The effect of Bacillus cell fractions as supplementary cementitious materials

The bacterial cell fractions (BCF) of Bacillus subtilis were studied as a promising supplementary cementitious material that modulates the carbonation process in cement mortar and modifies the viscosity of the cement paste. The live, heat-killed cells of Bacillus subtilis 168 and their cell fractions (BCF) were applied to the cement mortar to evaluate their roles in modulating the microbial induced CaCO₃ mineralization. The compressive strengths of the cement mortar samples amended with the BCF supplementary materials were observed to improve significantly. Subsequently, in vitro assay of CaCO₃ precipitation, SEM-EDX-XRD analysis, porosity test were conducted to elucidate the mechanisms of the observed significant improvement in compressive strengths. To investigate the viscosity modifying properties of BCF in concrete and whether polysaccharide based viscosity modifying admixtures (VMAs) modulate concrete carbonation, the viscosities of cement mortar samples amended with BCF or a commercial viscosity modifying admixture diutan gum were measured.

Our results indicated that the compressive strength of mortar samples increased significantly by 15.6% and 14.8% with the amendment of BCF at 3.3 mg/mL and 0.33 mg/mL, respectively, Porosities of the BCF amended mortar samples were found to decrease. In addition, carbonation of Ca(OH)₂ and formation of CaCO₃ in concrete were found to accelerate by the addition of BCF.

The apparent viscosity of cement paste amended with BCF of concentration 0.34% by mass of water increased up to 43.9%, which is comparable to commercial viscosity modifying admixture (VMAs) diutan gum of the same dosage. The shear thinning behavior, which is exhibited commonly in systems modified with a VMA, was also observed in our BCF amended cement paste. With BCF being a cost effective material requiring only physical separations during extraction while diutan gum needing expensive process of aerobic fermentation, these results demonstrated the feasibility of using BCF as a new VMA. Two parameters R_w/c and R_real were defined in quantifying the resistance of BCF to changes of water to cement ratio (W/C) and shear rate. The parameters demonstrated the sensitivity and accuracy in revealing the relationships among the dosage, W/C ratio and the shear rate for a VMA modified system.