Abstract
The cold bonding technology for producing lightweight aggregates has received significant international attention in the fields of waste treatment and the production of green building materials. In this study, sustainable lightweight aggregate concrete (LAC) is prepared by partially replacing natural coarse aggregate with cold-bonded sewage sludge ash lightweight aggregate. The properties and microscopic characteristics of concrete were studied. The results show that the density and compressive strength of LAC decreased to 1645 kg/m3 and 12.83 MPa when the replacement rate of lightweight aggregates from 0 to 100%, indicating that the LAC can be used for non-structural LAC. Structural LAC can also be designed when 75% lightweight aggregates and 25% natural coarse aggregate are used simultaneously. In addition, the results of Scanning Electron Microscope and Nuclear Magnetic Resonance show that high lightweight aggregate content is accompanied by higher porosity and pore size inside the LAC, which reduces the resistance of concrete to drying shrinkage and freeze-thaw cycles. This research provides theoretical support for the application of SSA in sustainable LAC and reducing the damage of SSA to the environment.
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Introduction
As a result of increasing industrialization and urbanization, the amount of municipal sewage sludge grows annually around the world1. Land, groundwater, and air are threatened by municipal sewage sludge, which has high organic content, high heavy metal content, and high levels of volatile carbon2,3. As the environmental policy applicable to municipal sewage sludge becomes more and more strict4 it is imperative to make environmentally friendly use of municipal sewage sludge. As a technology for greatly reducing the volume and organic pollutants of municipal sewage sludge, incineration has been widely used around the world5,6. However, sewage sludge ash (SSA) with high heavy metal content is produced after incineration7,8. The main mineral compositions of SSA are SiO2, Al2O3, Fe2O3, and P2O59,10. Due to the similar production process and composition of SSA to coal fly ash, the use of SSA in construction materials is considered the most environmental-friendly treatment11,12. However, the existing researches show that the utilization rate of SSA in concrete is still low. Therefore, exploring an efficient and environmental-friendly way to use SSA in the construction industry has always been the focus of researchers.
Concrete is the most common building material in roads, bridges, dams and other projects. More than 30 billion tons of concrete were produced worldwide each year13. However, the extensive use of concrete has led to the increasing depletion of resources such as sand and aggregate14,15. Aggregates in concrete account for up to 60%−75%16,17. The application of artificial lightweight aggregate in concrete significantly saves the non-renewable aggregates and decreases the weight of the concrete18,19,20. The production of lightweight aggregates by industrial by-products can meet the sustainable development of aggregates and reduce the economic cost related to the disposal of these pollutants21,22,23. Based on 175 studies, Chinnu concluded that lightweight aggregates prepared by industrial byproducts have great potential as substitutes for natural coarse aggregates24. The lightweight concrete, self-compacting concrete, and high-strength concrete prepared by lightweight aggregate have obtained many research achievements25,26. Therefore, the preparation of SSA into lightweight aggregate and its application in concrete is one of the most efficient and environmental-friendly treatment methods.
High-temperature sintering was one of the first technologies to transform waste into lightweight aggregate27,28. Lee produced lightweight aggregate of water treatment sludge using a rapid sintering process29. However, high-temperature sintering technology will consume a lot of energy and emit pollution gas30. The method of using cement and other cementing materials to prepare lightweight aggregates with waste is called cold-bonded technology. The amount of cement often needs to be increased to ensure the performance of the lightweight aggregate, which intensifies environmental issues related to cement manufacturing31. Geopolymer lightweight aggregates were developed by alkaline activation of different precursors, such as fly ash, granulated blast furnace slag, wood ash, and metakaolin31,32. Preparation of geopolymer lightweight aggregates requires that the precursor or additive has certain pozzolanic activity. The moderate pozzolanic activity of SSA has been confirmed, which indicates that geopolymer lightweight aggregate can be prepared from SSA with less binder33. However, there are few reports on the preparation of geopolymer lightweight aggregate by SSA34. In addition, when different lightweight aggregates are used in concrete, there will be significant differences in performance, and targeted exploration is required.
In this study, SSA, cement, and granulated blast furnace slag are mixed in a ratio of 1:0.15:0.15 to prepare lightweight aggregate by using an alkaline activator. The optimum ratio and performance of the SSA lightweight aggregate were obtained based on pilot tests. However, Study on the application of SSA in cold-bonded lightweight aggregates or lightweight aggregate concrete (LAC) is still in the exploratory stage. Therefore, to use SSA on a large scale in concrete, the lightweight aggregates made of SSA replaced natural coarse aggregates with different percentages (0%, 25%, 50%, 75% and 100%) to produce LAC in this study. The physical properties, mechanical properties, drying shrinkage, and resistance to freeze-thaw cycles of concrete were studied. The microscopic characteristics of concrete are used to characterize the mechanism of strength deterioration and damage failure. This research provides theoretical support for the application of SSA in sustainable LAC and reducing the damage of SSA to the environment.
Materials and experimental methods
Materials
The binder and supplementary cementitious material of concrete are Portland cement 42.5R and silica fume (SF). The specific density of cement and SF are 3140 kg/m3 and 2200 kg/m3, respectively. The Blaine specific surface area of cement and SF are 353 m2/kg and 25 m2/g, respectively. Table 1 shows the chemical composition of cement, SSA, and SF. Figure 1 shows the particle size distribution curves of the three raw materials.
The particle size distribution curves of SSA, cement and SF.
River sand with a 2.52 fineness is used as fine aggregate. SSA lightweight aggregates and natural granite were used as the coarse aggregates. According to previous studies35 the cylinder compressive strength and water absorption of SSA lightweight aggregate are 1.8 MPa and 24.58%, respectively, which meet the ACI-213R standard (Fig. 2). The preparation process of SSA lightweight aggregates is divided into the following steps. First, thoroughly mix SSA, cement and mineral powder, and then pour them into a granulator with a rotational speed of 36 rpm. Then, spray the alkaline activator solution onto the surface of the mixture through a spray bottle to make it into round particles. Finally, the aggregates with smooth surfaces and meeting the required particle size are taken out and placed in a standard curing box (20℃ and 95% relative humidity) for curing for 28 days. Size of lightweight aggregates and natural granite used in this study ranges from 4.75 to 16.0 mm. The coarse aggregates gradation complies with the requirements of standard ASTM C33. The bulk densities of SSA lightweight aggregates and natural granite aggregates are 655 kg/m³ and 1540 kg/m³, respectively. The apparent density and water absorption rate of natural granite aggregates are 2651 kg/m³ and 0.8%, respectively.
Coarse aggregate gradation curve.
Concrete mixes
According to the technical specification (JGJ-51-2002), LC30 LAC was prepared from SSA lightweight aggregates instead of traditional gravel coarse aggregates by equal volume substitution method36. According to the density grade of lightweight aggregates, cement in each mix was limited to 450 kg/m3. Silica fume was taken as 10% of the mass of binder and supplementary cementitious material. A total of five trial mixes (SSA0 to SSA100) were prepared as given in Table 2. Figure 3 shows the preparation of concrete samples. Considering that the higher water absorption rate of lightweight aggregates will affect the water-to-binder ratio of the mixes, lightweight aggregates were saturated with water absorption 24 h in advance and had a saturated dry surface before mixing.
The preparation and curing method of concrete.
Test method
According to the GB/T 11968 − 2020 standard, the drying density and water absorption rate of LAC were tested using 100 mm cubic specimens. According to the GB/T 50081 − 2002 standard, 100 mm cubic specimens were used to test the compressive strength (Fig. 4a) and splitting tensile strength (Fig. 4c) of LAC. According to the (EN 196-1 standard, the prism specimens (40 × 40 × 160 mm) were used for determining flexural strength (Fig. 4b) of LAC. According to the GB/T GB/T 50082 − 2009 standard, the prism specimens (40 × 40 × 160 mm) were used for determining drying shrinkage and resistance to freeze-thaw cycles tests36,37. The loading speed for compressive strength is 2400 N/s and the loading speed of 50 N/s is used for splitting tensile and flexural strength test. The resistance to freeze-thaw cycles and drying shrinkage of concrete are determined by a constant temperature and humidity test chamber (Fig. 4f) and a digital micrometer gauge (Fig. 4d), respectively. In the freeze-thaw test, the saturated concrete samples curing for 28 days were completely immersed in water. A single freeze-thaw cycle includes a 4-h freezing state (−18 °C) and a 4-h melting state (18 °C). The mass change of the samples is measured every 25 cycles. And the development of freeze-thaw failure of concrete is assessed through mass loss and apparent change38. The porosity of concrete is determined by a GeoSpec 2/150 NMR (Fig. 4e). All the data from the above tests are the average results obtained from three groups of samples. The microscopic image and interface transition zone (ITZ) of concrete observed by a SEM (Thermo Scientific Apreo 2 C).
(a) Compressive test, (b) Flexural test, (c) Splitting tensile test, (d) Drying shrinkage test, (e) Porosity test, and (f) Freeze-thaw test.
Results and discussion
Physical properties
To reduce the density of concrete and save natural aggregate resources, the use of lightweight aggregate in concrete is one of the mature methods39. The drying density and water absorption of LAC prepared from SSA lightweight aggregate is shown in Fig. 5. The results show that the drying densities of SSA0 and SSA100 are 2332 kg/m3 and 1645 kg/m3, respectively. Compared with SSA0, the drying density of SSA100 is reduced by 29.46%. When the substitution rate of SSA lightweight aggregate reaches 75%, the drying density of concrete meets the requirements of being lower than 1950 kg/m3 according to Standard JGJ/T 12-2019. However, lightweight aggregates with high water absorption generally increase the water absorption of LAC. This is due to the lightweight aggregate affecting the ITZ of concrete40. The results in Fig. 5(b) show that the 24 h water absorption of SSA0 and SSA100 are 1.89% and 11.13%, respectively. Compared to SSA0, the water absorption of SSA100 increased by 488.89%.
(a) drying density and (b) water absorption.
Mechanical properties
Compressive strength
Figure 6 shows the compressive strength of concrete under different curing ages. As shown in Fig. 6, the higher replacement rates of SSA lightweight aggregates decreased the compressive strength of LAC. According to the standards, the LAC with a greater compressive strength than 17.2 MPa can be used for structural LAC41. Therefore, the SSA75 with a 28-d strength of 18.24 MPa meets the strength requirements of structural LAC, while SSA100 with a 28-d strength of 12.83 MPa can only be used as non-structural LAC. The same phenomenon has also been found in other lightweight aggregate concrete, such as cold-bonded municipal solid waste incineration bottom ash lightweight aggregates42 rubberized concrete41 and diatomaceous earth concrete43. The weakening mechanism of SSA lightweight aggregate on concrete strength mainly has the following two reasons: On the one hand, the spherical shape of SSA lightweight aggregate and its low strength. In addition, SSA lightweight aggregate increases the porosity of concrete and reduces its compactness.
The 7 d, 28 d, and 96 d compressive strength of LAC.
Figure 7 shows the uniaxial compression stress-strain curves of concrete with different SSA lightweight aggregate contents. The result shows that the stress-strain curves of all concrete have obvious pore compaction stage, elastic stage, and yield stage. With the increase of SSA lightweight aggregate content, the strain variable in the compaction stage of concrete becomes larger, and the curve slope in the elastic stage decreases, indicating that the higher SSA lightweight aggregate content leads to the increase of the internal pores of concrete and the decrease of elastic modulus. After reaching the peak stress, the curve of SSA0 shows a tendency of straight-line decline and has no residual strength, indicating that traditional concrete is a brittle material. With the increase of the SSA lightweight aggregate content, the decline rate of the curve of concrete after reaching the peak stress significantly decreases, indicating that the plasticity of concrete is improved. The residual stress of SSA100 concrete can reach about 51.31% of the peak stress.
Stress-strain curve of LAC under uniaxial compression.
The SEM micrographs of concrete are used to analyze ITZ and pore characteristics inside concrete, as shown in Fig. 8. The granite and cement mortar in SSA0 has a dense structure, and the aggregate has good adhesion to the cement mortar, resulting in a small ITZ width. The cement mortar inside concrete containing SSA lightweight aggregates can be embedded in the aggregate surface due to the high roughness of the lightweight aggregate surface. This is because lightweight aggregate is formed by wrapping SSA particles under the action of a binder35. Compared to SSA0, the ITZ in SSA50 and SSA100 has no a through crack. In addition, the densification of mortar is better than that of SSA lightweight aggregate, resulting in the lightweight aggregate being damaged before mortar under the action of external forces.
SEM micrographs of the ITZ of concrete.
Splitting tensile and flexural strength
Figure 9 shows the splitting tensile and flexural strength development trend of concrete after 28 days. The results show that the two types of strength of concrete were reduced by increasing the content of SSA lightweight aggregates. The splitting tensile strength and flexural strength of SSA0 are 4.56 MPa and 9.98 MPa, respectively. However, the splitting tensile strength and flexural strength of SSA100 are only 0.94 MPa and 2.96 MPa, respectively. The reason for this phenomenon may be that the addition of lightweight aggregates weakens ITZ and increased the porosity of concrete44. Meanwhile, the cylinder compressive strength of the SSA lightweight aggregate is less than 2 MPa35 resulting in the lightweight aggregates failing before cement mortar under tensile stress. It is a good guess that the strength of concrete can be improved by increasing the strength of SSA lightweight aggregate. Previous studies have shown that the strength of lightweight aggregate can be highly strengthened by waterproofing and wrap-shell treatment based on maintaining low-density characteristics45 indicating that the application of the SSA lightweight aggregate in the LAC has broad prospects.
The development trend of (a) splitting tensile and (b) flexural strength.
The strength relationship between splitting tensile and compressive of LAC recommended by ACI 318–14 is plotted in Fig. 10(a), and the strength relationship between flexural and compressive of LAC is plotted in Fig. 10(b). The results show that based on lightweight, the splitting tensile strength of the LAC prepared by SSA lightweight aggregate is lower than the standard value, while the flexural strength is higher than the standard value. This may be due to the greater effect of lightweight aggregate on ITZ weakening in large-size samples. Similar results are also found in LAC prepared by diatomaceous earth lightweight aggregates43.
Strength relationship (a) between splitting tensile and compressive and (b) between flexural and compressive.
Figure 11 shows the failure surfaces of the LACs under different SSA lightweight aggregate content after splitting tensile test. The failure surfaces of SSA25 to SSA100 show that the SSA lightweight aggregates are evenly distributed inside the concrete, which is conducive to the production of LAC with more stable properties. In addition, almost all the SSA lightweight aggregates in the failure surfaces are damaged, indicating that the lightweight aggregate itself will be destroyed before the interface transition zone between the aggregate and the mortar under the action of tensile stress. Therefore, the strength of lightweight aggregate is also one of the main factors determining LAC performance.
Failure surfaces of the concrete after splitting tensile test.
Resistance to freeze-thaw and drying shrinkage
Resistance to freeze-thaw
When the ambient temperature is below 0 °C, the water inside the concrete and the SSA lightweight aggregates with high water absorption becomes ice and then produces the expansion force in pores, leading to increased cracks in the lightweight aggregates and the concrete46. The obvious freeze-thaw damage in concrete will be appeared with the increase and expansion of cracks. Figure 12 shows the mass loss changes of the concrete with different SSA lightweight aggregate contents with the increase of the number of freeze-thaw cycles. In the early stage of the freeze-thaw cycle, the continuous hydration and the increase in pore water content result in an increase in the mass of the samples. The long-term freeze-thaw cycles aggravates the damage of concrete and causes the surface mortar or aggregate to drop, which leads to its mass loss. High SSA lightweight aggregate content is associated with higher mass loss of concrete. This is because the higher lightweight aggregate content increase of the porosity of the LAC, and the SSA lightweight aggregate has poor freeze-thaw resistance35,47. However, the mass loss of the concrete with different SSA lightweight aggregate contents under 250 cycles is less than 5%, indicating that the concrete meets the freeze-thaw resistance grade of F250.
Mass loss of concrete with different SSA lightweight aggregate contents during the freeze–thaw test.
Figure 13 shows the surface damage of concrete with different SSA lightweight aggregate contents after 250 cycles. It is obvious that the freeze-thaw damage of concrete is positively correlated with the content of the SSA lightweight aggregates under the same freeze-thaw cycles. The SSA0 without SSA lightweight aggregate is damaged only at the corners. The SSA25 and SSA50 specimens exhibited surface damage and the exposure of SSA aggregates. However, the SSA75 and SSA100 show more obvious damage. This may be attributed to the poor freeze-thaw resistance of the SSA lightweight aggregates and the increased porosity of the cement mortar around the lightweight aggregates37,48.
The surface damage of concrete with different SSA lightweight aggregate contents after 250 cycles.
Figure 14 shows the micrographs of the SSA50 and SSA100 after 250 cycles. Obvious cracks appear in the mortar and the ITZ after 250 cycles. However, the SSA lightweight aggregate suffered more significant freeze-thaw damage, resulting in direct loss from the LAC (Fig. 13). This may be that when the pore water inside the mortar and SSA lightweight aggregates become ice, the expansion force is generated, which causes the SSA lightweight aggregates with lower bond and tensile strength appearing freeze-thaw damage before the mortar.
Micrographs of the SSA50 and SSA100 after 250 cycles.
Drying shrinkage properties of SSA LAC
As shown in Fig. 15, the LAC with different SSA lightweight aggregate content shows three different deformation characteristics in the drying shrinkage test. In the first 20 days, the drying shrinkage of the LAC increases rapidly. This phenomenon occurs in almost all LAC49. As the drying shrinkage test continued to the 80th day, the drying shrinkage of the LAC increased slowly due to the slow loss of moisture inside the lightweight aggregate and mortar. As the drying shrinkage test continued, the drying shrinkage of LAC increased at a very slow rate. At 163 days, the drying shrinkage of SSA0, SSA25, SSA50, SSA75 and SSA100 LAC are 693,75µε, 818.75µε, 912.50µε, 1197.92µε, and 1729.17µε, respectively. The drying shrinkage of apricot shell and peach shell concrete37 wood sand concrete50 and foamed concrete51 at three months are approximately 1300–1800µε, 1200–1900µε, and 2250–3000µε. Therefore, the drying shrinkage of LAC prepared by SSA lightweight aggregates in this study is close to or lower than that in other studies, indicating the feasibility of its application.
Drying shrinkage of the concrete with different SSA lightweight aggregate contents.
Figure 16 shows the schematic diagram of the drying shrinkage of LAC prepared by SSA lightweight aggregate. Before the drying shrinkage test, both the mortar and lightweight aggregates inside the LAC are saturated due to the soaking treatment of the SSA lightweight aggregates before the preparation of the LAC. As the internal relative humidity of the mortar and SSA lightweight aggregates decreases, the concrete exhibits obvious drying shrinkage, as shown in Fig. 13. Meanwhile, the porosity and water content of the LAC increased with the increase of lightweight aggregate content, increasing drying shrinkage52.
Schematic diagram of drying shrinkage of the LAC prepared by SSA lightweight aggregates.
Pore characteristics
Previous studies have shown that NMR technology can characterize the pore size, porosity, and pore content of concrete53,54. T2 spectrum relaxation curves of concrete with different SSA lightweight aggregate content were obtained, as shown in Fig. 17. The results show that the T2 spectrum relaxation curves of all specimens contain a main peak and a minor peak. Previous studies have shown that the main peak and the minor peak represent the transition pores and macropores, respectively55. In addition, the two peaks of the curves show obvious rightward and upward movement with the increase of lightweight aggregate content. This phenomenon indicates that the pore diameter and porosity of the concrete increase. The effective porosity of SSA0, SSA25, SSA50, SSA75 and SSA100 are 0.53%, 0.97%, 1.40%, 2.30% and 2.90%, respectively. This is consistent with the research results of physical properties, mechanical properties, and durability of concrete prepared by SSA lightweight aggregates.
T2 spectra distribution curves of the concrete.
Discussion
In this paper, SSA is prepared into lightweight aggregates in a green and environmental-friendly way and then made into LAC. Obviously, the addition of SSA lightweight aggregates can significantly reduce the density of concrete, which is conducive to reducing the self-weight of components and improving the utilization rate of SSA. However, the mechanical properties of concrete will also decrease with the increase of SSA lightweight aggregate content, which will become the biggest obstacle to the application of SSA lightweight aggregate in concrete. Furthermore, the addition of SSA lightweight aggregates will also increase the water absorption rate of the concrete. When high water-absorbing concrete is used outdoors, it may accelerate the infiltration of harmful components in water, thereby reducing the durability of the concrete. Therefore, it is advisable to avoid direct contact between concrete and water as much as possible. Fortunately, through microscopic feature analysis, the mechanism of damage to concrete performance caused by SSA lightweight aggregates was discovered, which is the low strength of the lightweight aggregates themselves and the increase in LAC porosity. These findings are helpful for clarifying the future research direction.
Conclusions
In this study, green sustainable concrete is successfully prepared by SSA lightweight aggregate replacement for traditional granite gravel coarse aggregate. The physical properties, mechanical properties, drying shrinkage, and resistance to freeze-thaw cycles of concrete were studied. The microscopic characteristics of concrete are used to characterize the mechanism of strength deterioration and damage failure. the following main conclusions were extracted.
(1) The LAC with a density of less than 1950 kg/m3 (JGJ/T 12 2019) was successfully prepared by SSA lightweight aggregates. The density of the SSA0 and SSA100 are 2332 kg/m3 and 1645 kg/m3, respectively.
(2) The increase of SSA lightweight aggregate content weakens the mechanical properties of concrete. The SSA75 with a 28 d strength of 18.24 MPa meets the strength requirements of structural LAC, while SSA100 with a 28 d strength of 12.83 MPa can only be used as non-structural LAC.
(3) The drying shrinkage and mass loss after the freeze-thaw cycles of the concrete are positively correlated with SSA lightweight aggregate contents. However, the LAC prepared by SSA lightweight aggregates has a freeze-thaw resistance grade of F250 and a drying shrinkage value within a reasonable range.
(4) The NMR results show that the addition of the SSA lightweight aggregates leads to an increase in the porosity and pore diameter of concrete, thereby leading to the deterioration of its mechanical properties and durability. The porosity of SSA0 and SSA100 are 0.53% and 2.90% respectively.
Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Funding was provided by Guizhou Province Science and Technology Cooperation Achievement Project, [2023] General project:777.
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“J.S. and X.L. wrote the main manuscript text and P. and G.C. prepared all figures. Y.T and B.X. reviewed the manuscript.”
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No, I declare that the Junsong Jiang, Xianliang Zhou, Yantao Zheng, Ping Wu, Guichuan Jiang and Bingxi Jian have no competing interests as defined by Nature Research, or other interests that might be perceived to influence the results and/or discussion reported in this paper.
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Jiang, J., Zhou, X.L., Zheng, Y.T. et al. Recycling of sewage sludge ash in cold-bonded artificial lightweight aggregate for sustainable lightweight concrete. Sci Rep 15, 27671 (2025). https://doi.org/10.1038/s41598-025-13133-w
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DOI: https://doi.org/10.1038/s41598-025-13133-w
