Chemical aspects of the process of concrete cracks elimination with the help of bacteria

Abstract

The article describes the chemical processes of biogenesis of calcium carbonate for self-healing of concrete, taking into account four main factors: the concentration of calcium, the concentration of soluble inorganic carbon, the pH value, the presence of the crystallization center. A number of bacteria that can be found in soil, sand and natural minerals have the ability to release calcium carbonate, both in natural and laboratory conditions. In the laboratory, calcium lactate (CaC6H10O6) was used as a starting material for the formation of calcium carbonate. In addition, urea necessary for bacteria as a source of urease enzyme and yeast extract as a source of carbon and nitrogen were added. The resulting pH was brought to 9 to avoid possible chemical deposition of calcium carbonate. To improve the production technology of biological concrete, specially selected bacteria of the genus Bacillus with a combination of nutrients were used to create a reducing agent in concrete. With the help of such self-healing concrete by means of bacteria, cracks more than 100 µm wide can be compacted. With this approach, the bacteria in the alkaline medium convert CO2 into carbonate ions, which then interact with the Ca ions from the concrete matrix. This leads to the formation of calcium carbonate crystals. In addition, CO2 directly reacts with the calcium hydroxide matrix, which leads to the formation of calcite precipitate. The appearance of calcium carbonate crystals of large size with the participation of bacteria incorporated into the self-healing concrete provides an excellent ability to self-healing compared to traditional or developed environmentally unsafe self-healing cement materials. That is why this area of research is a promising alternative to environmentally hazardous methods of repair using cement.