Bacterial calcification in cement pastes and mortars containing mineral admixtures
Recently, interest in using biomineralization for self-healing applications in cement-based materials has gained broader attention in the field. Biomineralization is a biochemical process in which microorganisms stimulate the formation of minerals, and in this particular case calcium carbonate (CaCO3). However, one of the main challenges in biomineralization applications for cement-based materials is that the cell death or insufficient metabolic activity to precipitate CaCO3 may occur when the cells are inoculated in the cement paste matrix. Concerns regarding the viability of the microorganisms within the restrictive and high pH environment of cementbased materials have led to researchers to propose incorporation of endospores rather than metabolically active (vegetative) cells or encapsulation of the endospores. However, a priori encapsulation of the microorganisms might be unnecessary, especially if the bacteria can form endospores. With the proper selection of bacteria and nutrient medium, vegetative microorganisms might also be able to survive for extended periods. This paper summarizes the results of a study undertaken to investigate the influence of vegetative bacteria, Sporosarcina pasteurii (S. pasteurii), when it is incorporated with its growth medium in cement paste. In addition the influence of incorporation of mineral admixtures (Limestone filler and C- type fly ash) on strength of biomineralized mortar was investigated. The efficiency of this vegetative inoculation approach was evaluated using thermogravimetric analysis (TGA), ethanol exchange porosity and compressive strength testing of the biomineralized mortars and metabolic state analysis of S. pasteurii cells by using the Most Probable Number (MPN). The results showed that the vegetative cells were able to induce CaCO3 within the cement paste, which led to a decrease in porosity and an increase in compressive strength. The increase in compressive strength was more pronounced when mineral admixtures were incorporated. Moreover, S. pasteurii cells were found viable in mortar samples up to 330 days.