Characterization and thermodynamic modelling of alkali-activated calcined clays: potentiality of Cameroon's laterites as eco-sustainable binders

Alkali-activated calcined clay may represent a successful alternative to ordinary Portland cement in order to maximize the potentiality of widely available raw materials as low-CO2 binders. The abundant presence of kaolinite-rich laterite soils at the sub-tropical latitudes may be an interesting alternative for the design of alkali-activated binders given that the rapid urban development in emerging countries is expected to push forward cement demand. The assessment of an appropriate mix design that ensure the improvement of mechanical performance of the calcined laterite from the surroundings of Yaounde (Cameroon), along with the research on the chemistry of the binding phase of clay-based alkali-activated materials, are the main goals of the thesis work. The phase composition of the laterite was characterized by X-ray diffraction and its thermal behaviour was assessed by thermogravimetric analysis. The relation ship between mechanical strength and intimately related calcination variables was evaluated by the Experimental Design. The microstructure of the clay-based alkali-activated materials was investigated by means of X-ray microtomography and scanning electron microscopy, and the phase composition by X-ray diffraction, implementing the PONKCS (partial or not known crystal structure) approach. The data collected from the spot chemical analyses (SEM-EDS) were compared with the data obtained from thermodynamic simulations using a Gibbs free energy minimization software (GEMS). An adequate compressive strength was obtained by carefully dosing the amount of alkali, although the presence of iron in the laterite negatively hinders the mechanical performance. The simulated composition of the geopolymeric reaction product formed in Na2SiO3 ·5H2O-activated calcined clay mortar, highlighted by ternary diagrams, is close to that of the matrix collected with SEM-EDS, by revealing that this aluminosilicate product has a chemistry similar to that of an ideal solid solution formed by zeolite end-members (NASH ss).