Bacteria-mediated self-healing concrete: Biomineralization refers to the process of mineral formation by living organisms at ambient temperatures. Most biomineralization processes are accomplished through biologically induced mineralization, in which the organism modifies its local environment to create appropriate conditions for mineral precipitation. Bacteria-mediated self-healing concrete based on biologically induced mineralization processes has been extensively studied. Bacteria precipitate calcium carbonate through three different pathways. (1) Certain species of ureolytic bacteria promote calcium precipitates by increasing the local pH via catalyzing the hydrolysis of urea to ammonium. This approach has proven to be quite successful. However, for each carbonate ion two ammonium ions are produced during the process, which may lead to excessive nitrogen oxide emission into the atmosphere. Moreover, excessive ammonium in the concrete may increase the risk of salt damage through its conversion to nitric acid. (2) Metabolic conversion of organic compound to calcium carbonate has also been tested. In this approach, aerobic oxidation of organic acids results in production of carbon dioxide, leading to carbonate production in an alkaline environment. The existence of calcium ions then leads to the calcium carbonate precipitation. This pathway is more sustainable due to the absence of ammonium. However, in the case of low concentration in oxygen, such as for most underground structures, the efficiency of this approach can be limited. (3) The third pathway is known as dissimilatory nitrate reduction. Bacterial denitrification is defined as a respiratory process to reduce nitrate stepwise to nitrogen gas. Minerals are precipitated via oxidation of organic compounds by the reduction of nitrate via denitrifying bacteria. The most important advantage of the denitrification approach is its application in anaerobic zones. Nonetheless, regarding the production of calcium carbonate, the efficacy of this approach is much lower than that of the ureolysis pathway.

Fungi-mediated self-healing concrete has also been explored recently. While bacteria promote mineral precipitation only through induced biomineralization processes, fungi are capable of both induced biomineralization and organomineralization processes, which involves an organic substrate to initiate or enhance crystal nucleation and growth. In this type of biomineralization, living organisms are not required and only the organic fraction matters as a template for nucleation. An important characteristic that places fungi in a different kingdom from bacteria is the chitin in their cell walls, which is a substrate that significantly reduces the required activation energy for nucleus formation so that the interfacial energy between the fungi and the mineral crystal becomes much lower than the one between the mineral crystal and the solution. Therefore, cation binding by fungi can occur by means of metabolism-independent binding of ions onto cell walls, which is an important passive property of both living and dead biomass. Bound calcium ions can interact with soluble carbonate ions, leading to calcium carbonate deposition on the fungal hyphae. Calcite formed in the aqueous phase could also nucleate onto the hyphae.

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