Factors Influencing Compressive Strength of Fly Ash-based Geopolymer Concrete

In recent years, geopolymer has been introduced as a novel and green alternative to Portland cement. Compressive strength is considered one of the important characteristics of concrete. In geopolymer concretes, according to the ingredients, several factors have been identified as important parameters affecting the compressive strength. Hence, in this experimental research, several factors affecting the compressive strength of fly ash-based geopolymer concrete including the type of alkaline activator solution, the weight ratio of water to solid material participated in geo-polymerization, sodium hydroxide concentration, the weight ratio of alkaline activator solution to aluminosilicate source, sodium silicate to sodium hydroxide weight ratio and time and temperature of curing, were studied. The obtained results indicated that using potassium hydroxide and potassium silicate as an alkaline activator solution, result in higher 28-day compressive strength compare to sodium-based alkaline activator solution. On the other hand, using sodium hydroxide and sodium silicate as an alkaline activator solution, result in higher 3- and 7-day compressive strengths and also, faster hardening. Furthermore, increasing the weight ratio of water to solid material results in significantly decreasing geopolymer concrete compressive strength. Also, compressive strength is increased with an increase in the concentration of sodium hydroxide up to 14 M, but for 16 M, there are no remarkable changes in compressive strength. The optimum ratio of alkaline activator solution to fly ash and sodium silicate to sodium hydroxide was measured 0.5 and 1.5, respectively. Increasing the time and temperature of curing results in significant increasing 3-and 7-day compressive strengths.