Effects of mix design parameters on heat of geopolymerization, set time, and compressive strength of high calcium fly ash geopolymer

Abstract In this study, the effects of key design parameters, such as SiO2/Na2O mole module (Module), concentration of solute in alkaline solution (Concentration), liquid-to-fly ash mass ratio (L/F), and curing temperature on the geopolymerization process, set time, and compressive strength of high calcium fly ash geopolymer mixes were investigated. The geopolymerization process of these mixes was monitored using a semi-isothermal calorimeter at testing temperatures of 23 °C and 50 °C for 24 h. The results showed two major exothermic peaks generated during the fly ash geopolymerization processes. The first peak, the dissolution peak, appeared within the first hour after the fly ash was in contact with a liquid activator, and it did not vary very much with mix design parameters, while the second peak, the polymerization peak, varied largely in its amount and emerging time with mix design parameters of the geopolymers. As Module increased, the set time was accelerated but the total heat generated from geopolymerization and the compressive strength of the geopolymer were reduced. As Concentration increased, the set time for mixes with 1.0 and 1.5 Module prolonged but it was shortened for the mixes with 2.0 Module, while the total heat of geopolymerization and strength of these mixes were increased. Elevated curing temperature decreased the polymerization peak time, increased the total heat of geopolymerization, and improved the strength of the geopolymers. Additionally, the close relationships between the calorimetric characterization and setting time/compressive strength were obtained. To achieve optimal strength and setting behavior, it is recommended that a geopolymer mix made with high calcium fly ash shall be designed to have an activator with Modules ≤ 1.5, Concentrations of 20–25%, and L/F ≤ 0.40, and elevated curing (e.g., at 50 °C) is preferred.