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

Abstract

Backround: Due to the ability of some bacteria to precipitate calcium carbonate, the cracks in the concrete can be healed. The biggest challenge of developing self-healing concrete is to ensure the viability of bacteria spores. Bacteria must survive harsh conditions such as high pH values of the early age concrete and mechanical stress during concrete hardening. Objectives: The viability of bacteria decrease drastically within several days after their incorporation into the biological concrete matrix. In this study, the influence of the cement type and composition on the viability of Bacillus strains was investigated. Methods: An expanded clay (EC) was impregnated with bacteria spores and nutrients under the vacuum. The concrete mix was obtained by mixing EC, sand, cement and water. The viability of spores was measured using the microbiological dilution-to-extinction method by CFU counting after plating on alkaline nutrient agar. The toxicity of metal oxides was assessed by determining the minimum inhibitory concentration (MIC) for Bacillus strains. Results: Out of five cement types tested, the best long-term viability of Bacillus pseudofirmus was obtained in a concrete mix using the white CEM-I cement. The study of the chemical composition of cement showed that it contains metal oxides with antimicrobial properties. Determined MIC values showed that Al2O3, Fe2O3, MgO, and TiO2 are non-toxic for Bacillus pseudofirmus, Bacillus cohnii and Bacillus halodurans strains. The MIC values of ZnO varied between 12.5 and 25 μg/ml. ZnO was found to be the most toxic metal oxide and causes the death of bacteria in biological concrete.