Effects of the n(H2O: Na2Oeq) ratio on the geopolymerization process and microstructures of fly ash-based geopolymers

Water plays a crucial role in the entire process of geopolymer synthesis. To clarify the effects of water on the geopolymerization process and microstructures of fly ash (FA) based geopolymers, the differences induced by varying n(H2O: Na2Oeq) ratio from 13–17 were investigated by ICC testing, XRD, FTIR, MIP, SEM and TGA-DSC analysis. A higher n(H2O: Na2Oeq) ratio contributes to the dissolution of the active component of fly ash, but hinders the polymerization reaction of monomeric and oligomeric gels. According to the total heat release and the standard enthalpies of formation of the reactants, the degree of reaction for the FA-based geopolymers from 0 to 12 h at 60 °C rose from 4.9% to 5.6% with an increase of the n(H2O: Na2Oeq) ratio. More water could increase both porosity and pore size of geopolymers from 32.3% to 45.6% and from 7.1 nm to 18.1 nm, respectively. All samples had a weight loss of ~12% associated with the evaporation of water after they are heated to 275 °C corresponded to ~75% of the total weight loss. Nearly all of the water was lost by ~700 °C as confirmed by TGA. The compressive strength of the geopolymers at all ages declined by 62% with an increase in the n(H2O: Na2Oeq) ratio from 13 to 17.