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The growing demand and use of geopolymer binders have been of high interest in recent times due to their sustainability and economic benefits. However, locally available materials must be used in the production of geopolymer binders in order to maximize their benefits. In this study, a laterite soil which is a locally available material in many parts of the world was used as the aluminosilicate precursor. In order to produce the geopolymer binders, the laterite soil was activated thermally through calcination and the resulting calcined laterite was activated with an alkali activator composed of sodium hydroxide (NaOH) and sodium silicate (Na2SiO3). The influence of calcination temperature and the concentration of NaOH on the properties of the developed geopolymers was investigated. Calcination temperature used ranged between 550 °C to 750 °C and the NaOH concentrations used were 8 M and 10 M. The results from this study indicate that some locally available laterite soils can be utilized in the production of geopolymer binders if they undergo calcination. Increasing the calcination temperature from 550 °C to 750 °C resulted in the transformation of phases and an increase in the reactivity of the laterites, resulting in material with improved properties. The use of laterite calcined at 750 °C and activated with 8 M NaOH solution resulted to an increase in the 28 days compressive strength by 35.3 MPa compared to when laterite calcined at 550 °C was used. Increasing the concentration of the NaOH solution was also found to yield higher material performance. Microstructural investigations were also carried out to confirm the macrostructural properties.

In this study, meta-halloysite (MH) mixed with an alkaline solution(8, 10, and 12 M) was used as a binder phase to produce geopolymer mortars with alkaline solution/MH ratios of 0.6, 0.7, and 0.8. The flow slump behaviour, setting time, and mechanical properties of the end products were studied at room temperature. The microstructural properties of the geopolymer mortars were evaluated using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results indicated that the cohesion between the geopolymer binder and quartz sand aggregates improved with an increase in the molarity and alkaline solution/meta-halloysite ratio from 0.6 to 0.8. The corresponding slump and flow values increased from 25 to 118 mm, 14 to 115 mm, and 12 to 102 mm, whereas the setting time increased from 77 to 163 min, 54 to 140 min, and 35to 121 min,respectively, with an increase in the alkaline solution concentration. Increasing the alkaline solution/MH ratio (from 0.6 to 0.8) improved the dissolution of the solid precursor and produced a sufficient amount of geopolymer,ensuring the formation of a dense and compact structure with few accessible voids, thus justifying the reduction in water absorption and porosity. A similar trend was observed in the compressive, flexural, and tensile strengths, which increased with the alkaline activator/solid precursor and curing time (7 and 28 days). The geopolymer mortar samples prepared using 0.8 and 12 M NaOH developed high compressive strength (65 MPa), lower porosity, and lower water absorption.

Kaolin and halloysite are aluminosilicate materials widely available in Cameroon. The present study aims at investigating the fresh and hardened properties of consolidated metakaolin- meta-halloysite blends geopolymer binders cured at room temperature (23 ± 3 ◦C). To produce the geopolymers, metakaolin was used as the main solid precursor and was replaced by meta-halloysite up to 50 wt%. The raw materials and synthesized products were characterized using BET, FX, XRD, FTIR, SEM/EDS, leachability, setting time, mechanical properties, water absorption, porosity and bulk density. The results revealed that both initial and final settings decreased with the rise of the meta-halloysite content. Such behaviour is related to the higher reactivity of meta-halloysite which led to a higher geopolymerization rate resulting in shorter setting and hardening times of the MK-based binder. Incorporating meta-halloysite increased the rate of release of aluminosilicate oligomers in the alkaline solution favourable for the development of a dense and strong matrix explaining the high mechanical performances. The compressive strength and bulk density increased from 30.3 to 65.2 MPa and 1.75 to 2.12 g.cm− 3, respectively. The highest performance was attained on sample GP50, which contained 50 wt% metakaolin and meta-halloysite (65.2 MPa) and also had a decreased porosity (13.02%) and water absorption (6.72%).

This paper investigates the resistance of alkali-activated binder from volcanic ash (VA) + Municipal Solid Waste Incinerator Fly Ash (MSWI-FA) system exposed into sulphuric acid solution and artificial seawater at 56 days. The effect of these aggressive media on the physical, structural, mechanical and microstructural properties of alkali-activated mortars was discussed. The degradation was studied using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The decrease of compressive strength after 56 days was 9.10%, 22.08%, 32.5% in artificial seawater, 2% H2SO4 and 4% H2SO4, respectively. The loss of strength in both media is primarily due to the fact that the water molecules present in these media penetrated into the pore cavities between binders and aggregates, making them weak. In an acidic medium, it is also due to the decalcification and depolymerization reactions whereas in seawater medium, it is attributed to the movement of content ions and probable formation of Friedel’s salt. The results disclose that VA + MSWI-FA based alkali activated materials resist better in seawater than in sulphuric acid solution. Synergistic use of volcanic ash and MSWI-FA for construction materials through alkaline activation looks like the upcoming trend to valorize these wastes.

This paper investigates the novel effect of ultrafine rice husk ash (URHA) on the properties of alkali-activated concrete. Rice husk ash was obtained as waste from biomass energy production and fur...

In this study, the effect of nano-silica (NS) and silica fume (SF) on workability, setting time, compressive strength and microstructural properties of fly ash-ground granulated blast furnace slag (FA-GGBFS) based geopolymer concrete (GPC) is investigated. Five mixtures of each containing 0.5%, 1.0%, 1.5%, 2.0% and 2.5% NS and SF are prepared for this investigation. The optimum GPC mixture with NS resulted in compressive strength of 63 MPa and the SF modified GPC achieved a compressive strength of 59.59 MPa after 28 days of outdoor temperature curing (Avg. temp. 31.4℃). The hardened concrete samples are analyzed through X-ray diffraction (XRD), X-ray fluorescence (XRF), field emission scanning electron microscope (FESEM), Fourier transform infrared spectroscopy (FTIR), and petrographic examination, for the better understanding of geopolymer mineralogy, mechanism and microstructure. Results indicate that both NS and SF facilitated a higher degree of geopolymerization, leading to the densification of the geopolymer matrix which led to the improvement of the properties of FA-GGBFS based GPC.

Vast amounts of rice husk ash (RHA) are generated annually, with most of them unutilized or disposed to landfills, resulting in serious environmental degradation. To avoid this, the use of RHA as precursors or co-binder in the development of alkali activated materials (AAMs) is viewed as a viable alternative to avert this significant problem. In this paper, the generation, properties, and utilization of RHA in AAMs are systematically and comprehensively reviewed. Furthermore, the physical, chemical, and mineralogical composition of several RHA were critically examined, and their impact on fresh and hardened state properties of AAMs and blended formulations are presented. Generally, the properties of RHA are influenced by the source material and the production process (pre-treatment, burning time, burning temperature, cooling rate, coolingduration and milling) which in turn affect their pozzolanic reactivity. Due to its high pozzolanic nature, low energy requirement and greenhouse gas emission during production, RHA is an environmentally friendly and cost-effective alternative cementitious material to produce AAMs. Various studies have reported the beneficial role of RHA on the mechanical, microstructural and durability properties of AAMs, especially when used at an optimal level. Overall, this review will provide valuable insight, direction, and recommendations for researchers and industrial sectors on the generation, recycling, characteristics, and utilization of RHA in the development of AAMs for potential construction applications.

In the last three decades, the gigantic demand for sustainable and environmentally friendly concrete with reduced environmental footprints has resulted in the development of low carbon concretes such as geopolymer concrete. Metakaolin which is commonly used as an admixture or partial replacement of cement owing to its most effective pozzolanic properties, which improve the microstructure and strengthen the mechanical and durability properties of cement concrete, has been investigated as a precursor in geopolymer concrete. Several studies have been conducted to comprehend the effect of metakaolin as an additive in geopolymer mortar and concrete prepared with various aluminosilicate sources as precursors such as fly ash and rice husk ash to enhance geopolymerization, densify microstructure, and elevate durability. The present paper recapitulates these investigations primarily concentrating on the various properties of metakaolin-based concrete. The effect of various factors such as alkali content, solids/liquids ratio, alkali reactant ratio, molar ratio, water content, and curing regime has been compiled. Most of them revealed that metakaolin is used as a precursor and yields better geopolymer products. XRD studies reported the peaks demonstrating the development of enhancement in hydration products in comparison to other precursors. Examination of SEM graphs reveals that the addition of a smaller quantity of silica-rich materials densifies the microstructure of geopolymers and produces higher mechanical strength. Durability studies reveal that metakaolin geopolymers possess better water resistance, thermal resistance, and anti-corrosion properties. The possible applications of metakaolin-based geopolymeric materials are also pointed out. The comprehensive knowledge presented here is expected to support the prospective researchers to decide their future course of the research area. Graphical abstract.

International audience; In this work, two different laterites (iron contents of 13.07 and 49.34 wt%, respectively, for lateritic clay, LAC, and iron-rich laterite, LAI) were selected and calcined at 600 °C. The obtained calcined laterites, namely LAI600 and LAC600, were separately mixed with an alkaline solution (silicate modulus of 1.35) or an acidic solution (phosphoric acid solution at pH ≤ 2) for the synthesis of inorganic polymer products. The fabricated products were cured at 20 °C (ambient) and 40 °C (oven). The obtained results showed that the compressive strength of each series of alkaline-based inorganic polymer binder increased with ageing time (7 and 28 days) for room temperature curing, while the reverse trend was noted for oven-cured specimens. The best mechanical performance was obtained when using a phosphoric acid solution, 38 and 52 ± 1 MPa; 62 and 65 ± 1 MPa at 28 days for LAC and LAI respectively. It appeared that a higher iron content within the laterite contributed to an increase in the compressive strength under acidic conditions (LAI (59 MPa) > LAC (48 MPa)) compared to the behavior obtained under alkaline conditions (LAI (6 MPa) < LAC (29 MPa)). Accordingly, the acidic products exhibited a dense structure (with lower porosity) and contained amorphous iron/aluminium phosphate phases such as berlinite (FePO4), iron hydrogen phosphate hydrate (Fe3H15(PO4)8·4H2O), ferrowyllieite (AlFe2Na2(PO4)3) and sodium iron phosphate (Na3Fe2(PO4)3) arising from the alteration of iron minerals in an acidic medium, confirmed by Mössbauer spectroscopy and electron paramagnetic resonance spectroscopy.

The present work carried out the influence of curing cycles on the performance of laterite-based geopolymer composites. To do so, the end products were obtained by altering laterite with 15, 20, and 25 wt% of rice husk ash (RHA). Alkaline solution in a constant solid/liquid ratio of 0.35 was added together with fine and coarse aggregates (representing equal and double weight of laterite, respectively). The different obtained matrices were treated in the following three curing cycles before characterization: room temperature curing (RTC), oven curing at 80 °C (OTC) and controlled humidity steam curing at 80 °C (STC). The mechanical tests carried out at 28 days give the following maximum values for each curing mode: 16.40, 28.82 and 56.41 MPa for RTC, OTC, and STC modes respectively. This means that when the samples, submitted in a moisture-controlled environment, the end products are more stable, less porous and resistant. Regarding the physical properties, the results show that the maximum value of open porosity is 11.62% corresponding to a matrix that was cured at room temperature without rice husk ash added, while the minimum value of 7% corresponds to a matrix that was cured under controlled humidity and containing 20% rice husk ash. The optimum and minimum absorption values are 2.70 and 4.60% respectively for the OTC and RTC curing modes. As for bulk densities, the optimum value is 2.64 g cm−3 for the matrix having 15% rice husk ash and the minimum value is 2.33 g cm−3 for a matrix having 20% rice husk ash, for OTC and STC curing modes respectively. The appropriate curing type for laterite-based geopolymer is when the humidity is controlled.

During the thermal power production, the combustion of coal results in coal ashes as waste products, viz. fly ash (FA) and bottom ash (BA). Considering the fact that the major part of the power comes from thermal power plants, a huge amount of coal ashes are generated that cause serious issues for its disposal. Extensive efforts have been made to utilize these ashes for different applications in bulk amount, and building material manufacturing is one of them. Therefore, in this chapter, the characterization and utilization of coal ashes in building material synthesis are discussed with providing a critique on the challenges and future trends of coal ash utilization.

Two calcined clays (halloysite and kaolinite clays at 700 °C) were used as solid precursors for geopolymer synthesis. This study compares the physicochemical properties of the both resulting geopolymer series heated at 200, 400, 600 and 800 °C. The end specimens were characterized using Optical Dilatometer, XRD, FTIR, MIP and SEM analyses. Results revealed that the flexural strengths were 18.10 and 21.74 MPa for meta-halloysite- and metakaolin-based geopolymers, respectively. After subjected to high temperatures, the flexural strength drastically decreased from 18.10 ± 1.06 to 6.7 ± 0.23 MPa and 21.74 ± 1.20 to 4.63 ± 0.24 MPa, respectively. The maximum shrinkage recorded on metakaolin and meta-halloysite-based geopolymers was 14 and 16% around 950 °C, respectively. The thermal conductivities decreased with increase in heating temperature from 0.78 to 0.19 Wm−1 K−1 and 0.96 to 0.26 Wm−1 K−1, respectively. This reduction is linked to the additional voids and microcracks that occurred within the geopolymer network. The cumulative intrusion in both geopolymers increased with increase in heating temperature up to 600 °C, leading to the degradation of geopolymer network that affected the mechanical strength evolution. Both synthesized geopolymer series are potential candidates for insulation materials or refractory applications.

This paper presents the results of experimental evaluation of curing conditions on the microstructure and performance of geopolymer binders developed from iron-rich laterite soils. Two calcined iron-rich laterites namely LB600 and LY600 were used as solid precursors in the preparation of geopolymer binders. The geopolymer samples were cured at 20, 60 and 80 °C. FTIR, XRD, EDS and DTA/TG were used to evaluate the microstructural properties of the prepared products. The performance of the binder was evaluated in terms of the compressive strengths, water absorption, porosity, bulk density and thermal conductivity. The findings from this study showed that the dissolution of the calcined laterites in 8 M NaOH increased the dissolution of Al, Si and Fe elements with increasing temperature from 20 to 80 °C. This higher dissolution of the monomers further resulted to an increase in the compressive strength of the binders at 7 and 28 days. It was also found out that curing the geopolymer in the dry state resulted in higher compressive strength at all ages compared to those cured in the wet and wet-dry state. Drying shrinkage evaluation of the geopolymer samples cured between 60 and 80 °C exhibited a lower linear shrinkage due to a high degree of geopolymerization. Microstructural investigation of the geopolymer samples cured at 80 °C showed a heterogeneous compact and dense structure resulting from high polycondensation. This densified microstructure also induced an increase in the thermal conductivity from 0.65 to 0.90 W/mK and 0.75 to 0.91 W/mK for LB600 and LY600, respectively. Nonetheless, both geopolymer binders made of LB600 and LY600 laterite powders performed well in dry, wet and wet-dry conditions, and can be used for various construction applications especially in the precast industry.

In this paper, four samples of meta-halloysite from widely available Cameroon halloysite clay calcined at 600, 650, 700 and 750 °C, respectively, and the resultant geopolymer binders were systematically characterized. Isothermal conduction calorimetry (ICC) was used to measure the reaction kinetics of meta-halloysite geopolymers at room temperature during 48 h. The increase in reaction rate and in heat released correlates with the thermal activation temperature. It was typically found that the increase of thermal activation temperature (from 600 to 750 °C) enhances the amorphous or reactive phase content from MH600 to MH750 samples. This results in an improvement of rheological behaviour and setting time of the fresh meta-halloysite based geopolymer pastes. The hardened meta-halloysite based geopolymers were amorphous, compact and dense according to X-ray diffractometry (XRD) and Scanning Electron Microscopy (SEM). The compressive strength of resultant products increased with the activation temperature, up to 750 °C (74 MPa at 180 days). Therefore, the best temperature in view of the highest strength of meta-halloysite geopolymer cured at room temperature is 750 °C, although the lower increase in mechanical performance (7.3%) gained between GPMH700 and GPMH750 is limited. Thus, thermally activated halloysite clay appears to be a promising candidate for geopolymer synthesis.

This study aimed to investigate the effect of Titanium Dioxide TiO2 (anatase and rutile) on mechanical and microstructural properties of meta-halloysite based geopolymer mortars namely GMHA and GMHR series. Meta-halloysite received 2.5, 5.0, 7.5 and 10 wt% of anatase or rutile as addition before calcination and geopolymerization. The raw materials and the end products were characterized using XRD, FTIR, ESEM and MIP analyses. The flexural strength increases from 6.90 to 9.13 MPa and from 6.90 to 12.33 MPa for GMHA and GMHR series respectively. The cumulative pore volume decreases from 102.2 to 84.2 mm3 g−1 and from 102.2 to 51.3 mm3 g−1 for GMHA and GMHR products respectively. Both matrices present micrographs with very low capillaries pores and fractured surfaces that confirmed the enhancement of the mechanical properties. It was concluded that TiO2 in both forms is beneficial for the reduction of porosity and densification of geopolymer matrices. Rutile enabled more compact and denser geopolymer structure compared to anatase. The aforementioned results showed the efficiency of both fine TiO2 particles to improve the geopolymer network significant for its durability.

This paper investigates the use of bauxite widely available in northern Cameroon as an additive in the optimization of some properties of metakaolin-based geopolymer. To do this, several geopolymer mixtures were prepared by substituting metakaolin (MK) by bauxite (BA) (from 0 to 50%) and partially kept at room temperature (28 °C), while others were sintered at 200, 800 and 1200 °C. The raw materials and resulting products were characterized using X-ray fluorescence spectrometry (XRF), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), densification parameters, mechanical properties as well as microstructural morphologies. The results revealed that the setting time of the geopolymer pastes increased with the bauxite content due to its low dissolution in alkaline medium at room temperature. The mechanical strength of samples decreased from 35.20 to 11.10 MPa at room temperature. At 1200 °C, the higher strengths (50, 98 and 70 MPa) were achieved in MKBA10, MKBA20 and MKBA30, respectively. These samples also exhibited dense and compact microstructure partially due to packing particles effect and the nature of bauxite known as refractory material. Thermal shrinkage and relatively high mass losses reflected the decomposition of chemical compounds within the system. Thus, the synthesized materials heated at 1200 °C could be used as a potential candidate for refractory applications.

The progressive addition of the reactive silica from rice husk ash (10–50 wt%) changes the phases evolution and the microstructure of the laterites based geopolymer composites. It was found that the presence of soluble silica enhanced the formation of Si–(A)-OH–Fe bonds and changed considerably the FTIR and XRD patterns of the products: polysialates, ferrosialates and ferrisilicates were present into the matrices. The solid/liquid ratio and the curing cycle influenced the pore network, the bulk density, the total pore areas, and the final microstructure. The relative low viscous paste that is maintained at ambient temperature for 1–2 h before curing at 90 °C gave better cohesion and low porosity resulting to the high concentration of strongest (Fe–O–Si) bonds compared to pastes cured directly after preparation or those with high viscosity. While polysialates and ferrosilicates are hindered in the case of quite treatment at 80 °C due to the rapid evaporation of water, polysialates, ferrosialates and polynuclear ferrisilicates complexes are formed at room temperature and polynuclear complexes can growth to more crystalline phases with the increase of the temperature. Ferrisilicates integrated the interlayers of the matrix improving the geopolymerization with significant modification of the microstructure. Those newly formed phases are promising components for the development of eco-friendly and sustainable high strength composites, porous matrices for filtration, heavy metal removal/immobilization, membranes for catalysis and template for new carbone replicate.

This work focuses on the comparison between the mechanical and microstructural properties of poly(sialate-siloxo) networks based on metakaolins from halloysite and kaolin. Poly(sialate-siloxo) networks were prepared using three metakaolins as aluminosilicate sources. Sodium waterglass from rice husk ash and commercial sodium waterglass were used as chemical reagents. The obtained results showed that metakaolins from kaolins have plate shapes with coarse particle sizes whereas the one from halloysite has a spherical morphology and smaller particle sizes. The IR spectra of poly(sialate-siloxo) networks from calcined halloysite indicate the higher value of the wavenumber of the main band. The XRD patterns of all poly(sialate-siloxo) networks show the broad hump structure with higher intensity between 18 and 40°(2θ). The XRD patterns of poly(sialate-siloxo) networks show the band of the unreacted metakaolin at about 20.45°(2θ). This band is more pronounced on the XRD patterns of geopolymer cements from calcined halloysite. The obtained poly(sialate-siloxo) networks based on metakaolins from halloysite and kaolin have a compact, homogenous and denser microstructures. The compressive strength values of the poly(sialate-siloxo) networks using calcined kaolin are ranging from 58.43 to 66.52 MPa whereas those using calcined halloysite are between 72.29 and 88.50 MPa. The compressive strength values of poly(sialate-siloxo) networks using calcined halloysite are higher compared to those from calcined kaolin. The higher compressive strength values of the geopolymer cements from calcined halloysite could be attributed to the fine and spherical particle sizes of calcined halloysite. This implies that the shape and the fine particle sizes of the raw materials influence the properties of the poly(sialate-siloxo) networks. Metakaolin from halloysite can be used as an aluminosilicate source for producing poly(sialate-siloxo) network with higher mechanical properties.

Laterite soils are one of the promising aluminosilicate materials that can be utilized as precursors in the synthesis of geopolymers. However, it is critical to understand how the composition of the laterites and the type of activator influence the corresponding performance of geopolymer composites. Hence, this study was undertaken to evaluate the influence of mineralogy and activator type on the rheology, microstructure and performance of laterite-based geopolymers pastes. In this study, calcined laterite alongside an alkali and acid solution was utilized for geopolymer synthesis. The physical and chemical properties of the raw laterite, calcined laterite and geopolymer pastes were evaluated using Brunauer Emmett and Teller method, X-ray diffraction, fluorescence X analyses, quantitative Rietveld analysis and infrared spectroscopy. The fresh geopolymer pastes were assessed in terms of slump flow, setting time and rheological properties. Microstructural investigations alongside the evaluation of mechanical and permeability properties of the hardened geopolymer pastes were also carried out. The findings from this study showed that the fresh and hardened properties of laterite-based geopolymers are dependent on the mineralogy of the calcined laterite and the type of activator used. Geopolymer pastes synthesized using acidic solution exhibited a slower reaction than those made with an alkali solution. However, geopolymer pastes activated with an acidic solution exhibited higher mechanical performance and lower permeability compared to those synthesized with an alkali solution.

The type of aluminosilicate precursor used in the synthesis of geopolymer binders plays a huge role in the resulting performance. Thus, it is critical to understand the properties of precursors and how they influence the corresponding performance of geopolymer binders. In this study, metakaolin and meta halloysite are used as the aluminosilicate precursor in the synthesis of geopolymer binders. These precursors are obtained locally in order to propel the sustainable development and application of geopolymers. The precursors were characterized and the corresponding influence on the reactivity, rheology and setting times of geopolymers was investigated. In addition to the influence of precursor type on the properties of the geopolymers, the effect of two silica moduli (i.e. 1.3 and 1.5) was also evaluated. The results from this study indicated that increasing the activator silica modulus from 1.3 to 1.5 extended the setting times and increased the stress strain of the geopolymer binders. Characterization of the precursors indicated that metakaolin has a higher amorphous content compared to that of meta halloysite. However, the finer particles of meta halloysite embodied it with the ability to participate in a faster geopolymerization and result in more formation of activation products.

In this study, an ecofriendly binder was developed by using rice husk ash (RHA) and sodium hydroxide as the activator alongside fly ash (FA) as the aluminosilicate precursor at ambient conditions. The developed binder was used in the production of mortars with varying proportions of sand. A total of three mortar mixtures were developed with sand to binder ratios of 0.55, 0.83 and 1.11. The corresponding behaviour of the mortars and influence of the sand proportions was assessed in terms of the compressive strength, water absorption, density, porosity. Microstructural investigations such as scanning electron microscopy, energy dispersive x-ray spectroscopy, infrared spectrum analysis and mercury intrusion porosimetry were also used to validate the physical properties. The findings from this study demonstrated that RHA can be used successfully as an activator component in the production of mortars. In terms of the sand content, it was found out that increasing the sand to binder ratio has detrimental effects on the performance of the mortars due to the reduction in the binder content primarily. The microstructure analysis of the mortars made with sand to binder ratio of 0.55 showed a compact and strong structure justifying the higher compressive strength achieved. The MIP analysis confirmed that the formation of C-A-S-H gel at the advanced ages allowed the pores refinement with a significant decrease in the fraction of the capillary and macroscopic pores within the geopolymer mortars.

Geopolymers were developed in this study by using locally available laterite soil as the aluminosilicate precursor. Rice husk ash (RHA) which is an agricultural waste was incorporated as an additional source of silica in the geopolymers by using it as a partial replacement of the laterite. In addition to the incorporation of RHA, quartz sand was also incorporated as filler in the developed geopolymers. The influence of the RHA, quartz sand and two initial curing temperatures (i.e. 40 and 80 °C) on the properties of the geopolymers developed was investigated. A comprehensive characterization of the aluminosilicate precursors used and the developed geopolymers were also carried out. The outcome of this study showed that the combined use of RHA and quartz sand in laterite-based geopolymer is beneficial in terms of performance enhancement. The incorporation of RHA and quartz sand increased the compressive strength by 15.5 MPa and reduced the water absorption by 4.1% for geopolymers cured at 40 °C. The RHA enriched the geopolymer network with silica resulting in the formation of additional geopolymer products and a corresponding enhancement in performance. Similarly, the quartz sand densified the geopolymer matrix thereby improving the properties of the geopolymer. It was also found out the use of higher initial curing temperature (i.e. 80 °C) is only beneficial for geopolymers incorporating RHA.

This paper present the results from a comprehensive study undertaken to investigate and develop alkali-activated binders (AABs) with laterite soil as the aluminosilicate precursor. In this study, the effect of the silica modulus (SiO2/Na2O) of the activator on the setting time, rheological properties and strength development were investigated. Iron-rich laterite sourced from West Africa was used as the aluminosilicate precursor alongside sodium silicate and sodium hydroxide for the production of the activator. The activators were prepared to have varying silica modulus of 1.3, 1.5, 1.7 and 2. The findings from this study showed that the silica modulus of the activator used in the synthesis of the laterite-based AABs has a significant influence on the resulting properties of the binders. It was found out the optimum silica modulus is 1.3 and increasing the silica modulus of the activator results in detrimental effects on the hardened properties of the AABs. In the same context, increasing the silica modulus of the activator from 1.3 to 2.0 extended the final setting of the binder time by 56.1% while the compressive strength at 56 days reduced by 57%. Microstructural investigation on the binders showed that the main products of alkali activation of the calcined laterite are quartz, ilmenite, hematite and maghemite. It was concluded that laterite-based AABs with good performance can be produced with a silica modulus of 1.3 and used as a possible alternative for Portland cement as a binder.

Alkali-activated composites are promising sustainable alternatives that can be utilized as a replacement for Portland cement materials. Alkali-activated composites are made with a binder obtained from the alkali activation of aluminosilicate precursors. However, the types of conventional activators used posed a huge challenge due to their high embodied energy and carbon. On the other hand, sodium sulphate which exists naturally in the environment and does not require complicated production systems can be utilized as a sustainable alternative activator in the production of alkali-activated composites. Compared to the use of conventional activators in the production of alkali-activated composites, the use of sodium sulphate is limited. Hence, in order to gear more research, developments, and applications of alkali-activated composites produced with sodium sulphate as an activator; this review was carried out. In this overview, the performance of sodium sulphate-activated composites in terms of its fresh and hardened properties is discussed.

The present project deals with the production of alkali activated concretes based on volcanic and calcined laterite under different curing temperatures (28–80 °C). A fraction of 0–40% calcined laterite was added to improve the reactive phase content and the resulting mixtures were labelled GL0, GL20 and GL40. The raw materials and hardened products were characterized using XRD, FTIR, optical microscopy, mechanical properties and durability. The results depicted that both calcined laterite addition (40%) and curing temperature (28–80 °C) have beneficial influences on the mechanical performance. At 28 days, flexural and compressive strengths ranged between 5.05-5.35 MPa and 39.68–48.80 MPa for GL40 specimens. Water absorption and porosity decreased from 7.23 to 4.51% and 9.70 to 6.01%; 16.70 to 10.32% and 22.20 to 13.80% respectively, for geopolymers GL40 and GL20 samples. Bulk densities values were 2398.44/2400.16; 2362.46/2364.16 and 2338.50/2340.20 kg/m3 for GL40 and GL20 specimens respectively. After 19 weeks of sulfuric acid attack (pH = 1), a significant mass loss (1.28–6.22%) was measured in the geopolymer concretes. This behavior is linked to formation of a strong and compact matrix. The above-mentioned alkali activated concretes appear to be a suitable candidate for engineering applications in extreme conditions.

The need to cure fly ash-based geopolymer at elevated temperatures has limited the practicability and sustainability of the composite. Hence, there is an imminent need to find ways at which fly ash-based geopolymers can be cured at ambient conditions. This paper presents the results from the experimental investigation of the effect of lime and silica fume on the properties of geopolymer concrete cured at ambient conditions. Lime and silica fume were used as the partial replacement of fly ash as an aluminosilicate precursor and the corresponding effects on the fresh, strength and microstructure of the geopolymer concrete were investigated. The findings from this study showed that the slump and setting times of the geopolymer concrete increases with increasing silica fume content and reduces with increasing lime content. Also, the use of lime and silica fume as 7.5% and 2% respectively, replacement of fly ash yielded the highest compressive strength. Microstructural investigation showed that the combined use of lime as silica fume in GPC resulted in a densified microstructure.

This study investigated the impact of calcium carbonate nanoparticles (nano-CaCO3) on the mechanical properties of geopolymer composites. To that end, varying contents of nano-CaCO3 were mixed with geopolymer paste. On the other hand, mechanical properties were investigated based on three aspects: flexural and compressive strengths, impact strength, and hardness. According to the findings of this study, nano-CaCO3 could be efficacious in improving all the mechanical properties of geopolymer. Microstructures of the geopolymer mixed with nano-CaCO3 were also evaluated by scanning electron microscopy (SEM). According to the findings, nanoparticles could potentially enhance the mechanical performance of the geopolymer nanocomposites. In addition, the SEM micrographs of fractured surfaces were found to exhibit a greater packed structure with a reduced number of pores of the geopolymer that contained nano-CaCO3; this, in turn, enhanced the strength of geopolymer nanocomposite specimens.

In this paper, the results of the experimental investigation were used to understand the effect of fine meta-halloysite on the reactivity, mechanical and microstructural properties of laterite-based geopolymers. Laterite was replaced by 0, 20, 30 and 50 wt% of meta-halloysite in order to improve the physico-chemical performance. Meta-halloysite was prepared by calcination of natural halloysite at 600 °C. The moduli (molar ratio SiO2/Na2O) of the activating solutions were 1.04, 0.92, and 0.75 with H2O/Na2O = 9.78, 10.45 and 12.04, respectively. The results indicated that calcined laterite has a high specific surface area (43.00 ± 0.12 m2/g), notwithstanding a high average particle size (d50 = 45.20 μm) compared to meta-halloysite with a smaller average particle size (d50 = 8.40 μm) and a specific surface (29.80 ± 0.16 m2/g). The compressive strength of geopolymers increased upon the addition of meta-halloysite from 12 MPa to 45 MPa at 28 days. While the setting time and water absorption decrease with increase in the of meta-halloysite content as well as with increase in Si/Al, Si/Fe, Al/Fe and Na/Al molar ratios used in the synthesis of geopolymers. The use of fine meta-halloysite resulted in better efficiency and improved mechanical performance of synthesized products.

Cameroonian Balengou clay has been characterized as precursor for inorganic polymer (geopolymer) binder. The lowest possible calcination temperature for this halloysite type clay was found to be 600 °C to convert it into meta-halloysite (MH), which is reactive enough for geopolymerization. This implies an energy saving compared to the 800 °C, previously reported in literature. The inorganic polymer has been produced using sodium hydroxide or sodium silicate solutions. A maximum dry compressive strength of 27.5 MPa after 28 days was obtained with a sodium silicate solution [Na2O·1.25SiO2·9.76H2O] resulting in a material with composition: Na2O·3.78SiO2·7.22Al2O3·9.76H2O. This inorganic polymer was amorphous according to XRD, showed a compact microstructure (scanning electron microscopy), with lower values of water absorption, porosity, and higher density. In addition the same composition showed better resistance to 5% sulfuric/hydrochloric solution than the ones made with higher and lower ratio of Na/Al. Meta-halloysite based geopolymers had a 20–30% reduction in strength after immersion in 5% sulfuric/hydrochloric acids for 7 days and even up to 62.5% after 28 days, due to the depolymerisation process of the geopolymer network. The results obtained demonstrated the suitability of Cameroonian meta-halloysite for geopolymer synthesis at room temperature.

This paper studies the alkali activation of iron-rich aluminosilicates (laterites). Three activating alkaline solutions were prepared from sodium hydroxide solution (8, 10 and 12 M) with sodium silicate (Na2SiO3) in order to obtain the sodium silicate solutions with moduli of SiO2/Na2O equal to 0.75, 0.92 and 1.04; H2O/Na2O = 9.78, 10.45 and 12.04. The effects of above-defined solutions on the setting time, physical and microstructural properties of geopolymer binders from calcined laterite (600 °C), containing metakaolinite, as the sole binder at room temperature are reported and discussed. A laterite from Eloumden and one from Odza were used. The synthesized products were labelled GPEL(i=1.04, 0.92 and 0.75) and GPOD(i=1.04, 0.92 and 0.75) series. The dry compressive strength measured after 7 and 28 days were 4–10 and 10–18 MPa, respectively. It was typically found that the geopolymer paste from sodium hydroxide with molar concentration 12 M and the molar ratio SiO2/Na2O of the silicate solution equal to 0.75 produced the highest compressive strength (~ 18 MPa). These samples also have a denser matrix. The dry bulk densities of both series increased with the decrease of silica moduli and were in the range 2.31–2.43 and 2.32–2.52 g/cm3 and the water absorptions were in the range of 8.21–11.40% and 7.23–13.03% for geopolymers GPEL and GPOD series, respectively. The setting time decreased with increasing molarity of NaOH solutions. The physicochemical properties and the mineralogy of both iron-rich aluminosilicates were influenced by the silicate modulus of activating solutions and the best compositions were achieved with characteristic SiO2/Na2O = 0.75 and H2O/Na2O = 9.78.