Novel approach to synthesize clay-based geopolymer brick: Optimizing molding pressure and precursors' proportioning

A novel approach has been proposed to synthesize clay-based geopolymer brick; series of experiments have been performed and three mandatory steps have been evolved. In Step-1, the reactivity of fly ash (FA) and clay was assessed through the modified Chapelle test. In Step-2, a semi-dry mixture of geopolymer paste was prepared and cylindrical specimens were formulated at molding pressures from 0 to 60 MPa at intervals of 10 MPa. And in Step-3, FA and clay contents were varied from 25 to 75% to form geopolymer paste, and specimens were prepared at two molding pressures (20 and 40 MPa), which is based on the results of Step-2. Physico-mechanical performance of specimens, cast in Step-2 and Step-3, was evaluated, and compared, and the rate of geopolymerization and microstructure was assessed through FTIR and SEM analysis, respectively. From the results, it was revealed that both precursors, clay and FA, were reactive, and molding pressure increases the wettability of both materials and resulted in an increase in compressive strength and density, and a decrease in porosity and water absorption. The amount of clay is attribute to the strength gain at high molding pressure due to its crystallinity load-bearing capacity and chemical surface compatibility with cementing properties of reactive mass. The strength gain mechanism was discussed at the micro-level as well as the molecular level. Finally, the proposed approach was validated by synthesizing the brick at the pilot scale. The physico-mechanical properties of brick were compared with the codes and cylindrical specimens, a close correspondence was observed between them, and also fulfil the criteria mentioned in standards.