Enhancing mortar mixtures with bacterial impregnation: Sustainable approaches and strength improvement

Abstract

Numerous studies have explored methodologies for incorporating bacteria into cementitious systems. However, the application of these methods in civil engineering has been hindered by several factors, including local availability, cost considerations, and the long-term preservation of bacterial cells. In this research, the integration of bacteria into mortar mixtures was achieved by impregnating the aggregates. The primary objective was to promote the preservation of natural resources and enhance sustainability. Bacillus subtilis bacteria, at a concentration of 105 cells/ml, were introduced into both recycled concrete aggregate (RCA) and pumice aggregate (PA), accompanied by a urea-calcium chloride nutrient medium (UCC-NM). The study aimed to assess the impact of this method on various properties of mortar mixtures, including compressive and flexural strength, void ratio, water absorption capacity, capillarity, ultrasonic pulse velocity, and crack-healing performance. It was evident that the addition of the UCC-NM, in conjunction with bacteria, played a pivotal role as a source of Ca2+ and urea, facilitating the precipitation of CaCO3. The findings revealed a remarkable improvement of up to 17% in compressive strength for mortar mixtures containing both bacteria and the nutrient medium. Additionally, there was a substantial reduction of 60% in total void ratio and water absorption values. Notably, cracks with a width of 0.3 mm and a depth of 30-35 mm exhibited a significant self-healing process between 15 and 28 days, attributed to CaCO3 precipitation induced by bacterial action. This study underscores the constructive role of microorganisms, not only in enhancing the strength of mortar mixtures but also in facilitating the autonomous recovery of cracks. Furthermore, the results emphasized that the bacterial impregnation of RCA and pumice aggregates led to an enhancement of their characteristic properties.