Geopolymer Gel Research Articles

The Influence of Particle Size and Calcium Content on Performance Characteristics of Metakaolin- and Fly-Ash-Based Geopolymer Gels.

This research systematically investigates the influence of raw material particle size and calcium content on the geopolymerization process to gain insight into the physical and mechanical properties of geopolymer gels, including setting time, fluidity, pore structure, compressive strength, and leaching characteristics of encapsulated Cr3+ heavy metal ions. Utilizing a diverse range of particle sizes of metakaolin (MK; 3.75, 7.5, and 12 µm) and fly ash (FA; 18, 45, and 75 µm), along with varied calcium levels, this study assesses the dual impact of these factors on the final properties of both metakaolin- and fly-ash-based geopolymers. Employing sophisticated analytical techniques such as Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), and Nuclear Magnetic Resonance (NMR), the research meticulously documents alterations in chemical bonding, micro-morphology, and pore structures. Key findings reveal that reducing the size of MK and FA particles to 3.75 and 18 µm, respectively, enhances the compressive strength of their matrices by 128.37 and 297.58%, respectively, compared to their original values (63.59 and 33.87 MPa, respectively) at larger particle sizes. While smaller particle sizes significantly bolster compressive strength, they adversely affect slurry flow and reduce the leaching rates of Cr3+ from MK- and FA-based matrices, reaching 0.42 and 0.75 mg/L at 3.75 and 18 µm, respectively. Conversely, increased calcium content markedly enhances setting times and contributes to the formation of dense microstructures through the production of calcium aluminate silicate hydrate (C-A-S-H) gels, thus improving the overall curing performance and durability of the materials. These insights underline the importance of fine-tuning particle size and calcium content to optimize geopolymer formulations, offering substantial benefits for varied engineering applications and promoting more sustainable construction practices.

Design of physical and functional properties of zeolitic porous monoliths by combining different foaming additives

Self-supporting zeolitic foams were synthesized by a geopolymer gel conversion process, employing silicon powder and hydrogen peroxide as foaming additives. The influence of silicon and hydrogen peroxide combination on the physical properties, mechanical characteristics, and pore distribution was studied. Correlations between these properties were drawn. This approach allows to manufacture porous monoliths tuned in terms of zeolite type and amount and characterized by bulk density ranging between 573 and 762 kg/m3 and thermal conductivity 0.33–0.53 W/mK. Pore structure, size, and distribution were dominated by the type of foaming agent: the foamed samples showed the presence of two classes of pores, the first of the order of magnitude of 1 μm, the second of the order of magnitude of 100 μm. The use of hydrogen peroxide enhanced the porosity in the meso-macropore range (0.1–5 μm). The hydrothermal treatment performed on the foamed geopolymer allowed to achieve the crystallization of zeolites within the solid geopolymer. Mainly FAU and LTA were the zeolitic phases present in the foams, whereas the use of silicon as foaming agent locally increased the Si/Al ratio on the geopolymer surface, promoting the nucleation of FAU.

Immobilization of radioactive borate liquid waste using calcined laterite–phosphoric acid–Fe3O4-based geopolymer waste forms

In this study, the feasibility of preparing a calcined laterite–phosphoric acid–Fe3O4-based geopolymer is investigated and its application for immobilizing radioactive borate liquid waste (RBLW) is explored. The addition of Fe3O4 enhances the compressive strength of the geopolymer and mitigates the retardation effects on the geopolymer caused by RBLW. Reactions between Fe3O4 and H3PO4 not only promote the geopolymerization process by generating heat and forming additional geopolymer gel, but also produce amorphous iron–phosphorus phases, contributing to higher compressive strength of the waste forms. Moreover, partial replacement of [AlO4] tetrahedra with [FeO6] octahedra creates stable –Fe–O–P–O–Al–O–Si– network structures. The geopolymer waste forms exhibit remarkable leaching resistance after Fe3O4 addition owing to the reduced open porosity, leading to radionuclide immobilization through physical encapsulation more efficiently. Furthermore, residual Fe3O4 may contribute to Co2+ immobilization through magnetic adsorption. Overall, the calcined laterite–phosphoric acid–Fe3O4-based geopolymer waste forms exhibit excellent performance in immobilizing RBLW, offering a new strategy for RBLW treatment.

PVA fiber reinforced red mud-based geopolymer for 3D printing: Printability, mechanical properties and microanalysis

Red mud-based geopolymers are an efficient alternative to resolve the high cement consumption in 3D printed concrete (3DPC). From a materials perspective, good printability and mechanical properties are critical to adopting red mud-based geopolymers in 3DPC. In this study, we selected bauxite tailings-red mud and blast furnace slag to prepare geopolymers. Polyvinyl alcohol (PVA) fibers with 0, 0.3 %, 0.6 %, 0.9 %, and 1.2 % volume fractions were used to reinforce 3D printed red mud-based geopolymers. To evaluate the influence of PVA fiber on the printability and mechanical properties of 3D printed red mud-based geopolymer. We tested the workability, extrudability, structural build-up ability, shape stability, and mechanical properties of the red mud-based geopolymer containing different volume fraction PVA fibers, and a microstructural analysis was performed to reveal the interface bond between the fibers and the geopolymer matrix. The results showed that adding PVA fibers impaired the workability of the red mud-based geopolymer mortar. The red mud-based geopolymer mortar showed a more than threefold increase in extrusion pressure compared to the control mixture (P0) when adding 1.2 % vol fiber. For all other mixtures, the increase was less than twice that of the control mixture. The PVA fiber had a significant positive impact on the structural build-up ability and shape stability of the 3D printable red mud-based geopolymer. In addition, PVA fibers promoted the bending strength of the hardened geopolymer specimens better than the compression strength, and the geopolymer gel on the fibers indicated the good interfacial bonding ability of the PVA fiber. This study demonstrates the feasibility of 3D printed red mud-based geopolymer and the reinforcement of 3D printed red mud-based geopolymer by PVA fibers, providing a material perspective to increase the range of red mud applications in 3D printed concrete.

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.

Efflorescence and mitigation of red mud–fly ash–phosphogypsum multicomponent geopolymer

In this study, the efflorescence formation of geopolymers synthesized from red mud (RM), fly ash (FA) and phosphogypsum (PG) was evaluated to investigate the influence of different proportions. The mechanical properties, leaching properties and microstructure of the geopolymers were studied through compressive strength test, visual observation of efflorescence, leach properties and a series of microscopic tests by changing PG content (0–25 %), reducing precursor alkali concentration (4–10 %) and changing FA content (0–300 g). The results show that PG rich in SO42- lead to the formation of a new efflorescence product Na2SO4, the presence of Aluminum (Al) in FA is beneficial to the polymerization degree and chemical stability of the geopolymer gel, and excessive or insufficient alkali content will affect the polymerization reaction and the efflorescence resistance of the geopolymer. The geopolymers are synthesized with 55 % RM, 25 % FA and 20 % PG as precursors and activated with 10 wt% Na2O, which shows good strength and resistance to efflorescence. Proper alkali content is conducive to the formation of geopolymer strength phase. The initial pH value required for the suitable effective strength and strength development of the multicomponent geopolymers in this study is about 10.5–11.3, which can provide a reference for the synergistic utilization of RM-FA-PG.

Surface modification of geopolymer coatings with polymethylhydrosiloxane-tetraethyl orthosilicate (PMHS-TEOS) for inhibiting efflorescence

Geopolymer coatings, as novel environmentally friendly inorganic coating, hold promising prospects in the field of concrete protection due to their characteristics such as corrosion resistance and fire resistance. However, its vulnerability to efflorescence can alter surface colouration and damage the structural integrity of the surface film, thereby diminishing its concrete protective efficacy. To inhibit efflorescence in geopolymer coatings, this study reports a surface modification method using polymethylhydrosiloxane-tetraethyl orthosilicate (PMHS-TEOS) for the first time. To explore the underlying mechanism, the effects of PMHS-TEOS on the geopolymer gel structure, mechanical properties, and ion leaching concentration were investigated. The experimental results indicate that PMHS-TEOS can penetrate the surface of geopolymer coatings and participate in the activation reaction, generating a new gel phase that effectively fills pore structure. Moreover, its hydrophobic groups facilitate the establishment of a waterproof structure, thereby inhibiting water infiltration and alkaline ion leaching. After PMHS-TEOS modification, the contact angle of the geopolymer coating reached 136.5°, demonstrating excellent wear resistance. The dissolution rate of alkaline cations in the modified geopolymer coatings was reduced by approximately 90 % and no efflorescence was observed in the long-term accelerated tests. This method is convenient for practical engineering of geopolymer coatings.

Experimental study on preparation of fly ash-based geopolymer blended with recycled calcium source

A large amount of waste eggshells raises environmental issues. This study uses eggshells as recycled calcium source to synthesize fly ash and eggshell geopolymer (FAEG). The effects of eggshell content, liquid-solid ratio, NaOH/Na2SiO3 ratio, and NaOH solution concentration on the strength of the FAEG were evaluated by unconfined compressive strength (UCS) test. The geopolymerization products and microstructure characteristics of the FAEG were analyzed by XRD, FTIR, TGA, and SEM test. The results show that eggshells acted as recycled calcium sources are capable of significantly enhancing the UCS of the FAEG. The optimal content of eggshells is 15%. The NaOH/Na2SiO3 ratio has a significant effect on the UCS of the FAEG, while the influence of liquid-solid ratio and NaOH concentration is slight. The optimal NaOH/Na2SiO3 ratio is 50:50 and the liquid-solid ratio is recommended to be 0.55. Eggshells take part in the geopolymerization by acting as a precipitating and seeding element in the reaction and favoring the generation of the geopolymeric gels in the FAEG.

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.

Effect of synthesis conditions on mechanical properties and microstructure of coal gasification slag-based geopolymer

Coal gasification slag (CGS) is an industrial solid waste produced during coal gasification, and it is mostly deposited on the land at present, resulting in serious environmental pollution. In order to promote its resource utilization, geopolymer was synthesized from CGS in this study. The influence of important factors containing the water-solid ratio, alkali activator dosage, activator modulus and curing temperature on the mechanical properties and microstructure of geopolymers were investigated. Micromorphology, chemical composition, mineralogy and chemical bonding of the geopolymer were assessed by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR), respectively. Results indicate that the compressive strength of geopolymer can be improved by reducing the amount of initial water. The optimal mechanical properties of geopolymer are obtained when the alkali activator dosage is 25 % and the activator modulus is 1. The moderate temperature (e.g. 60 °C) is the most favorable to the compressive strength development of geopolymer. Under these optimal conditions, the 90 days compressive strength of the CGS-based geopolymer (CGSG) can reach 58.3 MPa. The Si/Al molar ratio of the geopolymer gels is between 2.0 and 3.1, and the Al/Na molar ratio is around 1.0. These results can provide a reference for recycling of the CGS and production of eco-friendly building materials.

Molecular Insights into Adhesion at Interface of Geopolymer Binder and Cement Mortar.

The degradation of concrete and reinforced concrete structures is a significant technical and economic challenge, requiring continuous repair and rehabilitation throughout their service life. Geopolymers (GPs), known for their high mechanical strength, low shrinkage, and durability, are being increasingly considered as alternatives to traditional repair materials. However, there is currently a lack of understanding regarding the interface bond properties between new geopolymer layers and old concrete substrates. In this paper, using advanced computational techniques, including quantum mechanical calculations and stochastic modeling, we explored the adsorption behavior and interaction mechanism of aluminosilicate oligomers with different Si/Al ratios forming the geopolymer gel structure and calcium silicate hydrate as the substrate at the interface bond region. We analyzed the electron density distributions of the highest occupied and lowest unoccupied molecular orbitals, examined the reactivity indices based on electron density functional theory, performed Mulliken charge population analysis, and evaluated global reactivity descriptors for the considered oligomers. The results elucidate the mechanisms of local and global reactivity of the oligomers, the equilibrium low-energy configurations of the oligomer structures adsorbed on the surface of C-(A)-S-H(I) (100), and their adsorption energies. These findings contribute to a better understanding of the adhesion properties of geopolymers and their potential as effective repair materials.

Synthesis of geopolymer mortar from biomass ashes and forecasting its compressive strength behaviour

The global warming dilemma raised an exigency toward sustainability in the construction industry and compelled scientists to hunt for alternative binders in construction materials. The, geopolymerization as a spectacular technology has arisen as a looming solution to this dilemma. This work assesses the features of geopolymer mortar developed with both industrial and agricultural wastes at ambient temperature curing. The geopolymer mix proportion was evolved in this study using main precursor material as fly ash, ground granulated blast furnace slag (GGBFS), sugarcane bagasse ash (SCBA) or rice husk ash (RHA). Workability and mechanical properties, including compressive strength, flexural strength, and split tensile strength of different mortar mixes with various percentages of RHA and SCBA for 365 days, were evaluated. Geopolymer mortar with 10 % SCBA and 5 % RHA achieved improvements of 25 % and 13.91 % in compressive strength, 10.6 % and 3.5 % in flexural strength, and 35 % and 16.8 % in split tensile strength, respectively. Good geopolymer gel formation is clearly evident from the SEM images of SCBA mortar. The main polymeric bonds Si–O–Al and Si-O-Si are also identified using FTIR. To assess the efficacious use of biomass ashes, resistance to chemical attack using H2SO4, HCl, Na2SiO3, and NaCl solutions for 365 days was studied. It was revealed that SCBA incorporated geopolymer mortar specimens achieved good resistance compared with RHA geopolymer mortar. The relation between the ambient temperature cured geopolymer specimen compressive strength at 28 days and 365 days chemical solution curried geopolymer specimen compressive strength find out using python and obtained R2 values more than 95 %. Also, this research proposed various machine learning techniques such as long short-term memory, support vector regression, random forest regression, XGBOOST, and AdaBOOST algorithms for the prediction of compressive strength of geopolymer using 724 mixture proportions with diverse curing temperatures and varying ages. The XGBOOST algorithm achieved the highest R² value of 0.92, demonstrating its effectiveness for the strength prediction.

Exploring mechanical and fracture properties in geopolymer concrete with ternary precursor waste materials through laboratory investigations and statistical analysis

The prevalent excessive utilization of cement concrete, characterized by energy-intensive processes and notable carbon dioxide emissions, necessitates a shift towards sustainable construction materials. This study addresses the environmental challenges posed by conventional cement concrete by investigating the potential of recycled precursor geopolymer concrete as an environmentally friendly alternative. The research delves into the mechanical and fracture characteristics of ternary blend geopolymer (TBG) concrete incorporating an industrial by-product, red mud (RM), through comprehensive laboratory testing and statistical analysis. The investigation focuses on assessing the impact of varying RM replacement levels (0 %, 8 %, 15 %, 23 %, and 30 %) on mechanical properties and fracture behaviour in both Mode I and Mode II, utilizing semi-circular bend (SCB) specimens for fracture tests. Statistical methodologies, including Weibull fracture failure probability modelling and analysis of variance (ANOVA), were employed to interpret the results derived from 180 SCB test specimens. Furthermore, microstructural analysis through FTIR, SEM, and EDX testing was conducted on the diverse mixes. The outcomes revealed that the incorporation of RM in geopolymer concrete consistently diminished the compressive strength and flexural strength of the geopolymer concrete as the incorporation levels increased. For instance, the compressive strength decreases from approximately 40 MPa in the RM-free sample to about 25 MPa in the sample with 30 % RM. However, statistical analysis revealed that up to 8 % RM inclusion does not significantly affect the compressive strength and, in some cases, may even slightly increase it, highlighting the subtle effect of RM dosage. Additionally, Weibull modelling revealed that the probability of failure for samples with 30 % RM is about five times higher than for samples without RM. The microstructural analysis also depicted that high RM content may lead to decreasing homogeneity and the formation of deep cracks, negatively impacting mechanical properties. Elevated Fe and Al concentrations with higher RM content suggest potential reactions forming stable geopolymer gels and the presence of minerals like katoite.

Influence of Thermal Treatment on the Chemical and Structural Properties of Geopolymer Gels Doped with Nd2O3 and Sm2O3.

In this research, the influence of the thermal treatment of geopolymer gels at 300 °C, 600 °C and 900 °C when incorporated with 5% rare earth elements (REEs) in the form of (GP-Sm) Sm2O3 and (GP-Nd) Nd2O3 was investigated. Changes in the chemical and structural properties of the geopolymer gels during thermal treatment for 1 h were monitored. Physico-chemical characterization was performed using the following methods: diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), and X-ray photoelectron spectroscopy (XPS). Besides the characterization of the fundamental properties, some practical macroscopic properties were analyzed as well: sorptivity, open porosity, and Archimedean density. The stretching vibrations of Nd-O-Si and Sm-O-Si were confirmed at a value of around 680 cm-1and an Nd-O-Si absorption band at a higher value, together with the most dominant band of Si-O stretching vibration similar for all the samples. No significant chemical changes occurred. Structural analysis showed that for GP-Nd, the largest pore diameter was obtained at 900 °C, while for GP-Sm, the largest pore diameter was obtained at 600 °C. EDS confirmed the amount of dopant to be about 5%. X-ray photoelectron spectroscopy showed that for GP-Nd, the ratio of Si and Al changed the most, while for GP-Sm, the ratio of Si and Al decreased with increasing temperature. The contributions of both dopants in the GP-gel structure remained almost unchanged and stable at high temperatures. The atomic percentages obtained by XPS analysis were in accordance with the expected trend; the amount of Si increased with the temperature, while the amount of Al decreased with increasing temperature. The sorptivity and open porosity showed the highest values at 600 °C, while the density of both geopolymers decreased linearly with increasing temperature.

Utilization of industrial waste sodium sulfate for calcined clay-based sustainable binder

This study explored the utilization of industrial waste Na2SO4 to develop a calcined clay-based sustainable cementitious composite inspired by ancient Roman concrete. Na2SO4 with or without the addition of NaCl was utilized, keeping the total Na2O quantity constant, and their performances at the macro and micro scale were compared. A separate set of samples was prepared using seawater. A medium-grade calcined clay, along with lime, was utilized as the binder. The salts were first dissolved in water and then used as the mixing water for sample preparation. A diverse array of characterization methods was utilized, encompassing thermogravimetric analysis, chemical extraction, X-ray diffraction (XRD), and Backscattered Scanning electron microscopy (SEM). It was observed that the salt-containing batches achieved compressive strength faster than the seawater samples. However, the strength remained in a comparable range (∼30 MPa) for all the samples after 28 days of curing. The initial strength of salt-containing samples was attributed to the formation of gypsum, ettringite, and hydrocalumite. Additionally, the geopolymer gel and calcium aluminum silicate hydrate (C-A-S-H) gel phases were present in all samples. Finally, this study also showed that lime-clay mortars can achieve a reduction in carbon footprint of up to 50 % compared to ordinary portland cement (OPC) mortars.

Synthesis of geopolymer using glass fiber powder from retired wind blades: A new attempt to recycle solid waste in wind power industries

This study explores the potential of recycling and utilizing discarded wind turbine blades. It proposes a new method that upcycles the glass fiber powder (GFP) in these blades into geopolymer gel, offering a new solution for the recycling and treatment of retired wind blades. Analyses of the X-ray diffraction (XRD) pattern of the GFP raw material and its dissolution in concentrated alkali indicate a large content of vitreous and active silica-aluminum, suggesting its suitability for producing geopolymers through alkali activation. The effects of the alkali activator modulus (Ms), alkali-binder ratio (N/B), and water-binder ratio (w/b), raw material particle size, and initial curing temperature on the compressive strength of the specimens were comprehensively and systematically investigated. The reaction behavior, chemical structure, and microstructure of GFP-synthesized geopolymers were characterized by selective dissolution, microcalorimetry, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS). The results showed that gels were formed by the geopolymerization reaction between the GFP and alkali activator. The compressive strength of the specimens increased first and then decreased with increasing alkali activator Ms and N/B. However, increasing w/b led to a decrease in strength. Appropriately small particle sizes of the raw material and moderately increasing the initial curing temperature are beneficial to strengthen the compressive strength. The optimal 28 days compressive strength of 43.43 MPa was achieved with an activator Ms of 1.4, N/B of 10 %, w/b of 0.33, GFP with a D90 of 170 mesh, and an initial 24 h curing temperature of 60 °C. The results of this study can promote the utilization of waste wind blades in the production of sustainable building materials.

Study on the microscopic characteristics and hydration mechanism of thermo-alkali activated electrolytic manganese residue geopolymers

As a form of solid waste, electrolytic manganese residue (EMR) exhibits low reactivity, high sulfate content, and exceeds the standards for heavy metals. To address these drawbacks, a high-temperature thermal activation method was employed to enhance its reactivity and remove pollutants. The study investigated the physicochemical properties and leaching toxicity of EMR under different thermal activation conditions. Additionally, the thermally activated EMR was utilized in the preparation of geopolymer material, and the influence of EMR activation temperature, EMR content, sodium silicate content, and molar ratio on the mechanical properties, hydration mechanism, microstructure, and leaching toxicity of the EMR geopolymers was analyzed. The results indicated that after activation in the temperature range of 0–1200°C, the active content of silica-alumina in EMR increased, sulfate decomposition occurred, and the leaching of Mn2+ and NH4+-N decreased to 21.24 and 0.576 mg/L, respectively. When the activation temperature reached 1200°C, the compressive strength of the geopolymer, prepared with 50 % EMR, 50 % granulated blast furnace slag (GBFS), 30 % sodium silicate, and a molar ratio of 1.2, could reach 53.2 MPa at 28-day. Under activation in the range of 0–800°C, ettringite was abundant in the geopolymers, providing strength support. With activation in the range of 800–1200°C, the geopolymers became denser, exhibiting flaky and agglomerate gels. A diffraction peak appeared at 20°-40°(2θ) corresponding to C-(A)-S-H and N-S-A-H gels. Due to the slow depolymerization-polymerization kinetics of the EMR geopolymer and high sulfate content, the heat release rate and cumulative heat were significantly lower than ordinary portland cement (OPC). In the structure of EMR geopolymer, Mn2+ replaced Na+ and Ca2+ in the silicoaluminate structure or chemically bonded with anionic groups to the geopolymer. Simultaneously, Mn2+ participated in the formation of the EMR geopolymer gel, encapsulating within the gel. Therefore, the leaching of pollutants from EMR geopolymers met the standards of GB8978–1996.

In-depth analysis of macro-properties and micro-mechanism of eco-friendly geopolymer based on typical circulating fluidized bed fly ash

Geopolymer is an eco-friendly building material that significantly reduces energy consumption and CO2 emissions. The increase in circulating fluidized bed fly ash (CFBFA) production presents an opportunity to use CFBFA in geopolymer synthesis, mitigating environmental hazards and promoting a sustainable, low-carbon cement industry. However, the complex transformation of amorphous aluminosilicates in CFBFA into geopolymer gels and the impact of calcium (Ca) composition on the macro-properties and micro-mechanism of geopolymers are not well understood, hindering large-scale utilization of the CFBFA-based geopolymer. In this study, low-Ca CFBFA (LCFA) and high-Ca CFBFA (HCFA) were used as precursors, with NaOH as the activator. The workability, mechanical properties, and microstructure evolution of the geopolymer samples were systematically compared. The LCFA-based geopolymer slurry exhibited ideal workability. The high Ca content in HCFA increased the water demand of the slurry, thereby reducing its workability. Compared to LCFA-based geopolymer samples, although Ca-containing components in HCFA accelerated the geopolymerization processes, some gels transformed into irregular crystalline phases during the later stages of curing, compromising the dense structure and slowing the development of mechanical properties. The evolution of the Si–Al coordination structure showed that LCFA-based geopolymers favored the development of Si-rich gels (Q4(1Al) and Q4(2Al)), whereas HCFA-based geopolymers exhibited a preference for Al-rich gels (Q4(3Al) and Q4(4Al)). The results of this research can provide valuable insights for the synthesis of CFBFA-based geopolymers.

Effect of Ca content on the synthesis and properties of FA-GGBFS geopolymer: Combining experiments and molecular dynamics simulation

The elemental components in the precursor of geopolymers are different, and their mechanical properties are also different. As one of the major influencing factors, calcium content is worth further research and exploration. This paper aims to explore the impact of calcium content on fly ash (FA)-ground granulated blast furnace slag (GGBFS) geopolymer in a comprehensive and multi-scale manner through various research methods such as indoor experiments, microscopic experiments, and molecular dynamics simulations. Using GGBFS as the main source of calcium content in geopolymers, different calcium content samples were prepared by adjusting the GGBFS content. The consistency, setting time, compressive strength, flexural strength, elastic modulus, self-shrinkage value, and dry-shrinkage value of the test blocks were tested at corresponding ages. Analyze its microscopic characteristics through scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS). Finally, the structural models of four geopolymer gels with different Ca contents were constructed, and the mechanism of calcium influence was further characterized from the atomic level. The results indicate that an increase in calcium content is beneficial for the rapid hardening of geopolymer mortar, with varying degrees of increase in compressive strength, flexural strength, and elastic modulus. However, with the increase in calcium content, shrinkage is greater, which may lead to durability drawbacks such as cracking in practical engineering. Therefore, the calcium content should be appropriately controlled. The microscopic morphology showed that the polymer became denser with increasing calcium content, and the Ca/Si ratio in EDS also increases accordingly. Molecular dynamics simulation results also show that Ca can form Ca–O bonds with distant oxygen atoms, making the gel structure more stable.

Novel utilization of waste concrete powder in alkali-activated binder

This experimental study investigates a novel approach to utilize waste concrete powder (WCP) in conjunction with metakaolin as a precursor in the production of alkali-activated binder for sustainable consumption of construction and demolition waste. A Chapelle test confirms the presence of reactive silica in thermo-mechanically activated WCP. Different alkali-activated mixtures with metakaolin replacement ranging from 0 % to 80 % were prepared. The mixture with 40 % activated WCP, with a sodium silicate to sodium hydroxide ratio of 2, achieved better compressive strength than the reference sample without WCP. Mineralogical analysis of the mixture pastes revealed that activated WCP-based mixtures developed geopolymer gel and C-S-H gel, contributing to better strength properties in the case of the mixture with 40 % activated WCP. Life cycle analysis demonstrated that incorporating 40 % thermo-mechanically activated WCP by replacing metakaolin reduces carbon dioxide emissions by 49.5 % and 2.2 % compared to Portland cement and metakaolin-based binder, respectively.