Viability of Bacillus species in the biological self-healing concrete with different cement types

Abstract

Concrete is the most widely used building material in the world. However, one of its drawbacks is the inevitable opening of the cracks. Due to the ability of some bacteria to precipitate calcium carbonate, the cracks in the concrete can be healed. The biggest challenge in developing self-healing concrete is to ensure the viability of bacteria spores. Bacteria must survive harsh conditions, such as high pH values of early age concrete and mechanical stress during hardening of concrete. The viability of bacteria decreases drastically within several days after their incorporation into the biological concrete matrix. In this study, the influence of the type and composition of cement on the viability of Bacillus strains was investigated. Of the five types of cement tested, the best long-term viability of Bacillus pseudofirmus was obtained in a concrete mix using white CEM-I cement. The pH of all cement types studied was slightly different and fell into the range of 12.4 to 12.8. This indicates that pH does not significantly affect the viability of bacteria in the concrete matrix. Studying of the chemical composition of different types of cement showed that it contains metal oxides with antimicrobial properties. Those metal oxides are Al2O3; Fe2O3; TiO2, MgO, ZnO and CuO. The determined MIC values showed that Al2O3, Fe2O3, MgO and TiO2 are nontoxic for strains of Bacillus pseudofirmus, Bacillus cohnii and Bacillus halodurans. The MIC values of ZnO and CuO varied between 12.5 and 25 μg/ml, and between 1500 and 2000 μg/ml, respectively. ZnO was found to be the most toxic metal oxide and causes the death of bacteria in biological concrete. White CEM-I concrete does not contain CuO and ZnO metal oxides, making it the most suitable one for the production of self-healing concrete.