Effects of Curing Conditions and Sand-to-Binder Ratios on Compressive Strength Development of Fly Ash Geopolymer

This paper investigates the effects of curing conditions on a high-strength geopolymer material synthesized by activating different combinations of Class F fly ash (FA), ground-granulated blast-furnace (slag), and ultrafine fly ash (UFFA) with a unified mixture of D-grade sodium silicate (Na2SiO3) and 12-M sodium hydroxide (NaOH) solutions. The effect of ambient air and water curing on the strength properties, cumulative porosity, and microstructure of geopolymer mortar samples was evaluated at room temperature. Subsequently, the effects of two different sand-binder ratios, the amount of FA replacement with UFFA, and slag on the short-term mechanical properties and workability of geopolymer mixtures is also discussed. The results show that the geopolymer specimens cured in water possessed a higher total porosity in relation to the air-cured samples. Air curing resulted in additional polymerization and fewer microcracks were observed within the scanning electron microscope photographs, which led to an increased compressive strength. Moreover, the use of UFFA was observed to significantly improve the strength of FA-slag blended geopolymers, where reduced porosity and denser microstructure in FA geopolymers was observed with the inclusion of slag at higher quantities.