Linking the microstructural characteristics of mortar specimens involving non-ureolytic bacteria and their strength properties

Despite a growing interest in using bacteria to improve the performance of cementitious composites, the mechanisms by which bacteria favourably modify the composites remain unclear. This study investigated the microscale processes by which Bacillus cohnii (B. cohnii), a non-ureolytic bacterium, modified the properties of mortar focusing on the roles played by factors such as bacteria concentration, status (live or dead) of the cells, and water-cement ratio. Results show that the use of dead or live B. cohnii at 105 cells/ml of mixing water led to an increase in compressive and flexural strength at early (3 and 7) curing days, while 107 cells/ml resulted in a decrease in strength. After 28 days of curing, no significant strength improvement was observed from either dead or live cells of B. cohnii. Water-cement ratio did not have a significant influence on how B. cohnii impacted the strength of mortar. Raman spectroscopy analysis indicated the presence of calcite of higher intensity within the specimens with 105 dead cells/ml than within the control and the specimens with both dead and live cells at 107 cells/ml. F-SEM imaging confirmed the presence of calcite crystals within the specimens with 105 dead cells/ml. The dominant mechanism by which addition of B. cohnii cells impacted the strength of the mortar specimens was through pore-filling as evidenced by the reduction of porosity. The pore-filling benefits of B. cohnii addition within cementitious materials holds prospect for applications especially within aggressive environments where the need to reduce the ingress of deleterious substances can be critical.