Search Results (212)

Considering the current requirement for high temperatures and the significant energy consumption in the preparation of geopolymer-based cements, this paper presents a study on the compressive strength of metakaolin-based geopolymers containing various low-calcium fly ash admixtures, prepared at room temperature (25 ± 2 °C). The physical properties and microstructure of the geopolymers were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The type of alkaline cations, phase transformation, evolution of characteristic functional groups, and hydration characteristics of the microstructures were analyzed, and the hydration mechanism is discussed. The experimental results indicated that the fly ash content had a more significant impact on compressive strength than the alkaline cation type (Na⁺/K⁺). The optimal formulation (20% fly ash with 20% KOH activator) reached a compressive strength of 76.70 MPa at 28 days, which was around 6% higher than that of the NaOH-activated counterpart (72.34 MPa). Crystalline phase analysis in the transformation of mullite and microstructure analysis indicated that the increase in compressive strength could be attributed to the effective filling of the matrix interface by chemically inert fillers and the dense N-A-S-H and C-(A)-S-H multi-dimensional gel structures. These experiments prove the feasibility of using fly ash and metakaolin to prepare geopolymer materials with high compressive strength at room temperature. Full article

Geopolymers represent an innovative and environmentally sustainable alternative to traditional construction materials, offering significant potential for reducing anthropogenic CO₂ emissions. Among these, phosphoric acid-activated metakaolin-based systems have attracted increasing attention for their chemical and thermal resilience. In this study, we present a comprehensive structural and mechanical evaluation of metakaolin-based geopolymers synthesized across a wide range of Al/P molar ratios (0.8–4.0). Six formulations were systematically prepared and analyzed using X-ray powder diffraction (XRPD), small-angle X-ray scattering (SAXS), Fourier-transform infrared spectroscopy (FTIR), solid-state nuclear magnetic resonance (ssNMR), and complementary mechanical testing. The novelty of this work lies in the integrated mapping of composition–structure–property relationships across the broad Al/P spectrum under controlled synthesis, combined with the rare application of SAXS to reveal composition-dependent nanoscale domains (~18–50 nm). We identify a stoichiometric window at Al/P ≈ 1.5, where complete acid consumption leads to a structurally homogeneous AlVI–O–P network, yielding the highest compressive strength. In contrast, acid-rich systems exhibit divergent flexural and compressive behaviors, with enhanced flexural strength linked to hydrated silica domains arising from metakaolin dealumination, quantitatively tracked by ²⁹Si MAS NMR. XRPD further reveals the formation of uncommon Si–P crystalline phases (SiP₂O₇, Si₅P₆O₂₅) under low-temperature curing in acid-rich compositions. Together, these findings provide new insights into the nanoscale structuring, phase evolution, and stoichiometric control of silica–alumino–phosphate geopolymers, highlighting strategies for optimizing their performance in demanding thermal and chemical environments. Full article

Today, several conventional wastes (fly ash, ground granulated blast furnace slags, etc.) are used as valid precursors for geopolymer synthesis. However, there are several new wastes that can be studied to replace geopolymer precursors. This study investigates the behavior of four industrial wastes—suction dust (SW1), red mud (SW2), electro-filter dust (SW3), and extraction sludge (SW4)—as 20 wt.% substitutes for metakaolin in geopolymer synthesis. The objective is to assess how their incorporation before alkali activation affects the structural, thermal, mechanical, chemical, and antimicrobial properties of the resulting geopolymers, namely GPSW1–4. FT-IR analysis confirmed successful geopolymerization in all samples (the main Si-O-T band underwent redshift, confirming Al incorporation in geopolymer structures after alkaline activation), and stability tests revealed that none of the GPSW1–4 samples disintegrated under thermal or water stress. However, GPSW3 showed an increase in efflorescence phenomena after these tests. Moreover, compressive strength was reduced across all waste-containing geopolymers (from 22.0 MPa for GP to 12.6 MPa for GPSW4 and values lower than 8.1 MPa for GPSW1–3), while leaching tests showed that GPSW1 and GPSW4 released antimony (127.5 and 0.128 ppm, respectively) above the legal limits for landfill disposal (0.07 ppm). Thermal analysis indicated that waste composition influenced dehydration and decomposition behavior. The antimicrobial activity of waste-based geopolymers was observed against E. coli, while E. faecalis showed stronger resistance. Overall, considering leaching properties, SW2 and SW3 were properly entrapped in the GP structure, but showed lower mechanical properties. However, their antimicrobial activity could be useful for surface coating applications. Regarding GPSW1 and GPSW4, the former needs some treatment before incorporation, since Sb is not stable, while the latter, showing a good compressive strength, higher thermal stability, and leaching Sb value not far from the legal limit, could be used for the inner reinforcement of building materials. Full article

Geopolymers are typically cured either at ambient temperature (~25 °C) or subjected to short-term heat curing before being stored under ambient conditions until testing. However, in hot-arid regions, the daily ambient temperature may exceed 45 °C during summer months. Therefore, such conditions should also be considered as high ambient curing, and their influence on low-calcium geopolymer performance needs to be investigated. In this study, pumice- and fly ash-based geopolymer mortars were produced to evaluate the effects of different curing regimes. In the pumice-based mixtures, 10 wt% of pumice was replaced with metakaolin to enrich the alumina content. Three curing conditions were applied: ambient curing, high ambient curing, and heat curing. Setting times of geopolymers were determined based on each curing regime. Physical properties, including density, water absorption, and sorption coefficient, were assessed. Compressive strength development was evaluated over 90 days. In addition, durability performance was assessed through water resistance, freeze–thaw durability, and resistance against sulphuric and hydrochloric acid. Fourier transform infrared spectroscopy and X-ray diffraction confirmed that geopolymerisation reactions continued significantly up to 90 days under high ambient curing, while mercury intrusion porosimetry showed a reduction in porosity. These findings explain the continuous increase in compressive strength. Pumice-based geopolymers cured under this condition exhibited significantly better long-term strength than those cured under other regimes. High ambient-cured fly ash-based geopolymers, a 3-day strength of 40.3 MPa was achieved, eliminating the need for heat curing. Thus, high ambient curing enables the in situ use of these geopolymers and offers a cost-effective and eco-friendly alternative. Full article

The HYPEX^® process is a novel method for conditioning spent ion exchange resins from nuclear power plants, aiming to reduce final waste volume and carbon emissions by stabilizing the resins in metakaolin-based geopolymers. This study addresses the challenge posed by the natural variability of commercial metakaolin and defines a testing strategy to ensure consistent performance of the final matrix. The reactivity of two batches of metakaolin, characterized by comparable chemical composition and BET surface area, was evaluated by monitoring temperature evolution during geopolymerization at varying water-to-solid ratios. The resulting geopolymers were tested for compressive strength, water permeability, and strontium leachability to assess correlations between precursor properties and final matrix performance. Despite similar compositions, the two batches showed marked differences in compressive strength that could be linked to early thermal behavior. These findings demonstrate that conventional precursor characterization is insufficient to guarantee reproducibility and that thermal profiling is useful to predict mechanical performance. The results suggest the implementation of thermal response monitoring as a quality control tool to ensure the reliability of geopolymer wasteforms in nuclear applications. A simplified analytical model for the thermal evolution during geopolymerization was also developed, matching qualitatively the measured evolution, to suggest scale-up rules from laboratory specimens to full-scale drums, which should be achieved while preserving the thermal evolution. Full article

This article presents experimental studies on the characterization of geopolymer composites intended for applications in aquatic environments, with particular emphasis on underwater infrastructure. The motivation for conducting the research was the growing need to develop durable and ecological building materials that will be resistant to long-term exposure to moisture and aggressive chemical agents, typical for the underwater environment, where traditional cement concretes undergo gradual degradation due to long-term water impact, including hydrotechnical and underwater infrastructure. Geopolymer binders were produced based on metakaolin activated by alkaline solutions containing sodium hydroxide. Several series of mixtures with additives such as blast furnace slag, amphibolite and carbon fibers were developed to evaluate the effect of these components on mechanical strength, water absorption and chemical durability. The conducted studies showed that slag additions improved mechanical properties, for the best composition it across 50 MPa. In contrast, the addition of amphibolite had an unfavorable effect, which probably results from introducing inhomogeneity into the material structure. The presence of carbon fibers promoted matrix cohesion, but their uneven distribution could lead to local strength differences. Water absorption tests have shown that geopolymers reach full water saturation within 24 to 48 h, which indicates rapid establishment of capillary equilibrium and limited further water penetration. The conclusions from the work indicate that geopolymer composites with a moderate amount of blast furnace slag and subjected to appropriate curing conditions. High strength, water and chemical resistance make them suitable for, among others, the construction of marine foundations, protection and structural shields of submerged applications. Full article

Geopolymer concrete uses a geopolymer binder instead of traditional Portland cement; thus, it reduces carbon emissions by a significant amount. In this study, Edge-Oxidized Graphene Oxide (EOGO), a carbon-based nanomaterial, was added into a metakaolin-based geopolymer, and its effect on the mechanical and rheological properties of the mixture was investigated. EOGO was added into the mixture at 0% (control), 0.1%, 0.5%, and 1% of the metakaolin mass. Several experiments were conducted to characterize the properties of the metakaolin–EOGO (MKGO) geopolymer, including its compressive strength, free–free resonance column (FFRC), void content, water absorption, setting time, flow, and rheology. It was found that the compressive strength and stiffness showed their maximum values and the void content was minimized at 0.1% EOGO. In addition, as the EOGO addition rate increased, the setting time tended to shorten, and the fluidity tended to decrease. This suggests that 0.1% EOGO is the most optimal content in metakaolin paste. This study confirms that EOGO is an additive material that can improve the performance of metakaolin-based geopolymers and presents opportunities for the development of sustainable construction materials through optimization of EOGO addition. Full article

Geopolymers are an environmentally sustainable class of low-calcium alkali-activated materials (AAMs), distinct from high-calcium C–A–S–H gel systems. Synthesized from aluminosilicate-rich precursors such as fly ash, metakaolin, slag, waste glass, and coal gasification fly ash (CGFA), geopolymers offer a significantly lower carbon footprint, valorize industrial by-products, and demonstrate superior durability in aggressive environments compared to Ordinary Portland Cement (OPC). Recent advances in thermodynamic modeling and phase chemistry, particularly in CaO–SiO₂–Al₂O₃ systems, are improving precursor selection and mix design optimization, while Artificial Neural Network (ANN) and hybrid ML-thermodynamic approaches show promise for predictive performance assessment. This review critically evaluates geopolymer chemistry and composition, emphasizing precursor reactivity, Si/Al and other molar ratios, activator chemistry, curing regimes, and reaction mechanisms in relation to microstructure and performance. Comparative insights into alkali aluminosilicate (AAS) and aluminosilicate phosphate (ASP) systems, supported by SEM and XRD evidence, are discussed alongside durability challenges, including alkali–silica reaction (ASR) and shrinkage. Emerging applications ranging from advanced pavements and offshore scour protection to slow-release fertilizers and biomedical implants are reviewed within the framework of the United Nations Sustainable Development Goals (SDGs). Identified knowledge gaps include standardization of mix design, LCA-based evaluation of novel precursors, and variability management. Aligning geopolymer technology with circular economy principles, this review consolidates recent progress to guide sustainable construction, waste valorization, and infrastructure resilience. Full article

Numerous researchers have successfully made alkali-activated material or geopolymer using fly ash, ground granulated blast furnace slag, or metakaolin, either individually or in combination. However, few researchers first determined the reactive Si:Al of their solid precursor and then used this information to develop a formulation with a specific targeted Si:Al for their alkali-activated material. Even if a targeted Si:Al is chosen, few researchers check if the actual Si:Al of the geopolymer matches the targeted values. Characterisation of the precursor, setting target Si:Al values for the geopolymer and verifying target Si:Al values are present in the geopolymer are all part of quality control and essential if high quality products are to be manufactured. Quality control is critical but does not provide the target Si:Al value. This work presents results from a range of geopolymers made with different Si:Al values using sodium aluminate, sodium hydroxide and sodium silicate, either by themselves or in combination. Results reveal, surprisingly, for samples tested, that compressive strength exhibits a maximum for samples with Si:Al less than and greater than the starting Si:Al of the precursor. A strength minimum was found to be present close to the starting Si:Al of the precursor and between the strength maxima. This new information extends the usability range of aluminosilicate precursors and at the same time, makes available a broader range of applications based on Si:Al. Selection of an optimum Si:Al for a geopolymer based on strength can only be made when first a complete spectrum of Si:Al ratios have been evaluated. Full article

Alkali-activated aluminosilicate matrices are increasingly studied for their ability to stabilize hazardous metal contaminants via alkali activation at room temperature. In this study, metakaolin-based geopolymers were used to immobilize chromium and nickel salts, with systematic variation of key synthesis parameters, Na/Al molar ratio, metal concentration, anion type, and alkaline solution aging time, which have not been previously studied. A Design of Experiments approach was employed to study the effect of factors on metal leaching behavior and to better understand the underlying immobilization mechanisms. The analysis revealed that higher Na/Al ratios significantly enhance geopolymerization and reduce metal release, as supported by FTIR spectral shifts and decreased shoulder intensity. Notably, aging time had an influence on chromium behavior due to its effect on early silicate network formation, which can hinder the incorporation of chromium species. All tested formulations achieved metal immobilization rates of 98.8% or higher for both chromium and nickel. Overall, this study advances our understanding of geopolymer-based heavy metal immobilization. Full article

High demand for sustainable solutions in the construction industry determines the significant relevance of developing new eco-friendly composites with a reduced carbon impact on the environment. The main aim of this study is to investigate the possibility and efficiency of using technical sulfur (TS) as a modifying additive for geopolymer composites and to select the optimal content of polypropylene fiber (PF). To assess the potential of TS, experimental samples of geopolymer solutions based on metakaolin and fly ash were prepared. The TS content varied from 0% to 9% by weight of binder in 3% increments. In the first stage, the density, compressive and flexural strength, capillary water absorption and microstructure of hardened geopolymer composites were tested. The TS additive in an amount of 3% was the most effective and provided an increase in compressive strength by 12.6%, flexural strength by 12.8% and a decrease in capillary water absorption by 18.2%. At the second stage, the optimal PF content was selected, which was 0.75%. The maximum increases in strength properties were recorded for the composition with 3% TS and 0.75% PF: 8% for compression and 32.6% for bending. Capillary water absorption decreased by 12.9%. The geopolymer composition developed in this work, modified with TP and PF, has sufficient mechanical and physical properties and can be considered for further study in order to determine its competitiveness with cement composites in real construction practice. Full article

Cement-based building materials usually exhibit weak flexural behavior under high temperature or fire conditions. This paper develops a novel geopolymer with enhanced residual flexural strength, incorporating fly ash/metakaolin precursors and corundum aggregates based on our previous study, and further improves flexural performance using hybrid fibers. The flexural load–deflection response, strength, deformation capacity, toughness and microstructure are investigated by a thermal exposure test, bending test and microstructure observation. The results indicate that the plain geopolymer exhibits a continuously increasing flexural strength from 10 MPa at 20 °C to 25.9 MPa after 1000 °C exposure, attributed to thermally induced further geopolymerization and ceramic-like crystalline phase formation. Incorporating 5% wollastonite fibers results in slightly increased initial and residual flexural strength but comparable peak deflection, toughness and brittle failure. The binary 5% wollastonite and 1% basalt fibers in geopolymer obviously improve residual flexural strength exposed to 400–800 °C. The steel fibers show remarkable reinforcement on flexural behavior at 20–800 °C exposure; however, excessive steel fiber content such as 2% weakens flexural properties after 1000 °C exposure due to severe oxidation deterioration and thermal incompatibility. The wollastonite/basalt/steel fibers exhibit a positive synergistic effect on flexural strength and toughness of geopolymers at 20–600 °C. Full article

Metakaolin-based geopolymers with different molar ratios of Si/Al were synthesized and utilized as an efficient adsorbent for the removal of methylene blue (MB) as a model cationic dye from aqueous solution. Various analytical techniques were employed to characterize the synthesized geopolymers. The influence of the main operation conditions on the adsorption kinetics of MB onto the geopolymer was examined under various operating conditions. Results showed a significant maximum MB adsorption capacity at the temperature of 30 °C for all four types of geopolymers studied (designated as A, B, C, and D) up to 35.3, 23.6, 25.5, and 19.0 mg g⁻¹, respectively. The corresponding order of Si/Al ratio was A < C < B < D. Adsorption kinetics was so fast and reached equilibrium in 10 min, and the experimental results were described using the adsorption dynamic intraparticle model (ADIM). The equilibrium data for MB removal was in agreement with the Langmuir isotherm. Full article

This research explores the effectiveness of eggshell powder (ESP) and polypropylene (PP) fiber in geopolymer (GP) mortars. It examines how various doses of ESP, ranging from 0% to 25%, and two volumes of PP fibers, at 0.1% and 0.2% (by volume), impact the workability, mechanical and physical characteristics, and microstructure of GP mortars. Assessments were made for workability, apparent porosity, water absorption, and flexural and compressive strengths, along with microstructural evaluations. Using ESP as a substitute for metakaolin (MK) at 15% and 25% (by weight) improved the flexural and compressive strengths by 22.9%, 22.5%, 37.1%, and 50.7%, respectively. Using PP fiber resulted in flexural strength improvements of up to 97%. These findings deepen the understanding of ESP’s potential as a partial replacement for MK in geopolymer mortar, provide insights on material enhancement, and demonstrate superior mechanical and durability properties. Full article

Microwave curing has proven to be a highly effective method for enhancing the structural integrity, compressive strength, and heavy metal immobilization performance of geopolymer (GP) gels. For fly ash-based GP gels, optimal compressive strength (126.84 MPa) and minimal heavy metal ion leaching (0.01 mg/L) were achieved under microwave irradiation at 100 W for 75 s. Similarly, metakaolin-based GP gels reached peak compressive strength (76.84 MPa) and reduced heavy metal leaching (0.44 mg/L) under 440 W irradiation for 60 s. Microwave energy significantly accelerates geopolymerization by promoting the aggregation of dispersed particles, rapidly forming a dense, block-like matrix. This accelerated densification enhances the mechanical properties of GP gels within minutes. Moreover, the dense matrix structure effectively encapsulates heavy metal ions, minimizing their leaching through a combination of physical encapsulation and chemical bonding. In summary, microwave treatment significantly enhances both mechanical performance and heavy metal immobilization, offering a practical pathway for sustainable applications. Full article