Use of Gypsum as a Preventive Measure for Strength Deterioration during Curing in Class F Fly Ash Geopolymer System

This study discusses strength deterioration during the curing process of fly ash geopolymer and the use of CaSO4·2H2O (gypsum) as a deterioration remedy when the ash was synthesized using a 10M NaOH and Na-silicate solution. The strength decline was mainly due to the widespread formation of nanometer-sized cracks that were related to excessive Na and Si concentrations at an early age. Use of 2 wt% CaSO4·2H2O resulted in the best measured strength by temporarily reducing Na and Si concentrations; Na was absorbed by SO42−, up to 11% in the matrix within one day, and formed Na2SO4 (thenardite), which gradually dissolved over time, slowly releasing Na ions. However, more than 4% gypsum suppressed overall strength development because too many Na ions were locked into Na2SO4 and could not participate in geopolymerization. The addition of gypsum impeded glass dissolution and even halted the process when more than 4% gypsum was used.