Exploiting Bacillus pseudofirmus and Bacillus cohnii to promote CaCO3 and AFt phase formation for stabilizing waste concrete fines

In this study, we explored the potential of microbiologically induced calcite precipitation (MICP) for enhancing the microstructural integrity of waste concrete fines (WCF). Traditionally, ureolytic bacteria, such as Sporosarcina pasteurii, have been widely used in MICP due to their ability to produce calcium carbonate via urea hydrolysis, but this process generates ammonia, raising environmental concerns. As an alternative, we employed the carbonic anhydrase pathway using Bacillus pseudofirmus and Bacillus cohnii to induce biomineralization without ammonia byproducts. We examined three types of WCF materials and found that samples containing gypsum facilitated the formation of AFt phases (ettringite/thaumasite crystals) when treated with these bacteria, significantly increasing WCF cohesion and forming strong conglomerates. Comparative analysis revealed that facultative anaerobes exploiting the carbonic anhydrase pathway outperformed ureolytic bacteria in strengthening the material. Investigations into pre-compaction and feather fiber reinforcement did not yield improvements in strength and stiffness.