Geopolymer Paste Research Articles

Cement-Free Geopolymer Paste: An Eco-Friendly Adhesive Agent for Concrete and Masonry Repairs

This study aimed to investigate the feasibility of using geopolymer paste (GP) as an adhesive agent for (i) anchoring steel bars in concrete substrates, (ii) repairing concrete, and (iii) repairing limestone and granite masonry blocks commonly found in historic buildings. In this investigation, seven cement-free GP mixes were developed with different combinations of binder materials (slag, silica fume, and metakaolin). The mechanical properties, adhesive performance, and production cost of the developed GP mixes were compared to those of a commercially epoxy adhesive mortar (EAM). The results obtained from this study indicated that the use of GPs enhanced the bonding between steel bars and concrete substrates, achieving bonding strengths that were 19.7% to 49.2% higher than those of control specimens with steel bars directly installed during casting. In concrete repairs, the GPs were able to restore about 60.6% to 87.9% of the original capacity of the control beams. Furthermore, GPs exhibited a promising performance in repairing limestone and granite masonry blocks, highlighting their potential suitability for masonry structures. The best adhesive performance was observed when a ternary binder material system consisting of 70% slag, 20% metakaolin and 10% silica fume was used. This combination, compared to the investigated EAM, showed comparable adhesive properties at a significantly low cost, indicating the viability of GPs as a cost-effective, eco-friendly adhesive agent.

The effect of artificial metakaolin coarse aggregate on the mechanical and durability properties of lightweight geopolymer concrete

<p style="text-align:justify; margin-bottom:11px"><span style="font-size:11pt"><span style="line-height:150%"><span style="font-family:Calibri,"sans-serif""><span lang="PT-BR" style="font-size:12.0pt"><span style="line-height:150%"><span style="font-family:"Times New Roman","serif""><span style="color:#131413">This study presents a unique methodology for investigating the properties of  Metakaolin Coarse Aggregates (MCA). The NaOH ratios were varied as 8M, 10M, 12M, and 14M, while the NaOH to Na<sub>2</sub>SiO<sub>3</sub> ratio was maintained at 0.5 entire study. The liquid-to-binder (L/B) ratios set at 0.3, 0.35, and 0.4. Testing was conducted on the MCA to determine its physical and mechanical properties, comparing them with those of Natural Coarse Aggregate (NCA). Notably, the 8M mix with a 0.4 L/B ratio was found to be economical for producing MCA and was selected for further research. This chosen mix was then employed to prepare geopolymer concrete with 50% MK and 50% PA, with L/B ratios set at 0.4, 0.45, and 0.5. The NaOH ratio and Na<sub>2</sub>SiO<sub>3</sub> to NaOH ratio were maintained at a constant 8M and 2, respectively. Mechanical and durability properties were compared between Geopolymer concrete with NCA and Geopolymer concrete with MCA. The test results demonstrate that MCA in geopolymer concrete can effectively replace NCA in geopolymer concrete, making it a viable option for structural members. Additionally, microstructural characterization was performed on Geopolymer concrete with MCA mixes with varying L/B ratios of 0.4, 0.45, and 0.5, at 28 days. The study specifically focuses on the Interfacial Transition Zone (ITZ) between MCA and geopolymer paste, revealing an improvement in ITZ and microstructure in the GMCA sample with a 0.4 L/B ratio compared to other mixes.</span></span></span></span></span></span></span></p>

Effect of emulsification step on calcium carbonate encapsulated eicosane and incorporation into geopolymer

AbstractThe effect of the emulsification process of the organic n‐eicosane as a phase change material (PCM) in an aqueous solution of sodium dodecyl sulfate (SDBS) as a surfactant has been studied regarding the properties of the CaCO3 microcapsules. Such microcapsules aim to limit interactions between the PCM and the matrix (i.e., leakage and unwanted reactions). Optical and scanning electron microscopy (SEM) shows that the mean size of the n‐eicosane capsule is of the order of 5 μm. However, large non‐spherical objects which could be clusters of flocculated capsules or the results of encapsulation of coalescing n‐eicosane droplets, are observed, particularly when mechanical stirring (MS) is used rather than when an ultra‐turrax (UT) or sonotrode (S) is used in the initial emulsification step. These large, partially encapsulated objects could be the cause of the poor results in leakage tests. It is possible to encapsulate n‐eicosane inside a calcium carbonate shell, with a greater than 50% encapsulation ratio. The yield of encapsulated n‐eicosane measured by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) is similar and shows that 55% n‐eicosane core in CaCO3 shell can be produced. Up to 33 wt.% CaCO3 microcapsules (with 55% n‐eicosane content) have been successfully incorporated into fresh geopolymer paste. This incorporation of 18% (55% × 33%) of PCM does not modify the hardening conditions of the geopolymer since demolding was possible after 2 days at room temperature. DSC curves confirm that the melting reaction with n‐eicosane is conserved inside the hardened geopolymer.

The Potential of Seawater in Geopolymer Mixtures – Effect of Alkaline Activator, Seawater, and Steam Curing on the Strength of Geopolymer Paste

The Potential of Seawater in Geopolymer Mixtures – Effect of Alkaline Activator, Seawater, and Steam Curing on the Strength of Geopolymer Paste

Recycling and optimum utilization of GFRP waste into low-carbon geopolymer paste for sustainable development

To provide an eco-friendlier alternative to traditional ordinary Portland cement (OPC) binder, this study utilized glass fiber reinforced polymer (GFRP) waste powder as a precursor material to prepare geopolymer paste (GP). Through a series of mechanical, thermal, and mineral admixture activation procedures, efforts are made to augment the reactivity of GFRP powder-based geopolymers. The research reveals that subjecting GFRP powder to a 30-min grinding process amplifies the compressive strength of GP by up to 37 %. Moreover, elevating curing temperatures to 60 °C leads to a substantial enhancement in the strength of GFRP powder-based GP, attributable to the formation of more amorphous N-A-S-H and C-(A)-S-H gels. However, prolonged grinding durations yield only marginal alterations in particle size, while curing at 80 °C demonstrates a detrimental effect on strength. The choice of resin exhibits a discernible impact on both workability and strength. Notably, pure glass fiber powder-based GP displays superior compressive strength but exhibits increased brittleness, whereas GFRP powder-based GP excels in flexural strength. Moreover, the inclusion of ground granulated blast furnace slag (GGBS) in the mixture accelerates setting time and reduces flowability of GFRP powder-based GP. Strength shows a positive correlation with GGBS content, although the impact on flexural strength is less pronounced compared to compressive strength. Life cycle assessment (LCA) showed GFRP powder-based GP with 50 % GGBS replacement had a 35 %–48 % lower Global Warming Potential (GWP)/strength ratio compared to OPC pastes. Furthermore, this formulation demonstrates a 15 % decrease relative to GGBS-based GP, and a reduction of 13 %–25 % compared to GGBS/Fly Ash (FA)-based GPs. These results underscore its potential as an environmentally preferable building material.

Development of a ternary high-temperature resistant geopolymer and the deterioration mechanism of its concrete after heat exposure

A geopolymer paste (GP) synthesised using fly ash, slag, and metakaolin was developed to enhance the heat resistance of industrial by-product-based geopolymers. The GP exhibited a 28-day strength of 60 MPa and maintained at least 80 % of its initial strength and elastic modulus even at temperatures up to 900°C. Subsequently, the GP was blended with aggregates (sands and stones) to produce geopolymer concrete (GPC) with a 28-day strength of 45 MPa. Compressive tests on the GPC after exposure to 100–1000°C revealed that the GPC retained 90 % of its compressive strength up to 500°C, but experienced a 60 % loss in elastic modulus and a fourfold increase in peak strain after 300°C exposure. Stress-strain relation, physical properties, and microstructure were analysed to explore the property evolution mechanisms after heat exposure. The combined effect of geopolymer shrinkage (100–780°C), stone expansion (> 200°C), and stone decomposition (> 500°C) was the primary factor for the GPC property decline. At 800°C, a complete loss of mechanical properties in both the stone and interface transition zones led to significant deterioration of the GPC. These findings offer valuable insights for developing durable construction materials suitable for high-temperature applications, ensuring structural integrity and performance under extreme thermal conditions.

Comparative Study of the Structural, Microstructural, and Mechanical Properties of Geopolymer Pastes Obtained from Ready-to-Use Metakaolin-Quicklime Powders and Classic Geopolymers.

This study compares the structural, microstructural, thermal, and mechanical properties of geopolymer pastes (GPs) created through traditional methods and those derived from ready-to-use powders for geopolymer (RUPG) materials. The metakaolin (MK) precursor was activated using a sodium silicate solution or CaO and MOH (where M is Na or K). Various ratios of precursor/activator and Na2SiO3 or CaO/MOH were tested to determine the optimal combination. For RUPG, the MK precursor was activated by replacing the sodium silicate solution with quicklime. Metakaolin, alkaline hydroxide, and quicklime powders were mixed at different CaO ratios (wt%) and subjected to extensive ball milling to produce RUPG. The RUPG was then hydrated, molded, and cured at 20 °C and 50% relative humidity until testing. Analytical methods were used to characterize the raw and synthesized materials. Classic geopolymers (CGPs) activated with quicklime burst after one hour of molding. The results indicated slight amorphization of GP compared to raw MK, as confirmed by X-ray diffraction analysis, showing N(K)-A-S-H in CGP and N(K)-A-S-H with calcium silicate hydrate (C-S-H/C-A-S-H) in RUPG. The compressive strength of MK-based geopolymers reached 31.45 MPa and 34.92 MPa for GP and CGP, respectively, after 28 days of curing.

Influence of aluminum waste, hydrogen peroxide, and silica fume ratios on the mechanical and thermal characteristics of alkali-activated sustainable raw perlite based geopolymer paste

Geopolymer concretes have great potential in the sustainable construction industry in the future, as they can be produced without using cement and waste materials can be used as binders. This paper explores the use of sustainable, lightweight geopolymer pastes as construction materials using raw perlite as the primary ingredient. It achieves this by alkaline activation with additives such as waste aluminum lathe (Al), Hydrogen peroxide (H2O2) and silica fume (SF). The experiments were conducted in four groups: Al added in the 1st group, Al + Silica fume (SF) in the 2nd group, H2O2+SF in the 3rd group, and Al + H2O2 in the 4th group. Apparent density, compressive strength, flexural strength, and thermal conductivity tests were performed at 7, 14, and 28 days. SEM and EDX analyses were also conducted. The results showed that as the ratio of Al and H2O2, which have a aerating effect, increased, decrease in apparent density and strength was observed. However, when Al + SF were added together, significant improvements of 10%–20% were observed in compressive and flexural strengths, as well as apparent density. 35% water/pearlite ratio and 0.50% Al addition reduced compressive strength by 5% and lightened apparent density by 15%. At 40% water/pearlite ratio, 0.50% Al addition reduced compressive strength by 10% and lightened apparent density by 5%. According to these results, the best performance is obtained with 35% water/pearlite ratio and 0.50% Al addition Furthermore, the addition of lightweight SF resulted in good geomerization and reduced crack formation. This effect was evident in the internal structure based on SEM analysis. Interestingly, despite aluminum's overall high thermal conductivity, its incorporation into raw perlite-based pastes resulted in reduced thermal conductivity.

Effect of waste travertine powder on properties of rhyolitic tuff-based geopolymer

This investigation explores the potential of geopolymer technology as an eco-friendly substitute for conventional construction materials by emphasizing the innovative use of naturally occurring green rhyolitic tuff and repurposed travertine powder in geopolymer paste formulations. Replacement levels of tuff with travertine at 40 %, 45 %, and 50 %, along with an alkaline solution with a NaOH molarity range of 8.2 M–22.1 M, have been analyzed for their impact on flowability, water absorption, porosity, compressive strength, and unit weight of the geopolymers over curing periods of 2, 28, and 90 days. The flowability measurements show that adding 40 %, 45 %, and 50 % travertine leads to approximately 7 %, 31 %, and 31 % reductions in the flowability of the geopolymer, respectively. It is demonstrated that adding 40 % travertine significantly improves the geopolymers’ compressive strength with an 11.5 M NaOH concentration, showing substantial increases of approximately 15.5, 9.0, and 2.4 times at the curing duration of 2, 28, and 90 days, respectively, relative to the geopolymer without travertine. As an optimum points, an 18.5 M NaOH and an 0.7 solution-to-powder ratio decrease the long-term apparent porosity and water absorption of the geopolymer without travertine by about 14 % and 19 % compared to those at the same solution-to-powder ratio with 11.5 M NaOH, respectively. Microscopic examinations were employed to validate the development of sodium aluminosilicate hydrate, calcium aluminosilicate hydrate, and calcium silicate hydrate gels, underscoring the advantageous contribution of the elevated CaO content in the travertine to the geopolymer matrix. This research not only highlights the environmental benefits of repurposing waste materials but also contributes to the development of more sustainable and durable geopolymer pastes, offering a promising approach to enhancing environmental stewardship in material science practices.

Filling Fresh Metakaolin‐Based Geopolymer Paste with Wood Ash: Chemical and Mechanical Characterization

AbstractWood ash is the powdery residue remaining after wood combustion. Today, only some of the wood ashes are recycled, while most are disposed of in landfills. Increasing recycled wood ash can limit environmental damage and save transportation costs and waste disposal. This paper presents including 10% of wood ashes as a filler material for geopolymers. The ashes, coming from a local Italian restaurant, are sieved between 65 and 79 µm and are incorporated into a metakaolin‐based geopolymer matrix, cured at room temperature (25 °C). Ionic conductivity, pH, and Infrared Spectroscopy analysis, conducted after 7, 14, 28, and 56 days of curing time, reveal the occurrence of the geopolymerization process. Furthermore, the mechanical strength results allow the comparison of these materials with Portland cement, one of the most common polluting competitors.

Characterization and Properties of Binder and Nano-Filler in Geopolymer Paste with Graphene Oxide

Globally, current research has developed new cement-based materials to meet increased demands for performance, energy efficiency, and environmental protection. Geopolymer, cost-effective, and high-early strength concrete binder alternative, has grown in popularity. Geopolymer reduces emissions by 80% during production while maintaining strength levels comparable to Ordinary Portland Cement (OPC). In their natural state, geopolymer binders have a microstructure that is cross-linked and significantly more brittle than OPC. To improve the properties of geopolymer, several approaches were adopted. However, recent study suggests that incorporating nanomaterials such as graphene oxide (GO) to geopolymer has shown an improvement in physical and mechanical properties. Graphene oxide is an inorganic nanomaterial that improves the mechanical characteristics of various composite materials by showing substantial filling effects on composite materials that significantly improve composite material integrity. The investigation into the potential of GO to improve the efficacy of geopolymer composite materials in various engineering applications has garnered considerable attention in recent years. The simplified hummer’s method was employed to synthesise the GO. Various characterization techniques involving SEM, XRD, and FTIR were utilized to understand the crystalline structure and microstructure of GO nanoparticles. GO powder were used as 0.05%, 0.10%, 0.15%, 0.20%, and 0.25% by weight of binder that includes fly ash class F and steel slag with an optimum binder modification of 40% fly ash and 60% steel slag. The influence of GO on Bulk density, microstructure, and mechanical strength were determined. The compressive strength results indicated an improvement of compressive strength by 21.48% in geopolymer paste with 0.15% GO after 28 days of curing.

Unveiling the high strength and ductile character of reinforced CNT/geopolymer paste subjected to elevated temperatures

A significant challenge in developing suitable advanced materials in the construction sector is urgent face to the natural disasters affecting our infrastructure and endanger the world population. At this stage, geopolymers renowned as a green and low-carbon material has received considerable attention. Here, novel properties are revealed for Na-based and K-based metakaolin geopolymer paste reinforced by carbon nanotubes with various dosage, exposed to high temperatures. Results show that adding 1.08 wt% of CNT makes the geopolymer matrix thermally stable and resistant up to 1327 °C. It comes out that geopolymer nanostructure with low CNT concentration became tough to shear deformation along (001) plane up to 1127 °C and is easier to shear along (100) plane for highest CNT content. Calculations predict that low CNT dosage double the ductility of Na-based and K-based geopolymer matrix at room temperature. Heating up to 1327 °C enhances the stiffness of both nanostructures. The micro-hardness doubles at 800 °C for Na-based rather than K-based geopolymer matrix when adding low CNT dosage.

Rheology and microstructure of lithium slag/metakaolin geopolymer pastes: Insights from particle packing and water dynamic evolution

The high viscosity of alkali activator and physicochemical characteristics of metakaolin particles impair the rheological properties of fresh geopolymer pastes, becoming one of the urgent problems. To better understand and control the rheological properties of geopolymer pastes, this study investigates the particle packing of fluid suspensions and water dynamic evolution in pastes. The incorporation of lithium slag (LS) reduces the water required for the geopolymerization reaction, contributing to the increase of water film thickness between particles and the free water in pastes. Consequently, the internal friction between particles is reduced and the workability is improved. The results obtained identify the relevance of water film thickness and water occurrence state to qualitatively estimate the rheological properties, and confirm the applicability of LS for geopolymer manufacture. This work presents a theoretical approach towards assessing rheology and proposes an efficient method to develop more workable metakaolin-based geopolymer concrete.

Direct evidence for the mechanism of early-stage geopolymerization process

Geopolymer, an alternative construction material to cement, acquires strength and endurance through the geopolymerization process. However, the study on the early-stage geopolymerization is rare since it occurs rapidly and involves liquid phase, viscous slurry, and gel-like material, making the observation of this process complicated. This research addresses the challenge of observing this process by providing real-time evidence through in situ techniques: Fourier-transformed infrared (FTIR) spectroscopy, X-ray absorption spectroscopy (XAS), and environmental scanning electron microscopy (E-SEM). These methods consistently confirm the formation of aluminosilicate oligomers, short chains of silicate and aluminate species, in the polycondensation Stage I within the initial 5 minutes. This stage involves the reaction of Si-O and Al-O monomers, released during a dissolution process, with available Na2SiO3 from alkaline solutions in the system. In the subsequent polycondensation Stage II, aluminosilicate oligomers polymerize, creating a three-dimensional network of aluminosilicate chains through the crosslinking of Si-O-Si and Si-O-Al bonds. This process continues beyond the initial 5 minutes, gradually nearing completion around 30 minutes, as consistently confirmed by various experimental techniques. Rheological studies on the geopolymer paste flow rate support these interpretations. The study provides compelling direct evidence into the mechanisms of early-stage geopolymerization, significantly contributing to the research field and paving the way for widespread utilization in geopolymer applications.

Comprehensive assessment of engineering and environmental attributes of geopolymer pervious concrete with natural and recycled aggregate

This study aims to conduct comprehensive assessment of engineering and environmental attributes of geopolymer pervious concrete with natural and recycled aggregate. Pervious concrete mixtures were fabricated with different paste and aggregate types for laboratory characterization at different scales. As compared to traditional cement pervious concrete, mechanical strengths of geopolymer pervious concrete increase significantly when either natural coarse aggregate (NCA) or recycled concrete aggregate (RCA) is used. The replacement of NCA with RCA reduces concrete strength but the advantage of geopolymer pervious concrete is maintained. Micro-scale observations indicate that geopolymer pastes have better bonding to RCA. Life-cycle assessment (LCA) results show that the lower global warming potential (GWP) and eutrophication potential (EP), but the greater energy consumption (EC) and acidification potential (AP) are achieved by geopolymer-based pervious concrete. Photochemical ozone creation potential (POCP) is found greater for fly ash-based but smaller for metakaolin-based geopolymer pervious concrete. The use of RCA brings environmental benefit at raw materials and transportation stages, in addition to avoiding landfilling and preserving natural aggregate. The pervious concrete with metakaolin-based geopolymer achieves the best overall performance considering multi-criteria of strength and environmental indicators regardless of aggregate type.

Performance of geopolymer paste under the different NaOH solution concentrations

With the characteristics of low carbon, environmental protection, energy saving, and emission reduction, geopolymer pastes can effectively alleviate environmental and economic problems caused by the construction materials industry. The mechanical properties of geopolymer pastes are greatly affected by the composition of alkaline activators, while the existing research on their influence rules is not uniform. This study investigated the effects of alkaline activators composed of varying concentrations of NaOH solution under standard curing conditions on the mechanical properties and material characteristics of fly ash and slag-based geopolymer pastes. The results of the study indicated that under standard curing conditions, the mechanical properties of geopolymer pastes increased with the increase of NaOH solution concentration in alkaline activators. Under the influence of alkaline activators with 16 M NaOH solution, the flexural strength and compressive strength of geopolymer pastes reached the highest values at 28 d, up to 5.43 MPa and 39.13 MPa, respectively. Increasing the NaOH solution concentration in alkaline activators can promote the production of gel products such as N-A-S-H, forming a dense microstructure with a lower porosity, leading to higher mechanical properties.

Recycled foam concrete masonry and porcelanite rocks-based lightweight geo-polymer concrete at elevated temperatures

This study investigated the mechanical and microstructural properties of lightweight aggregate geo-polymer concrete (LWAGC) produced by alkali-activating glass powder (GP) and Fly Ash (FA) at elevated temperatures ranging from 200 to 800°C. It also examined the effects of incorporating crushed foam masonry (RFA) and crushed porcelanite rock aggregates (PA) into FA and GP-based geo-polymer concrete, both before and after exposure to ambient and high temperatures. A low-calcium type of FA was used as a binder in the geo-polymer concrete paste, with a 10 % replacement of glass powder. The concrete samples were heated at temperatures of 200°C, 400°C, 550°C, and 800°C for a duration of 60 minutes, with a heating rate of 7°C per minute. It was observed that the inclusion of weaker coarse aggregate resulted in a reduction of the compressive strength of the concrete. The geo-polymer concrete was subjected to tests for water absorption, mass loss, cracking, and microstructure analysis at elevated temperatures. The findings indicate that at heating temperatures of 400°C and above, the geo-polymer concrete underwent degradation and dehydration. The test findings also revealed residual compressive strengths of 104.9 %, 97.2 %, 81.8 %, and 64.2 % for the (RFA) types, and 107.3 %, 94.8 %, 78.3 %, and 58.8 % for the (PA) types. Additionally, the density decreased by 1.02 %, 4.88 %, 8.10 %, and 13.88 % for (RFA) and (PA) types, respectively, and by 0.27 %, 1.91 %, 4.67 %, and 10.79 % overall. The results indicate that the compressive strength of the concretes increased after exposure to elevated temperatures of 35°C and 200°C. However, when exposed to temperatures ranging from 400°C to 800°C, the strength of the LWAGC started to degrade and decline. Based on the obtained findings, the present study recommends performing laboratory tests on construction waste generated during demolition while developing and evaluating numerical models that predict the behavior of the resulting demolition materials when incorporated in the production of geo-polymer concrete.

The Utilisation of Rice Husk Ash Leachates for The Synthesis of Eco-Friendly Geopolymers

Geopolymers are inorganic polymers projected as feasible alternatives to Portland cement due to their lower CO2 emissions and high mechanical properties. In recent times, however, their impact on other environmental indices such as abiotic depletion, freshwater toxicity, marine ecotoxicity, terrestrial ecotoxicity, acidification, and human toxicity has become questionable, thus the need to investigate alternative eco-friendly precursors and activators. This work aims to manufacture an eco-efficient geopolymer by utilising biomass ash leachate as an alternative to the conventional alkali silicate solution. The leachate was obtained by soaking the rice husk ash (RHA) in a 10 M concentration of NaOH solution and filtering to obtain a clear solution. The effects of the calcination temperature of the kaolin and the RHA content in the alkaline solution were investigated, and a factorial design of experiments was developed considering three levels of calcination temperature (700°C, 800°C, and 900°C) and five levels of RHA content (0 g, 5 g, 10 g, 15 g, and 20 g). Physical and mechanical tests were performed on the synthesised geopolymer pastes, and chemical analysis was performed using X-ray diffraction and X-ray spectroscopy. The impacts of replacing the synthetic alkali silicates with rice husk ash-based activators were compared. The results showed that the calcination temperature of the kaolin and the content of the RHA both contributed significantly to the flexural and compressive strength at the 0.01 level of significance. A compressive strength of 10.4 MPa was obtained for the MK915 binders, which showed a 100% increase from samples without RHA content. This research proves that utilising RHA-based activators in metakaolin-based geopolymers can be feasible for less critical applications where a very high compressive strength will not be required.Keywords: alkaline activators, biomass, geopolymers, leachates, rice husk ash

Enhancing the mechanical and thermal properties of geopolymers prepared by refractory brick dust: An investigation into optimal mix ratios and curing conditions

This study proposes the development of a novel geopolymer by investigating the feasibility of using refractory brick dust (RBD), a waste product from brick manufacturing. The focus was on exploring the trade-off between waste incorporation and high-temperature resistance in geopolymer pastes. Throughout the experiments, the NaOH solution concentration was consistently maintained at 10 M. The primary goal was to examine how varying the ratios of Na2SiO3/NaOH (NS/NH) and RBD/alkaline activator (AS) on the geopolymers’ characteristics. A series of meticulous tests were conducted using various curing methods, ranging from room temperature to high temperatures between 100 °C and 140 °C, with curing times extending up to 72 h. It was observed that geopolymers exhibit the highest compressive strength after being precured at 100 °C for 72 h, achieving a maximum compressive strength of 15.40 MPa. SEM microstructure analysis revealed a correlation between the curing regime, material density and porosity. Specifically, at 140 °C, the data indicated that more porous structures have lower compressive strength. Geopolymers exhibited good mechanical properties at low curing temperatures, but exposure to temperatures above 1200 °C resulted in significant deterioration. For example, Mix 3.0RBD2.0, with a higher RBD/AS and NS/NH ratio, experienced the greatest compressive strength loss of 61.3% and a mass loss of 15.5% at these elevated temperatures. Interestingly, a geopolymer composition of 2.0RBD0.5 showed high-temperature-induced physical changes that might alter its strength and stability, yet it retained its overall structure. This study provides an in-depth explanation of the importance of optimising waste utilization to develop resilient geopolymer materials. It demonstrates how the composition – specifically, a Si/Al ratio of approximately 2.26, which is preferred based on EDS results – along with the curing methods, significantly influences the performance and high-temperature resistance of geopolymers.

Multistep kinetics of the thermal dehydration/decomposition of metakaolin-based geopolymer paste

This study investigated the kinetics of the multistep thermal dehydration/decomposition of the metakaolin-based geopolymer paste. The component two reaction steps were characterized by the evolution of water vapor and the simultaneous evolution of water vapor and CO2, respectively. In a stream of dry N2, the kinetics of the first and second reaction steps were characterized by the apparent activation energy (Ea) values of 92 and 166 kJ mol−1, respectively. Both reaction steps exhibited a diffusion-controlled rate behavior. In a stream of wet N2, the mass loss curves systematically shifted to higher temperatures with an increase in the water vapor pressure (p(H2O)). The first reaction step was significantly influenced by p(H2O), and the apparent Ea increased to 175 kJ mol−1 at p(H2O) = 11.4 kPa. The second reaction step was less sensitive to the atmospheric water vapor, as characterized by its Ea of ∼165 kJ mol−1, irrespective of the p(H2O).