Physio-mechanical and micro-structural properties of cost-effective waste eggshell-based self-healing bacterial concrete

Concrete is versatile but prone to cracking, which weakens its strength and durability. Self-healing concrete can automatically repair cracks, thereby preventing their occurrence. Previous studies have focused on improving self-healing efficiency in concrete to regulate cracks and minimize their effects. Unfortunately, the initial cost of self-healing concrete concerning calcite precipitation by bacterial actions is high. The current study implemented cost-reduction measures by synthesizing calcium lactate from waste eggshells and lactic acid using a more affordable bacterial growth medium made of yeast extract and molasses. To make self-healing concrete specimens, a mixture of OPC, sand, gravel, water, calcium lactate, and Bacillus subtilis bacterial solution was mixed directly at a concentration of (9.84 x 10⁶ and 4.56 x 10⁸) cells/mL. The study found that the workability of bacterial concrete exceeds that of conventional concrete, attributed to the addition of calcium lactate, which acts as a retarding agent and improves the mix's fluidity. Over a 28-day curing period, bacterial concrete with a dosage of 20 mL at a concentration of 9.84 x 10⁶ cells/mL enhanced compressive strength by 14.37 % and reduced water absorption by 23.05 %. This may be due to the calcite precipitation by bacteria that fills voids and micro-cracks inside the concrete matrix. The study also discovered that cracks smaller than 0.5 mm were fully healed within 14 days due to calcite formation produced by bacterial activity. Images from scanning electron microscopes and X-ray diffraction verified the existence of calcite in these cracks. Additionally, the current study highlighted cost reductions in waste eggshell-based self-healing bacterial concrete compared to other study-related findings. Overall, the study emphasizes the advantages of using bacterial self-healing concrete: eco-friendly, cost-effective, enhances workability, strength, and durability, and can autonomously repair cracks without human intervention.