There are several self-healing mechanisms, both natural and artificial, applied to cementitious materials. In recent years, microbially induced calcite precipitation (MICP) technology has garnered special attention in the fields of Microbiology and Civil Engineering. The technology involves the synthesis of calcium carbonate crystals at ambient temperatures in calcium rich systems. Biocementation occurs as active microbes diffuse through the cracks and micro-pits generating calcitic deposits owing to their metabolic pathway. The calcifying bacterial cultures produce urease or carbonic anhydrase enzyme which is capable of precipitating calcium in the surrounding micro environment as CaCO3. The bacterial degradation of urea locally increases the pH and stimulates the microbial deposition of carbonate. The calcium carbonate produced binds the soil particles together, thus cementing and clogging the grains, and hence improves the strength and reduces the hydraulic conductivity of the unconsolidated sands. Moreover, these precipitated crystals can thus fill the cracks and enhance the durability of cement, mortar, and concrete. Incorporating calcifying bacteria is the essence of developing a self-healing material or "bio-cementing" technology as bacteria behaves as a long-lasting healing agent.
The calcifying microbes can be isolated from different sources like water springs, soil, ocean, environments with high pH values or the cement itself. The purified strains can be grown for a 24-hour period in the laboratory and then blended with the cement or other materials depending on the desired application. A cheap carbon source like glycerol/molasses is supplemented to the mixture triggering fast bacterial multiplication. It was found that after the curing time of 28 days, tensile strength, micro-crack healing capacity, and durability increased significantly. The process is as simple as mixing bacteria into a cement paste. The technique for creating a high strength cement in a permeable starting material involves combining the starting material with effective amounts of (1) a urease producing micro-organism with a high urea hydrolysis rate; (2) urea; and (3) calcium ions, under standard conditions of 0.5-50 mM urea hydrolyzed min-1. Scientists found that after injecting the bacterial cementitious solution for a period of one month, the spores of three particular bacteria where still viable. Harmless bacteria such as Bacillus genus remains dormant until water enters the cracks. In this case, formation water, or water from fracturing fluids or any source can be used to trigger the bacteria. Moreover, the process does not require oxygenation.
In this paper, self-healing approaches based on bacteria will be thoroughly reviewed. The concept of biomineralization, bioclogging, and biorepair and its applications in improving the engineering properties of sands and cement is tackled. Based on the aforementioned aspects of self-healing in cementitious materials, recommendations for further research in self-healing engineering applications are proposed. This method is a green and eco-friendly way and the use of bacteria can lead to substantial savings. The following presents major practical applications for the oil and gas industry. Via the microbial calcification theory, solidifying the sea beds before drilling for oil, preventing hole cavings and wellbore enlargements or washouts, sealing undesirable leakage pathways near wellbores to achieve fracture plugging and permeability reduction, plugging sands to diminish water absorption and porosity are all lucrative potential applications the industry is in dire need of.