Exploring calcination parameters: Impacts on reaction kinetics, mechanical strength, and microstructural evolution in laterite-derived geopolymers

Calcined laterite offers a promising alternative to traditional precursors in geopolymer synthesis. Despite its potential, knowledge regarding the effects of calcination conditions, particularly the heating rate, on its properties remains sparse. This research delved into the impact of different calcination heating rates on laterite and the subsequent geopolymer products by calcining laterite at 600°C for 4 hours, with heating rates ranging from 1.5 to 10°C/min. Analytical techniques including isothermal conduction calorimetry (ICC), setting time determination, XRD, TG/DTA, XF, FTIR, Mössbauer spectroscopy and SEM/EDS were employed to characterize both raw and calcined laterite as well as the resulting geopolymer products. Key findings indicate that higher heating rates augment the amorphous phase in the precursors, leading to decreased setting times. This reduction is directly associated with heightened reaction activity and increased heat release, as noted from ICC results. Notably, mechanical properties improved at a heating rate of 7.5°C/min, giving a compressive strength of 51 MPa. The mentioned heating rate also resulted in the best porosity and water absorption values of 14% and 8%, respectively, suggestive of a more consolidated microstructure. The data ultimately designates 7.5°C/min as the optimal heating rate for crafting geopolymers with superior mechanical and microstructural attributes. Mössbauer spectroscopy revealed details of the iron site geometry and oxidation state, showing an increase in Fe 3+ content within the silicoaluminate phases. This effect was slightly more pronounced in samples synthesised with a heating rate of 7.5°C/min.