Effects of Elevated Temperature and Activation Solution Content on Microstructural and Mechanical Properties of Fly Ash-based Geopolymer

This paper examined the effects of sodium silicate to sodium hydroxide (SS/SH) and fly ash (FA) to alkaline solution (FA/AS) ratios on the thermal and mechanical properties of FA-based geopolymers. To tackle this, two factors were varied including the SS/SH (1.5, 2.5, and 3.5) and the FA to the AS (1.5, 2.5 and 3.5). The elaborated nine specimens were characterized employing several techniques including FT-IR, XRD, 29Si and 27Al MAS NMR, and SEM analysis. The thermogravimetric analysis (TGA) was utilized to study the thermal stability of geopolymer specimens. The fire resistance of the geopolymer specimen was evaluated by weight change and residual compressive strength before and after being subjected to fire testing (up to 1,000°C). A decrease in the resistance recorded when heated at 200 and 400°C was related to the vaporization of free water was observed. Meanwhile, a gain in strength was achieved for the geopolymer samples after being subjected to 600 and 800°C which was attributed to the viscous flow of silicate-rich phases that eventually bridged the fissures or defects occurring during its solidification. Samples with a high FA/AS ratio showed good fire resistance after high-temperature exposure.