Effect of blending GGBS and silica fume on the mechanical properties of geopolymer concrete

Abstract

This study investigates the mechanical properties of geopolymer concrete made with ground granulated blast furnace slag (GGBS) and silica fume (SF) as binders. The influence of varying binder proportions and sodium silicate-to-sodium hydroxide (SS-to-SH) ratios of 1.5 and 2.0 in the alkali-activated solution was examined. Experimental tests evaluated slump, compressive strength, modulus of elasticity, and splitting tensile strength at 1, 7, and 28 days. Increasing SF content up to 50% in the binder with a solution ratio of 1.5 improved the 28-day compressive strength by 50% compared to mixes made solely with slag. However, further increase in SF reduced splitting tensile strength and compressive strength by 79 and 56%, respectively, at 28 days. Increasing the solution ratio from 1.5 to 2.0 enhanced compressive strength for slag-dominant mixes by up to 63% but reduced strength for SF-rich mixes by up to 87%. The highest modulus of elasticity, 18.7 GPa, was achieved with slag-only binders and a solution ratio of 2.0, marking a 240% increase over its counterpart mix with a lower solution ratio. Equal GGBS and SF blends improved splitting tensile strength compared to SF-rich mixes but were surpassed by GGBS-rich mixes in terms of overall structural performance.

Introduction

Concrete is one of the most widely used construction materials globally, with consumption rates increasing to meet the growing demand for infrastructure and urban development¹. However, its primary component, cement, is produced through energy-intensive processes that utilize non-renewable resources and emit substantial quantities of greenhouse gases. Cement production reached 3.27 billion metric tons in 2020 and is projected to rise to 4.4 billion metric tons by 2030, with China, India, and the United States collectively contributing over 80% of global output². Each ton of cement emits nearly an equivalent ton of CO₂, accounting for 5–7% of global anthropogenic emissions, and this figure could rise to 10% in the near future³. These emissions significantly contribute to climate change, leading to severe weather events such as storms, floods, and droughts^4,5. Geopolymer technology has emerged as a promising alternative to traditional cement-based concrete, offering an environmentally friendly solution that can drastically reduce CO₂ emissions while maintaining or even enhancing mechanical performance.

Geopolymer concrete replaces cement with binders made from aluminosilicate precursors activated by alkaline solutions. Common precursors include fly ash, metakaolin, rice husk ash, Silica Fume (SF), and granulated blast furnace slag (slag). The mechanical performance of geopolymer concrete depends on several factors, including the chemical composition of the precursor, the alkaline activator ratio, curing conditions, and the interactions between materials⁶. Among these precursors, SF and slag have garnered attention due to their unique chemical properties and their ability to form gels that contribute to the strength and durability of geopolymer concrete.

SF, a byproduct of silicon and ferrosilicon alloy production, is characterized by its ultrafine particle size and high amorphous silica content, making it highly reactive in alkali-activation processes. Studies have demonstrated its potential to act as a primary binder in geopolymer concrete, primarily forming sodium aluminosilicate hydrate (N-A-S–H) gels that densify the concrete matrix. For instance, Zannerni et al.⁷ reported that SF-based geopolymer concrete achieved compressive strengths of up to 65 MPa, a 20–30% improvement over fly ash-based systems, due to the formation of a dense microstructure with minimal porosity. This densification enhances not only compressive strength but also resistance to chemical attack and permeability, contributing to the long-term durability of the concrete. However, SF’s high surface area increases water demand, which can reduce workability if not addressed through optimized mix designs⁸. Additionally, studies by Payakaniti et al.⁹ highlighted that SF-based geopolymer pastes exhibit excellent thermal resistance up to 200°C, but their performance significantly degrades at higher temperatures. In contrast, slag, a byproduct of the steel industry, is a calcium-rich material that has been extensively studied as a primary binder for geopolymer concrete. Upon alkali activation, slag predominantly forms calcium silicate hydrate (C-S–H) and calcium aluminum silicate hydrate (C-A-S–H) gels, which provide rapid early strength development. Kumar et al.¹⁰ found that slag-based geopolymer concrete achieved compressive strengths exceeding 60 MPa within 7 days, with strengths often surpassing 70 MPa at 28 days under ambient curing conditions. Bellum et al.¹¹ (2018) reported that curing slag-based geopolymer concrete at elevated temperatures (70°C) with a 14 M sodium hydroxide solution further enhanced early strength, achieving over 60 MPa within 24 h. Slag-based systems are also noted for their high resistance to acid attack and their ability to maintain structural integrity under harsh environmental conditions. However, these benefits come with trade-offs, as slag-based mixes may exhibit increased shrinkage and creep under prolonged exposure to high temperatures¹².

While both SF and slag have been studied extensively as individual binders, their combined use as co-binders in geopolymer concrete remains relatively underexplored. Preliminary findings suggest that combining slag and SF can lead to hybrid systems where N-A-S–H and C-S–H gels coexist, potentially enhancing both early and long-term mechanical performance. Zannerni et al.⁷ demonstrated that incorporating 10–15% SF into a slag-based geopolymer matrix increased compressive strength by 15–20%, achieving values of up to 72 MPa. This improvement was attributed to the synergistic effects of the hybrid gel structure, which increased matrix density and reduced microcracking. Similarly, Hadi et al.¹³ observed that SF improved the workability and setting time of slag-based geopolymer concrete, although compressive strength was slightly reduced compared to slag-only systems. SF and slag demonstrate significant potential as primary and complementary binders in geopolymer concrete, offering strength and durability enhancements while addressing the environmental challenges posed by traditional cement. SF excels in enhancing long-term durability and reducing porosity, achieving compressive strengths of up to 65 MPa, while slag provides rapid early strength development, often exceeding 70 MPa at 28 days^8,14. Increasing the SS-to-SH ratio in the activator solution further enhances compressive strength, with values exceeding 80 MPa, albeit at the cost of reduced workability¹⁵. The combination of slag and SF as co-binders introduces a synergistic effect, leveraging the strengths of both materials and resulting in hybrid gel structures that improve overall performance¹⁶. However, the existing literature, including the work of Zannerni et al.⁷ who utilized sodium hydroxide as the sole activator in SF and GGBS- and FA-based geopolymer mixes, where the SF inclusion was caped to 15%, suggests that the combined influence of SF in slag-geopolymer with varying SS-to-SH ratios remained unexplored.

The trends in Table 1 demonstrate the influence of SF in different geopolymer concrete systems, particularly regarding the average change in workability (∆Slump), compressive strength (∆f’c), and splitting tensile strength (∆fsp). Singh et al.¹⁴ confirmed that increasing SF content up to 20% in FA/GGBS blended geopolymer concrete mixture enhances compressive strength by densifying the matrix, though this improvement is offset by a reduction in workability due to SF’s high surface area and increased water demand​. Sundararaj et al.¹⁷ further emphasized that SF inclusion in FA/GGBS blended geopolymer system, significantly increases compressive strength, with a 5% SF mix showing the highest performance, though workability issues became evident at higher dosages of SF. This trend is consistent with Venkat et al.¹⁸, who found that SF incorporation by 10% in cement-based concrete mixtures improved split tensile strength, further addition of SF resulted in diminishing returns set in due to reduced flowability and compaction issues. The role of alkali activation is also pivotal, as Zannerni et al.⁷ demonstrated that single-activator (NaOH) systems yielded lower strength improvements compared to hybrid alkali solutions, indicating the importance of optimizing the activator composition to maximize SF and slag reactivity. Eltantawi et al.¹⁹ corroborated these findings, showing that increasing the sodium silicate-to-sodium hydroxide (SS-to-SH) ratio in geopolymer mix made of ternary binder of SF, FA, and GGBS, where SF inclusion ranged between 5–15%, improved strength but at the cost of workability, reinforcing the need for balanced activator proportions​. Similarly, Hamed et al.²⁰ confirmed that SF inclusion up to 30% from the total binder in an FA-based geopolymer system enhances compressive strength due to its pore-filling ability but reported that SF beyond 30% leads to polymerization inefficiencies and reduced tensile capacity​. The importance of hybrid binder compositions was also highlighted by Limbachiya et al.²¹, who found that binary blends of SF and GGBS at 10–15% and 25–30%, respectively, achieved optimal compressive strength and durability, underscoring the necessity of proportion control​. Mohamed et al.²² demonstrated that in ultra-high-performance concrete, 15% SF yielded the highest strength due to its pozzolanic reaction with calcium hydroxide, further emphasizing the micro-filler effect of SF in dense matrix formation. Mohan et al.²³ reported similar trends, with SF inclusion up to 10% improving overall mechanical properties but diminishing performance at higher levels due to excessive fineness affecting dispersion and compaction​. Premkumar et al.²⁴ reinforced the notion that SF reduces workability across all mixes while improving compressive strength up to 40% SF, though at 60% SF, excessive fineness led to microstructural inconsistencies and reduced strength gains​.

Table 1 Influence of SF incorporation on the mechanical properties of geopolymer concrete mixtures across various studies.

The ratio of sodium silicate (SS) to sodium hydroxide (SH) in the alkaline activator significantly influences the geopolymerization process and the resulting mechanical properties of geopolymer concrete. Higher SS-to-SH ratios generally enhance compressive strength by providing additional silicate ions that facilitate stronger gel formation and cross-linking. El-Hassan et al.²⁵ reported that increasing the SS-to-SH ratio from 1:2 to 2:1 improved compressive strength by 25–30%, with strengths exceeding 80 MPa. However, higher SS content can increase the viscosity of the activator solution, reducing workability and making the mix more difficult to handle without adjustments in water content or admixtures. Conversely, lower SS-to-SH ratios improve workability but may compromise compressive strength due to insufficient gel crosslinking and lower matrix density^16,26. Optimizing the SS-to-SH ratio is therefore critical to achieving a balance between strength and workability.

These findings collectively highlight the research gap concerning the mechanical properties of geopolymer systems with SF content beyond 60%, and the interaction between SF, GGBS, and alkaline activators. Limited studies have systematically explored the interplay between SF and slag as main binders in forming geopolymer concrete and the influence of activator composition on hybrid gel formation^{27,28,29,30,31,32,33}. Additionally, most of the studies discussed herein lacked results related to splitting tensile strength (${\text{f}}_{\text{sp}}$), modulus of elasticity (${\text{E}}_{\text{c}}$), the compressive stress–strain response, the cube compressive strength (${\text{f}}_{\text{cu}}$), and the slenderness ratio (${\text{f}}_{\text{c}}{\prime}$/${\text{f}}_{\text{cu}}$) of SF-GGBS geopolymer concrete mixtures, which remain underexplored and undetermined. Addressing these gaps through a systematic evaluation of high SF content in hybrid geopolymer systems and the optimization of SS-to-SH ratios will provide a more comprehensive understanding of the mechanical performance of geopolymer concrete and the possibility for GGBS-SF blended geopolymer concrete to be used for structural applications. To address these gaps, the current study evaluates the effects of varying proportions of slag and SF as binders on the mechanical properties and workability of geopolymer concrete. Additionally, it investigates the influence of two distinct SS-to-SH ratios in the alkaline activator solution. The findings aim to identify optimal mix designs that balance strength, durability, and workability, contributing to the development of sustainable, high-performance geopolymer concrete that reduces reliance on traditional cement and its associated environmental impact.

Materials and methods

Materials

The geopolymer binding materials included ground granulated blast furnace slag (GGBS) and silica fume (SF). Table 2 summarizes the chemical compositions of the as-received materials. GGBS is mainly composed of calcium oxide (CaO) and silica (SiO₂), while SF is mainly composed of silica (SiO₂). Their morphologies of the as-received materials obtained through JEOL JSM 6390A scanning electron microscopy (SEM) are shown in Fig. 1. The particle sizes of GGBS and SF were in ranges of 2–80 and 0.1–0.2 µm, respectively.

Table 2 Chemical and Physical Properties of the as-received materials.

Fig. 1

SEM Morphologies of the as-received materials for (a) GGBS, (b) SF, and (c) dune sand.

Dune sand was used as fine aggregates in the geopolymer concrete mixes. The dune sand is mainly abundant with SiO₂ in the form of angular particles. As shown in Table 2, it is characterized by a specific surface area of 117 cm²/g and a specific gravity of 2.77. Furthermore, natural crushed dolomitic limestone aggregates with a nominal maximum size of 20 mm were used as coarse aggregates. Table 3 presents their physical properties.

Table 3 Physical properties of Natural Aggregates.

The alkaline activator solution, used in initiating the geopolymerization process, was formulated by mixing sodium silicate (SS) and sodium hydroxide (SH). The SS solution was grade N with a chemical composition of 26.3% SiO₂, 10.3% Na₂O, and 63.4% H₂O. The SH solution was prepared by dissolving 97% of pure SH flakes in a specific content of water to attain molarity of 14 M, as recommended by previous studies^19,20. In this work, a total of two groups of mixes with similar binder composition of GGBS and SF were made, but with different SS-to-SH (Na₂SiO₃/NaOH) ratios of 1.5 and 2.0, denoted herein as an alkaline activator solution (AAS) ratio. Furthermore, a polycarboxylic ether-based polymer-based superplasticizer (SP) and additional water were incorporated into the mix to maintain adequate fresh workability of the geopolymer concrete.

Mixture proportioning

Table 4 presents the mixture proportions of the investigated GGBS/SF-based geopolymer concrete mixtures. The mixes were designed to attain at least a cube compressive strength of 20 MPa and a slump of 50 mm and investigate the effect of binder composition at GGBS:SF ratios of 1:0, 3:1, 1:1, 1:3, and 0:1. Moreover, to investigate the effect of an AAS (i.e., SS-to-SH) ratio, two similar sets of mixes with similar proportions were prepared but with AAS ratios of 1.5 and 2.0, named groups A and B, respectively. Other proportions such as dune sand, coarse aggregates, SH molarity, SP, and water contents remained fixed for all mixes at 725 kg/m³, 1200 kg/m³, 14 M, and 12 kg/m³ (4% of total binder mass), respectively. A sufficient amount of water content of 100 kg/m³ was added to ensure enhanced workability of all mixtures and remained constant, as concluded by preliminary trial mixes and as recommended by Zuaiter et al.³⁰.

Table 4 Mixture proportions of GGBS/SF-based geopolymer concrete mixtures (kg/m³).

Sample preparation

The geopolymer concrete mixes were prepared and poured in a laboratory setting with a temperature of 24 ± 2 °C and relative humidity of 50 ± 5%. Initially, SH flakes were mixed with water to create a 14 M SH solution. Once the solution reached room temperature, SS was added to the SH solution. The heat produced by the resulting AAS was allowed to dissipate overnight. Additional water and SP were then mixed with the AAS before gradually adding it to the dry ingredients (GGBS, SF, coarse aggregates, and dune sand). The mixture was blended for three minutes to ensure a uniform consistency. The freshly mixed geopolymer concrete was then poured into 100 mm cubes and 100 × 200 mm (diameter × height) cylinders and compacted on a vibrating table for 10 s. Plastic sheets were placed over the samples for 24 h before they were demolded and left at room temperature until testing.

Performance evaluation

A slump test was used to evaluate the effect of binder composition and AAS ratio on the workability of the GGBS/SF-based-geopolymer concrete mixtures. The test was conducted on GGBS/SF-based-geopolymer concrete mixtures at their fresh state following ASTM C143³⁹. Additionally, the 28-day hardened concrete densities were determined according to ASTM C642⁴⁰.

The cube (${\text{f}}_{\text{cu}}$) and cylinder (${\text{f}}_{\text{c}}{\prime}$) compressive strength of GGBS/SF blended geopolymer concrete was determined using a Universal Testing Machine with a load capacity of 3000 kN following BS EN 12,390–3⁴¹ and ASTM C39⁴², respectively.

An axial load was applied at a loading rate of 7 kN/sec. The cube compressive strengths of geopolymer concrete mixes were obtained at 1, 7, and 28 days, while the cylinder compressive strengths were obtained at 28 days only. A total of three samples were tested per mix to obtain an average. To ensure the accuracy of our measurements on low-strength materials using a high-capacity compression machine, we implemented several rigorous checks. The machine was calibrated to confirm precision across all load levels, including lower loads. We conducted preliminary tests on dull specimens with additional verification using a 500 KN Material Testing System (MTS) Universal Testing machine, applying displacement control to enhance measurement reliability. Statistical validation was performed to ensure consistency, and results were cross-referenced with existing literature and theoretical predictions to verify accuracy. These measures effectively mitigate the limitations associated with testing low-strength materials on high-capacity equipment. Additionally, two 60-mm long strain gauges were attached vertically at mid-height and diametrically opposite points of the cylinder circumference to record the axial strains, as illustrated in Fig. 2. The load cell and the strain gauges were connected to a data acquisition system to obtain the stress–strain response.

Fig. 2

Orientation of 60-mm strain gauges on the circumference of the cylindrical specimens.

Subsequently, the experimental stress–strain curves were developed to obtain the peak stress ($\sigma$), peak strain ($\varepsilon$), and modulus of elasticity ${\text{E}}_{\text{c}}$, according to ASTM C469⁴³. The modulus of elasticity, ${\text{E}}_{\text{c}}$, was obtained as the slope of the chord connecting the stress corresponding to 40% of the ultimate stress (S₂) and the corresponding stress of a strain of 0.00005 (S₁). The corresponding strain of S₂ is denoted as ${\upvarepsilon }_{{2}}$ Equation (1) was employed in determining the modulus of elasticity of GGBS/SF-based geopolymer concrete mixes. An average of three samples were taken per mix to represent ${\text{E}}_{\text{c}}$.

$$E_{c} = \frac{{S_{2} - S_{1} }}{{\varepsilon_{2} - 0.00005}}$$

(1)

The splitting tensile strength (${\text{f}}_{\text{sp}}$) of GGBS/SF blended geopolymer concrete mixes was determined at 28 days following ASTM C496⁴⁴. The load was applied across the entire length of the cylindrical specimens of 100 mm diameter and 200 mm height, at a loading rate of 1 kN/sec. Triplicate specimens were tested per mix to obtain an average. It is worth noting that in this manuscript, GGBS-dominant mixes are defined as those with at least 50% GGBS in the total binder, while SF-dominant mixes have at least 50% SF from the total binder.

Results and discussion

Slump

The slump test was performed to evaluate the workability of GGBS/SF blended geopolymer concrete mixes with two different AAS ratios. Figure 3 presents the slump values, ranging from 34 to 64 mm. The highest workability was observed in the mix with GGBS only (i.e., G100S0). The effect of blending SF with GGBS was found by comparing the mix with 100% GGBS. The effect of increasing the AAS ratio was demonstrated by comparing Group A mixes with their counterparts in Group B. Table 5 presents the percentages of change in workability.

Fig. 3

Slump values of Group A with AAS = 1.5 and Group B with AAS = 2.0.

Table 5 Slump values of GGBS/SF-blended geopolymer concrete mixes.

Group A mixes with an AAS ratio of 1.5 show a trend of decreasing slump values as SF content in the binder increases. Specifically, slump reductions were 14%, 18%, and 18% for blends of 1:3, 1:1, and 3:1 (GGBS: SF), respectively, compared to the GGBS-only mix (G100S0). Using only SF as a binder in forming the geopolymer concrete resulted in the highest reduction in workability, with a 40% decrease. This indicates that increasing SF content in the GGBS/SF-blended binder decreased the workability of geopolymer concrete mixes. In Group B, with an AAS ratio of 2.0, workability is generally lower than in Group A. Blending GGBS and SF at 1:3, 1:1, and 3:1 (GGBS: SF) in Group B results in workability reductions of 28%, 31%, and 34%, respectively, compared to G100S0-A2.0. However, using only SF as a binder (G0S100-A2.0) results in a 37% reduction in workability, compared to G100S0-A2.0. These findings align with other studies indicating that SF content above 20% degrades workability in both conventional and geopolymer concrete^43,44,45. In a similar study by Premkumar et al.²⁴, where they investigated the effect of silica fume (SF) and recycled aggregates on slag-based geopolymer concrete with an alkali-activator solution (AAS) ratio of 1.5. Their findings indicated that the incorporation of 40% and 60% SF in slag-based geopolymer concrete resulted in a 5% and 14% reduction in workability, respectively, which was comparatively lower than the reduction observed in the present study. This difference can likely be attributed to the presence of recycled aggregates and the use of a lower sodium hydroxide molarity of 12 M in the alkaline activator solution, both of which may have influenced the rheological properties of the geopolymer mixture.

Increasing the AAS ratio from 1.5 to 2.0 in GGBS/SF-blended geopolymer concrete mixes generally decreased workability, except in the GGBS-based geopolymer concrete mix (G100S0-A2.0), where workability improved by 11%. This could be due to lower reactivity from reduced SH content in the alkaline activator solution, resulting in delaying the reaction of GGBS particles in the GGBS-based geopolymer concrete mix (G100S0-A2.0)⁴⁶. Compared to Group A, Group B mixes (with an AAS ratio of 2.0) showed reduced workability, whereas geopolymer concrete mixtures made of GGBS and SF blends at 3:1 and 1:1 (GGBS:SF) showed a reduction in workability by 6 and 11%, respectively. Interestingly, increasing the AAS ratio to 2.0 in the SF-based geopolymer concrete mix (i.e., G0S100-A2.0) enhanced workability by 18%, likely due to reduced SH content in the alkaline activator causing a delayed reaction with SF particles at the onset of mixing¹⁶. A similar observation was found when increasing the AAS ratio from 1.5 to 2.5 in SF-slag blended geopolymer concrete made with recycled aggregates, where the workability resulted in a maximum reduction by 10% with the inclusion of 40% SF from the total binder²⁴. Additionally, similar trends were observed in fly ash/SF blended geopolymer concrete^51,52.

As such, increasing the amount of water in GGBS/SF blended geopolymer concrete mixtures would still be required to attain adequate workability. SF-based mixes have lower workability compared to GGBS-based mixes due to finer particles of SF. Increasing the AAS ratio (i.e., reducing SH content in the alkaline activator solution) does not significantly affect the workability of blended mixes but is found beneficial for mixes with a sole binder, such as G100S0 and G0S100. A higher AAS ratio in forming the alkaline solution, i.e., lower SH content, delays the reaction in GGBS- and SF-based geopolymer mixes, which assists in enhancing the workability¹⁵.

Hardened density

Figure 4(a,b) presents the hardened densities $\left( {{\uprho }_{{\text{h}}} } \right)$ of Group A and B mixes over 1-, 7-, and 28-day periods, illustrating varying trends. All mixes exhibited increased ${\uprho }_{{\text{h}}}$ with time. Group A mixes exhibited ${\uprho }_{{\text{h}}}$ ranging from 2297–2410 kg/m³, 2306–2472 kg/m³, and 2330–2480 kg/m³, while Group B mixes ranged from 2290–2465 kg/m³, 2313–2520 kg/m³, and 2358–2557 kg/m³ over the same periods, indicating an overall increase in density with time. Notably, hardened densities of Group A mixes increased slightly when blending SF up to 50% from the total binder with GGBS, i.e., up to a ratio of 1:1 (GGBS: SF). However, Group B mixes showed a consistent density decrease with increasing SF contents in the GGBS/SF blend.

Fig. 4:

1-, 7-, and 28-day Hardened Densities $\left( {{\uprho }_{{\text{h}}} } \right)$ for GGBS/SF-blended geopolymer concrete mixes made with (a) AAS of 1.5, and (b) AAS of 2.0.

In Group A, blending GGBS and SF elevated ${\uprho }_{{\text{h}}}$ over time, with up to 50% SF substitution insignificantly altering density. However, a notable decrease occurred with 75% SF and 25% GGBS mix, indicating increased porosity. Conversely, SF-based mixes exhibited lower densities compared to GGBS-based counterparts due to the lack of calcium oxide (CaO) and alumina (Al₂O₃) found in SF, where these are crucial for the formation of denser geopolymer matrix⁴⁸. Equal amounts of GGBS and SF in forming the geopolymer concrete mix yielded the highest densities across all ages. When GGBS interacts with an alkali activator solution containing sodium hydroxide (NaOH) and sodium silicate, a series of chemical transformations occur⁴⁶. Initially, the components of GGBS, primarily silicates and aluminates, dissolve in the alkaline solution, releasing reactive silicon (Si) and aluminum (Al) ions. These ions are then activated by the presence of alkali, undergoing condensation reactions to form polymeric species. Subsequently, these polymeric species polymerize and cross-link to create a three-dimensional network structure known as the geopolymer gel, which serves as the binder in geopolymer concrete. Similarly, when SF, consisting mainly of amorphous silicon dioxide (SiO₂) particles, is introduced to the same alkali activator solution, it undergoes hydrolysis⁴⁹. This hydrolysis reaction results in the dissolution of SF particles and the release of silicate ions (SiO₃²⁻) into the solution. These silicate ions, along with the alkalis present in the solution, participate in subsequent polymerization reactions, ultimately contributing to the formation of the geopolymer gel⁵⁰. While both GGBS and SF are combined and exposed to the alkali activator solution, they undergo simultaneous dissolution processes. GGBS releases silicon and aluminum ions (SiO₂ and Al₂ O₃), while SF generates silicate ions (SiO₃²⁻) through hydrolysis. These dissolved species are then activated by the alkalis and undergo polymerization reactions, leading to the formation of the geopolymer gel. The synergistic effect of GGBS and SF enhances the overall reactivity and binding properties of the resulting geopolymer concrete^25,57. A different trend was found in Group B mixes where mixes with higher contents of GGBS in the binder tend to result in higher 1-, 7-, and 28-day $\rho_{{h^{\prime}}}$ while a further increase of SF in the binder tends to reduce the ${\uprho }_{{\text{h}}}$ at all ages. This could be due to the lower amount of NaOH (AAS = 2.0) in the alkali activator solution, whereas NaOH plays a crucial role in activating the SF particles and initiating the geopolymerization reaction⁵³. Reducing the amount of NaOH in the alkali activator solution with the presence of SF may slow down the geopolymerization process, potentially leading to longer setting times and lower strength development in the hardened concrete⁵⁴. Additionally, insufficient NaOH can result in incomplete activation of SF particles, affecting interfacial bonding, microstructure, and overall mechanical properties of the geopolymer concrete⁵⁵.

Cube compressive strength

The compressive strength of GGBS/SF blended geopolymer concrete mixes with different binder proportions and AAS ratios of alkaline activator solution are evaluated through testing cube specimens at 1, 7, and 28 days. The cube compressive strength (${\text{f}}_{\text{cu}}$) development profiles are shown in Fig. 5. The percent of increase in ${\text{f}}_{\text{cu}}$ at early age (1- to 7-days) and later age (7- to 28-days) are presented in Table 6. All mixes showed increased strength over time, with varying rates of increase at early and later ages. For Group A mixes with ≥ 75% GGBS, percentages of early-age strength development were higher, whereas mixes with more SF showed greater strength development at later ages. Group B mixes generally exhibited higher percentages of strength development at later ages.

Fig. 5

Compressive strength development profiles with time for all GGBS-based, SF-based, and GGBS/SF blended-geopolymer concrete mixes in (a) Group A and (b) Group B.

Table 6 Percentage increase of compressive strength development at early and later ages for all geopolymer concrete mixes.

In Group A, GGBS-dominant mixes had higher percentages of early-age strength development. For instance, a 100% GGBS mix had compressive strengths of 14, 19, and 21 MPa at 1, 7, and 28 days, respectively, with 33% and 11% increases from 1 to 7 days and from 7 to 28 days. A mix with 75% GGBS and 25% SF showed a 16% increase from 1 to 7 days and a 0.6% increase from 7 to 28 days, indicating that SF hinders early strength development. The equal GGBS/SF blend displayed consistent percentages of strength development, with a G50S50 mix showing 25% and 26% increases at early and later ages. The SF-abundant mixes had the highest percentages of strength development, especially at later ages. For example, a 100% SF mix showed 37% and 73% increases at early and later ages, respectively, despite having lower absolute strength values. In Group B, GGBS-abundant mixes showed larger percentages of strength development at later ages due to lower NaOH content. For instance, a 100% GGBS mix in Group B showed 14% and 28% increases at early and later ages, respectively. Substituting GGBS with SF up to 50% resulted in lower early-age and higher percentages of later-age strength development. The G50S50 mix in Group B showed consistent percentages of strength development of 25% and 26% at early and later ages, similar to its Group A counterpart. Geopolymer concrete mix made with SF only Showed the highest percentages of strength development, with early and later strength development percentages of 63% and 139%, respectively, due to the high reactivity of SF.

Increasing the AAS ratio from 1.5 to 2.0 in Group B led to higher percentages of strength development, particularly at later ages. For example, the G0S100 mix in Group B showed significantly higher early and later strength development percentages (63% and 139%) compared to its Group A counterpart (37% and 73%). This is owed to the reduced amount of sodium hydroxide in forming the alkaline-activated solution, where reducing AAS tends to enhance strength development at later ages^47,61. However, the continued reaction and further densification of the matrix over time result in substantial late-age strength development. The high surface area and amorphous structure of SF facilitate ongoing polymerization and refinement of the microstructure, filling pores and reducing porosity, which enhances the mechanical properties over time. Therefore, the unique properties of SF, including its ability to act as a pozzolan and its high reactivity, ensure that the G0S100 mixture continues to develop strength significantly beyond the initial stages, highlighting the importance of SF in optimizing the long-term performance of geopolymer concrete. Similar findings were found in high-performance concrete upon the addition of SF^62,63. Compared to its counterpart mix in Group A, the percentages of strength development at early and later ages were greater by 26 and 66%, respectively. This indicates that increasing the AAS ratio from 1.5 to 2.0 results in greater percentages of strength development. This is due to the higher alkali activator solution (AAS) ratio enhancing the dissolution of aluminosilicate precursors, leading to a continuous geopolymerization process. A higher AAS ratio provides more reactive species, such as sodium or potassium ions, which facilitate the breakdown of the fly ash and slag particles, resulting in the formation of a more extensive and interconnected gel network at an earlier stage⁵⁸. Additionally, the increased AAS ratio promotes the continuous growth and densification of the geopolymer matrix over time, reducing porosity and increasing the overall structural integrity, which significantly boosts later-age strength development⁵².

The 1-, 7-, and 28-day ${\text{f}}_{\text{cu}}$ results of mixes with an AAS of 1.5 are presented in Fig. 6(a). At day 1, the G100S0 mix (100% GGBS) shows moderate early strength around 12 MPa, indicating rapid geopolymerization reaction with GGBS. The G75S25 mix (75% GGBS, 25% SF) exhibits slightly higher early strength, suggesting additional reactive silica from SF accelerates the initial setting, as proven by slump results. The G50S50 (50% GGBS, 50% SF) mix achieves higher early strength around 15 MPa, likely due to the optimal synergy between GGBS and SF. In contrast, the G25S75 mix (25% GGBS, 75% SF) and the G0S100 mix (100% SF) show lower early strengths, around 10 MPa and 5 MPa respectively, due to slower initial reaction rates with higher SF content. By day 7, the G100S0 mix increased to about 20 MPa, while the G75S25 mix reached 23 MPa, demonstrating the continued beneficial impact of SF on strength development. The G50S50 mix shows a higher strength of 27 MPa, reflecting the optimal interaction between GGBS and SF. The G25S75 mix and the G0S100 mix exhibit slower strength gains, achieving 15 MPa and 10 MPa respectively, highlighting the delayed reaction and strength gain due to higher SF contents. At day 28, the G100S0 mix attains a compressive strength of 25 MPa, while the G75S25 mix reaches 28 MPa, maintaining a slightly higher strength by 12% due to the additional SF. The G50S50 mix achieves the highest compressive strength of around 32 MPa, indicating the most efficient long-term strength development with a balanced ratio of GGBS and SF by 28%, compared to the G100S0 mix. The G25S75 mix continues to develop strength, reaching about 20 MPa, although still lower than mixes with higher GGBS content. The G0S100 mix shows significant improvement from early ages, attaining 15 MPa, but remains the lowest in strength among the mixes due to the slower geopolymerization process.

Fig. 6:

1-, 7-, and 28-day compressive strengths of GGBS-based, SF-based, and GGBS/SF blended-geopolymer concrete mixes with (a) AAS ratio of 1.5 and (b) AAS ratio of 2.0.

The 1-, 7-, and 28-day ${\text{f}}_{\text{cu}}$ results of mixes with an AAS of 2.0 are presented in Fig. 6(b). It shows that ${\text{f}}_{\text{cu}}$ results tend to decrease with increasing SF contents in Group B mixes. At day 1, the G100S0 mix shows the highest early compressive strength around 25 MPa, demonstrating rapid geopolymerization with AAS = 2.0. The G75S25 mix follows closely with a compressive strength of approximately 20 MPa, indicating the beneficial contribution of SF to early strength gain. The G50S50 mix achieves moderate early strength around 15 MPa, suggesting that while SF contributes to strength, the balance of GGBS and SF affects early reaction kinetics. The G25S75 and G0S100 mixes show lower early strengths of around 10 MPa and 5 MPa, respectively, due to slower initial reaction rates with higher SF content. By day 7, the ${\text{f}}_{\text{cu}}$ for G100S0, G75S25, and G50S50 mixes showed substantial increases to about 30, 27, and 20 MPa, respectively, reflecting ongoing strength development. Similar findings were observed in GGBS-based alkali-activated concrete mixes with up to 15% SF. The G25S75 mix shows a modest increase to around 15 MPa, while the G0S100 mix reaches about 10 MPa, highlighting the slower reaction and strength gain of SF. On day 28, the ${\text{f}}_{\text{cu}}$ for G100S0, G75S25, and G50S50 mixes were 35, 32, and 25 MPa, respectively. The G50S50 mix achieved efficient long-term strength development with a balanced ratio of GGBS and SF. The G25S75 mix continues to develop strength, reaching around 20 MPa, though still lower than the mixes with higher GGBS content. The G0S100 mix, with 100% SF, shows a final strength of around 15 MPa, reflecting the lowest strength among the mixes due to the slower geopolymerization process.

The ${\text{f}}_{\text{cu}}$ development of geopolymer concrete mixes with varying proportions of GGBS and SF and different AAS ratios of 1.5 and 2.0 were evaluated at 1, 7, and 28 days, as illustrated in Figs. 7(a–c). Figure 7a demonstrates that increasing the AAS ratio from 1.5 to 2.0 significantly enhances early compressive strength for most mixes. For the G100S0 mix, the ${\text{f}}_{\text{cu}}$ increased by approximately 100% with the higher AAS ratio, while the G75S25 mix showed an increase of about 43%. However, mixes with SF contents of 50% and above exhibited lower ${\text{f}}_{\text{cu}}$ values upon increasing the AAS ratio. Specifically, the ${\text{f}}_{\text{cu}}$ decreased by 85% and 63% for the G50S50 and G25S75 mixes, respectively. The ${\text{f}}_{\text{cu}}$ of the G0S100 mix remained unchanged. Figure 7(b) presents the 7-day ${\text{f}}_{\text{cu}}$ results for mixes with AAS ratios of 1.5 and 2.0. The G100S0 and G75S25 mixes showed increases in ${\text{f}}_{\text{cu}}$ of at least 44% and 25%, respectively, indicating that a higher AAS ratio was more effective in GGBS-based geopolymer concrete mixes. Conversely, the G50S50 and G25S75 mixes exhibited decreases in 7-day ${\text{f}}_{\text{cu}}$ by 75% and 40%, respectively. Interestingly, the G0S100 mix showed a 20% increase in ${\text{f}}_{\text{cu}}$ with the higher AAS ratio. Figure 7(c) illustrates the 28-day ${\text{f}}_{\text{cu}}$ results. The G100S0 mix showed a 65% increase in strength with the higher AAS ratio, while the G75S25 mix exhibited a 31% increase. However, the higher AAS ratio was detrimental to mixes with higher SF content, with the G50S50 and G25S75 mixes showing 28-day ${\text{f}}_{\text{cu}}$ decreases of 42% and 23%, respectively. Notably, the G0S100 mix exhibited a 64% increase in 28-day ${\text{f}}_{\text{cu}}$ with the higher AAS ratio. As such, reducing the amount of sodium hydroxide in the AAS (i.e., increasing the AAS ratio) reduces the compressive strength of GGBS/SF blended geopolymer mixes, particularly those with more than 50% SF in the binder, due to the essential role sodium hydroxide plays in the geopolymerization process. SH creates a highly alkaline environment necessary for dissolving aluminosilicate precursors like GGBS and SF⁵⁹. When its concentration is reduced, the pH level decreases, slowing down the dissolution rate of these precursors and resulting in fewer reactive species for the geopolymerization process⁴⁸. SF, which is rich in amorphous silica, requires a sufficient amount of sodium hydroxide to break down its structure and contribute effectively to the geopolymer matrix⁴⁵. Insufficient sodium hydroxide leads to incomplete geopolymerization, producing a less cohesive and less dense microstructure with fewer C-S–H and N-A-S–H gels, especially in high SF mixes⁵¹. This reduction in sodium hydroxide also means fewer nucleation sites for gel formation, leading to higher porosity and weaker structural integrity⁶¹.

Fig. 7

The results of ${\text{f}}_{\text{cu}}$ for GGBS-based, SF-based, and GGBS/SF blended-geopolymer concrete mixes at (a) day 1, (b) day 7, and (c) day 28.

The current findings correlate with the $\rho_{h}$ at all ages, where mixes with an AAS of 1.5 tend to result in higher ${\uprho }_{{\text{h}}}$ by blending up to 50% SF with GGBS in the binder. However, mixes made with an AAS of 2.0, and the addition of SF tend to decrease the ${\uprho }_{{\text{h}}}$ across all ages. As such, a correlation exists between the 28-day $f_{cu}$ and $\rho_{h}$, presented in Fig. 8. It can be indicated that a linear relationship (Eq. (2)) was developed using regression analysis to predict ${\text{f}}_{\text{cu}}$ from $\rho_{h}$ and vice versa with good accuracy (R² = 0.90). Such correlation makes it possible to predict the 28-day cube compressive strength (${\text{f}}_{\text{cu}}$) from the measured 28-day ${\uprho }_{{\text{h}}}$ for SF-GGBS blended geopolymer concrete presented in this work. However, this relationship is limited to ρ_h values ranging between 2290 and 2557 kg/m³ and to the mixes examined in this study.

$$f_{cu} = 0.13(\rho_{h} - 2290.4)$$

(2)

Fig. 8

Relationship between $f_{cu}$ and $\rho_{h}$ at 28-days.

Cylinder compressive strength

The 28-day cylinder compressive strength, ${\text{f}}_{\text{c}}{\prime}$, for GGBS-based, SF-based, and GGBS/SF blended-geopolymer concrete mixes are presented in Fig. 9. Similar to the 28-day ${\text{f}}_{\text{cu}}$, increasing the SF content up to 50% in GGBS/SF blended geopolymer concrete mixes made with an AAS ratio of 1.5 enhanced the 28-day ${\text{f}}_{\text{c}}{\prime}$. Whereas increasing the amount of SF contents of more than 25% in similar mixes made with an AAS ratio of 2.0 decreased the 28-day ${\text{f}}_{\text{c}}{\prime}$. The use of GGBS and SF in forming geopolymer concrete mixes with an AAS ratio of 1.5 resulted in a 28-day ${\text{f}}_{\text{c}}{\prime}$ of 8, 9, 12, 7, and 4 MPa for G100S0, G75S25, G50S50, G25S75, and G0S100, respectively. This indicates that the inclusion of SF in the binder of up to 50% of the total binder was effective in enhancing the 28-day ${\text{f}}_{\text{c}}{\prime}$. However, increasing the AAS ratio to 2.0 led to a 28-day ${\text{f}}_{\text{c}}{\prime}$ of 13, 13, 7, 4, and 3 MPa for the same mixes. This corresponds to respective changes of + 63, + 44, -41, -43, and -25%, compared to their counterpart mixes of Group A. These results indicate that increasing the AAS ratio significantly enhances the 28-day ${\text{f}}_{\text{c}}{\prime}$ in GGBS dominant mixes (i.e., G100S0 and G75S25), and decreases the 28-day ${\text{f}}_{\text{c}}{\prime}$ in SF dominant mixes (i.e., G50S50, G25S75, and G0S100). The mix with 75% GGBS and 25% SF (G75S25) consistently achieved the highest ${\text{f}}_{\text{c}}{\prime}$ with a higher AAS ratio of 2.0, demonstrating that a balanced binder composition of GGBS and SF optimizes the geopolymerization process. Furthermore, the mix with 100% GGBS (G100S0) and an AAS ratio of 2.0 also showed a substantial 28-day ${\text{f}}_{\text{c}}{\prime}$ of 13 MPa. On the other hand, mixes with higher SF content (G50S50, G25S75, and G0S100) with an AAS ratio of 2.0, exhibited decreased 28-day ${\text{f}}_{\text{c}}{\prime}$, which is owed to the slower reactivity and dissolution rate of SF compared to GGBS. The high silica content in SF requires a more prolonged activation period to fully participate in the geopolymerization process. Consequently, their overall strength remains lower than mixes with higher GGBS content due to the less efficient and slower reaction kinetics of SF in the stages of geopolymer formation.

Fig. 9:

28-day cylinder compressive strength, ${\text{f}}_{\text{c}}{\prime}$, for all mixes.

 The 28-day cylinder compressive strength, ${\text{f}}_{\text{c}}{\prime}$, for GGBS-based, SF-based, and GGBS/SF blended-geopolymer concrete mixes are presented in Fig. 9. Similar to the 28-day ${\text{f}}_{\text{cu}}$, increasing the SF content up to 50% in GGBS/SF blended geopolymer concrete mixes made with an AAS ratio of 1.5 enhanced the 28-day ${\text{f}}_{\text{c}}{\prime}$. Whereas increasing the amount of SF contents of more than 25% in similar mixes made with an AAS ratio of 2.0 decreased the 28-day ${\text{f}}_{\text{c}}{\prime}$. The use of GGBS and SF in forming geopolymer concrete mixes with an AAS ratio of 1.5 resulted in a 28-day ${\text{f}}_{\text{c}}{\prime}$ of 8, 9, 12, 7, and 4 MPa for G100S0, G75S25, G50S50, G25S75, and G0S100, respectively. This indicates that the inclusion of SF in the binder of up to 50% of the total binder was effective in enhancing the 28-day ${\text{f}}_{\text{c}}{\prime}$. However, increasing the AAS ratio to 2.0 led to a 28-day ${\text{f}}_{\text{c}}{\prime}$ of 13, 13, 7, 4, and 3 MPa for the same mixes. This corresponds to respective changes of + 63, + 44, -41, -43, and -25%, compared to their counterpart mixes of Group A. These results indicate that increasing the AAS ratio significantly enhances the 28-day ${\text{f}}_{\text{c}}{\prime}$ in GGBS dominant mixes (i.e., G100S0 and G75S25), and decreases the 28-day ${\text{f}}_{\text{c}}{\prime}$ in SF dominant mixes (i.e., G50S50, G25S75, and G0S100). The mix with 75% GGBS and 25% SF (G75S25) consistently achieved the highest ${\text{f}}_{\text{c}}{\prime}$ with a higher AAS ratio of 2.0, demonstrating that a balanced binder composition of GGBS and SF optimizes the geopolymerization process. Furthermore, the mix with 100% GGBS (G100S0) and an AAS ratio of 2.0 also showed a substantial 28-day ${\text{f}}_{\text{c}}{\prime}$ of 13 MPa. On the other hand, mixes with higher SF content (G50S50, G25S75, and G0S100) with an AAS ratio of 2.0, exhibited decreased 28-day ${\text{f}}_{\text{c}}{\prime}$, which is owed to the slower reactivity and dissolution rate of SF compared to GGBS. The high silica content in SF requires a more prolonged activation period to fully participate in the geopolymerization process. Consequently, their overall strength remains lower than mixes with higher GGBS content due to the less efficient and slower reaction kinetics of SF in the stages of geopolymer formation.

Table 7 presents the ratio of ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ for all geopolymer concrete mixes. The ratio of the mixes ranged between 0.24 and 0.52, indicating a significant difference between cylinder and cube compressive strengths. Similar low ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratios were also reported in previous studies for plain slag, slag-fly ash, and SF/slag-based geopolymer concretes [67–70]. It indicates that geopolymer concrete is more sensitive to the slenderness effect than cement-based concrete. Such observation should be considered by practitioners and researchers when designing based on the values of ${\text{f}}_{\text{cu}}$.

Table 7 The cube (${f}_{cu}$) and cylinder (${f}_{c}{\prime}$) compressive strength at 28 days for all mixes.

In Group A mixes, made with an AAS ratio of 1.5, the ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratio increased upon increasing SF contents. This indicates that the slenderness effect was reduced in mixes dominant with SF, compared to those mixes that are dominant in GGBS. GGBS-based geopolymer concrete mix resulted in a ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratio of 0.39. However, blending GGBS with SF by 25, 50, and 75% led to ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratios of 0.41, 0.42, and 0.45, respectively, whereas a full substitution of SF by GGBS resulted in ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratio of 0.52. It can be observed from the presented results that increasing SF content from 25 to 50% in forming GGBS/SF blended geopolymer concrete did not significantly influence the ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratio.

Meanwhile, increasing the AAS ratio from 1.5 to 2.0, represented by mixes of Group B, led to different ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratios. For instance, the ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratio of the G100S0 mix decreased to 0.37, and upon AAS ratio increased to 2.0. GGBS/SF blended geopolymer concrete mixes made with 25 and 50% SF resulted in greater ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratios of 0.47 and 0.46, respectively, presenting an increase from their counterpart mixes made in Group A (with an AAS ratio of 1.5). Conversely, mixes made with 75% and 100% SF (i.e., G25S75 and G0S100) resulted in lower ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratios of 0.3 and 0.24, respectively. This is owed to the lowered ${\text{f}}_{\text{cu}}$ values when an AAS ratio of 2.0 was employed, where increasing AAS to 2.0 tends to degrade the cube and cylinder compressive strength of SF-dominant geopolymer concrete mixes.

A correlation exists between cylinder and cube compressive strengths. As presented in Fig. 10, a linear relationship was developed using regression analysis to predict ${\text{f}}_{\text{c}}{\prime}$ from ${\text{f}}_{\text{cu}}$ and vice versa with good accuracy (R² = 0.9), presented by Eq. (3). The presented correlation is tailored to the SF-GGBS blended geopolymer concrete mixes investigated in this study. However, this relationship is limited to ${\text{f}}_{\text{cu}}$ values exceeding 7.5 MPa and to the mixes examined in this study.

Fig. 10

Relationship between the cylinder and cube compressive strengths at 28 days.

$${\text{f}}_{\text{c}}{\prime}\text{=0.41}\left({\text{f}}_{\text{cu}}\right)$$

(3)

Compressive stress–strain response

Figure 11(a-b) illustrates the stress–strain curves for GGBS/SF blended geopolymer concrete mixes at 28 days. The peak stresses and their corresponding strains for all mixes are also summarized in Table 8. All stress–strain responses resulted in a linear increase up to the peak load, followed by a decrease until failure. The peak stresses tend to decrease with increased amounts of SF in the binder, while their corresponding strains tend to increase in Groups A and B.

Fig. 11

Compression stress–strain curves of cylinder concrete specimens for GGBS/SF-blended geopolymer concrete mixes with (a) AAS = 1.5, and (b) AAS = 2.0.

Table 8 Maximum stresses and their corresponding strains for all geopolymer concrete mixes.

In Group A, the G100S0 mix (100% GGBS) demonstrated a peak stress of 8.2 MPa, serving as the baseline for comparison. The addition of 25% SF (G75S25) slightly increased the peak stress by 7.3% to 8.8 MPa, with a notable reduction in strain at peak stress by 28.3%. Further increasing SF to 50% (G50S50) significantly improved peak stress by 41.5% to 11.6 MPa, indicating an optimal blend of GGBS and SF. This mix also exhibited a 39.1% decrease in strain at peak stress. Conversely, mixes with higher SF content, such as G25S75 and G0S100, showed reduced peak stresses by 11.0% and 52.4%, respectively, compared to G100S0. Strain at peak stress for these mixes also decreased, indicating reduced ductility. In Group B, the G100S0 mix displayed a peak stress of 12.9 MPa, reflecting a 57.3% increase due to the higher AAS ratio. Strain at peak stress decreased by 51.8%. For G75S25, peak stress marginally increased by 1.6% to 13.1 MPa, with strain at peak stress increasing by 79.8%. G50S50 showed a significant reduction in peak stress by 42.6% to 7.4 MPa, despite a higher AAS ratio, and an increase in strain at peak stress by 71.3%. The mixes with higher SF content (G25S75 and G0S100) exhibited substantial decreases in peak stresses by 71.3% and 76.7%, respectively. These mixes also showed increased strains at peak stress, indicating higher ductility compared to their Group A counterparts.

The effect of AAS modification from 1.5 to 2.0 is found for each mix in Group B, compared to their counterpart mix in Group A. Increasing the AAS ratio from 1.5 to 2.0 generally improved peak stresses for mixes with higher GGBS content. For instance, the peak stress of G100S0 increased from 8.2 MPa to 12.9 MPa, a 57.3% enhancement. For the G75S25 mix, the higher AAS ratio led to a modest peak stress increase of 48.9% and a significant rise in strain at peak stress by 20.8%. In contrast, the G50S50 mix experienced a 36.2% reduction in peak stress when the AAS ratio increased, although strain at peak stress increased by 35.5%. The high SF content mixes (G25S75 and G0S100) saw considerable decreases in peak stresses by 49.3% and 23.1%, respectively, with only marginal changes in strain at peak stress.

The reduction in peak stress for mixes with higher SF content can be attributed to the lower reactivity of SF compared to GGBS, resulting in a less cohesive geopolymer matrix. The higher silica content in SF necessitates more extensive geopolymerization reactions, which may not be fully achieved within the same curing period, leading to lower stress capacity and higher susceptibility to cracking. The increased strains at peak stress for GGBS dominant mixes with higher AAS ratios indicate enhanced ductility, potentially due to the more extensive dissolution and polymerization processes facilitated by the higher alkaline environment. Overall, higher GGBS content and optimal AAS ratios contribute to improved peak stresses and balanced ductility, whereas excessive SF content tends to reduce strength and increase brittleness.

Figure 12(a–e) illustrates the effect of increasing the AAS ratio from 1.5 to 2.0 on the stress–strain response for each geopolymer concrete mix. The compressive stress–strain response is shown to be sensitive to changes in the AAS ratio across all mixes. For mixes dominant in SF (with SF content of 50% and above), as shown in Fig. 12(a–c), increasing the AAS ratio to 2.0 hindered the stress–strain response, resulting in lower peak loads than when using an AAS ratio of 1.5. Specifically, for mixes with SF contents of 50%, 75%, and 100% (G50S50, G25S75, and G0S100), increasing the AAS ratio led to reductions in peak stresses by 36%, 49%, and 23%, respectively, and increases in corresponding strains by 36%, 22%, and 5%, respectively. This is attributed to the higher AAS ratio being less effective in activating the higher SF content compared to GGBS. SF requires more intensive and prolonged activation to fully participate in the geopolymerization process. The dissolution rate of SF may not significantly increase with a higher AAS ratio, leading to incomplete geopolymerization and a weaker overall matrix. Moreover, the high surface area of SF particles can increase water demand, potentially resulting in a less cohesive matrix with more porosity. This results in lower compressive strength and higher strain at peak stress, indicating a less stiff and more deformable material.

Fig. 12

Typical compression stress–strain curves of cylinder concrete specimens for (a) G0S100, (b) G25S75, (c) G50S50, (d) G75S25, and (e) G100S0.

The reduced efficiency in forming a strong geopolymer network in high SF mixes, even at a high AAS ratio, underscores the importance of optimizing the balance between GGBS and SF to achieve the desired mechanical properties. For the stress–strain response of the G75S25 mix illustrated in Fig. 12d, an increase in peak stress by 49% was observed. However, the corresponding strain also increased by 21% when the AAS ratio was adjusted to 2.0. The increased strain at peak stress indicates that the mix not only gains strength but also exhibits greater ductility, allowing for better energy absorption and deformation before failure. This combination of higher strength and increased strain suggests that the higher AAS ratio facilitates a more flexible and resilient microstructure, capable of sustaining greater loads and deformations, which is particularly beneficial for structural applications requiring both high strength and toughness. In the GGBS-based geopolymer concrete mix (G100S0) shown in Fig. 12e, increasing the AAS ratio from 1.5 to 2.0 led to a 57% increase in peak stress and a 52% decrease in corresponding strain. This indicates that a higher AAS ratio enhances the strength and stiffness of the geopolymer concrete. The significant reduction in strain at peak stress suggests improved material stiffness and reduced deformability under load.

Modulus of elasticity

Figure 13 illustrates the modulus of elasticity of 28-day GGBS-based (G100S0), SF-based (G0S100), and GGBS/SF-blended geopolymer concrete mixes with AAS ratios of 1.5 and 2.0. It can be indicated that the ${\text{E}}_{\text{c}}$ tends to increase in mixes of Group A up to an inclusion of 50% SF in the binder. However, ${\text{E}}_{\text{c}}$ of Group B mixes tends to decrease with increasing amounts of SF in the binder. Values of ${E}_{c}$ ranged between 3.8 and 11.3 GPa and between 1.8 and 18.7 GPa for mixes made with AAS ratios of 1.5 and 2.0, respectively.

Fig. 13

Modulus of elasticity (${\text{E}}_{\text{c}}$) of geopolymer concrete mixes with AAS = 1.5 and 2.0.

The G100S0 mix (100% GGBS) exhibits an ${E}_{c}$ of 5.5 GPa with an AAS ratio of 1.5. However, the addition of SF in the G75S25 mix (75% GGBS, 25% SF) resulted in an almost similar ${E}_{c}$ of 6 with an AAS ratio of 1.5, representing an increase of 8% in ${E}_{c}$ from the G100S0 mix. This indicates that the addition of SF up to 25% in a GGBS/SF blended geopolymer concrete could lead to potential enhancements in ${\text{E}}_{\text{c}}$. An ultimate ${\text{E}}_{\text{c}}$ of 11.3 GPa was found in a GGBS/SF blended geopolymer concrete mixture made with equal amounts of GGBS and SF (50% GGBS and 50% SF) and an AAS ratio of 1.5, representing an enhancement of 105%, compared to G100S0 mix. Further addition of SF in forming the GGBS/SF blended geopolymer concrete with an AAS ratio of 1.5 tends to hinder ${\text{E}}_{\text{c}}$, where G25S75 and G0S100 mixes resulted in ${\text{E}}_{\text{c}}$ of 5.4 and 3.8 GPa, respectively, representing a respective decrease of 3 and 32%, from the G100S0 mix.

In mixes made with an AAS ratio of 2.0, counter-observation can be indicated, whereas increasing the addition of SF tends to decrease ${\text{E}}_{\text{c}}$. A mix made with GGBS only (G100S0) and an AAS ratio of 2.0 resulted in a maximum ${\text{E}}_{\text{c}}$ at 28-day of 18.7 GPa. Blending SF with GGBS by 25, 50, and 75% in GGBS/SF blended geopolymer concrete mix made with an AAS ratio of 2.0, resulted in significant reductions in the 28-day ${\text{E}}_{\text{c}}$ by 48, 80, and 87%, respectively, compared to the G100S0 mix made with an AAS ratio of 2.0. This indicates that the higher AAS ratio significantly benefits mixes with higher GGBS content, enhancing the geopolymerization process and resulting in a denser and stiffer matrix⁶². However, increasing SF content reduces the effectiveness of the AAS in activating the geopolymerization, leading to a less cohesive matrix and lower modulus of elasticity. The results suggest that while a higher AAS ratio can improve stiffness in GGBS-dominant mixes, it is less effective for mixes with higher SF content.

The ${\text{E}}_{\text{c}}$ presented herein was correlated to the 28-day cylinder compressive strength (${\text{f}}_{\text{c}}{\prime}$), as illustrated in Fig. 14. Equation (4) presents a formula to estimate ${\text{E}}_{\text{c}}$ from ${\text{f}}_{\text{c}}{\prime}$ with high accuracy, given the correlation coefficient of R² = 0.88. Note that the proposed relationship is tailored to the SF-GGBS blended geopolymer concrete mixes investigated herein, and is limited to values of ${\text{f}}_{\text{c}}{\prime}$ exceeding 3.0 MPa and to the concrete mixes investigated in this study.

Fig. 14

Relationship between ${\text{E}}_{\text{c}}$ and ${\text{f}}_{\text{c}}{\prime}$ for geopolymer concrete mixes at 28 days.

$${\text{E}}_{\text{c}}\text{=1.293}{\text{e}}^{\text{0.18(}{\text{f}}_{\text{c}}{\prime}\text{)}}$$

(4)

The developed relationship in Eq. (4) was compared to other codified equations developed by ACI 318–19³⁸, CEB-FIP⁶⁴, and AS3600⁶⁵, presented in Table 9. Figure 15 presents the predicted versus experimental values of ${\text{E}}_{\text{c}}$. The $\rho_{h}$ of geopolymer concrete mixes presented in Sect. 3.2 were used in equations developed by ACI 318–19 and AS3600. For the mixes investigated herein, it appears that the model developed in this work, Eq. (4), could more accurately predict ${E}_{c}$ than the codified equations, as scatter plots better converge around the 45-degree line. Nevertheless, it can be indicated that the models developed by CEB-FIP and ACI 318–19 overestimated ${\text{E}}_{\text{c}}$, albeit the latter provided the least deviation from the experimental values. Furthermore, the codified equation developed by AS3600 also overpredicted ${\text{E}}_{\text{c}}$ but to a lower limit, compared to the other codified equations. As such, the codified equations are considered unsuitable for geopolymer concrete produced in this work.

Table 9 Codified equations relating the ${\text{E}}_{\text{c}}$ to ${\text{f}}_{\text{c}}{\prime}$.

Fig. 15

Comparison between codified and developed models for experimental versus predicted ${\text{E}}_{\text{c}}$ of geopolymer concrete mixes.

Splitting tensile strength

Figure 16 illustrates the splitting tensile strength (${\text{f}}_{\text{sp}}$) of all geopolymer concrete mixes at 7 and 28 days, along with the percentage of strength development over time. The ${\text{f}}_{\text{sp}}$ generally increases with curing duration across all mixes, albeit at varying rates. For instance, the G100S0 mix (100% GGBS) with an AAS ratio of 1.5 exhibits ${\text{f}}_{\text{sp}}$ values of 7.4 MPa at 7 days and 9.8 MPa at 28 days, indicating robust early-age performance due to rapid geopolymerization of GGBS. In contrast, introducing SF into GGBS/SF blends with an AAS ratio of 1.5 tends to decrease ${\text{f}}_{\text{sp}}$ at both 7 and 28 days, as depicted in Fig. 16(a). Specifically, blending GGBS with SF by 25%, 50%, 75%, and 100% results in respective reductions in ${\text{f}}_{\text{sp}}$ values at 7 days by 6%, 25%, 50%, and 79%, and at 28 days by 9%, 27%, 51%, and 56%. This demonstrates that SF additions of 50% and higher substantially reduce both early and 28-day ${\text{f}}_{\text{sp}}$. Similar trends are observed in mixes with an AAS ratio of 2.0, shown in Fig. 16(b), where increasing SF content generally decreases ${\text{f}}_{\text{sp}}$, particularly at 28 days. For instance, blending GGBS with SF by 25%, 50%, 75%, and 100% leads to reductions in ${\text{f}}_{\text{sp}}$ at 7 days by 0.4%, 48%, 72%, 73%, and 86%, respectively, and at 28 days by 35%, 37%, 65%, and 68%, respectively. This is attributed to SF’s slower initial reactivity and higher water demand, resulting in a less cohesive and porous matrix initially. Over time, SF gradually contributes to strength development, although not as rapidly as GGBS.

Fig. 16

The 7- and 28-day Splitting tensile strength and their percentages of strength development for geopolymer concrete mixes made with (a) AAS = 1.5, and (b) AAS = 2.0.

The development percentages of ${\text{f}}_{\text{sp}}$ from 7 to 28 days reflect these dynamics. Mixes with an AAS ratio of 1.5 (Fig. 16(a)) show relatively consistent ${\text{f}}_{\text{sp}}$ development percentages averaging 31%, except for G0S100, which exhibits a significantly higher percentage of 187%. In contrast, mixes with an AAS ratio of 2.0 (Fig. 16(b)) demonstrate varied ${\text{f}}_{\text{sp}}$ development percentages, with notable differences between SF-containing mixes compared to those with GGBS dominance. Overall, while GGBS-dominant mixes achieve higher early and overall ${\text{f}}_{\text{sp}}$, SF-dominant mixes show greater relative strength development from 7 to 28 days. However, the absolute ${\text{f}}_{\text{sp}}$ values of SF-dominant mixes remain lower. These findings underscore the trade-offs between early and long-term strength development in geopolymer concretes based on GGBS and SF blends.

The impact of increasing the AAS ratio from 1.5 to 2.0 across all geopolymer concrete mixes is evident from Fig. 17(a-b). Initially, on day 7, this adjustment enhanced the splitting tensile strength (${\text{f}}_{\text{sp}}$) by 5% and 11% for GGBS-dominant mixes G100S0 and G75S25, respectively. In contrast, mixes with 50% SF and higher experienced a decrease in 7-day ${\text{f}}_{\text{sp}}$, with reductions of 28%, 41%, and 32% observed in G50S50, G25S75, and G0S100, respectively. By day 28, a similar trend of decreased ${\text{f}}_{\text{sp}}$ was noted in mixes with higher SF content when the AAS ratio was increased to 2.0. Conversely, GGBS-based geopolymer concrete benefited with a 33% increase in ${\text{f}}_{\text{sp}}$ at 28 days compared to the mix with an AAS ratio of 1.5. Mixes containing 25%, 75%, and 100% SF showed lower ${\text{f}}_{\text{sp}}$ by an average of 6% due to the higher AAS ratio. Interestingly, the 2.0 AAS ratio significantly improved the 28-day ${\text{f}}_{\text{sp}}$ by 13% in GGBS/SF blended geopolymer concrete mixes with equal parts GGBS and SF. This enhancement is attributed to the balanced synergy between GGBS and SF in the 50/50 blend, where the increased alkalinity activated both components effectively, promoting enhanced geopolymerization and a denser, more cohesive matrix. Overall, the higher AAS ratio optimized the dissolution and reaction of both GGBS and SF, leading to improved microstructure and mechanical properties.

Fig. 17

Splitting tensile strengths, ${\text{f}}_{\text{sp}}$, for geopolymer concrete mixes made with different AAS ratios at (a) 7-day, and (b) 28-day.

The ratio of ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ is obtained and presented in Table 10. The ratio ranged between 0.62 and 1.4 for all geopolymer concrete mixes. In Group A mixes, which use an AAS ratio of 1.5, the ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratios show considerable variation across different binder compositions. The G100S0 mix exhibits a high ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratio of 1.20, indicating significant tensile strength relative to its compressive strength. This suggests that GGBS greatly enhances the tensile properties of the concrete. The G75S25 mix has a balanced ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratio of 1.01, reflecting proportional improvements in both tensile and compressive strengths with the inclusion of SF up to 25%. However, the G50S50 mix shows a lower ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratio of 0.62, indicating that this blend may not be optimal for tensile performance despite its relatively high compressive strength. The G25S75 mix has a ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratio of 0.67, demonstrating the reduced effectiveness of high SF content in enhancing tensile properties. Lastly, the G0S100 mix shows a ratio of 1.08, indicating a relatively high tensile strength despite low compressive strength, likely due to the intrinsic properties of SF.

Table 10 28-day compressive and splitting tensile strengths for all geopolymer concrete mixes.

In Group B mixes, which are made with an AAS ratio of 2.0, the higher AAS ratio generally enhances both compressive and tensile strengths but results in different trends for the ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratios. The G100S0 mix has an ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratio of 1.00, showing an equal enhancement in both tensile and compressive strengths, reflecting the effectiveness of the higher AAS ratio in activating GGBS. The G75S25 mix shows a ${\text{f}}_{\text{sp}}$-to-${\text{f}}_{\text{c}}{\prime}$ ratio of 0.64, suggesting that the increased AAS ratio enhances ${\text{f}}_{\text{c}}{\prime}$ more significantly than ${\text{f}}_{\text{sp}}$. Conversely, the G50S50 mix (50% GGBS, 50% SF) has an increased ratio of 1.10, indicating a substantial improvement in ${\text{f}}_{\text{sp}}$ relative to ${\text{f}}_{\text{c}}{\prime}$, demonstrating that the higher AAS ratio effectively balances the activation of both GGBS and SF. The G25S75 mix shows a ratio of 1.22, indicating higher ${\text{f}}_{\text{sp}}$ relative to ${\text{f}}_{\text{c}}{\prime}$. The G0S100 mix has the highest ratio of 1.40, showing a significant relative increase in ${\text{f}}_{\text{sp}}$ compared to ${\text{f}}_{\text{c}}{\prime}$, underscoring the substantial impact of the higher AAS ratio on the tensile properties of SF-dominant mixes. This suggests that a higher AAS ratio is particularly beneficial for enhancing the tensile properties of geopolymer concrete, especially in mixes with significant SF content.

Based on the results of ${\text{f}}_{\text{sp}}$ and ${\text{f}}_{\text{c}}{\prime}$, an equation was developed correlating the two properties, as shown in Eq. (5) and Fig. 18(a). An acceptable correlation exists between the two properties, with a correlation coefficient, R², of 0.85. Nevertheless, the codified equations presented in Table 11, are used to examine their applicability for GGBS/SF-blended geopolymer concrete mixes in this study. The experimental and predicted ${\text{f}}_{\text{sp}}$ results obtained from the proposed Eq. (4) are compared to the codified equations presented in Table 11, and plotted in Fig. 18(b). The models developed by ACI 318–19⁶³, CEB-FIB⁶⁴, and AS3600⁶⁵ significantly underestimated ${\text{f}}_{\text{sp}}$, while those of Eq. (5) converged on the 45°-line. The ACI 318–19, CEB-FIP, and AS3600 models result in, on average, 80, 84, and 87% lower values, respectively. Consequently, the codified equations can be adjusted to suit the geopolymer mixes presented herein by introducing correction factors, presented in Table 11. Yet, the proposed Eq. (4) and the modification presented in Table 11 are limited to the geopolymer concrete mixes investigated in this study.

Fig. 18

(a) Relationship between ${\text{f}}_{\text{sp}}$ and ${\text{f}}_{\text{c}}{\prime}$ for all geopolymer concrete mixes at 28 days, (b) Experimental versus predicted splitting tensile strength, ${\text{f}}_{\text{sp}}$, and (c) Relationship between ${f}_{sp}/\sqrt{{f}_{c}{\prime}}$ ratio with SF content % with previous studies.

Table 11 Codified Equations used to predict the ${\text{f}}_{\text{sp}}$ from ${\text{f}}_{\text{c}}{\prime}$.

$${\text{f}}_{\text{sp}}\text{=2.76}\sqrt{{\text{f}}_{\text{c}}{\prime}}$$

(5)

Few studies have investigated the incorporation of significant amounts of SF in GGBS-based geopolymer concrete systems. Figure 18(c) illustrates the variation of the normalized tensile strength ratio ${f}_{sp}/\sqrt{{f}_{c}{\prime}}$ with increasing SF content for different AAS ratios across studies, highlighting key differences between this study and those by Premkumar et al.²⁴ and Hamed et al.²⁰. In this study, for AAS = 1.5, ${f}_{sp}/\sqrt{{f}_{c}{\prime}}$ decreases as SF content increases up to 75%, after which it improves, likely due to compaction and workability challenges at higher SF levels. For AAS = 2.0, the ratio is consistently higher with SF content of at least 50% or more of the total binder, demonstrating enhanced geopolymerization and improved tensile-to-compressive strength performance at higher activator concentrations. In contrast, Premkumar et al.²⁴ reported much lower ratios for both AAS = 1.5 and 2.5, likely due to the inclusion of recycled aggregates, which may have weakened the aggregate-paste bond and limited tensile performance. Similarly, Hamed et al.²⁰, for AAS = 2.5 in a 100% SF geopolymer mix, observed stable but lower values compared to this study, likely due to excessive SF affecting workability and reducing matrix homogeneity. These findings highlight the superior performance of SF-GGBS geopolymer concrete in this study, emphasizing the importance of optimized AAS ratios, mix design, and aggregate selection in achieving favorable tensile-to-compressive strength relationships.

Performance Index for individual criteria

The performance index (PI) was used as an indicative metric to evaluate various aspects, including fresh, mechanical, and durability properties, as well as cost. It serves as a management tool that consolidates multiple data sets into a single assessment measure⁶⁶. In this study, the properties of the proposed GGBS/SF blended geopolymer concrete mixes were compared to cement-based concrete with properties recommended by ACI 318–19⁶³. The benchmark concrete is to be used for structural purposes, with a slump, ${\text{f}}_{\text{c}}{\prime}$, ${\text{E}}_{\text{c}}$, and ${\text{f}}_{\text{sp}}$ values of at least 75 mm, 25 MPa, 25 GPa, and 5 MPa, respectively. Note that the mechanical properties considered herein are at 28 days.

Initially, the individual performance criterion for all three mechanical properties (i.e., ${\text{f}}_{\text{c}}{\prime}$, ${\text{E}}_{\text{c}}$, and ${\text{f}}_{\text{sp}}$) and slump was assigned a weighted ranking (${\text{W}}_{\text{i}}$), in which the best value in each criterion was given a value of 1.0, and the remaining were proportioned with respect to the best value, as depicted in Eq. (6). For example, ${\text{W}}_{\text{i}}=1$ for the benchmark concrete mix because it has an optimum slump value, after which the PI of other mixes was proportioned to the slump value of the benchmark concrete mix. The weighted ranking (${\text{W}}_{\text{i}}$) presented in Eq. 6 is computed by dividing the measured performance value (${\text{V}}_{\text{m}}$) over the best performance value (${\text{V}}_{\text{b}}$). The ${\text{V}}_{\text{m}}$ is the value obtained experimentally, which is an average of 3 tested specimens, while ${\text{V}}_{\text{b}}$ is the best value for each property, recommended by ACI 318–19⁶³. The ${\text{V}}_{\text{b}}$ values for a slump, ${\text{f}}_{\text{c}}{\prime}$, ${\text{E}}_{\text{c}}$, and ${\text{f}}_{\text{sp}}$ are presented in Table 12, which represents the properties of concrete for structural applications, as recommended by ACI 318–19.

Table 12 Properties of concrete recommended by ACI 318–19 for structural applications.

$${\text{W}}_{\text{i}}\text{=}\frac{{\text{V}}_{\text{m}} }{{\text{V}}_{\text{b}}}.$$

(6)

$${\text{V}}_{\text{m}}\text{: Measured Performance Value}$$

$${\text{V}}_{\text{b}}\text{: Best Performance Value}$$

The PIs for the individual criteria of each mix are presented in Fig. 19. The figure presents the individual performance criteria (${\text{W}}_{\text{i}}$) for geopolymer concrete mixes across four key properties: slump, cylinder compressive strength (${\text{f}}_{\text{c}}{\prime}$), modulus of elasticity (${\text{E}}_{\text{c}}$), and splitting tensile strength (${\text{f}}_{\text{sp}}$). The cost was not considered in the analysis because Group A and B mixes had similar proportions with variations in sodium hydroxide, and the costs of GGBS and SF were similar. It is worth mentioning that Group B mixes had slightly lower costs than Group A mixes. This evaluation was based on the local market and does not apply internationally. Figure 19 presents the performance indices for the individual criteria of each mix, compared to the properties of concrete mix recommended by ACI 318–19 [72], denoted here as the benchmark. The recommended properties were given a value of ${\text{W}}_{\text{i}}$ = 1, whereas values above this limit indicate better performance compared to the benchmark.

Fig. 19

Individual Performance Criteria for all mixes compared to the properties of benchmark concrete mix recommended by ACI 318–19⁶³ for structural applications.

All geopolymer concrete mixes did not meet the benchmark slump of 75 mm, with slump values decreasing as SF content increases. The G50S50 mix with AAS = 1.5 shows improved ${\text{f}}_{\text{c}}{\prime}$ (PI = 1.113), while the G100S0 mix with AAS = 2.0 achieves the highest compressive strength (PI = 1.387). However, mixes with high SF content, like G25S75 and G0S100, have reduced compressive strengths in both groups. For ${\text{E}}_{\text{c}}$, none of the mixes surpass the benchmark, with the G100S0 mix with AAS = 2.0 having the highest value (PI = 0.748). All geopolymer concrete mixes with at least 50% GGBS in groups A and B had at least 45% greater values of ${\text{f}}_{\text{sp}}$ than the benchmark. ${\text{f}}_{\text{sp}}$ is highest in the G100S0 mix with AAS = 2.0 (i.e., PI = 2.6), with several other mixes also attaining the recommended values of ${\text{f}}_{\text{sp}}$. Overall, the G100S0 mix with AAS = 2.0 shows the best performance in ${\text{f}}_{\text{c}}{\prime}$ and ${\text{f}}_{\text{sp}}$, though it has reduced stiffness and workability, highlighting the need to balance SF and admixture content for optimal properties.

The properties of the produced geopolymer concrete mixes are compared to the recommended standards for structural applications. Mixes that do not meet the minimum structural requirements can still be utilized for infrastructural applications.

Conclusion

The hardened properties and workability of geopolymer concrete mixes made with varying binder proportions of GGBS and SF and AAS ratios were investigated. Based on the experimental test results, the following conclusions can be drawn:

• SF substitution for GGBS generally reduces workability, with higher SF content and increased AAS ratio leading to lower slump values, except in mixes made with 100% SF.

• The ${\uprho }_{{\text{h}}}$ increased by 7.9% at 28 days with up to 50% SF substitution at an AAS ratio of 1.5, but a 75% SF substitution led to an 11.6% density decrease. Conversely, at an AAS ratio of 2.0, densities consistently decreased with higher SF content, with a maximum reduction of 11.6% at 28 days when 100% SF replaced GGBS, indicating increased porosity.

• Compressive strength development varied; the G100S0 mix in Group A showed a 33% increase from 1 to 7 days and an 11% increase from 7 to 28 days, while the G50S50 mix demonstrated balanced strength development (11% early, 33% later). Group B mixes, with higher AAS ratios, exhibited greater late-age strength development, with G100S0 showing a 28% increase from 7 to 28 days compared to 11% in Group A.

• Increasing the AAS ratio from 1.5 to 2.0 significantly enhanced early compressive strength in GGBS-based mixes: G100S0 showed a 100% increase and G75S25 showed a 43% increase at day 1. Conversely, the same increase in the AAS ratio led to substantial decreases in compressive strength for mixes with 50% SF and above G50S50 and G25S75 experienced reductions of 85% and 63%, respectively.

• Increasing SF content up to 50% in GGBS/SF blended mixes at an AAS ratio of 1.5 enhanced 28-day cylinder compressive strength (${\text{f}}_{\text{c}}{\prime}$) by up to 50%. However, at AAS ratio 2.0, G100S0 and G75S25 mixes showed increases of 63% and 44%, respectively, while SF-dominant mixes (G50S50, G25S75, G0S100) exhibited decreases of 41%, 43%, and 25%, respectively.

• The ${\text{f}}_{\text{c}}{\prime}$-to-${\text{f}}_{\text{cu}}$ ratio ranged from 0.24 to 0.52 for GGBS/SF blends, with higher SF content reducing the slenderness effect. Increasing the AAS ratio generally increased the ratio for mixes up to 50% SF but decreased it for SF-dominant mixes due to lower compressive strengths.

• Increasing the AAS ratio from 1.5 to 2.0 in GGBS-based geopolymer concrete (G100S0) increased peak stress by 57% and decreased corresponding strain by 52%. Conversely, mixes with higher SF content (50%, 75%, and 100%) showed decreased peak stresses of 36%, 49%, and 23%, respectively, and increased strains of 36%, 22%, and 5%.

• Increasing the AAS ratio from 1.5 to 2.0 significantly enhanced ${\text{E}}_{\text{c}}$ in GGBS-based mixes: G100S0 showed a 240% increase from 5.5 GPa to 18.7 GPa. However, increasing SF content in GGBS/SF blends decreased ${\text{E}}_{\text{c}}$, with reductions of 80%, 87%, and 90% for G50S50, G25S75, and G0S100 mixes, respectively.

• The ${\text{f}}_{\text{sp}}$ generally decreased with increasing SF content and AAS ratio, except for balanced GGBS/SF blends. At an AAS ratio of 2.0, GGBS-based mixes showed increases in 28-day ${\text{f}}_{\text{sp}}$ by 33%, while mixes with equal GGBS and SF showed increases by 13%. SF-dominant mixes exhibited decreases averaging 6%, highlighting SF’s negative impact on early and long-term tensile strength.

• All geopolymer concrete mixes fall short of the benchmark slump, but the G100S0 mix with AAS = 2.0 surpasses the benchmark in compressive strength and splitting tensile strength, despite none exceeding the benchmark for modulus of elasticity, highlighting the need to balance SF and admixture content for optimal properties.

Future research could focus on optimizing SF activation, enhancing workability through additives, and further investigating the long-term durability of SF-abundant geopolymer mixes.