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Acid-based geopolymers exhibit interesting properties, such as higher temperature resistance (up to 1450 °C) and better mechanical properties, as compared to alkali-based geopolymers. The aim of this work was to highlight the origin of water and thermal resistances of acid-based geopolymers, demonstrated in our previous study, by 27Al, 29Si and 31P NMR and XRD experiments. Different networks were indeed identified in consolidated materials, such as Al-O-P, Si-O-P, vitreous silica and Si-O-Al (only for Al/P = 4, due to unreacted metakaolin) depending on processing conditions. The final properties are induced by the proportion of these networks which are controlled by the Al/P ratio and the setting temperature. It was shown that a higher amount of Al-O-P bonds and the absence of unreacted metakaolin conduct to a thermally resistant sample. A more organized structure with a low proportion of Q0(1Al) and a preference for Q1(2Al), as seen by 31P NMR, induces a great water resistance of geopolymers. After heating at 1000 °C, different AlPO 4 polymorphs crystallize, as well as mullite in the case of Al/P = 4. Resistance of acid-based geopolymers with associated structural and working properties. Image 1 • Acid-based geopolymers display different processing-dependent working properties. • Acid-based geopolymers are mostly composed of Al-O-P and Si(OH) 4 networks. • The quantities of Al-O-P and unreacted metakaolin govern the thermal resistance. • The increase of the Young's modulus is related to a higher thermal resistance. • Ordered structures and low amounts of physisobed water lead to water resistance. [ABSTRACT FROM AUTHOR]

Here, the valorization of Callovo-Oxfordian (COx) argillite from the geological layer focused by the Cigéo project (Industrial Center for Geological Disposal of radioactive waste) is studied by Andra. Its reuse by alkali-activation is studied in order to develop thermally resistant binders. The thermal properties of various argillite-based samples are studied at 800 °C, and they present a good resistance to a slow firing environment but a lower resistance to thermal shock. This article discusses the influence of the sample compositions on the thermal resistance. First, the thermal activation of argillite has a significant impact on the thermal resistance of the alkali-activated samples. Indeed, a higher resistance is reached with the use of furnace-calcined argillite due to the complete dehydroxylation of clay minerals and decomposition of carbonates. However, the use of flash-calcined argillite (partial dehydroxylation and decompositions) leads to different species present in the mixture and therefore may be conducive to higher structural evolutions during sample firing. Then, the influence of the nature of the alkaline silicate solution is highlighted. It appears that the use of sodium-potassium silicate powder-based solution (Si-Na-K) is conducive to lower reactivity and more viscous flow than a liquid-based potassium solution (Si-K). Finally, the addition of filler, sand (SiO 2 ) and calcined clay (Al 2 O 3 -SiO 2 ) allows for better thermal resistance but limits the development of mechanical properties. [ABSTRACT FROM AUTHOR]

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. • Laterite, aggregates and rice husk ash were used to produce geopolymer composites. • Geopolymer composites were cured at Room curing temperature (RTC), Oven curing and Steam curing (STC). • Applying the steam curing (STC) under controlled humidity (>65%) optimized the geopolymerization reaction. • High mechanical (56.41 MPa) was achieved on sample containing 25 wt% of RHA. • This sample also exhibits compact and dense structure with few accessible voids. [ABSTRACT FROM AUTHOR]

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. • Polysialates and Ferrosialates reinforced with ferrisilicates for sustainable and low cost porous system. • Development of curing system for the production of iron-rich aluminosilicates based geopolymers. • Cleaner product of ferrisilicates based functionalizing matrices for catalytic and filtration processes. [ABSTRACT FROM AUTHOR]