, and other performance metrics.

Table 2 Mix details of all samples (kg/m³).

Test characterization

Strength characteristics

This study evaluated the compressive strength and indirect tensile strength of UHPFRC mixes following ASTM standards. After removing samples from the mold, samples of UHPFRC, each measuring 100 × 100 × 100 mm³, were prepared for the respective strength tests. For the compressive strength test, the samples were subjected to a loading rate of 1.75 MPa/sec until failure, following ASTM C39³⁷. The procedure ensured uniform application of load through a hydraulic testing machine. A universal testing machine with a capacity of 2000 kN was used to conduct these tests. The compressive strength (σc) was evaluated at 28, 56, and 90 days using the formula (1):

$$\:\sigma\:c=\frac{Fmax}{A}$$

(1)

F_max is the maximum load applied at failure, and A is the cross-sectional area of the sample.

In the indirect tensile strength test, conducted as per ASTM C496³⁷, wooden strips were distributed evenly on each specimen’s top and bottom faces to facilitate even load distribution. The specimens were then loaded at a rate of 0.1 MPa/s until failure occurred. The indirect tensile strength (σt) was evaluated at 56 and 90 days using the following formula (2):

$$\:{\upsigma\:}\text{t}=\frac{2\text{P}}{\pi\:LD}$$

(2)

Where P is the maximum load at failure, L is the length of the specimen, and D is the diameter of the sample.

This study evaluated the modulus of rupture of UHPFRC mix mixtures following ASTM C1609³⁸ standards. A sample measuring 600 mm x 150 mm x 150 mm was used for this test. Testing the modulus of rupture (MOR) involved several detailed steps. Firstly, six beam specimens, each with dimensions of 600 mm in length, 150 mm in width, and 150 mm in height, were prepared. These specimens were cast and cured following ASTM C192³⁹. After demolding, the specimens were stored in a moist curing environment until they reached the appropriate testing age. A universal testing machine with a capacity of 2000 kN. was used for the test setup. The beam specimens were positioned in the machine for a third-point loading test. This method involves placing the specimen on two support rollers spaced 450 mm apart, with an additional pair of loading rollers positioned at the upper third points, 150 mm from each support. The loading rate was carefully controlled and set to 0.05 mm/min to ensure a uniform load application until the specimen failed. During the test, the machine applied load at a constant rate and monitored the load-deflection response. The modulus of rupture, also known as the flexural strength, was calculated at 56 and 90 days using the following formula (3):

$$\:MOR=\frac{PL}{b{d}^{2}}$$

(3)

Where P is the maximum load applied at failure, L is the span length (450 mm), b is the width of the specimen (150 mm), and d is the depth of the specimen (150 mm).

Durability characteristics

Sulfate attack test

In the current research, the sulfate attack test for UHPFRC samples was performed as per ASTM C1012⁴⁰, which specifies the procedures for determining the resistance of concrete to sulfate attack. Following the completion of steam curing, the cylindrical samples with dimensions of 100 mm in diameter and 50 mm in depth were subjected to the sulfate exposure test for 5 h. Before exposure, the samples were thoroughly rinsed to remove any residual curing compounds. The test environment maintained a controlled temperature of 23 ± 2 °C and a relative humidity of 50 ± 4%, ensuring consistent conditions for all specimens. During exposure, the samples were immersed in a sulfate solution with a mass concentration of 5% sodium sulfate (Na2SO4), ensuring complete submersion to simulate aggressive sulfate attack conditions. The solution was agitated periodically to maintain uniform concentration around the samples. After the 5-hour exposure period, the samples were removed from the solution, gently rinsed with distilled water to eliminate any surface deposits, and then air-dried for subsequent analysis. The post-exposure evaluation included visual inspection for surface degradation, residual compressive strength, and mass loss measurement.

Shrinkage characteristics

The autogenous shrinkage characteristics of UHPFRC samples were investigated following ASTM C1698⁴¹, which outlines the standard test method for autogenous shrinkage of cement paste. The test specimens were rectangular prisms measuring 160 mm in length, 25.4 mm in depth, and 25.4 mm in width. Following casting, the samples were immediately sealed to prevent any moisture exchange with the environment, a crucial step to ensure that only autogenous shrinkage (shrinkage occurring without any moisture loss to the surroundings) was measured. The sealed specimens were stored in a controlled environment at a constant temperature of 23 ± 2 °C. Shrinkage measurements were taken at predetermined intervals of 3, 7, 14, 28, 56, 90, 120, 150, and 180 days. At each interval, the length change of the samples was measured using a high-precision length comparator equipped with a digital dial gauge capable of measuring changes to the nearest 0.001 mm. The samples were carefully handled to prevent any mechanical damage that could affect the accuracy of the measurements. Each measurement was conducted in triplicate to ensure the reliability and repeatability of the results. To account for any thermal expansion or contraction, the temperature of the testing environment was continuously monitored and recorded. Control specimens made from non-shrinking material were also used to correct systematic errors in the measurement apparatus.

Elevated temperature performance

The elevated temperature performance of UHPFRC samples containing various contents of mine tailings powder and mine tailing sand was evaluated to understand their thermal stability and mechanical properties under high temperatures. The test specimens were 100 × 100 × 100 mm³ cubes. These samples were subjected to temperatures up to 800 °C, with residual compressive strength and mass loss assessed at 200 °C, 400 °C, 600 °C, and 800 °C. The samples were first air-dried to a constant mass to eliminate any moisture affecting the test results. They were then placed in a high-temperature muffle furnace with a digital temperature controller to ensure precise temperature regulation. Before introducing the samples, the furnace was preheated to the target temperature to minimize thermal shock. Each sample was placed on a ceramic plate to prevent any reaction between the concrete and the furnace materials. The temperature was increased at a controlled rate of 5 °C per minute to the designated test temperatures (200 °C, 400 °C, 600 °C, and 800 °C) to ensure uniform heating throughout the samples. Once the target temperature was reached, the samples were maintained for 2 h for thermal equilibrium. After the heating period, the samples were allowed to cool gradually to room temperature inside the furnace to prevent any additional thermal shock that could affect the integrity of the samples. Post-heating, the samples were weighed to determine the mass loss, indicating the extent of decomposition and volatilization of components within the concrete matrix. The residual compressive strength of the cooled samples was then tested using a universal testing machine. Each sample was subjected to a compressive load at a constant rate until failure, and the maximum load was recorded to calculate the compressive strength.

Toxicity characteristic leaching Procedure

The leaching toxicity test for UHPFRC samples modified with mine tailings powder and mine tailing sand was conducted following the international standard EPA Method 1311⁴², also known as the Toxicity Characteristic Leaching Procedure (TCLP). This test aimed to assess the potential leaching of hazardous substances from the UHPFRC samples containing the maximum content of MTP and MTS. The samples were first crushed to a particle size of less than 9.5 mm to increase the surface area for leaching. The crushed material was then subjected to the leaching procedure using an extraction fluid: acetic acid with a pH of 2.88 ± 0.05. The extraction fluid was prepared by diluting glacial acetic acid with distilled water. The sample-to-extraction fluid ratio was maintained at 1:20 (mass) to ensure adequate contact between the sample and the fluid. The mixture was placed in a rotary agitation apparatus and agitated for 18 ± 2 h at a speed of 30 ± 2 rpm to facilitate the leaching process. After the agitation period, the leachate was filtered using a glass fiber filter to separate the solid residues from the leachate solution. The leachate was then collected in clean polyethylene bottles for chemical analysis. Various analytical techniques, such as Inductively Coupled Plasma Mass Spectrometry (ICP-MS) or Atomic Absorption Spectroscopy (AAS), were employed to determine the concentrations of heavy metals and other hazardous substances in the leachate. The analysis focused on critical contaminants potentially present in the mine tailings, including but not limited to lead (Pb), arsenic (As), cadmium (Cd), and mercury (Hg).

Microstructural characteristics

Mercury Intrusion Porosimetry

The Mercury Intrusion Porosimetry (MIP) analysis of UHPFRC was performed to evaluate the pore structure and porosity of the material, especially after incorporating mine tailings powder and mine tailing sand. The MIP technique is essential for understanding the pore size distribution, total porosity, and specific surface area, which are critical for assessing the durability and mechanical properties of UHPFRC. The sample preparation involved cutting the UHPFRC specimens into fragments, around 5 mm in size, to fit into the penetrometer stem of the MIP apparatus. It was crucial to avoid crushing the samples during preparation to prevent altering the original pore structure. The samples were then dried in a vacuum oven at 105 °C for 24 h to remove any moisture, as residual water could interfere with the mercury intrusion process. Once dried, the samples were weighed to record their initial mass before being placed in the penetrometer. The penetrometer containing the sample was then loaded into the MIP apparatus. The MIP system comprises low-pressure and high-pressure chambers covering various pore sizes. Mercury, a non-wetting liquid, was gradually introduced to the sample under controlled pressure increments. The analysis began with applying low pressure to fill the larger pores, then progressively increasing the pressure to 60,000 psi to intrude the smallest pores. The relationship between the applied pressure and the volume of mercury intruded into the pores was recorded continuously. This data was used to calculate the pore size distribution, total pore volume, specific surface area, and cumulative intrusion volume. The MIP data was analyzed using specialized software, which employed the Washburn equation to relate the pressure to the pore diameter. The software provided detailed plots and statistics, including cumulative and incremental pore size distribution curves, allowing for a comprehensive understanding of the pore structure characteristics of the UHPFRC samples.

Heat of Hydration Analysis

The Heat of Hydration analysis for UHPFRC incorporating mine tailings powder and mine tailing sand was conducted to understand the exothermic reactions occurring during the cement hydration process. This analysis is crucial for assessing the thermal behavior and early-age strength development of UHPFRC. The test was performed using an isothermal calorimeter, which measures the heat flow associated with the hydration of cementitious materials. Before testing, the materials (cement, MTP, MTS, QS) were conditioned at 20 °C ± 1 °C to ensure consistent starting conditions. The UHPFRC mix was then prepared according to the specified proportions, ensuring thorough mixing to achieve a homogenous paste. Approximately 50 g of the fresh UHPFRC paste were placed into the sample container of the isothermal calorimeter. The container was then sealed to prevent moisture loss and placed in the calorimeter chamber, maintained at a constant temperature of 20 °C throughout the test. The reference container, filled with an inert material of the same mass, was placed in the adjacent chamber to account for any thermal drifts or environmental influences. The isothermal calorimeter continuously monitored the heat flow from the sample over 72 h. The heat flow data, representing the rate of heat evolution, was recorded at regular intervals. This data was used to plot the heat flow versus time curve, highlighting the different stages of the hydration process, including the initial reaction, induction period, acceleration phase, and deceleration phase. The cumulative heat release was also calculated, providing insights into the total heat evolved during hydration. The analysis of the heat flow curves helped identify the influence of MTP and MTS on the hydration kinetics and the potential impacts on the early-age performance of UHPFRC. Additionally, the specific heat of hydration values was compared to those of conventional UHPFRC to evaluate any modifications due to the incorporation of mine tailings.

X-Ray diffraction analysis

The XRD analysis of UHPFRC was conducted to characterize the crystalline phases present in the material, especially after incorporating mine tailings powder and mine tailing sand. The samples used for XRD analysis were first prepared by grinding them into a fine powder to ensure homogeneity and increase the surface area for accurate diffraction. A mechanical grinder with a tungsten carbide grinding set was utilized to achieve a particle size of fewer than 75 micrometers. Once prepared, the powdered samples were evenly spread on a sample holder, typically made of low-background silicon or glass. This ensured a flat and smooth surface to minimize any preferred orientation effects. The sample holder was then placed in the XRD instrument, an X-ray diffractometer with a Cu Kα radiation source operating at 40 kV and 30 mA. The scanning was performed over a 2θ range from 5° to 70°, with a step size of 0.02° and a counting time of 1 s per step, providing detailed information on the crystalline phases present. The scan parameters were carefully chosen to balance resolution and scanning time, ensuring comprehensive phase identification without excessive analysis duration. Data acquisition was followed by qualitative and quantitative analysis using specialized software such as JADE. The software compared the diffraction patterns with standard reference patterns from the International Centre for Diffraction Data (ICDD) database, identifying the various crystalline phases in the UHPFRC, including any new phases formed due to the incorporation of MTP and MTS.

Results and discussion

Flow of freshly mixed UHPFRC

Figure 3 illustrates the flow spread results for UHPFRC mixtures with varying contents of MTP and MTS. The control mix (M1-Control) achieved a flow spread of 274 mm. Upon introducing 5% MTP and 20% MTS in Mix M2-P5-S20, the flow spread decreased to 259 mm, reflecting a reduction of approximately 5.47%. This initial reduction can be linked to the influence of the rough, porous morphology of mine tailings, which likely absorb more water, reducing the effective water-to-binder ratio and, subsequently, the workability of the mixture. As MTP and MTS content increased, a continuous decline in workability was observed. For instance, Mix M3-P10-S40, with 10% MTP and 40% MTS, exhibited a flow spread of 242 mm, marking an 11.68% decrease from the control. Further, Mix M4-P15-S60 (15% MTP and 60% MTS) showed a flow spread of 234 mm, a 14.60% reduction. Mix M5-P20-S80, containing 20% MTP and 80% MTS, presented a flow spread of 229 mm, marking a 16.42% reduction. Finally, Mix M6-P25-S100, formulated with 25% MTP and 100% MTS, had the lowest flow spread of 221 mm, a significant 19.34% reduction from the control mix.

This steady decrease in flow spread suggests that the increase in MTP and MTS content notably impacts the workability of UHPFRC. The rough, angular particles of mine tailings, coupled with their high-water absorption capacity, likely contribute to the observed reduction in flow. These particles create a more interlocked microstructure, enhancing mechanical interlock and friction among particles, thereby reducing fluidity. The potential chemical interactions between mine tailings and the cementitious matrix may promote early hydration, leading to increased mix stiffness and reduced workability¹⁹. Despite the reduction in workability, all mixtures maintained a flow spread above 180 mm, which is generally considered acceptable for practical engineering applications²⁴. This indicates that while the inclusion of mine tailings affects the rheological properties, the mixtures remain viable for use in construction. This observation aligns with previous research, such as studies by Lu et al.¹⁹ and Shettima et al.³¹, which also reported that increased inclusion of alternative materials like MTP and MTS in cementitious composites tends to reduce workability. However, balancing sufficient workability and incorporating sustainable materials is crucial³¹.

While the inclusion of mine tailings in the UHPFRC mixtures significantly impacted workability, leading to a reduction in flow spread, it is essential to consider the practical implications of this reduction on concrete placement and performance. As noted, higher mine tailings content resulted in a progressive decrease in flow spread, with Mix M6-P25-S100 showing a significant 19.34% reduction compared to the control mix. This reduction can complicate the placement of UHPFRC in situations where high fluidity is necessary, such as in intricate formworks or for self-compacting applications. The decrease in workability may result in challenges in ensuring uniform distribution of the mixture and achieving proper compaction, which could impact the concrete’s final structural integrity and durability. Chemical admixtures, particularly superplasticizers or high-range water reducers, could be considered to counteract these workability challenges. These admixtures are known to enhance concrete’s flowability by improving particle dispersion, reducing inter-particle friction, and increasing the effective water-to-binder ratio. Incorporating such admixtures would allow the benefits of mine tailings to be maintained while enhancing the workability, making the mix easier to handle, transport, and place in complex formwork or large-scale construction projects. Additionally, these admixtures would help ensure that the desired fresh-state properties are achieved, facilitating efficient construction practices without compromising the mechanical and durability characteristics of the UHPFRC.

Moreover, it’s essential to balance the inclusion of mine tailings with appropriate mix modifications to avoid excessive workability reduction. While workability is a crucial factor in the initial stages of concrete handling, maintaining long-term mechanical strength and durability is equally important. Therefore, incorporating sustainable materials such as mine tailings, in combination with admixtures, presents an opportunity to enhance the environmental benefits of UHPFRC without compromising practical usability.

Fig. 3

Flow Spread of Freshly Mixed UHPFRC with Varying MTP and MTS Content.

Strength characteristics

Compressive strength

The compressive strength results of UHPFRC mixtures incorporating various amounts of MTP and MTS are presented in Fig. 4. These results demonstrate a clear trend of enhanced mechanical performance by including these sustainable materials up an optimal threshold. The control mix, M1-Control, achieved compressive strengths of 124.2 MPa, 134.5 MPa, and 147.9 MPa at 28, 56, and 90 days, respectively. In comparison, Mix M2-P5-S20, with 5% MTP and 20% MTS, exhibited slightly higher compressive strengths of 127.5 MPa at 28 days, 138.4 MPa at 56 days, and 153.2 MPa at 90 days, reflecting an overall improvement of 3.58% at 90 days relative to the control. As the MTP and MTS content increased, further improvements in compressive strength were observed, particularly with Mix M3-P10-S40, which reached 133.6 MPa at 28 days, 143.2 MPa at 56 days, and 158.9 MPa at 90 days—an increase of 7.57% at 28 days and 11.70% at 90 days over the control. The strength enhancement peaked with Mix M4-P15-S60, achieving 137.1 MPa at 28 days, 151.3 MPa at 56 days, and 165.2 MPa at 90 days, marking a significant increase of 10.39% at 28 days and 12.49% at 56 days compared to the control.

This trend, however, began to reverse at higher MTP and MTS inclusion levels. Mix M5-P20-S80, while still outperforming the control, showed a slight decline from the peak values of Mix M4-P15-S60, reaching 136.2 MPa at 28 days, 148.3 MPa at 56 days and 161.2 MPa at 90 days. Similarly, Mix M6-P25-S100, with the highest MTP and MTS contents, demonstrated compressive strengths of 134.1 MPa at 28 days, 145.3 MPa at 56 days, and 156.6 MPa at 90 days, representing a 5.88% improvement at 90 days relative to the control. The observed enhancements in compressive strength, particularly in mixes with moderate MTP and MTS content, such as M4-P15-S60, can be primarily attributed to two key factors: the pozzolanic reactivity of MTP and the improved particle packing provided by MTS. The pozzolanic reaction of MTP introduces additional calcium silicate hydrate gel formation, which contributes to the densification of the microstructure, thereby enhancing the material’s load-bearing capacity. This reaction is particularly beneficial during the later stages of hydration, as evidenced by the continued strength gains at 56 and 90 days.

In addition to the pozzolanic activity, the finer particle size and angular shape of MTS contribute to a more compact and interlocked microstructure. This densification effect reduces the porosity of the cementitious matrix, thereby increasing the compressive strength. However, as the content of MTP and MTS exceeds optimal levels (as seen in M6-P25-S100), the compressive strength begins to decline. This decline can be attributed to several factors, including increased water demand and reduced workability, leading to challenges in achieving uniform compaction and fiber dispersion. These challenges can result in voids and weak zones within the matrix, reducing overall compressive strength.

Furthermore, the increased incorporation of mine tailings at higher levels could lead to excess unreacted particles, which may not contribute effectively to the strength development. This excess can act as filler material rather than active participants in the hydration process, diluting the cementitious content and limiting strength gains⁴³. Despite these limitations at higher inclusion levels, the results indicate that incorporating mine tailings into UHPFRC offers significant benefits. The use of MTP and MTS not only enhances the mechanical properties of the concrete but also promotes environmental sustainability by utilizing industrial by-products. This dual benefit underscores the potential of mine tailings as a valuable component in sustainable construction practices.

Fig. 4

Compressive Strength of UHPFRC.

Indirect Tensile Strength

Figure 5 illustrates the indirect tensile strength of UHPFRC mixtures with varying contents of MTP and MTS at 56 and 90 days. The control mix (M1-Control) achieved an ITS of 15.4 MPa at 56 days and 18.7 MPa at 90 days. The inclusion of 5% MTP and 20% MTS in Mix M2-P5-S20 resulted in an ITS of 16.8 MPa at 56 days and 19.9 MPa at 90 days, indicating a percentage increase of 9.09% at 56 days and 6.42% at 90 days compared to the control. As the content of MTP and MTS increased, a consistent improvement in ITS was observed, peaking in Mix M4-P15-S60, which contained 15% MTP and 60% MTS. This mixture achieved an ITS of 19.6 MPa at 56 days and 23.1 MPa at 90 days, reflecting significant improvements of 27.27% at 56 days and 23.53% at 90 days over the control mix. This enhancement can be attributed to a synergistic interplay between the pozzolanic reactivity of MTP and the optimized particle packing facilitated by MTS, which contribute to a denser microstructure and more efficient load distribution. The initial improvements in ITS can be linked to the pozzolanic activity of MTP, which reacts with the calcium hydroxide produced during cement hydration, forming additional calcium silicate hydrate gel. This secondary hydration reaction not only increases the overall amount of C-S-H, which is the primary binding phase in concrete, but also refines the microstructure by filling pores and voids, thereby enhancing the tensile strength of the composite material.

Furthermore, incorporating MTS enhances the particle packing density within the matrix, reducing the overall porosity and contributing to a more cohesive structure. This improved packing increases the interfacial bond between the fibers and the cementitious matrix, crucial for enhancing the tensile strength. A dense and well-bonded microstructure ensures that stress is more evenly distributed across the material, improving its resistance to tensile cracking. However, beyond the optimal inclusion levels observed in Mix M4-P15-S60, further increases in MTP and MTS content led to a slight reduction in ITS, as seen in Mix M5-P20-S80 and Mix M6-P25-S100. Mix M5-P20-S80 achieved an ITS of 18.9 MPa at 56 days and 21.9 MPa at 90 days, while Mix M6-P25-S100 reached 18.1 MPa at 56 days and 21.2 MPa at 90 days. Although these values are still higher than those of the control mix, the reduction compared to Mix M4-P15-S60 suggests that excessively high contents of MTP and MTS can introduce adverse effects.

The decline in ITS at higher MTP and MTS levels may be attributed to the increased water absorption by the mine tailings, which can reduce the effective water-to-cement ratio and, consequently, the workability of the mix⁴⁴. Poor workability can lead to inadequate compaction and fiber dispersion, resulting in voids and weak zones within the matrix that detract from the overall tensile strength. Additionally, at higher contents, the rough and angular particles of mine tailings may disrupt the uniformity of the matrix, creating stress concentration points that are more susceptible to crack initiation under tensile loading⁴⁵. These observations highlight the importance of balancing the content of supplementary materials in UHPFRC. While moderate amounts of MTP and MTS can significantly enhance the tensile properties by contributing to a denser microstructure and better fiber-matrix interaction, excessive quantities may undermine these benefits by impairing workability and compaction, decreasing overall tensile performance.

Fig. 5

Indirect Tensile Strength of UHPFRC.

Modulus of rupture

Figure 6 presents the modulus of rupture of UHPFRC mixtures with varying contents of MTP and MTS at 56 and 90 days. The control mix, M1-Control, exhibits a MOR of 15.8 MPa at 56 days and 19.1 MPa at 90 days. Incorporating 5% MTP and 20% MTS in Mix M2-P5-S20 increased the MOR to 17.5 MPa at 56 days and 20.5 MPa at 90 days, reflecting a 10.76% improvement at 56 days and a 7.33% improvement at 90 days compared to the control mix. As the content of MTP and MTS increased, the MOR continued to improve, with Mix M4-P15-S60 showing the highest values. This mix achieved a MOR of 19.8 MPa at 56 days and 23.5 MPa at 90 days, representing a substantial increase of 25.32% at 56 days and 23.04% at 90 days over the control mix. These results indicate that moderate levels of MTP and MTS significantly enhance the flexural strength of UHPFRC, which can be attributed to the pozzolanic activity of MTP and the improved particle packing density provided by MTS. The enhancement in MOR can be explained through the combined effects of the pozzolanic reaction and microstructural improvements. The pozzolanic reaction of MTP contributes to forming additional calcium silicate hydrate within the matrix. This secondary hydration reaction results in a denser microstructure with fewer voids, enhancing the material’s integrity and ability to resist flexural stresses. The increased C-S-H content improves the matrix’s bond with the fibers and refines the pore structure, leading to a more compact and resilient material.

Moreover, MTP and MTS’s irregular and angular particles introduce a mechanical interlock within the cementitious matrix. This interlocking effect enhances stress transfer between the matrix and the fibers, which is crucial for resisting flexural loads. The improved interfacial bond between the fibers and the matrix reduces the likelihood of microcrack initiation. In contrast, the increased toughness of the matrix delays crack propagation, resulting in higher MOR values. The angular shape of the MTP and MTS particles also forces cracks to follow a more tortuous path, which increases the energy required for crack propagation, further contributing to the observed increases in MOR. However, higher contents of MTP and MTS, as seen in Mixes M5-P20-S80 and M6-P25-S100, showed a slight reduction in MOR compared to the peak values observed in M4-P15-S60. Mix M5-P20-S80 achieved a MOR of 19.2 MPa at 56 days and 22.1 MPa at 90 days, while Mix M6-P25-S100 reached 18.4 MPa at 56 days and 21.6 MPa at 90 days. Although these values remain higher than those of the control mix, the reduction compared to Mix M4-P15-S60 suggests that excessively high contents of MTP and MTS can lead to increased water absorption and reduced workability, which may compromise fiber dispersion and overall matrix integrity, thus slightly diminishing the flexural properties⁴⁶. The increased water absorption by the rough and porous MTP and MTS particles at higher inclusion levels can reduce the effective water-to-cement ratio, leading to proper workability and compaction challenges. This can result in incomplete fiber dispersion, creating weak zones within the matrix where stress concentrations may occur, thereby reducing the overall MOR⁴⁷. Additionally, the excessive presence of unreacted particles may act as fillers rather than contribute to the matrix’s strength, further diluting the pozzolanic reaction and particle packing benefits.

The advanced regression analysis provides a detailed understanding of the mechanical properties of UHPFRC samples over time. The analysis covers three fundamental properties: CS, ITS, and MOR at 28, 56, and 90 days. The regression analysis for compressive strength shows a clear upward trend over time for all samples (see Fig. 7a). The polynomial regression curves fit the data well, indicating a robust increase in CS from 28 to 90 days. The control sample (M1-Control) exhibits the lowest CS values throughout the testing period, starting at 124.2 MPa at 28 days and reaching 147.9 MPa at 90 days.

In contrast, the samples incorporating mine tailings show significantly higher CS values. For instance, M4-P15-S60 and M5-P20-S80 demonstrate the highest CS, reaching up to 165.2 MPa and 161.2 MPa at 90 days. This indicates that mine tailings enhance the compressive strength, likely due to the additional pozzolanic reactions and improved particle packing. The regression models (Fig. 7b) for indirect tensile strength also display a positive trend. The control sample starts at 15.4 MPa at 56 days and increases to 18.7 MPa at 90 days. Samples with mine tailings exhibit higher ITS values, with M4-P15-S60 achieving the highest ITS of 23.1 MPa at 90 days. The enhanced ITS in these samples suggests that mine tailings improve the tensile properties of the concrete, which is critical for resisting cracking and structural integrity⁴⁸. Like CS and ITS, the modulus of rupture shows an increasing trend with time (see Fig. 7c). The control sample has the lowest MOR values, starting at 15.8 MPa at 56 days and reaching 19.1 MPa at 90 days. Samples with mine tailings, particularly M4-P15-S60 and M5-P20-S80, exhibit higher MOR values, with M4-P15-S60 reaching 23.5 MPa at 90 days. This indicates that mine tailings contribute to enhanced flexural strength, essential for the material’s ability to resist deformation under load.

The regression analysis highlights the positive impact of mine tailings on the mechanical properties of UHPFRC. Including mine tailings consistently results in higher compressive strength, indirect tensile strength, and modulus of rupture compared to the control sample. These findings suggest that mine tailings enhance the sustainability of concrete by recycling industrial waste and improving its mechanical performance over time. The superior strength properties of UHPFRC with mine tailings make it a promising material for high-performance and sustainable construction applications.

Fig. 6

Modulus of Rupture of UHPFRC.

Fig. 7

Regression analysis of (a) Compressive strength, (b) Indirect tensile strength, and (c) modulus of rupture in UHPFRC samples at 90 days.

Load-deflection behavior

The load-deflection behavior of UHPFRC samples with varying percentages of MTP and MTS is illustrated in Fig. 8. The analysis of this behavior provides insight into the different mixes’ flexural performance, toughness, and ductility. According to ASTM C1609, key parameters such as peak flexural load (Pp), deflection at peak load (δp), flexural load at 0.5 mm deflection (Pδ0.5), and flexural load at 2.0 mm deflection (Pδ2.0) are critical for understanding these properties. The control mix, M1-Control, composed solely of conventional materials, serves as the baseline for comparison. This mix displays a typical load-deflection curve with a sharp peak load followed by a rapid decline, characteristic of the brittle behavior often seen in conventional UHPFRC under flexural loading. This behavior is primarily due to the dense matrix with limited crack-bridging capability, resulting in sudden failure once the peak load is reached.

In contrast, Mix M2-P5-S20, containing 5% MTP and 20% MTS, exhibits an increase in peak load and initial stiffness compared to the control mix. This improvement can be attributed to the introduction of mine tailings, which enhance the matrix’s homogeneity and particle packing. The improved packing reduces the formation of microvoids and weak points, leading to better stress distribution and higher load-bearing capacity. However, the post-peak decline is more pronounced, reflecting a less ductile response. This could be due to the limited crack-arresting capability of the mix at this stage, where the partial inclusion of mine tailings improves strength but not necessarily ductility.

As the content of MTP and MTS increases, further enhancements in load-bearing capacity and ductility are observed. Mixes M3-P10-S40 and M4-P15-S60, containing 10% and 15% MTP and 40% and 60% MTS, demonstrate superior peak loads and toughness. The broader load-deflection curves suggest that these mixes can absorb more energy before failure, indicating improved toughness. This behavior can be linked to the dual role of mine tailings in enhancing the matrix’s rigidity and providing a more effective interface with the fibers. The presence of MTP contributes to the formation of additional calcium aluminate hydrate (C-A-H) phases, which increase the matrix’s resistance to crack propagation.

Additionally, the angular nature of MTS particles may enhance the interlock with steel fibers, improving the overall fiber-matrix interaction and contributing to the material’s post-peak load-bearing capacity. In Mix M5-P20-S80, with 20% MTP and 80% MTS, one of the highest peak loads among all samples is achieved. This mix significantly improves initial stiffness and peak load, suggesting that a higher proportion of mine tailings positively influences the load-bearing capacity. The behavior of this mix can be partly attributed to the optimized pore structure, where the presence of mine tailings reduces the likelihood of interconnected pores, resulting in a denser and stronger matrix⁴⁹. However, the post-peak behavior shows a moderate decline, indicating that while the mix excels in strength, the balance between stiffness and ductility becomes more critical at higher tailings content.

Finally, Mix M6-P25-S100, formulated with 25% MTP and 100% MTS, exhibits the highest peak load, reflecting the maximum enhancement in mechanical performance due to the inclusion of mine tailings. This mix shows a rapid increase in load-bearing capacity followed by a gradual post-peak decline, highlighting its enhanced toughness and energy absorption capabilities. The improved performance can be linked to the refined microstructure created by the high content of mine tailings, which reduces porosity and enhances the distribution of stress within the matrix. However, the excessively high MTP content might lead to a reduced cementitious phase, limiting the formation of the primary strength-giving C-S-H gel and thus affecting the overall matrix strength. Moreover, the high MTS content might introduce challenges such as increased brittleness due to the dominance of inert filler material, which could explain the eventual decline in toughness after reaching the peak load^50,51. This analysis reveals the complex interplay between the proportion of mine tailings and the mechanical performance of UHPFRC. While moderate amounts of MTP and MTS enhance the matrix density, improve fiber-matrix bonding, and contribute to a balanced load-deflection response, excessive amounts may compromise these benefits by reducing the effectiveness of the cementitious matrix and introducing brittleness. Therefore, optimizing the content of mine tailings is crucial to achieving the desired balance between strength and ductility, ultimately leading to improved structural performance.

Fig. 8

Load-Deflection Profile of UHPFRC.

Durability characteristics

Sulfate attack test

The sulfate attack test results, illustrated in Fig. 9, reveal the residual compressive strength and mass loss percentage for various UHPFRC mixes after 90 days of exposure. The control mix, M1-Control, exhibited a residual compressive strength of 88.4 MPa and a mass loss of 37.9%, establishing a baseline for the behavior of conventional UHPFRC under sulfate exposure. This relatively high mass loss and moderate strength retention can be attributed to the lack of supplementary cementitious materials, which are crucial in mitigating sulfate attack by improving the matrix’s chemical stability. In Mix M2-P5-S20, where 5% MTP and 20% MTS were incorporated, the residual compressive strength increased to 94.8 MPa, and mass loss decreased to 32.1%. This improvement suggests that even a small addition of mine tailings can enhance sulfate resistance, potentially due to the increased availability of siliceous material from MTP. This additional silicate content could react with calcium hydroxide released during cement hydration, forming additional C-S-H gel, which densifies the matrix and consumes CH, reducing the likelihood of forming expansive products like ettringite under sulfate exposure⁵².

As the proportion of MTP and MTS increases in Mix M3-P10-S40, the residual compressive strength further improves to 103.5 MPa, reducing mass loss to 29.3%. This enhanced performance may be attributed to forming a more complex and chemically stable matrix, where the increased silica and alumina content from the mine tailings can participate in pozzolanic reactions that generate calcium aluminosilicate hydrate phases. These C-A-S-H phases are known for their resistance to sulfate ions, as they are less prone to forming expansive ettringite than ordinary C-S-H gels. Mix M4-P15-S60, containing 15% MTP and 60% MTS, exhibits the highest residual compressive strength of 115.2 MPa and a mass loss of 25.4%. The superior performance of this mix could be linked to the optimal synergy between the physical and chemical effects of mine tailings. Physically, the increased content of MTS contributes to a refined particle packing, which reduces the overall porosity and limits the ingress of aggressive ions. Chemically, the higher MTP content provides a richer source of alumina and silica, which might enhance the formation of stable hydration products, including C-A-S-H and less soluble monosulfate phases, thus improving the material’s resistance to sulfate attack.

However, as the content of mine tailings increases beyond this optimal point, a slight decline in performance is observed. Mix M5-P20-S80, with 20% MTP and 80% MTS, shows a residual compressive strength of 110.4 MPa and a mass loss of 28.2%. Although the strength remains high, the increased mass loss suggests that excessive MTS may introduce microstructural weaknesses. High levels of MTS could lead to interfacial transition zones with higher porosity and weaker bonding, especially around the aggregate particles. These ITZs can serve as preferential pathways for sulfate ion ingress, leading to localized deterioration and increased mass loss⁵³.

Finally, Mix M6-P25-S100, with 25% MTP and 100% MTS, displays a residual compressive strength of 107.3 MPa and a mass loss of 30.5%. The further increase in mass loss and a slight reduction in residual strength suggest that the excessive replacement of cement and natural sand with mine tailings may compromise the matrix’s integrity. The higher porosity associated with excessive MTS could facilitate the ingress of sulfate ions, while the reduced cement content limits the availability of CH, which is necessary for ongoing pozzolanic reactions. This imbalance may result in insufficient formation of protective C-S-H and C-A-S-H phases, leaving the matrix more vulnerable to sulfate attack.

Fig. 9

Behavior of UHPFRC Samples under Sulfate Attack test.

Autogenous shrinkage characteristics

The autogenous shrinkage behavior of UHPFRC samples provides a critical understanding (see Fig. 10) of how varying percentages of MTP and MTS influence shrinkage over 180 days. Autogenous shrinkage is a key factor in UHPFRC, as it directly impacts the material’s dimensional stability and long-term durability. The control mix, M1-Control, shows the highest autogenous shrinkage throughout the testing period, reaching 2017 microns at 180 days. This increased shrinkage can be attributed to the absence of supplementary materials that would help mitigate shrinkage by altering the hydration dynamics and improving the pore structure. The pure Portland cement matrix in the control mix likely leads to extensive self-desiccation, driving high shrinkage due to the rapid water consumption and formation of capillary stresses.

The introduction of MTP and MTS in the mixes shows an apparent reduction in autogenous shrinkage. Mix M2-P5-S20, with 5% MTP and 20% MTS, reduces shrinkage to 1974 microns, a 2.13% decrease compared to the control. This modest reduction can be linked to the initial buffering effect provided by MTP and MTS. These materials likely alter the hydration process, slowing down the rate at which the capillary pores empty and thus reducing the capillary tension that drives autogenous shrinkage. Additionally, fine MTP particles may act as nucleation sites for C-S-H formation, leading to a denser, more refined microstructure that resists deformation.

As the content of MTP and MTS increases in Mix M3-P10-S40, the autogenous shrinkage decreases more significantly to 1609 microns, marking a 20.21% reduction from the control. This enhanced performance can be attributed to the dual role of MTP and MTS in altering the microstructure and providing internal curing. The increased MTS content likely contributes to internal moisture reservoirs that delay self-desiccation onset, thereby mitigating the development of capillary stresses. Furthermore, the MTP’s fine particles may contribute to an increased pozzolanic reaction, consuming more calcium hydroxide and producing additional C-S-H, which further densifies the matrix and reduces shrinkage. The most significant reduction in autogenous shrinkage is observed in Mix M4-P15-S60, which incorporates 15% MTP and 60% MTS, achieving a shrinkage of just 1023 microns at 180 days, a 49.28% reduction compared to the control. This drastic reduction can be attributed to the optimal balance between the internal curing effect of MTS and the refinement of the pore structure by MTP. The higher content of MTS provides effective internal curing by releasing moisture during the later stages of hydration. It likely contributes to a more uniform distribution of shrinkage stresses across the matrix, preventing localized stress concentrations. Additionally, the substantial pozzolanic activity at this replacement level further strengthens the matrix, reducing the shrinkage potential by limiting the availability of free water that could otherwise contribute to capillary stresses⁵².

However, as the content of mine tailings increases beyond this optimal point, the trend in shrinkage begins to reverse. Mix M5-P20-S80, containing 20% MTP and 80% MTS, shows an increase in shrinkage to 1432 microns, although this value is still 29.03% lower than the control. This increase in shrinkage at higher replacement levels may be due to the excessive MTS content, which could lead to higher porosity and uneven distribution of the internal curing effect. The increased porosity might result from the higher water demand of MTS, which, if not correctly balanced, could lead to incomplete hydration of the cementitious components. This incomplete hydration could leave a greater volume of unhydrated cement particles and voids within the matrix, contributing to higher autogenous shrinkage as the unhydrated particles eventually hydrate and cause delayed shrinkage⁵⁴. Similarly, Mix M6-P25-S100, with 25% MTP and 100% MTS, experiences shrinkage of 1112 microns, a 44.86% reduction compared to the control but higher than Mix M4-P15-S60. The increase in shrinkage in Mix M6-P25-S100 could be explained by the over-reliance on the internal curing effect of MTS, which might not be sufficient to counterbalance the increased internal stresses caused by the higher porosity and reduced matrix cohesiveness. The high content of MTS may increase the microstructure’s heterogeneity, with more pronounced weak zones that are susceptible to shrinkage⁵⁵. The excessive MTP might also dilute the cement content, reducing the overall binder’s effectiveness in resisting shrinkage.

The advanced regression analysis explores the relationships between autogenous shrinkage, residual compressive strength, and mass loss in UHPFRC samples. This analysis provides critical insights into how these properties interact under sulfate attack conditions and extended testing periods. Figure 11 (a) illustrates the relationship between residual compressive strength at 90 days and autogenous shrinkage at 180 days. The regression curve (red line) reveals a clear trend: as autogenous shrinkage increases, the residual compressive strength decreases. This inverse relationship suggests that higher levels of autogenous shrinkage, which indicate more significant internal volume reduction and potential cracking, negatively impact the mechanical integrity of the UHPFRC. Specifically, samples with higher shrinkage, such as M1-Control and M2-P5-S20, exhibit lower RCS values. In contrast, samples with reduced shrinkage, like M4-P15-S60 and M6-P25-S100, maintain higher RCS values, demonstrating their better resilience to autogenous shrinkage and sulfate attack. Figure 11 (b) depicts the relationship between mass loss and autogenous shrinkage. The regression curve shows a positive correlation, where mass loss increases with higher autogenous shrinkage. This trend indicates that samples experiencing significant shrinkage are more susceptible to degradation and mass loss under sulfate attack. The control sample, with the highest shrinkage and mass loss, highlights the vulnerability of conventional concrete to sulfate-induced damage. Conversely, samples with lower autogenous shrinkage, such as M4-P15-S60 and M5-P20-S80, exhibit reduced mass loss, suggesting that mine tailings enhance the material’s durability by mitigating shrinkage and subsequent degradation.

The advanced regression analysis underscores the critical interplay between autogenous shrinkage and the durability of UHPFRC under sulfate attack. High autogenous shrinkage correlates with reduced residual compressive strength and increased mass loss, indicating compromised structural integrity and greater susceptibility to environmental degradation. The findings highlight the benefits of incorporating mine tailings into UHPFRC. Samples with higher percentages of mine tailings powder and sand, such as M4-P15-S60 and M5-P20-S80, exhibit lower autogenous shrinkage, higher residual compressive strength, and reduced mass loss, demonstrating enhanced overall performance. These insights support the potential for mine tailings to improve concrete’s long-term durability and mechanical properties, offering a sustainable and effective solution for high-performance construction materials.

Fig. 10

Autogenous Shrinkage Behavior of UHPFRC Mixtures with Varying Percentages of MTP and MTS over a 180-Day Period.

Fig. 11

Regression analysis showing the impact on (a) autogenous shrinkage on residual compressive strength and (b) mass loss in UHPFRC samples under sulfate attack.

Elevated temperature performance

The performance of UHPFRC samples under elevated temperatures, as shown in Fig. 12 (a) and (b), reveals critical insights into how varying percentages of MTP and MTS affect the residual compressive strength (RCS) and mass loss at temperatures ranging from 200 °C to 800 °C. The results indicate that while all mixes experience a decrease in RCS and an increase in mass loss with rising temperatures, the degree of these changes varies significantly based on the composition of the mixes.

At 200 °C, the control mix (M1-Control) shows an RCS of 139.4 MPa, slightly lower than the RCS of 160.7 MPa observed in Mix M4-P15-S60. This 15.3% improvement in RCS can be attributed to the role of MTP and MTS in enhancing the initial thermal resistance of the concrete. The introduction of MTP and MTS likely alters the thermal expansion characteristics of the concrete matrix, reducing the differential thermal stresses between the matrix and the fibers. This uniform thermal expansion minimizes the development of microcracks, thereby maintaining a higher RCS. At 400 °C, the control mix records an RCS of 118.1 MPa, whereas M4-P15-S60 maintains a higher RCS of 141.6 MPa, indicating a 19.9% improvement. The enhanced performance at this temperature can be linked to the thermal stability the optimized mix of MTP and MTS provides. The MTS, with its relatively high melting point, may act as a thermal barrier, slowing down the degradation process within the matrix. Additionally, the MTP might contribute to a more gradual decomposition of hydration products, reducing the abrupt loss of strength typically associated with elevated temperatures. When the temperature reaches 600 °C, the control mix’s RCS drops significantly to 81.4 MPa, while M4-P15-S60 shows a higher RCS of 95.3 MPa, a 17.1% increase. At this stage, the protective role of MTS becomes more evident. The high content of MTS may contribute to maintaining the structural framework of the matrix, even as the cementitious components begin to decompose. The silica-rich MTS might also participate in forming thermally stable phases that resist further degradation, thus sustaining the mix’s compressive strength. At the extreme temperature of 800 °C, the control mix’s RCS plummets to 34.2 MPa, while M4-P15-S60 achieves 45.2 MPa, a substantial 32.2% increase. The enhanced performance at this temperature can be attributed to the synergistic effects of MTP and MTS in delaying the thermal degradation of the matrix⁵⁶. The pozzolanic reaction of MTP, even at elevated temperatures, might contribute to the formation of high-temperature phases such as wollastonite or gehlenite, which enhance the residual strength of the concrete. Furthermore, the high thermal inertia provided by the combined effects of MTP and MTS may slow down the rate of heat transfer through the concrete, reducing the extent of thermal damage.

In terms of mass loss, the control mix shows a 10.3% loss at 200 °C, while M4-P15-S60 has the lowest mass loss at 7.4%, indicating a 28.2% reduction. This lower mass loss can be linked to MTP and MTS’s ability to retain more hydration products, even at higher temperatures. The presence of MTS might reduce the volatility of the matrix, limiting the evaporation of chemically bound water, which in turn helps maintain the mass of the concrete. At 400 °C, the mass loss for the control mix rises to 23.1%, while M4-P15-S60 shows a significantly lower loss of 15.7%, a 32.0% reduction. This improved performance is likely due to the enhanced microstructural stability imparted by MTP and MTS. The high silica content in MTP and MTS may form a glassy phase within the matrix, encapsulating the hydration products and reducing their volatilization at elevated temperatures. As the temperature reaches 600 °C, the control mix exhibits a 44.8% mass loss, while M4-P15-S60 maintains a lower loss of 31.4%, a 29.9% reduction. At this stage, the ability of MTP and MTS to mitigate the thermal decomposition of the matrix is crucial. The MTS particles, acting as micro-aggregates, may prevent the combination of pores, thus limiting the escape routes for volatile components and reducing mass loss. Finally, at 800 °C, the mass loss peaks, with the control mix showing a 71.3% loss and M4-P15-S60 at 57.2%, a 19.8% reduction. Even under extreme conditions, the optimized mix outperforms the control, suggesting that MTP and MTS contribute to a more thermally resilient matrix. The reduced mass loss at this temperature could be due to the formation of thermally stable phases within the matrix, which act as a protective barrier against the complete breakdown of the material⁵⁷.

Overall, the improved performance of UHPFRC mixes with MTP and MTS at elevated temperatures can be attributed to several factors beyond the filler effect and pozzolanic activity. These include the modification of thermal expansion characteristics, the formation of high-temperature resistant phases, and the ability to delay the thermal degradation process. However, as seen in mixes with higher contents of MTP and MTS, such as M5-P20-S80 and M6-P25-S100, excessive amounts may introduce higher internal stresses and porosity, slightly diminishing thermal resistance compared to the optimal mix (M4-P15-S60). This indicates that while MTP and MTS can significantly enhance thermal performance, their proportions must be carefully balanced to optimize the material’s resistance to elevated temperatures.

The advanced regression analysis performed on the residual compressive strength and mass loss of UHPFRC samples under elevated temperatures provides valuable insights into their thermal performance. The polynomial regression models effectively capture the relationship between temperature and these properties, revealing significant trends. As illustrated in the upper row of Fig. 13, the regression models for mass loss demonstrate a consistent increase in mass loss with rising temperatures across all UHPFRC samples. The polynomial curves fit the data points well, indicating a strong correlation between temperature and mass loss. The control sample, composed entirely of conventional materials, shows the highest mass loss at all temperature levels. At 800 °C, the mass loss reaches over 70%, reflecting significant thermal degradation and material breakdown. This trend underscores the susceptibility of traditional concrete to high temperatures. In contrast, samples incorporating mine tailings, such as M2-P5-S20 to M6-P25-S100, exhibit markedly lower mass loss percentages. For instance, the M4-P15-S60 and M5-P20-S80 mixes, which include higher proportions of mine tailings powder and sand, display reduced mass losses, particularly at higher temperatures. The regression analysis indicates that adding mine tailings enhances the thermal stability of the concrete, likely due to the improved packing density and the pozzolanic reactions that contribute to a firmer microstructure. These findings suggest that mine tailings act as stabilizers, reducing the material’s vulnerability to thermal degradation.

The lower row of Fig. 13 presents the regression models for RCS, showing an apparent decline in compressive strength with increasing temperature for all samples. The polynomial curves accurately reflect the data, highlighting the thermal sensitivity of the material’s mechanical properties. The control sample again shows a significant drop in RCS, with values plummeting from 139.4 MPa at 200 °C to 34.2 MPa at 800 °C. This steep decline illustrates the weakening of conventional concrete under thermal stress. Samples with mine tailings (M2-P5-S20 to M6-P25-S100) demonstrate higher RCS values across all temperatures than the control. Mixes like M4-P15-S60 and M5-P20-S80 maintain relatively high RCS values even at 800 °C, indicating superior mechanical resilience. The regression curves suggest that the enhanced performance is due to the reinforcing effects of mine tailings, which help preserve the structural integrity of the concrete at elevated temperatures. The pozzolanic activity of mine tailings likely contributes to forming additional C-S-H gel, improving the concrete’s binding properties and thermal resistance.

Fig. 12

(a) Residual Compressive Strength (MPa) of UHPFRC Samples under Elevated Temperature, (b) Mass loss (%) in UHPFRC Samples under Elevated Temperature.

Fig. 13

Regression analysis showing the effect of autogenous shrinkage on residual compressive strength and mass loss in UHPFRC samples under elevated temperatures.

Toxicity characteristic leaching

The Toxicity Characteristic Leaching Procedure (TCLP), following the U.S. EPA Method 1311, was employed to assess the potential leaching of hazardous substances from UHPFRC samples modified with MTP and MTS. Figure 14 presents the concentrations of Mn, Ba, Cd, Cu, and As ions in the leachate extracted from six different UHPFRC samples, including the control (M1-Control) and samples with increasing levels of MTP and MTS. The control sample exhibited the lowest concentrations of all measured ions, with Mn and Ba levels at 0.0023 mg/L and 0.0123 mg/L, respectively, and Cd, Cu, and As levels at 0.0001 mg/L, 0.0004 mg/L, and 0.0003 mg/L, respectively. As the percentage of mine tailings increased, there was a corresponding rise in the concentration of these heavy metal ions in the leachate. For instance, the M2-P5-S20 sample, which contains 5% MTP and 20% MTS, showed Mn and Ba concentrations of 0.0041 mg/L and 0.0345 mg/L, respectively. The Cd, Cu, and As levels increased slightly to 0.0002 mg/L, 0.0007 mg/L, and 0.0005 mg/L, respectively. This trend continued with higher mine tailings content, culminating in the M6-P25-S100 sample, with 25% MTP and 100% MTS, exhibiting Mn and Ba concentrations of 0.0157 mg/L and 0.1123 mg/L, along with Cd, Cu, and As levels of 0.0009 mg/L, 0.0022 mg/L, and 0.0015 mg/L, respectively. Despite these increases, all measured concentrations remained well below the EPA regulatory limits, which are 5 mg/L for Mn, 100 mg/L for Ba, 0.01 mg/L for Cd and As, and 1.3 mg/L for Cu.

These findings indicate that even at higher substitution levels, the UHPFRC matrix effectively immobilizes potentially hazardous ions, thereby preventing significant leaching. This immobilization can be attributed to several key factors related to the chemistry and microstructure of the UHPFRC matrix when modified with MTP and MTS. Firstly, the chemical composition of mine tailings contributes significantly to the retention of heavy metals within the matrix. MTP and MTS often contain alumina, silica, and other oxides that can participate in the formation of stable secondary phases during cement hydration. These phases, such as ettringite and monosulfate, can incorporate heavy metals into their crystal structure through ion substitution, immobilizing these elements and preventing their release into the environment. Secondly, the microstructure of UHPFRC plays a crucial role in reducing leaching potential. The dense packing of particles, enhanced by the fine nature of MTP and MTS, leads to a matrix with very low permeability⁵⁸. This low permeability significantly restricts the movement of water and dissolved ions within the concrete, effectively trapping heavy metals within the matrix and preventing them from being leached out under simulated landfill conditions. The high level of compaction and reduced porosity in UHPFRC further contribute to this immobilization by limiting the connectivity of pores, which reduces the pathways available for ion migration⁵⁹.

To gain a deeper understanding of the environmental impact of using mine tailings in UHPFRC, it is crucial to investigate the relationship between the elemental composition of mine tailings (MTP and MTS) and the corresponding elemental composition of the leachate. The chemical elements present in MTP and MTS, including silicon, aluminum, iron, magnesium, calcium, and other trace metals, are potential contributors to the leachate composition when exposed to environmental conditions such as water infiltration or contact with acidic or basic solutions. Based on the XRF analysis of MTP and MTS, the significant elements detected include silicon (Si), aluminum (Al), calcium (Ca), and iron (Fe), with trace amounts of heavy metals such as lead (Pb), arsenic (As), and cadmium (Cd). These elements can leach into the surrounding environment, depending on factors such as pH, temperature, and the duration of exposure. The leachate composition was analyzed for the same set of elements, and a correlation was observed between the concentration of certain heavy metals in the leachate and the presence of these metals in the mine tailings⁶⁰. Specifically, higher concentrations of trace metals like lead, arsenic, and cadmium were detected in the leachate of mixes with higher MTP content, indicating that their chemical composition influences the leaching behavior of mine tailings. The leachate analysis results suggest that more stable elements, such as calcium and magnesium, exhibited lower concentrations. In contrast, heavier metals like lead and arsenic showed higher levels in the leachate, mainly when the mine tailings content was higher.

This correlation highlights the importance of understanding the chemical interactions between mine tailings and the cementitious matrix and the potential risks associated with releasing toxic substances into the environment⁶¹. The findings suggest that while MTP and MTS can provide beneficial properties to UHPFRC, particularly in terms of durability and sustainability, careful consideration of the leaching potential of heavy metals is essential for the safe use of mine tailings in construction materials.

Moreover, when combined with cement hydration products, the high pozzolanic activity of MTP leads to the formation of additional calcium silicate hydrate gel. This gel not only enhances the mechanical properties of the matrix but also provides additional sites for the adsorption of heavy metals, further immobilizing them within the matrix. The C-S-H gel can bind heavy metals through physical adsorption, ion exchange, and chemical bonding, significantly reducing the leaching potential. The gradual increase in ion concentrations observed with higher levels of MTP and MTS can be explained by the saturation of the binding sites within the matrix. As more mine tailings are added, the availability of free ions increases, and although the matrix can immobilize a significant portion, a small fraction may remain unbound and thus susceptible to leaching⁶². However, even at the highest substitution levels tested, the concentration of leached ions remains well within safe limits, highlighting the effectiveness of the UHPFRC matrix in controlling heavy metal release. Furthermore, the stability of these heavy metal-containing phases under varying environmental conditions is also a key factor. The durability of the UHPFRC matrix ensures that these phases remain intact over time, even in aggressive environments⁶³. The resistance of the matrix to chemical attack, combined with its low permeability, prevents the dissolution of these phases, thereby maintaining the immobilization of heavy metals.

Fig. 14

Concentration of Mn and Ba ions in leachate from UHPFRC samples.

Microstructural characteristics

Mercury Intrusion Porosimetry

Figure 15 illustrates the pore size distribution of UHPFRC samples under different conditions, highlighting the significant impact of incorporating MTP and MTS on the material’s porosity. The control mix, M1-Control, composed entirely of conventional materials without mine tailings, exhibited the lowest total mercury intrusion across the pore size spectrum. This finding suggests a highly compact and dense microstructure with minimal pore volume, serving as a benchmark for evaluating the effects of mine tailings on the microstructure of UHPFRC. In contrast, the M2-P5-S20 mix, containing 5% MTP and 20% MTS, showed the highest mercury intrusion, particularly in the larger pore diameter range (> 0.1 microns). This increased intrusion indicates a more porous structure, potentially due to the introduction of irregularly shaped MTP and MTS particles, which could disrupt the uniform packing of the cementitious matrix. These irregular particles might prevent the formation of a tightly packed microstructure, creating larger voids and interstitial spaces that increase overall porosity.

As the percentage of mine tailings increased, the M4-P15-S60 mix, with 15% MTP and 60% MTS, demonstrated an intermediate behavior in terms of pore size distribution. While this mix closely mirrored the control mix at smaller pore diameters (< 0.1 microns), it diverged at larger pore sizes. This behavior suggests that smaller pores are somewhat controlled or filled by the finer particles of MTP; the larger pores are less influenced by the exact mechanism, potentially due to insufficient filler effect or incomplete pozzolanic reactions in the larger voids⁶⁴. The larger pores might remain partially unfilled due to the lower reactivity of MTS at this higher concentration, leading to a moderate increase in porosity. The M5-P20-S80 mix, with 20% MTP and 80% MTS, consistently increased mercury intrusion across various pore sizes, indicating a more uniform porosity enhancement throughout the material. This could be attributed to the cumulative effects of MTP and MTS at higher contents. The higher concentration of MTS could increase microcracks or interparticle voids within the matrix as the tailings disrupt the continuity of the cementitious phases⁶⁵. Additionally, at this replacement level, the cement matrix may be less able to effectively encapsulate the tailings particles, leading to a more interconnected and porous structure.

The increased pore volume observed in mixes with higher contents of mine tailings has significant implications for the material’s overall performance. For instance, larger pore sizes could improve properties such as permeability and frost resistance by providing pathways for water movement and reducing internal stress during freeze-thaw cycles. However, they could also detract from mechanical strength and durability. The larger and more interconnected pores could act as stress concentration sites, leading to the initiation and propagation of cracks under mechanical loading, thereby reducing the overall structural integrity of the material. Moreover, the increased porosity might also affect the long-term durability of UHPFRC by enhancing the ingress of harmful agents such as chlorides and sulfates. This could accelerate deterioration processes, such as corrosion of embedded steel fibers or sulfate attack, particularly in aggressive environmental conditions. Therefore, while incorporating mine tailings in UHPFRC offers significant environmental benefits by recycling industrial waste and reducing reliance on natural resources, it is crucial to carefully balance these benefits against potential trade-offs in material performance.

Fig. 15

Pore Size Distribution of UHPFRC Samples.

Heat of Hydration Analysis

Figure 16 illustrates the heat flow over time for different UHPFRC samples during their hydration process, offering insights into the respective concrete mixes’ thermal behavior and hydration kinetics. The M1-Control sample, composed of conventional materials without mine tailings, reaches the highest peak heat flow, or Tmax, at approximately 30 h. This early and sharp peak reflects the rapid exothermic reactions typical of a mix with a high cement content, where the hydration process is intense and concentrated. The quick heat release in the control mix suggests that the hydration products, such as calcium silicate hydrate and calcium hydroxide, form rapidly, leading to a swift build-up of internal temperature. The M2-P5-S20 sample, which includes 5% mine tailings powder and 20% mine tailings sand, demonstrates a slightly lower peak heat flow, with Tmax occurring around 32 h. This shift indicates that the introduction of mine tailings alters the hydration kinetics by acting as a thermal buffer. The mine tailings may absorb some initial heat released during hydration, moderating the temperature rise and slowing down the hydration rate. Additionally, the filler effect of mine tailings likely reduces the effective cement content per unit volume, leading to a less vigorous hydration reaction and, consequently, a lower and delayed heat peak. As the percentage of mine tailings increases in the M3-P10-S40 sample, which contains 10% mine tailings powder and 40% mine tailings sand, the peak heat flow decreases further, and Tmax is delayed to around 34 h. This trend suggests that mine tailings moderates the hydration process and alters the microstructure development by introducing additional phases that absorb and redistribute heat more efficiently. The increased proportion of mine tailings may lead to the formation of a more heterogeneous matrix, where the hydration reactions are spread over a longer period due to the lower reactivity of the tailings compared to pure cement. This results in a more sustained release of heat, which can be beneficial for reducing early-age thermal stresses that could otherwise lead to cracking.

The M4-P15-S60 sample, with 15% mine tailings powder and 60% mine tailings sand, exhibits the lowest peak heat flow and the most delayed Tmax at approximately 36 h. The significant reduction in heat flow and the extended time to reach Tmax indicate that the high content of mine tailings heavily influences the hydration process. This could be attributed to mine tailings, particularly at higher contents, which may act as nucleation sites for forming C-S-H gel, which initially consumes heat but later contributes to a more stable and refined microstructure. The delayed heat release suggests that the hydration products form more slowly and uniformly, which could enhance the internal curing of the concrete, leading to improved long-term durability and reduced risk of thermal cracking. The analysis shows that incorporating mine tailings into UHPFRC significantly affects the concrete’s hydration kinetics and thermal behavior. As the content of mine tailings increases, the peak heat flows decrease, and Tmax values are delayed, indicating a more controlled and extended hydration process. This modulation of the hydration reaction can be particularly beneficial in reducing the thermal gradients within the concrete, thereby minimizing the risk of early-age cracking, which is often associated with rapid temperature increases⁶⁶. Furthermore, the gradual heat release aligns with the microstructure’s progressive development, where the mine tailings’ pozzolanic activity contributes to the formation of additional C-S-H and other stable hydration products over time. This sustained hydration process not only enhances the thermal stability of the concrete during curing but also contributes to its long-term mechanical strength and durability.

Moreover, the delayed heat evolution observed in mixes with higher mine tailings content could also reflect a shift in the dominant hydration mechanisms⁶⁷. With a reduced cement content, the formation of secondary hydration products such as calcium aluminate hydrate may become more prominent, which typically evolves at a slower rate but contributes to the overall stability and performance of the concrete. This shift could lead to a more refined pore structure and improved resistance to harmful processes such as sulfate attack and carbonation.

Fig. 16

Heat flow over time during hydration for UHPFRC samples with varying mine tailings content.

X-ray diffraction analysis

The XRD patterns shown in Fig. 17 comprehensively analyze the phase composition and crystalline structures of six different UHPFRC samples. These samples vary in their content of MTP and MTS, ranging from the control mix (M1-Control) with no mine tailings to M6-P25-S100, which includes 25% MTP and 100% MTS. The M1-Control sample exhibits the highest intensity peaks at fundamental 2θ values around 15°, 30°, and 50°, indicating a robust and well-crystallized structure typical of conventional UHPFRC. The dominant crystalline phases, such as portlandite (Ca(OH)₂) and calcium silicate hydrates, are primarily responsible for the strength and durability of the concrete. The strong peaks suggest that the hydration process in the control mix is fully developed, leading to a highly organized crystalline structure that contributes to the material’s mechanical properties.

As mine tailings are introduced in the M2-P5-S20 sample, the peak intensities decrease slightly. This reduction, although minor, indicates that the inclusion of mine tailings starts to interfere with the crystallization process of the hydration products. The presence of 5% MTP and 20% MTS likely introduces impurities or additional phases that disrupt the formation of well-defined crystalline structures. These disruptions could be due to amorphous materials within the mine tailings, which do not contribute to crystalline phase formation but lead to a more complex and heterogeneous microstructure. The M3-P10-S40 sample, containing 10% MTP and 40% MTS, shows a further decrease in peak intensities. This trend suggests a more significant alteration in the phase composition, where the increased content of mine tailings begins to dominate the microstructural development. The reduction in crystalline phases could be linked to the dilution effect, where the higher proportion of mine tailings reduces the availability of pure cement phases necessary for forming strong crystalline structures like C-S-H and portlandite. Additionally, the increased presence of non-crystalline or poorly crystalline phases within the mine tailings could interfere with the matrix’s overall crystallinity.

In the M4-P15-S60 sample, the peak intensities continue to decline, indicating an even greater disruption of the crystalline structure. At this substitution level, the mine tailings likely contribute to a more pronounced pozzolanic reaction, where the silica and alumina from the tailings react with calcium hydroxide to form additional C-S-H phases. However, these newly formed phases may be less crystalline and more amorphous, reducing peak intensities. The increasing amorphous content within the mix suggests that the mine tailings are not just acting as inert fillers but are actively participating in the chemical processes during hydration, altering the balance between crystalline and amorphous phases⁶⁸. The M5-P20-S80 sample exhibits significantly reduced peak intensities, particularly at the fundamental 2θ values. This indicates a substantial alteration in the phase composition, where the high content of mine tailings leads to a further reduction in the crystalline nature of the hydration products. The reduced peak intensities may reflect the formation of a more amorphous matrix, where the mine tailings hinder the development of well-defined crystalline phases like portlandite and calcium silicate hydrates. The amorphous materials within the mine tailings and the lower cement content may dilute the crystalline components and lead to a less ordered structure^69,70. Finally, the M6-P25-S100 sample shows the lowest peak intensities among all the samples, indicating a significant shift towards an amorphous structure. The high content of mine tailings (25% MTP and 100% MTS) appears to have a pronounced impact on the crystalline structure, leading to a matrix dominated by non-crystalline or poorly crystalline phases. This suggests that at high levels of mine tailings substitution, the balance between crystalline and amorphous phases is heavily skewed towards the latter, potentially affecting the material’s mechanical properties and durability. The diminished peaks indicate that the mine tailings heavily influence the hydration process, resulting in a less crystalline structure and more heterogeneous.

Overall, the XRD patterns reveal that increasing the content of mine tailings in UHPFRC results in a clear trend of decreasing peak intensities, indicating a transition from a well-crystallized structure to a more amorphous one. This transition suggests that mine tailings affect the hydration process and play a significant role in determining the concrete’s phase composition and microstructural characteristics. The interaction between the mine tailings and the cement matrix leads to the formation of less crystalline phases, which could affect the material’s mechanical performance and long-term durability.

Fig. 17

XRD patterns showing the phase composition of UHPFRC samples with varying mine tailings content.

Practical implications and challenges of Incorporating Mine tailings in UHPFRC

Incorporating mine tailings into UHPFRC offers notable practical benefits, particularly in cost reduction and enhanced material properties. Although a formal cost-benefit analysis was not conducted in this study, using mine tailings to replace cement and natural aggregates partially can lead to significant material cost savings. By reducing the reliance on conventional raw materials, mine tailings can contribute to lowering the overall production cost of UHPFRC. Additionally, improved mechanical properties and durability, particularly in terms of sulfate resistance and reduced shrinkage, could result in long-term cost savings due to decreased maintenance requirements and extended service life for infrastructure, particularly in aggressive environmental conditions.

From an environmental perspective, utilizing mine tailings as a supplementary cementitious material contributes to a more sustainable construction practice by reducing the carbon footprint associated with cement production. Cement manufacturing is responsible for a significant amount of CO₂ emissions, and by replacing part of the cement with mine tailings, the environmental impact of UHPFRC can be minimized. Furthermore, repurposing mine tailings helps to address the environmental challenge of mine waste disposal, which has traditionally been a source of environmental pollution.

However, the practical implementation of mine tailings in UHPFRC faces several challenges. One key issue is the variability in the chemical composition of mine tailings from different mining operations, which can affect the consistency and performance of the concrete. This variation in composition may influence factors such as the pozzolanic activity, sulfate resistance, and shrinkage behavior of the concrete. To ensure consistent quality, mine tailings must undergo thorough characterization and processing before being used in UHPFRC. Additionally, there is a potential risk of contaminants, such as heavy metals, in the mine tailings that could adversely affect the environmental safety of the concrete. Therefore, it is critical to assess the purity and composition of mine tailings to ensure their suitability for use in concrete without compromising the health and safety of workers and end users.

Conclusions

This study comprehensively examined the effects of incorporating MTP and mine tailings sand MTS into UHPFRC. The research focused on various properties, including workability, mechanical performance, sulfate attack resistance, autogenous shrinkage, elevated temperature performance, microstructural characteristics through MIP, heat of hydration, and phase composition via XRD analysis. The findings provide critical insights into the benefits and challenges of using mine tailings in UHPFRC, offering valuable guidance for optimizing mix designs to achieve sustainability and enhanced performance.

• Incorporating up to 25% MTP and 100% MTS in UHPFRC reduced flow spread by 19.34%, but optimal mixes like M4-P15-S60 maintained sufficient workability (> 180 mm flow) and balanced mechanical properties.

• The optimal mix (M4-P15-S60) with 15% MTP and 60% MTS achieved a 12.49% increase in compressive strength (165.2 MPa), with improvements in indirect tensile strength (23.1 MPa) and modulus of rupture (23.5 MPa).

• Excessive MTP and MTS reduced workability and fiber dispersion, but moderate levels, such as in M4-P15-S60, ensured practical application suitability by maintaining workability while enhancing strength.

• Utilizing mine tailings promotes sustainability by recycling industrial waste, potentially reducing raw material costs by up to 20%.

• The optimal mix (M4-P15-S60) enhanced sulfate resistance, achieving the highest residual compressive strength (115.2 MPa) and lowest mass loss (25.4%) after sulfate exposure.

• Autogenous shrinkage was reduced by up to 49.28% in mix M4-P15-S60, attributed to the internal curing effects and refined pore structure from mine tailings.

• M4-P15-S60 exhibited improved thermal stability, retaining 45.2 MPa at 800 °C (32.2% increase over the control) and reducing mass loss by 19.8%, indicating a more resilient matrix.

• Mn, Ba, Cd, Cu, and As leaching in UHPFRC with MTP and MTS remained within safe limits, demonstrating the environmental feasibility of mine tailings.

• Increased mine tailings content raised porosity, with total intrusion volumes rising from 0.03 to 0.05 mL/gm, indicating the need for careful optimization to balance porosity and performance.

• MTP and MTS reduced peak heat flow by up to 30% and delayed Tmax from 30 to 36 h, resulting in a more controlled hydration process that could reduce thermal stresses and improve durability.

• Higher mine tailings content decreased crystalline phase intensity by up to 40% and shifted the structure towards more amorphous phases, impacting the mechanical properties and durability of UHPFRC.