Strength Development of Soil–Fly Ash Geopolymer: Assessment of Soil, Fly Ash, Alkali Activators, and Water

In this study, fly ash was added to residual soil to produce soil–fly ash geopolymer bricks. This study investigated the effects of fly ash/soil, alkali activator/ash, Na2SiO3/KOH (or NaOH), additional water content, curing condition, and curing temperature on the compressive strength of soil–fly ash geopolymer. The results showed that the optimum compressive strength was obtained when the ratios of alkali activator/ash, Na2SiO3/KOH (or NaOH) and additional water were 0.6, 0.5, and 10% respectively. A higher amount of alkali activators was required for strength development in soil–ly ash geopolymer than conventional fly ash-based geopolymers. The formation of the rigid structure at low ratios of alkali activator/ash and Na2SiO3/KOH (or NaOH) was unlikely. Compressive strength decreased when additional water was increased. High curing temperature and long curing duration showed a negative effect on strength development. The compressive strength of the soil–fly ash geopolymer varied as different mixing sequences of raw materials were used, indicating the importance of the formation of geopolymer gel in the structure. Compressive strength results obtained in this study demonstrate that soil–fly ash geopolymer can be a potential alternative to traditional clay-fired brick.