Search Results (142)

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

This paper presents a comprehensive review of dealuminated metakaolin (DK), a hazardous industrial by-product generated by the aluminium sulphate (alum) industry and evaluates its potential as a component in cementitious systems for the partial or full replacement of Portland cement (PC). Positioned within the context of waste valorisation in concrete, the review aims to establish a critical understanding of DK formation, properties, and reactivity, particularly its pozzolanic potential, to assess its suitability for use as a supplementary cementitious material (SCM), or as a precursor in alkali-activated cement (AAC) systems for concrete. A systematic methodology is used to extract and synthesise relevant data from existing literature concerning DK and its potential applications in cement and concrete. The collected information is organised into thematic sections exploring key aspects of DK, beginning with its formation from kaolinite ores, followed by studies on its pozzolanic reactivity. Applications of DK are then reviewed, focusing on its integration into SCMs and alkali-activated cement (AAC) systems. The review consolidates existing knowledge related to DK, identifying scientific gaps and practical challenges that limit its broader adoption for cement and concrete applications, and outlines future research directions to provide a solid foundation for future studies. Overall, this review highlights the potential of DK as a low-carbon, circular-economy material and promotes its integration into efforts to enhance the sustainability of construction practices. The findings aim to support researchers’ and industry stakeholders’ strategies to reduce cement clinker content and mitigate the environmental footprint of concrete in a circular-economy context. 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

This paper explores the role of one-part alkali-activated cement, utilizing metakaolin as a precursor, in the long-term stabilization of mining tailings. Investigating three key factors (Si/Al and Na/Si ratios and curing period), this study reveals insights into the mechanical performance and microstructure of alkali-activated cemented iron ore tailings. Unconfined compressive strength test, statistical analysis, and Scanning Electron Microscopy analysis with Energy Dispersive Spectroscopy were performed. Findings indicate that the Si/Al ratio significantly influences strength, with an optimal ratio of 3.5. The Na/Si ratio introduces complexity, affecting alkali availability and reactivity, leading to nuanced strength variations. Extended curing periods consistently enhance the strength of alkali-activated cement, highlighting its dynamic nature. Notably, the 7-day specimens exhibit a less homogeneous distribution, weaker bonding, and decreased structural integrity compared to their 60-day counterparts. This research underscores the intricate nature of alkali-activated cement hydration, emphasizing the interdependence of Si/Al and Na/Si ratios. The observed strengthening effect with prolonged curing suggests the potential for tailoring these materials to specific applications. Addressing a research gap, especially in applying alkali-activation to mining tailings stabilization, this study highlights metakaolin’s role as a suitable precursor. Full article

This study explores the feasibility of utilizing granite sawing sludge (FC) as a precursor to produce alkali-activated materials (AAMs). To enhance the reactivity of the system, metakaolin (MK) was added and binary mixtures were synthetized. A multidisciplinary approach, including mineralogical, chemical and mechanical analysis, was employed to assess the suitability of these precursors to produce AAMs. X-Ray diffraction (XRD) and Fourier-Transform Infrared spectroscopy (FT-IR) confirmed the occurred activation reaction with the consequent increase in the amorphous content. Raman spectroscopy was used to further explore the mineralogical composition of the consolidated specimens, helping in the detection of salts, whose formation is ascribed to secondary carbonatation processes. Morphological analysis (SEM-EDS) displayed relatively uniform microstructures for all specimens. Compressive strength tests revealed that MK rich samples achieved best values compared to FC rich formulations, which exhibited reduced strength resistance. This study highlights, for the first time, the benefits of incorporating Cuasso al Monte granite sawing sludges into alkali-activated binders. Results suggested that the incorporation of FC is recommended for both environmental and economic advantages. Full article

This study investigates the mechanical and microstructural performance of three alkali-activated geopolymer formulations, constituted of metakaolin (MK), blast furnace slag (BFS), and a ternary blend of MK, BFS, and fly ash (MIX), for the immobilization of simulated radioactive liquid organic waste (RLOW). Thermal ageing tests were performed to evaluate geopolymer durability, including fire exposure (800 °C) and climatic chamber cycles (from −20 to 40 °C). Characterization through thermogravimetric analysis (TGA), compression tests, and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) was carried out to assess material degradation after thermal ageing. Preliminary results showed substantial strength and microstructural degradation in oil-loaded specimens after cyclic climatic ageing, while fire-exposed blank matrices retained partial mechanical integrity. BFS matrices exhibited the best thermal resistance, attributable to the formation of Ca-Al-Si-hydrate (C-A-S-H) gels. These findings support the use of optimized geopolymer formulations for safe RLOW immobilization, while contributing to the advancement of knowledge on sustainable and regulatory-compliant direct conditioning technology. Full article

This work focuses on the use of municipal waste incineration bottom ash (MSWI) for the development and production of products suitable for use as construction products. The generation of these ashes is increasing every year due to the incineration of municipal waste. There are currently three incineration plants operating in major cities in Lithuania. The non-hazardous bottom ash remaining from the incineration process is stored in dedicated sorting and aging sites until it is used as an inert form of aggregate for the installation of road foundations. However, it has been observed that these ashes have a tendency to bind and cement when exposed to atmospheric precipitation at the storage site. Based on this characteristic, it was decided in this study to use alkaline activation of the ash to accelerate the bonding process and to create a dense, non-porous composite concrete structure. This activation method is known to create another problem during ash bonding, where the presence of metallic aluminum particles in the ash leads to the release of hydrogen gas and makes the structure of the cured samples porous. For the purposes of the study, it was decided to create a completely different mixture structure and not to use additional water in the mixtures tested. A very low water/solids ratio (W/S) of <0.08 was used for the alkaline activation of the mixtures. All the water required for ash activation was obtained from sodium silicate and sodium hydroxide solution. Metakaolin waste (MKW) was used to adjust the SiO₂/Na₂O/Al₂O₃ ratio of the mixtures. Vibro-pressing was used to form and increase the density of the samples. And for the formation of the concrete structure, 0/4 fraction sand was used as aggregate. The final alkali-activated sample obtained had properties similar to those of the very widely used vibro-pressed cementitious paving tiles and did not exhibit hydrogen evolution during alkali activation due to the very low W/S ratio. The best results were achieved by samples with a highest compressive strength of 40.0 MPa and a tensile strength of 5.60 MPa, as well as a density of 1950 kg/m³. It is believed that this alkaline activation and vibro-pressing method can expand the use of MSWI ash in the development of building products. Full article

This research investigates whether metakaolin can be used as a partial substitution for slag to mitigate significant volume changes in alkali-activated slags. Its effect on compressive strength and workability (as well as on isothermal calorimetry, autogenous strain, and coefficient of thermal expansion (CTE)) were found to depend on both the type and concentration of the alkaline activator. When using 8 M and 10 M sodium hydroxide (NaOH), increasing the substitution rate increased the compressive strength. With sodium silicate (Na₂SiO₃), compressive strength decreased as the substitution increased. Isothermal calorimetry revealed metakaolin’s dilution effect at 10% substitution. With 8 M NaOH, a third reaction peak appeared, whose magnitude increased with the substitution rate, while the second peak decreased. The swelling was increased at 10% substitution, followed by constant shrinkage in case of NaOH-activation. Shrinkage was mitigated with Na₂SiO₃-activation. Higher substitutions with 8 M NaOH resulted in a significant increase in the shrinkage rate and CTE, occurring when the third reaction peak appeared. A 10% substitution delayed the CTE increase but resulted in higher later-age values (dilution effect). The 20% substitution led to a similar final CTE value at 300 h, while 30% substitution resulted in a decrease in CTE after the initial increase. Full article

The incorporation of mineral admixtures plays a crucial role in enhancing the performance and sustainability of geopolymer systems. This study evaluates the influence of fly ash (FA), silica fume (SF), and metakaolin (MK) as typical mineral admixtures on slag–Yellow River sediment geopolymer eco-cementitious materials. The impact of varying replacement ratios of these admixtures for slag on setting time, workability, reaction kinetics, and strength development were thoroughly investigated. To understand the underlying mechanisms, microstructural analysis was conducted using thermogravimetric–differential thermal analysis (TG-DTA), X-ray diffraction (XRD), scanning electron microscopy–energy dispersive spectroscopy (SEM-EDS), and mercury intrusion porosimetry (MIP). The results indicate that the incorporation of FA, SF, and metakaolin delayed the initial reaction, prolonged the induction period, and reduced the acceleration rate. These effects hindered early strength development. At 30% FA content, the matrix exhibited excellent flowability and sustained heat release. The 28-day splitting tensile strength increased by 42.40%, while compressive strength decreased by 2.85%. In contrast, 20% SF significantly improved compressive strength, increasing the 28-day compressive and splitting tensile strengths by 11.19% and 6.16%, respectively. At 15% metakaolin, the strength improvement was intermediate, with 28-day compressive and splitting tensile strengths increasing by 3.55% and 10.59%, respectively. However, dosages exceeding 20% for SF and metakaolin significantly reduced workability. The incorporation of FA, SF, and metakaolin did not interfere with the slag’s alkali-activation reaction. The newly formed N-A-S-H and C-S-H gels integrated with the original C-A-S-H gels, optimizing the pore structure and reducing pores larger than 1 µm, enhancing the matrix compactness and microstructural reinforcement. This study provides practical guidance for optimizing the use of sustainable mineral admixtures in geopolymer systems. Full article

In the context of the disposal of spent radioactive fuel, heat-emitting radionuclides such as Cs and Sr are of utmost concern, as they have a major influence on the distance at which disposal galleries should be spaced apart and, thus, the cost of a disposal facility. Therefore, certain scenarios investigate the partitioning and transmutation of spent fuel to optimize the disposability of both Cs- and Sr-rich waste streams and the remaining fractions. In this study, the Cs- and Sr-rich waste stream, a nitrate-based solution, was immobilized in metakaolin and blast furnace slag-based alkali-activated matrices. These matrices were chosen for immobilization because they are known to offer advantages in terms of durability and/or heat resistance compared with traditional cementitious materials. The goal of this study is to develop an optimal recipe for the retention of Cs and Sr. For this purpose, recipes were developed following a design-of-experiments approach by varying the water-to-binder ratio, precursor, and waste loading while respecting matrix constraints. Leaching tests in deionized water showed that the metakaolin-based matrix was superior for the combined retention of both Cs and Sr. The optimal recipe was further tested under accelerated leaching conditions in an ammonium nitrate solution, which revealed that the leaching of Cs and Sr remained within reasonable limits. These results confirm that alkali-activated materials can be effectively used for the immobilization and long-term retention of heat-emitting radionuclides. Full article

The addition of biochar to Portland cement composites has been proven to increase some of the material properties. The effect on alkali-activated materials has not been fully investigated. In this study, different recipes of metakaolin pastes at different biochar amounts are tested. Their physical and mechanical properties are analyzed to understand if any beneficial effects can be found even for alkali-activated binders. The results show that the addition of small amounts of biochar (<2 wt%) increases the compressive strength of metakaolin pastes (+15% after 28 days) and decreases the water absorption by capillarity, possibly leading to increased durability. Higher biochar content decreases the mechanical properties but provides higher dimensional stability and reduces the formation of efflorescence. Full article

The formation of an aluminosilicate gel structure made of alkali-activated materials (AAMs) was conducted through an alkali-activation reaction of the solid precursors (fly ash, metakaolin, and wood ash). Fly and wood ash are by-products of the burning process of coal and wood, respectively. Alkali-activated materials of aluminosilicate origin, made from the different ashes, fly and wood, are very attractive research targets and can be applied in various technological fields due to their thermal stability, resistance to thermal shock, high porosity, high sustainability, and finally, low energy loss during production. In this paper, we evaluate physico-chemical properties, microstructure, and radiological environmental impacts when wastes that contain elevated levels of naturally occurring radionuclides (NORs) such as fly ash and wood ash are made into “green cements” such as AAMs. The determination of radionuclide content was performed by means of gamma-ray spectrometry. Results showed that the AAMs have a lower value in the activity concentration of radionuclides than raw materials. The external absorbed gamma dose rate was 74.7–107.3 nGy/h, and the external radiation hazard index values were in range of 0.445–0.628 Bq/kg. The results of the activity concentration measurements for alkali-activated materials indicate the potential of their safe application in building construction. In terms of the structural characterizations, the obtained alkali-activated materials were examined using XRD, DRIFT, FESEM, and TEM analyses. Full article

Microwave technology in geopolymer synthesis offers a transformative, sustainable alternative to traditional methods, enhancing material properties and production efficiency. However, the effects of microwave-induced changes on pore structure and their relationship with mechanical strength and environmental performance, such as heavy metal leachability, are not fully understood. This study investigates the impact of microwave pre-curing on geopolymers, focusing on how microwave power and duration influence their pore structure and environmental performance. A total of 48 mixtures were prepared using sodium silicate and sodium hydroxide as alkali activators, with metakaolin and fly ash as raw materials. The modulus was adjusted to 1.5, and the liquid-to-solid ratio was set at 1.6 for metakaolin and 0.7 for fly ash. Microwave irradiation power settings of 100 W, 300 W, 440 W, 600 W, and 800 W were tested. The heating times ranged from 30 s to 90 s at intervals of 15 s. Our findings reveal that optimal microwave settings (100 watts for 45 s) can significantly enhance mechanical properties, with compressive strengths reaching 15.9 MPa for fly ash-based and 9.094 MPa for metakaolin-based geopolymers. However, excessive microwave energy leads to increased porosity, with adverse effects on structural integrity. Moreover, microwave pre-curing effectively reduces heavy metal leachability. Chromium (III) was used in leaching tests and it was demonstrated that ion concentrations as low as 0.097 mg/L enhance environmental safety. Advanced techniques like Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and X-ray CT were applied for the analysis of the atomic bonding, phases and pore structure of the studied geopolymers along with their ability to withstand compression (MPa). Chromium (III) was encapsulated and its leached concentration was measured by ICP-MS to evaluate the performance of the synthesized geopolymers. These results underscore the need for precise control over microwave irradiation parameters to maximize the benefits while mitigating negative impacts. This study provides valuable insights into the controlled use of microwave technology for geopolymer synthesis, recommending optimal irradiation conditions for improved performance and sustainability and advancing sustainable construction materials. The developed geopolymers show promise for applications in construction, waste stabilization, and heavy metal immobilization, contributing to more sustainable and environmentally friendly materials in these industries. Full article

Efflorescence, a time-dependent and water-driven phenomenon, is a major concern in alkali-activated materials (AAMs), impacting their practical use and preservation in a time-frozen state for post-characterisation. Although a method for stopping chemical reactions in conventional cements exists, it is time-consuming and not chemical-free. Therefore, this study explored the effects of low-power microwave-induced dehydration on efflorescence, mechanical performance, and structural integrity in AAMs, to create an alternative and more “user-friendly” dehydration method. For this purpose, several mixtures based on secondary raw (slag, fly ash, glass wool, and rock wool) and non-waste (metakaolin) materials were activated with a commercial Na-silicate solution in ratios that promoted or prevented efflorescence. Characterisation techniques, including Fourier-transform infrared spectroscopy and X-ray diffraction, showed that microwave dehydration effectively removed water without altering crystallinity, while mercury intrusion porosimetry and compressive strength tests confirmed increased porosity. In addition to being an efficient, time-saving, and solvent-free manner of stopping the reactions in AAMs, microwave irradiation emerged as an innovative, chemical-free method for evaluating curing finalisation and engineering foams in a stage when all other existing methods fail. However, the artificially provoked efflorescence in aged dehydrated AAMs connected the slipperiness of AAM with the instant extraction of Na, which raised the need for further research into alternative alkali replacements to evaluate the practical use of AAM. Full article