Bacterial Concrete: Biomineralisation

The bacterial concrete makes use of calcite precipitation by bacteria. The phenomenon is called microbiologically induced calcite precipitation (MICP). The pioneering work on repairing concrete with MICP is reported by the research group of Prof. Ramakrishnan V. and others (see bibliography) at the South Dakota School of Mines & Technology, USA.

The MICP is a technique that comes under a broader category of science called biomineralization. It is a process by which living organisms or bacteria form inorganic solids. Bacillus Pasteruii, a common soil bacterium, can induce the precipitation of calcite. Under favorable conditions Bacillus Pasteruii, when used in concrete, can continuously precipitate a new highly impermeable calcite layer over the surface of the already existing concrete layer. The precipitated calcite has a coarse crystalline structure that readily adheres to the concrete surface in the form of scales. In addition to the ability to continuously grow upon itself, it is highly insoluble in water. It resists the penetration of harmful agents (chlorides, sulphates, carbon dioxide) into the concrete thereby decreasing the deleterious effects they cause. Due to its inherent ability to precipitate calcite continuously, bacterial concrete can be called as a “Smart Bio Material” for repairing concrete. The MICP comprises of a series of complex biochemical reactions. It is selective and its efficiency is affected by the porosity of the medium, the number of cells present and the total volume of nutrient added. The phosphate buffer or urea-CaCl2 have been found effective as nutrients. The bacteria precipitate calcite in the presence of nutrients. The optimum pH for growth of B. Pasteurii is around 9. The alkaline environment of concrete with pH around 12 is the major hindering factor for growth of bacteria. However, B. pasteurii has the ability to produce endospores to endure an extreme environment, as observed by Prof. Ramakrishnan and the research team.

The microbial modified mortar or concrete has become an important area of research for high-performance construction materials. Ghosh et. al. investigated the effects of incorporating a facultative anaerobic hot spring bacterium on the microstructure of a cement–sand mortar. Environmental scanning electron
microscopic (ESEM) views and image analysis (IA) of the bacteria modified mortar (thin-section) showed significant textural differences with respect to the control (without bacteria) samples. X-ray diffraction (XRD) study confirmed the formation of new phases of silicates (Gehlenite) within the matrix of such mortar material, which cause an improvement in the strength of the material. Electron probe microstructure analysis (EPMA) suggested that the bacterial treatment promoted uniform distribution of silicate phases and increased the calcium/silicon (C/S)ratio within CSH gel of the matrices. The bacterium is found to leach a novel protein, which is capable of isolating silica from its source. The addition of such isolated protein, instead of the bacteria, into mortar also improves the strength of mortar.

Reference:

Ghosh S., Biswas M., Chattopadhyay B. D., Mandal S., "Microbial activity on the microstructure of bacteria modified mortar", Cement & Concrete Composites, Vol. 31, 2009, pp 93–98