Geopolymerization Reaction Research Articles

Effect of sintering temperature on the properties of ceramic binders synthesized from coal gangue-based geopolymer

ABSTRACT As a by-product of the coal industry, the effective use of coal gangue presents significant implications for environmental protection and sustainable development upon effective utilization. Given the rich content of silicon and aluminum in coal gangue, this study converts it into a precursor for ceramic binders used in grinding wheels through the geopolymerization reaction process. Ceramic binders of the SiO2-B2O3-Al2O3-RO-R2O system were prepared at sintering temperatures ranging from 590 to 740°C, and the performance of each sample was tested. The research found that the flexural strength of the ceramic binder reached a peak of 31 MPa when sintered at 690°C. Additionally, the hardness was observed to be the highest at a sintering temperature of 640°C, reaching 211.8 hV. It was observed that as the sintering temperature rose, wettability tended to decrease. Using techniques including XRD, SEM, and EDS, the effect of sintering temperature on the microstructure and phase transformation of the material was analyzed, and these changes determined the properties of the material. This study not only confirmed the potential value of coal gangue in the abrasive tools industry but also opened up new strategies and technical solutions for the field of solid waste resource utilization.

Influence of alkali molarity on compressive strength of high-strength geopolymer concrete using machine learning techniques based on curing regimes and temperature

The compressive strength behavior of high-strength geopolymer concrete (HSGPC) has been studied in this research work with varying alkali concentration using the novel machine learning techniques. The alkali concentration in the activation solution plays a significant role in the geopolymerization process and affects the resulting compressive strength. In this research work, the range between 4 M and 16 M for alkali molarity (M), 18 kg/m3 and 160 kg/m3 for NaOH and 41 kg/m3 and 229 kg/m3 for NaSi was collected from literature and used in the various design mixes of this exercise. This was necessary because higher alkali concentrations promote a more efficient dissolution and activation of the aluminosilicate compounds, leading to increased geopolymerization and the formation of more calcium silicate hydrate (C-S-H) gel. The increased C-S-H gel content contributes to improved strength development. However, there is an optimal alkali concentration range for the sustainable production of geopolymer concrete, and exceeding this range can have a negative impact on compressive strength and ecofriendly handling of concrete. A total of fifty-three records were collected from literature and deployed in modeling the compressive strength (Fc) considering various curing regimes. Three symbolic machine learning techniques such as genetic programming (GP), evolutionary polynomial regression (EPR), and the artificial neural network (ANN) are used in this research model. The relative importance values for each input parameter were also evaluated, which indicated that all factors have significant impacts on (Fc), but Age (i.e., curing regime) has the most influence compared to FA, NaOH, and CAg then the other inputs. From the model relations between the calculated and predicted values, it can be shown that the decisive model, ANN produced line of parametric equation of y = 0.995x, and produced performance indices; MAE of 2.13 MPa, RMSE of 2.86 MPa and R-squared of 0.981, which makes the ANN the most reliable model in agreement with previous applications of the technique. These are against the poor performance of the EPR and GP, which produced R-squared less than 0.8 with higher error rates. The Taylor chart and the variance distribution, which further compares the accuracy and variances of the developed models support the outcomes. Generally, alkali molarity has shown its potential in the production of HSGPC due to its role in the reactivity phases of the concrete formulation; hydration, activation, pozzolanic, and geopolymerization reactions producing the gel needed for the strength gain in HSGPC.

Performance optimization and environmental assessment of novel alkali-activated binders containing carbonated recycled concrete powder

This study investigates the influence of carbonated recycled concrete powder (CRCP), alkali content (AC), and silicate modulus (SM) on the compressive strength, chemical composition, and environmental benefits of CRCP-blast furnace slag blended alkali-activated binders (AABs). The results show that increasing CRCP content reduces the compressive strength of AABs due to lower geopolymeric reactivity. Using response surface methodology, the optimal values for AC and SM in AABs containing 50 wt.% CRCP were determined as 14.28% and 0.92, respectively, resulting in predicted and actual strengths of 49.9 MPa and 46.8 MPa. Chemical analyses reveal that calcite and vaterite are the predominant crystalline phases in AABs, while both Si-Al gels and calcite from CRCP participate in geopolymeric reactions. The effects of AC and SM on AAB strength are further elucidated through the combined analysis of precursor dissolution amount, geopolymeric gel formation amount, and calcium carbonate decomposition amount. Moreover, CRCP demonstrates negative carbon emissions, with a global warming potential of -0.16 kg CO2 eq/kg. The optimally proportioned AAB containing 50 wt.% CRCP shows global warming potential and carbon intensity values significantly lower than those of Portland cement, at 54.4% and 50.8%, respectively. These findings emphasize that AABs containing 50 wt.% CRCP not only meet the 43-grade strength classification but also offer notable environmental benefits. In summary, this study contributes to the development of a novel 43-graded low-carbon binder containing high-volume CRCP.

Hydration process and fluoride solidification mechanism of multi-source solid waste-based phosphogypsum cemented paste backfill under CaO modification

The large-scale, environmentally friendly utilization of phosphogypsum (PG) remains a global challenge. PG cemented paste backfill (PCPB) is a promising method to manage PG, but using ordinary Portland cement as the binder has drawbacks such as high cost, low mechanical strength, and high fluoride leaching risk. This paper presents a multi-source solid waste-based PCPB (MPCPB) material that enhances mechanical properties and reduces fluoride leaching risks. In MPCPB, industrial waste residues like steel slag (SS) and ground granulated blast furnace slag (GBFS) are used as precursors (SS: GBFS = 1:2). Additionally, 4–8 wt% CaO (relative to the dry weight of PG) is used as a neutralizing modifier and alkaline activator. The results indicate that an optimal amount of CaO can neutralize the residual acidity of PG, provide sufficient Ca(OH)2 for MPCPB hydration, and react with PG to produce significant amounts of AFt. Furthermore, CaO promotes the geopolymerization reaction between SS and GBFS, generating more calcium silicate hydrate (C-S-H) and calcium aluminate silicate hydrate (C-A-S-H) gels. Fluoride stabilization in MPCPB results from synergistic effects involving hydration reactions, complexation, ionic mobility, rearrangement, and physical adsorption. Notably, CaO enhances the conversion of free fluoride ions into stable compounds like fluorapatite, fluorite (CaF2), [AlF6]3-, and [FeF6]3- complexes. This approach offers a cost-effective, environmentally friendly, and efficient solution to the PG stockpiling challenge.

Materials design and strength development of a novel cold recycled mixture with asphalt emulsion and geopolymer

ABSTRACT Cold recycling of asphalt pavement with asphalt emulsion is an important approach for achieving low-carbon and sustainable development in road engineering. Enhancing the performance of cold recycled mixtures has become a key research focus. The objective of this paper is to develop a novel cold-recycled asphalt mixture stabilised with asphalt emulsion and geopolymer (CRM-GE). Firstly, the effects of mixing sequence, geopolymer content, asphalt emulsion content and curing conditions on the strength of the mixture and its time-dependent law are studied. Then, the microstructure and composition of the mixture are analysed by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The results show that the optimum amount of asphalt emulsion is different under different geopolymer contents. The indirect tensile strength of the mixture specimen increases with the increase in the curing time and curing temperature. The microstructure analysis indicates that geopolymer reaction and asphalt emulsion demulsification in CRM-GE with 7-day curing are relatively limited, while asphalt film and the geopolymer reaction products are intertwined to achieve a denser microstructure in CRM-GE with 28-day curing. In addition, the element analysis further verifies the result of materials design and micromorphology analysis.

Fracture characteristics and microscopic mechanism of slag-based marine geopolymer mortar prepared with seawater and sea-sand

This study utilized seawater, sea sand, and ternary solid waste to produce marine geopolymer mortar (MGM). 23 groups of MGM samples with varying alkali content and basalt fiber (BF) and polypropylene fiber (PPF) volume fractions, as well as marine cement mortar (MCM), were tested for fracture characteristics. Three initial crack length ratios (a0/h) (0.2, 0.3, and 0.4) were designed. The analysis included load-crack mouth opening displacement (P-CMOD) curves for all specimens, focusing on critical crack length (ac), critical crack tip opening displacement (CTODc), fracture toughness (KSIC), fracture energy (GF), and facture brittleness index (FBI). Results indicated that increasing alkali content enhanced strength and elastic modulus, peak fracture load, critical crack extension length (Δac), and CTODc, while decreasing final CMOD. This was attributed to the increased alkali content, which enhanced the mortar’s density. However, excessive alkali content also led to increased shrinkage damage within the matrix, making it prone to stress concentrations and resulting in brittle failure. Based on the research, it is recommended that the alkali content be controlled within the range of 4–5 %. Hybrid fibers showed superior overall toughness enhancement, increasing KSIC, GF, and FBI by 54.3 %, 95.6 %, and 427.4 %, respectively. This improvement was due to the synergistic effect of BF and PPF, which increased the specimen’s elastic modulus, reduced crack propagation speed, and sustained load-bearing capacity during the later stages of crack development. The fracture toughness of conventional geopolymer mortar was positively correlated with the Ca/Si ratio and was lower than that of MGM. In contrast, MGM’s Ca/Si ratio showed an inverse correlation with KSIC, indicating that phase assemblage governs the fracture resistance of MGM. This is because Cl- in seawater intensified geopolymerization reactions. Interwoven gel structures in MGM, with surface depositions like magnesium aluminosilicate hydrate (M-A-S-H), magnesium silicate hydrate (M-S-H), and silica gel, filled pores, and improved homogeneity, resulting in its superior fracture toughness over conventional geopolymer mortar.

A novel use of incineration bottom ash for H2 aeration in the production of artificial lightweight aggregates

Cold bonding is an energy-efficient method for producing artificial aggregate (AA). However, the relatively high density of AA restricts its use in lightweight concrete products. This study aims to fully upcycle aluminum (Al) content in municipal solid waste incineration bottom ash (IBA) for H2 aeration in the production of lightweight artificial aggregate through alkali activation. To achieve this, the glass particles (GP) in IBA were ground into powder and used as the precursor for NaOH, while the remaining IBA (RIBA), rich in Al, served as the foaming agent. The results showed that RIBA particles (<300 μm) can be used to produce lightweight AA with a loose bulk density ranging from 780 to 840 kg/m3, comparable to sintered fly ash aggregates, and a wide pore size distribution ranging from 0.02 mm to over 1 mm. The inclusion of finer RIBA particles (<75 μm) enhanced the homogeneity of the pore structure, resulting in aggregate strengths of 0.4–0.6 MPa. Moreover, increasing the GP content in IBA up to 40% further boosted the aggregate strength to 1.1 MPa by refining the pore structure and promoting more (C)-N-A-S-H gels with Q4 structures. To ensure sufficient geopolymerization reactions and achieve high strength in the AAs, the alkali concentration should be kept above 3 mol/L. The proposed strategy for lightweight AA production reduced the global warming potential by about 20% compared to the conventional sintered method, offering a more environmentally friendly approach.

Nanosilica-enhanced geopolymer cementitious materials: Mechanistic insights from experimental and computational simulations

To mitigate the pollution associated with municipal solid waste incineration fly ash (MSW-IFA), solidification/stabilization (S/S) techniques have garnered considerable attention. This study systematically investigates the modification effects of nanosilica (NS) on the properties of geopolymer functional cementitious materials (GFCM) derived from waste fly ash. By integrating experimental data with simulation-based calculations, the influence of NS on the macroscopic properties and microstructure of GFCM, including mechanical performance, heavy metal immobilization mechanisms, and hydration processes, was examined. The results indicate that NS enhances mineral phase and pore structure through pozzolanic and filling effects, promoting additional C-S-H gel formation and pore filling, which significantly accelerates the hydration process and improves mechanical properties. Furthermore, NS increases the proportion of heavy metals in a stable state, thereby reducing their leaching potential. Density functional theory (DFT) calculations further revealed that NS's modification effects primarily occur via filling, nucleation, and pozzolanic activity, which supported the construction of a geopolymer reaction network. This study offers a validated approach for advancing the understanding of NS modification mechanisms, the development of geopolymer reaction networks, and the mechanisms governing heavy metal S/S.

Innovative assessment of reactivity in fly ash: The role of particle size distribution characteristics

The relationship between the intrinsic properties of seven different types of fly ash and the compressive strength of the resulting geopolymers was investigated. A comprehensive examination of the effect of chemical and mineralogical compositions, particle size distribution, on the compressive strength of the fly ash-based geopolymers were performed. Results revealed that particle size distribution had a more significant impact on reaction activity than amorphous phase content or average particle size alone. Therefore, a novel concept of 'reaction volume' based on the geopolymerization reaction mechanism was proposed, which reveals a high correlation between the reactivity of fly ash and the compressive strength of geopolymers, thereby enhancing the predictability of the process. The degree of reactivity, formulated through the integration of reaction volume and amorphous silica-aluminum content, was identified as a robust predictor of compressive strength, which can serve as a crucial tool for evaluating fly ash for the production of alkali-activated materials.

Preparation of solid alkali activator for geopolymer synthesis using vanadium-bearing shale tailing

The vanadium-bearing shale tailing (VST) with large reserves pollutes the environment and urgently needs to be utilized as a resource. Due to its main component being silica, VST has great potential as a raw material for geopolymer. Therefore, this study uses VST to prepare the solid alkali activator (SAA) by thermochemical methods. Analysis show that the temperature and NaOH dosage of thermochemical reaction significantly affect the alkaline activation effect of SAA. Temperature affects the silicate content and the amount of NaOH affects the soluble silicate content in SAA. When the reaction temperature is high (such as above 900 ℃) and the amount of sodium hydroxide is low (such as the mass ratio of sodium hydroxide to VST is 0.6), quartz almost transforms into silicates, but the silicates have a stable cyclic chemical structure and are difficult to dissolve during the geopolymerization reaction. When the reaction temperature is low (such as less than 700 ℃) and the amount of sodium hydroxide is high (such as the mass ratio of sodium hydroxide to VST is 0.9), quartz in VST is difficult to fully convert into silicates, there are unreacted quartz and sodium hydroxide in SAA, and SAA has poor alkaline activation performance. The optimal synthesis process conditions for SAA are as follows: synthesis temperature is 900 °C and the mass ratio of sodium hydroxide to VST is 0.95. The main silicate components of SAA are Na2SiO3 and Na15.6Ca3.8Si12O36. During the synthesis of one-part geopolymer (OPG), Na2SiO3 dissolution generates a strong alkaline reaction environment and provides active silicon-oxygen tetrahedron monomers or oligomers, Na15.6Ca3.8Si12O36 is bonded with geopolymer gel structure by stable cyclic silicate structure, which makes OPG have a compact integrated structure. When the mass ratio of SAA to MK is 1:1.3, the 7-day compressive strength of the prepared OPG is about 49 MPa, SAA has the potential as a substitute for commercial sodium silicate to prepare geopolymers.

Effect of Calcium on the Setting Time and Mechanical Property of a Red Mud-Blast Furnace Slag-Based Geopolymer.

This study aims to compare the effects of three calcium compounds on the workability, setting time and mechanical properties of red mud (RM)-blast furnace slag (BFS)-based geopolymers. The crystalline phase, hydration process and microstructure of RM-BFS-based geopolymers were characterized by X-ray diffraction (XRD), heat evolution, X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) tests. The results showed that an appropriate amount of calcium compounds can improve the flowability and compressive strength of the geopolymers, but the excessiveness causes a decrease in strength due to rapid hardening. Other than calcium carbonate, both calcium oxide and calcium chloride played important roles in accelerating the setting times of RM-BFS-based geopolymers. The acceleration in the setting times of geopolymers could be attributed to the calcium hydroxide produced by the dissolution of the calcium compounds, which also provides nucleation sites for the geopolymerization reaction. This study gives new insights into the effect of calcium on the setting times and mechanical properties of geopolymers in the geopolymerization process.

Strength, durability, and toxicity characteristics of water treatment sludge based geopolymers for subgrade application

This study examined the strength, durability and toxicity characteristics of geopolymer made from water treatment sludge (WTS) using fly ash (FA) and ground granulated blast furnace slag (GGBS) as binding materials for the subgrade application. The mechanical properties of WTS-FA-GGBS geopolymers were assessed considering various parameters such as binder proportions, curing period, curing condition, Sodium silicate (SS) to Sodium hydroxide (SH) ratio (i.e. SS/SH), and Sodium hydroxide (NaOH) concentration. The results showed that increasing binder proportions, NaOH concentration, and curing period positively impacted the strength of the WTS-FA-GGBS geopolymer. The unconfined compressive strength (UCS) of the geopolymer samples with an SS/SH ratio of 2.5 showed higher compressive strength than those at an SS/SH ratio of 1. WTS-FA-GGBS geopolymers cured at 100°C showed increased UCS values after 1 to 3 days, but strength decreased after 7 days due to a faster geopolymerisation reaction, resulting in excessive moisture loss and microcracks in the geopolymerized samples. The study also found that all the geopolymer samples met the minimum strength criteria for cement-stabilized mix with a maximum mass loss of 8.17 % are well within the specified limits (<14 %) as per IRC:37–2018. The California Bearing Ratio (CBR) values of WTS-FA-GGBS geopolymers, which range from 41.11 % to 260.32 %, surpass the minimum criteria for cement stabilized subgrade or subbase material, as specified by IRC: SP:89–2010. Additionally, the changes during geopolymerization were confirmed by FESEM, EDS, and XRD analysis. The toxicity characteristic leaching procedure (TCLP) test results showed that the amounts of heavy metals in the geopolymer leachate were within the USEPA-mandated limits.

Study on the performance mechanism of alumina-alkali activator synergistic solidified lunar soil simulant

The utilization of lunar in-situ resources is an important way to realize the construction and operation of Moon scientific research base. The effect of alumina-alkali activator on the mechanical properties of solidified lunar soil simulant was studied by using basaltic lunar soil simulant as raw material, adding alumina and alkali activator for solidification treatment. Characterisation of hydration products in the simulated lunar soil using X-ray diffraction (XRD), scanning electron microscopy with energy spectroscopy (SEM-EDS), fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG-DTG) and X-ray photoelectron spectroscopy (XPS). The solidified mechanism of lunar soil simulant under the synergistic effect of alumina-alkali activator was discussed. The results showed that the compressive strength and splitting tensile strength of the solidified lunar soil simulant show an increasing trend, and the highest compressive strength was 17.29MPa, which was 57% greater than that of the control group. The energy evolution process inside the specimen can be divided into four stages: damage initiation, damage increase, damage mutation and damage acceleration. The incorporation of alumina can promote the geopolymerization reaction between the alkali activator and the lunar soil’s mineral composition to generate plenty of (N,C)-A-S-H gels that can fill the pores in the particles, thereby improving the mechanical strength of the solidified lunar soil simulant. Finally, the microscopic reaction mechanism model of alumina-alkali activator synergistic solidified lunar soil simulant was established.

Permeability and self-healing behavior of alkali activated geopolymers for well cementing applications

Geopolymers, also identified as alkali-activated materials (AAM), are alternative well-cementing materials with a lower carbon footprint than traditional ordinary Portland cement (OPC). Geopolymers are formed by combining an aluminosilicate precursor, such as fly ash (FA), with an alkaline activator solution. Among their merits is an ability to self-heal after becoming damaged, which can be exploited to restore permeability, sealing ability, zonal isolation, and well integrity in wells that suffer from cracking, fracturing, or debonding in their annular cement sheaths and abandonment plugs.This study investigates the self-healing ability of various alkali-activated Class F fly ash-based geopolymers. Liquid sodium hydroxide (LSH), liquid sodium silicate (LSS), and solid sodium silicate (SSS) were used as activators for the preparation of geopolymer specimens, while Class H OPC was used as the control material. Permeability was characterized using pressure transmission tests (PTT). The tested samples were of different compositions, curing ages (7-day and 28-day), damage stages (pristine, damaged after freeze-thaw cycling, self-healed after post-damage curing), and damage severity (low, medium, and high). Fourier-transform infrared-spectroscopy (FTIR) and loss on ignition (LOI) tests were performed to examine the geopolymer reaction over time, in an attempt to relate the various permeability results to the geopolymerization process. The experimental results show that geopolymers possess low permeability, comparable to – or lower than – OPC, and that effective permeability restoration occurs in geopolymers, even after high degrees of damage. The implication of this work is that geopolymers are promising alternative barrier materials to OPC with a superior ability to guarantee low barrier permeability and appropriate zonal isolation, keys to wellbore integrity over the entire lifecycle of wells, even if those wells suffer from damage.

Low-carbon coal fly ash/superfine iron tailings-based three-dimensional printed geopolymer (CS-3DPG) rapid prototyping: The role and impact mechanism of carbonate series accelerator (CSA)

The large accumulation of superfine iron tailings (SIT) as solid waste may lead to significant environmental and safety risks, urgently requiring resource utilization. The preparation of geopolymers using SIT is high-valued and environmentally friendly method. Therefore, a low-cost, alkali-containing solid carbonate series accelerator (CSA) was used to accelerate the early geopolymerization process of coal fly ash (CFA)/SIT-based three-dimensional printed geopolymers (CS-3DPG). The optimal addition time and content of CSA were 25 min and 6 %, respectively, which significantly shortened the setting time of CS-3DPG to 9 min and increased its early compressive strength to 24.18 MPa. The addition of CSA enhanced the rheological properties of CS-3DPG, improving its printability and buildability. These results provide assurance for achieving rapid prototyping of CS-3DPG. Combined with microscopic characterization, selective chemical extractions (SCE) and isothermal conduction calorimetry (ICC), CSA effectively promoted the CS-3DPG geopolymerization reaction process, transitioning from interactions at the phase boundary (I) to diffusion (D). Additionally, the CO2 emissions of CS-3DPG were reduced by 76 % compared to Ordinary Portland Cement (OPC). This study successfully reveals the role and impact mechanism of CSA in 3DPG, providing an important theoretical reference for regulating 3DPG buildability.

The Role of Water Content and Binder to Aggregate Ratio on the Performance of Metakaolin-Based Geopolymer Mortars

Geopolymers are in many applications a perfect alternative to standard cements, especially regarding the sustainable development of green building materials. This experimental study therefore deals with the investigation of different factors, such as the water content and the binder to aggregate ratio, and their influence on the workability of fresh mortar and its mechanical properties and porosity on different size scales. Although increasing the water content improved the workability and flow behaviour of the fresh mortar, at the same time, a reduction in compressive strength in particular and a lesser reduction in flexural strength could be demonstrated. This finding can be attributed to an increase in capillary porosity, as demonstrated by capillary water uptake and mercury intrusion porosimetry measurements. At the same time, the increasing water content led to an improved deaeration effect (low air void content) and to initial segregation (see the µXCT measurements). An alternative approach to enhance the compressive and flexural strengths of the mortar specimens is optimization of the binder to aggregate ratio from 1 to 0.25. This study paves the way for a comprehensive understanding of the underlying chemistry of the geopolymerization reaction and is crucial for the development of sustainable alternatives to cementitious systems.

The Effect of CaO in the Immobilization of Cd2+ and Pb2+ in Fly Ash-Based Geopolymer

The use of geopolymers for the solidification/stabilization (S/S) of municipal solid waste incineration fly ash (MSWI FA) is promising because the Cao in MSWI FA can provide an alkaline environment to facilitate geopolymer reactions and help to form the gel phase in the solidified body. This study investigated the role of CaO in MSWI FA in immobilizing common heavy metals, especially Cd2+ and Pb2+. Tests were performed to evaluate the effect of CaO on the unconfined compressive strength (UCS) of the polymer and the leaching of heavy metals. The findings revealed that as the CaO content increased, the UCS of the geopolymer samples also rose, reaching a maximum 28-day UCS of 24.8 MPa at a CaO content of 31.5%. Additionally, higher CaO levels resulted in lower leaching concentrations of heavy metals in the stabilized material. When the CaO level is 32%, the levels of heavy metals that leach out are very low, with Pb2+ at 0. 02 mg/L and Cd2+ at 0. 01 mg/L, achieving a stabilization rate of over 93.6% for these ions. Moreover, the geopolymer’s characteristics were analyzed by XRD, FTIR, and SEM, and the immobilization mechanisms of Cd2+ and Pb2+ were identified as gelation, physical encapsulation, and chemical substitution.

Curing-dependent behaviors of sustainable alkali-activated fiber reinforced composite: Temperature and humidity effects

Despite alkali-activated fiber reinforced composite (AAFRC) attracting increasing attention due to the advantages of low-carbon and superior tensile performance, the curing-dependent behaviors in practical environments are lack of knowledge. This paper aims to fill this gap by comparably investigating the macro-mechanical properties and pore structures of AAFRC under the different temperatures of −5, 20, and 60 °C and humidities of 60 % and 95 % RH. The loss on ignition, scanning electronic microscope (SEM), and fourier transform infrared spectroscopy (FT-IR) tests were performed to decouple the influence mechanisms. The results showed that the high-temperature curing slightly decreased the tensile and compressive strength but increased the tensile strain capacity with the main reason of decreasing the matrix toughness by forming more micro-cracks. The low-temperature curing deteriorated both the strength and tensile strain capacity by hindering its geopolymerization reaction. A decrease in humidity during the curing process was unconducive to the mechanical strength and slightly affect the tensile strain capacity of AAFRC, which resulted from the loss of water impeded its reaction to proceed. The normalized energy consumption and carbon emission of the AAFRC cured under the standard curing decreased by 22.6 % and 70.2 % than the classic engineered cementitious composite (ECC-M45), respectively, showing great sustainability.

Influence of foaming additives on the porous structure and properties of lightweight geopolymers based on ash–slag waste

Lightweight geopolymer is a promising thermal insulation material that conserves energy used for heating and cooling buildings. This study investigates foaming agents that influence the porous structure and properties of geopolymer foams based on ash–slag waste from thermal power plant using a 12 M sodium hydroxide solution, waterglass, and foaming agents (30 % hydrogen peroxide solution, aluminum, silicon, and zinc powders). The study demonstrates the feasibility of foaming geopolymers by water evaporation without foaming additives, where the best density of 949 kg/m3 and compressive strength of 4.2 MPa were achieved in samples foamed by microwave radiation, but such properties cannot attribute geopolymers as lightweight. Foaming with H2O2 is hard to synchronize with geopolymerization reactions. Aluminum and silicon powders lead to rapid reactions in alkaline conditions, resulting in uneven pore sizes. Zinc powder was the most effective foaming agent, producing materials with a density of 331 kg/m3 and compressive strength of 1.17 MPa, and it also integrates into the geopolymer gel structure. The foaming process should occur at room temperature. Elevated temperatures increase curing rates and result in incomplete geopolymerization, preventing uniform pore formation. Synthesized geopolymers are suitable for building insulation, basements, underground pipelines, and utility networks.

Improving geopolymer concrete performance with hazardous solid waste phosphogypsum

Recycling the hazardous solid waste phosphogypsum (PG) in concrete is a popular trend. However, the inherent properties of PG used alone in concrete tend to limit the strength of the concrete. In this study, PG was used as a precursor for the production of phosphogypsum-based geopolymer concrete (PGC), which effectively improved the performance of geopolymer concrete by utilising the synergistic effect of different solid wastes in geopolymer concrete. The effects of PG content (0, 10, and 20 %) and alkali modulus (0.8, 0.95, 1.1, and 1.25) on the axial compressive properties and resistance to chloride penetration of PGC were investigated. The results show that 10 % PG improved the properties of geopolymer concrete the most, with a maximum increase in axial compressive strength of 17 %. Due to the addition of SO42−, the PG content also affected the optimal alkaline modulus of PGCs. Chemical composition and microstructural analyses revealed that the promotion of the geopolymer reaction by PG is crucial for PGC performance enhancement. In addition, leaching tests and carbon emission assessment, confirmed that PGCs are an environmentally friendly option. Based on the analysis of axial compression behaviour, resistance to chloride ion permeability and environmental impact, PGC with 10 % PG content and 1.1 alkali modulus is an environmentally friendly alternative to conventional concrete. This study shows that hazardous solid waste PG can improve the performance of geopolymer concrete and create a new type of solid waste concrete.