A Critical Assessment of Microbially and Enzymatically Induced Carbonate Precipitation for Geotechnical Works

In the last decade, data from the IPCC has shown an alarming need to reduce CO2 emissions in the atmosphere. There is an urgent need to find long term sustainable, environmentally friendly materials to replace cementitious materials for civil engineering. One promising approach is inducing the precipitation of calcium carbonate through techniques like Microbially Induced Carbonate Precipitation (MICP) and Enzymatically Induced Carbonate Precipitation (EICP) within the void space of porous media. The former utilizes bacteria that release enzymes that induce the precipitation of carbonate minerals. The latter utilizes the enzyme directly. The most common enzyme used for these processes is urease since it causes the hydrolysis of urea resulting in the release of ammonium and carbonate ions into solution while creating a slightly alkaline environment. The presence of calcium in MICP or EICP results in calcium carbonate solid cement, transforming loose soils to achieve compressive strengths greater than 1 MPa. Given that the carbonate is derived from an organic source, the formation of calcium carbonate can act as a carbon capture mechanism. This is the major reason for the interest in this approach. However, some issues with this technique need to be addressed, which are illustrated in this paper. For example, for the process to be carbon negative, a readily available low carbon source of calcium is required. The most common source CaCl2 is generated via the dissolution of calcium carbonate, which releases CO2 as a by-product. In addition, the conversion of ammonium to ammonia causes acidity which weakens carbonate cements if the media is not washed out during precipitation. Overall, both MICP and EICP promise to be sustainable alternatives to cementitious grouts, but research needs to focus on these techniques' remaining issues.