Effect of Geopolymerization Reaction on the Flexural Strength of Kaolin-Based Systems

Geopolymers exhibit broad application prospects, including construction and radiation shielding, which require excellent mechanical performances. However, investigations on the nature of geopolymerization reactions and their consequential impact on mechanical performance are still vague. In this study, the effect of the major factors of Si/Al ratio and curing time on the geopolymerization reaction and flexural strength were studied based on the microstructure evolution and chemical bonding formation analyzed using the SEM, FTIR, peak deconvolution, and XRD methods. The microstructure of geopolymers was transferred from initially layered smooth particles of kaolinite to a 3D network porous structure, corresponding to sodalite. A spectrum exclusive to the geopolymer structure occurred at 973 cm−1, corresponding to the sodium aluminum silicate hydrate (N-A-S-H) links, the integral area of which represents the degree of geopolymerization reaction. Furthermore, a controllable reaction degree was achieved by adjusting the Si/Al ratio and curing time, where the maximum reaction degree of 55% was achieved at a Si/Al ratio of 1.94 when cured for 7 d. The correlation between the flexural strength and reaction degree was found to follow a proportional relationship, achieving a flexural strength of 21.11 MPa with a degree of 45%. This study provides insight into the development of mechanical strength through controlling the reaction process.