The role of bacterially induced calcite precipitation in self-healing of cement paste

Abstract Self-healing processes in cementitious materials have been intensively studied in recent years. They present a promising way to increase the durability of concrete and extend the service life of concrete structures, thus mitigating the impacts of the construction industry on public budgets and the environment. The role of bacterially induced calcite precipitation (BICP), the basis of one of the processes, is not fully understood yet, even though many researchers have reported improved mechanical properties or water tightness of cracked cementitious composites self-healed in the presence of bacteria. Therefore, the authors of this paper are focused on detailed characterization of BICP with the purpose of determining the direct impact of bacteria on the self-healing in cement composites . Three different bacterial strains, namely Sporosarcina pasteurii (SP), Bacillus cohnii (BC) and Bacillus pseudofirmus (BP) were studied. Their viability and metabolic activity in optimal and sub-optimal conditions were compared. Several analytical methods (XRF, TGA, TCD , FTIR) were used to characterize the attainable BICP depending on the particular strains. BP proved to be the most suitable bacterium, showing excellent viability and sufficient calcite production, even in sub-optimal conditions (10 °C temperature). Microscopic analysis of the self-healing products in artificially cracked cement paste showed that the application of BP with precipitation media lead to complete sealing of the crack by very dense calcite crystals (less than 1 μm), while porous structure of larger crystals (up to 30 μm) was formed in reference samples not treated with the bacteria, where only spontaneously induced calcite precipitation (SICP) occurred. As a result, it was proved that the bacteria directly contribute to the self-healing of cementitious materials and that BICP leads to the formation of a superior crack-sealing material.