Investigation of the hydration mechanism and optimal mix proportions of granulated blast-furnace slag/fly ash based geopolymers

Cement production and usage results in excessive carbon dioxide emissions and depletion of non-renewable resources. In this study, a granulated blast-furnace slag/fly ash based geopolymer (GFBG) system was developed using alkaline activation technology. The effect of the relative proportions of granulated blast-furnace slag (GBFS) and fly ash (FA) on setting time and flowability was investigated. Subsequently, the effects of the mass fraction of sodium oxide (MFSO), water glass modulus (WGM) and water/binder (w/b) ratio on compressive strength were analysed, and the response surface method was used to optimise the influencing factors. Finally, the hydration mechanism of GFBG was investigated under optimal conditions. The results showed that when the GBFS:FA ratio was 3:2, the flowability of fresh slurry was 171 mm and the final setting time was 32 min. For the same mix, when the w/b ratio was 0.450, the MFSO was 11.792 and the WGM was 1.206, the highest 28-day compressive strength was 51.48 MPa. The hydration of GFBG was basically completed in 7 days, and was accompanied by carbonisation. The hydration products were mainly sodium silicate aluminate hydrate (N-A-S-H) and cross-linked calcium silicate aluminate hydrate.