Unravelling the processes of microbially induced calcite precipitation (MICP) in recycled concrete aggregates

Microbially induced calcite precipitation (MICP) is a widespread biochemical process in the environment (soils, caves, freshwater, marine sediments etc.). It occurs through major metabolic processes, including urea hydrolysis, denitrification, dissimilatory sulfate reduction, and photosynthesis. MICP has been shown to have potential as an environmentally and sustainable solution in self-healing repair of concrete, encapsulation of heavy metals in soils and cementation of fugitive toxic dusts from soils. However, the microbiology of MICP is not well studied, especially the synergies witihin a microbial consortium in this process. Here, we investigate the ability of the indigenous microbes in recycled concrete aggregate or construction and demolition waste (C&DW), to precipitate calcium carbonate and cause cementation of new construction products. This process was achieved in a column microcosm by pumping a stimulation recipe for ureolytic bacteria and subsequently a cementation recipe to ensure calcite precipitation. In addition, we incorporated bender elements into the column to monitor the bio-cementation of the material by measuring the shear wave velocity. The presence of calcite was determined by X-ray diffraction (XRD) and thermogravimetric analysis (TGA). To characterise the microbial community, we employed both culture-dependent and culture-independent approaches to understand the abundance and diversity of microbes involved in this process. Preliminary results of microbial isolation show the presence of Microbacterium, with M. phyllosphaerae being the dominant species that has been shown to have urease activity. This study serves as a proof-of-concept for the microbial stabilisation of environmental contaminants, especially in C&DW, and the potential for the application of MICP-treated recycled materials.
Keywords: Microbes, Calcite precipitation, Crushed concrete, Bio-cementation.