Effect of Alkali and Water Content on Setting Time and Strength of Fly Ash Based Geopolymer

Abstract

The current work presents the effect of alkali concentration and water content on fly ash based geopolymer material. In this paper geopolymer material was prepared where Si/Al ratio (1.85: 1) is fixed and solid to water ratio (S/W) and NaOH concentration (4M, 8M, 12M) were varied. All samples were tested to establish the roles of water and alkali on the setting time and strength. At low solid to water ratio, (S/W= 3:1), the water quantity is high and the hardening of gel phase take more time. However, as water content is reduced from S/W 3:1 to 4:1 ratio, the setting point decreased significantly and only small change in setting time is observed when S/W shifted from 4:1 to 5:1. Sample prepared with 8M of NaOH showed the highest compressive strength. In this regards, Fourier transform infrared spectroscopy (FTIR) results support this observation where broad band (asymmetric strectching band) become sharper and shift towards lower wavenumber (1082.36 cm-1 to 985.16 cm-1). This finding shows that, to obtain required processing time at desired mechanical strength (20-30 Mpa) for a specific application of geopolymer, the suitable amount of water must be established.