Identification of genes in the bacterial isolates involved in microbial induced calcite precipitation (MICP) and investigation of various calcium sources for enhanced calcite precipitation and improved mechanical properties

Abstract

Microbially induced calcite precipitation (MICP), has an important place in construction industry. It is a modern technique which is very cost effective and environment friendly. MICP uses naturally occurring bacteria to strengthen soil and to enhance the properties of concrete. This modern persistent healing system is at risk to microbial depletion in the highly alkaline cementitious environment. Therefore, only alkali resistant calcifying microbes are used and these alkaliphilic microbes can be isolated from soil. In the present study, the genes which are responsible for calcite production were identified by performing 16S rRNA sequencing. Gel electrophoresis confirmed presences of calcifying genes (IcfA, etfA and etfB) in B.subtilis and B.safensis. Calcium carbonate was formed using five different calcium sources (calcium hydroxide, calcium gluconate, calcium formate, calcium nitrate and calcium chloride) at three different pH values (pH 7, pH 10 and pH 12). Precipitated calcite was further confirmed via SEM and EDX analysis. Bacterial inoculation and calcium sources were investigated in cementitious system through determination of compressive and flexural stresses. Furthermore, mortar specimens were prepared consists of bacterial solution and calcium source and pre-cracked. After 7 days of curing, self-healing of mortar cement cracks was achieved in mortar sample with a crack width of 0.7 mm consist of calcium hydroxide and B.safensis. After 16S rRna sequencing Bacillus sp. were most common in identified strains and also gave higher calcite precipitation than rest of the species.Among all calcium sources, calcium hydroxide possessed higher calcite secretion potential and comprises higher mechanical strength. Presence of etfA, etfB and icfA genes are confirmed in B. safensis same like in B. subtilis. The study reveals that Calcium hydroxide is a potent source for self-healing in concrete. B. safensis along with calcium hydroxide holds greater calcite secretion potential and comprises higher mechanical strength.