Effects of Bacillus subtilis on Crack Remediation in Thermally Degraded Limestone Calcined Clay Cement Mortars

Abstract

This paper reports experimental findings on the effect of Bacillus subtilis on crack remediation in thermally degraded Limestone Calcined Clay Cement (LC3) mortars. Mortar prisms measuring 160 mm × 40 mm × 40 mm were cast using LC3 at water/cement (w/c) ratio of 0.5 and cured for 28 days. Half of the 28-day cured mortar prisms were thermally degraded by heating them at 1000 °C in a furnace to induce the cracks while the other half was used as a control. Both cracked and un-cracked mortar prisms were subjected to compressive strength, porosity and accelerated chloride ingress tests. Moreover, half of the cracked and un-cracked mortars were immersed in bacterial solution containing Bacillus subtilis while the other half was separately immersed in curing water until the 90th day. Compressive strength, porosity and chloride ingress tests were also repeated on the 90th day. Microstructural changes in cracked LC3 mortars were carried out using Scanning Electron Microscope (SEM) before and after immersion in bacterial solution. Results showed that at 28 days of curing, un-cracked mortars exhibited higher compressive strength, lower porosity and lower apparent chloride diffusion coefficients than cracked mortars. However, compressive strength, porosity and apparent chloride diffusion coefficients of both cracked and un-cracked mortars were equivalent after 90 days of curing in bacterial solution. SEM images showed visible micro-cracks after thermal treatment and healed cracks with calcite deposition after curing in bacterial solution. In conclusion, Bacillus subtilis was found to improve the crack healing capacity of thermally cracked mortars.