Bacterial Self-healing Concrete with Limestone Calcined Clay: Mechanical Properties, Durability, and Digital Image-Based Evaluation

Abstract Conventional cement production creates a huge amount of CO₂ emission, and concrete is vulnerable to the formation of microcracks that shorten the service life. Bacterial self healing and Limestone Calcined Cement (LC³) are some sustainable solutions proposed. However, the combined effect of LC³ and bacterial self-healing has not been systematically optimized, and quantitative data on crack healing kinetics, pore refinement, and statistical validation are limited. The ordinary Portland cement (OPC) was partially substituted by LC³ at 4%, 8% and 12% replacement rates in this study. A bacterial solution of Bacillus sphaericus (OD₆₀₀ = 1.0; ~10⁹ cells/mL) was added at the rates of 10, 20 or 30 mL per concrete mix. Each experiment was repeated three times ( n = 3). Mechanical properties (compressive, split tensile, and flexural strength), durability characteristics (water absorption and sorptivity), and microstructural characteristics (XRD, SEM and EDS) were assessed and crack healing was assessed by digital image processing (DIP). One way ANOVA, Tukey’s HSD test and Response Surface Methodology (RSM) were conducted. The optimum mix (LC08B20: 8% LC³ + 20 mL bacteria) demonstrated a compressive strength of 60.30 MPa for 28 days which was about 11.7% higher than the control mix. The increase in split tensile and flexural strength was 27.8% and 18.9%, respectively. The water absorption and sorptivity had a decrease of 19.5% and 28, respectively, which is a sign of higher durability. The ANOVA results showed that the observed effect was statistically significant with a p value < 0.05. The optimal dosage of both LC³ (8.2%) and bacteria (19.5 mL) was predicted by RSM with model reliability (R² = 0.94–0.96). After 7 days of continuous water immersed test, it was found that about 73% of the cracks which were initially 0.45 mm wide were healed using self healing. Effective pore refinement was revealed by DIP analysis with a reduction in pore area by 50–55%. Moreover, the LC08B20 mix showed an average decrease of nearly 2.6% in CO₂ emissions when compared with the control mix. In summary, the LC³– Bacillus sphaericus system is a long-term, cost-efficient and environmentally friendly approach to self healing concrete.