Effects of initial SiO₂/Al₂O₃ molar ratio and slag on fly ash-based ambient cured geopolymer properties

Geopolymers cured at ambient temperature are of high interest since they provide promising environmentally friendly alternative to cement. This paper discusses the effect of the initial molar ratio of SiO₂/Al₂O₃ and fly ash replacement with ground granulated blast furnace slag (GGBFS) on the properties of fly ash-based geopolymers. Geopolymer mixtures developed in this study are user-friendly (SiO₂/Na₂O larger than 1.4) and suitable for producing self-compacting geopolymer concrete at ambient temperature. Results showed that the increase in SiO₂/Al₂O₃ ratio accelerated the setting of fly ash-slag geopolymer systems, which was quite different from that reported for metakaolin-based geopolymers. An increasing–decreasing trend in compressive strength was observed by increasing the SiO₂/Al₂O₃ ratio. The maximum compressive strength achieved when the SiO₂/Al₂O₃ ratio was 3.37 (Si/Al = 1.68). The higher compressive strength at this ratio could be related to the alumina-silica bondings in the amorphous area, which comprised more than 81%-84% of geopolymer microstructure. Furthermore, the study revealed that the decrease in fly ash replacement with GGBFS prolonged the setting of geopolymer mixtures and reduced their compressive strength significantly.