Effect of Silica Moduli on the Thermal Degradation Mechanisms of Fly Ash–Based Geopolymer Mortars

Abstract

This study investigates the effect of silica moduli (SiO2/Na2O ratio) of alkaline activators on the degradation mechanisms of fly ash–based geopolymer (FAG) mortars that are exposed to elevated temperatures of up to 1,000°C. The evolution of the mechanical properties, microstructure, mineralogy, and atomic structure of the FAG with three different silica moduli after the thermal treatment are studied. The results show that silica modulus affects the compressive strength and crack development of geopolymer mortars. Geopolymer mortars with the highest silica modulus have the best thermal stability and volume stability after exposure to high temperatures, as confirmed by the latest cracking initiation and lowest cracking intensity. The mechanism of decreased strength in geopolymer mortars with low silica moduli (owing to crack development) is different from that of increased strength in geopolymer mortars with high silica moduli (owing to further geopolymerization) before 400°C. The crystalline phases of nepheline and albite were observed to appear in the specimen at 800°C and 1,000°C, respectively, which was possibly due to Na+ entering into the crystal lattice of mullite and promoting the change of mullite into new crystal phases. Heating processes cause varying degrees of destruction of FAG and form a highly porous and weak structure owing to the sintering process above 800°C. Silica moduli are not an important factor for the gel phase structure of FAG at 1,000°C, which may be related to the same content of Na2O.