The mechanical properties improvement of environmentally friendly fly ash-based geopolymer mortar using bio-mineralization

The reasonable disposal and utilization of fly ash is an essential environmental issue for coal energy stage. As a novelty green cementation method, biomineralization has been innovatively applied to improving engineering performance of loose materials by generating cementing. This paper reported the feasibility of using microbially induced carbonate precipitation (MICP) partly instead of cement as a bonder to producing fly ash-based geopolymer mortar. The influence of mixture proportion, bio-chemical solution formula on engineering performance, water absorption, and dry density of composites were evaluated. The microstructure characteristics and mineral composition of mortar were also investigated. The results showed that MICP significantly improved the strength by 16%–45% of high-volume fly ash based geopolymer mortar with appropriate MICP formula. However, excessive chemical solution concentration significantly weakened the strength by even −14%. The water absorption was even reduced by 16.8% due to the production of CaCO3. The reduction of residual Ca2+ concentration, SEM, and EDS analysis of microstructure and composition confirmed the formation of CaCO3, which was fundamental to strength improvement and water adsorption reduction of mortar. XRD and FTIR analysis demonstrated the diversity of calcium carbonate crystal forms induced by different MICP formula, which affected the engineering performance of mortar. The BET results showed that appropriate MICP formula reduced the cumulative pore volume, leading to the more compact structure. Although the maximum pH of mortar reached 13, MICP still played a role in strength improvement. This study demonstrates that MICP can be selected as a binder, partially replacing cement, for making fly ash-based eco-friendly geopolymer mortar. However, it should be note that the strength of geopolymer mortar is still relatively low in the current formula. More research is needed to further improve the mechanical properties of the mortar. In addition, the relationship between bio-chemical concentration ratios on mechanical properties as well as the viability of bacteria in fly ash based geopolymer mortar also need to be further investigated.