Performance assessment of sustainable biocement mortar incorporated with bacteria-encapsulated cement-coated alginate beads

Abstract

Despite the extensive research on bacteria that are directly added to concrete to promote self-healing, studies on encapsulated bacteria have not been sufficient to comprehend crack healing. By employing Bacillus megaterium enclosed in cement-coated alginate beads, this research helps to improve the understanding of crack closure and the strength of self-healing mortar, which may be applied to the concrete. Autogenous healing occurs naturally, which will only repair micro-cracks. Moreover, it is a time-consuming process. Therefore, autonomous healing can assist in repairing little wider cracks with the addition of healing agents. Bacillus megaterium MTCC 8510 was used as the healing agent in the current study due to its ability to induce calcite precipitation (MICP) microbially. This was enclosed with alginate beads, and then coated with cement to form cement-coated alginate beads (CCAB). When a crack propagates, these beads break and generate CaCO3, which clogs up the crack domain. Several tests, including compressive strength, water permeability, FESEM, surface healing and ultrasonic pulse velocity (UPV), have been carried out to understand the healing performance and other characteristics thoroughly. To determine the optimal amount of CCAB, these hardened cement-coated beads were mixed in mortar in different percentages of 10%, 15%, 20%, and 25% as a replacement for fine aggregate (FA). The reduced compressive strength, anticipated due to the addition of fragile beads, was compensated by adding nano-silica (NS) to maintain the minimum strength. The calcite precipitation was collected from the healed specimen and was observed under FESEM to analyse its microstructure. For 25% aggregate replacement, a healing percentage of 92.64% was attained in the internal domain of the crack with water permeability test, whereas 93.96% of the crack core was filled when checked using the UPV test each after 56 days. Specimens with 20% CCAB and 5% NS also satisfied the minimum criteria mentioned. Therefore, it is concluded that 20% sand replacement with CCAB containing 5% nano-silica is optimal for both strength and healing.