Statistical and experimental investigation on self-healing of microcracks in cement mortar by encapsulation of calcite precipitating bacteria into expanded perlite

Healing of micro cracks in concrete using calcite precipitating microorganisms has become an interesting research for the past two decades. In this study, an attempt was made to encapsulate strong and efficient calcite precipitating spore-forming bacteria named Bacillus megaterium MTCC 8510. The encapsulation and survival of bacteria in expanded perlite was made successfully by dual coating using sodium silicate and cement. The mechanical properties and healing efficiency of the coated expanded perlite were tested after it was integrated into the cement mortar matrix. The encapsulated bacterial expanded perlite particles were added into cement mortar in 10%, 15%, 20%, and 25% replacement of fine aggregate. The results of 20% replacement of fine aggregate by expanded perlite showed an improvement in healing of microcracks of size varying from 0.154 mm to 0.626 mm. But due to the presence of expanded perlite, there was a decrease in compressive strength. To counteract the reduction in strength 5% of nano-silica was added along with cement, which had given improvement in compressive strength of mortar without compromising in healing. The improvement in healing was observed visually by width repair rate method and the healing in the depth of cracks was determined by Ultrasonic Pulse Velocity (UPV) test method. The crystal shapes and elemental analysis of calcite precipitation were observed using FESEM, and confirmed using XRD analysis. The regression equations were developed for each test procedure by considering the factors which influence the compressive strength and healing efficacy. Statistical model using ANOVA was employed to find the significance of factors and the optimum percentage of expanded perlite replacement. With a p-value of less than 0.05, the factors proved to be significant. A high R2 value nearing 100 percent and acceptable precision was used to determine the model's predictive efficiency. The model has shown that 20 percentage replacement of fine aggregate by expanded perlite is optimum for healing efficacy without compromising the compressive strength.

Automated summary

In this study, Bacillus megaterium MTCC 8510 was successfully encapsulated in expanded perlite for healing micro cracks in concrete. The addition of 20% of encapsulated perlite in cement mortar improved crack healing but led to a decrease in compressive strength. To mitigate the strength reduction, 5% nano-silica was added, maintaining both healing efficacy and strength.