Strength Optimization of Self-Compacting Concrete with Cement Kiln Dust Incorporating Sporosarcina Pasteurii Bacteria Using the Response Surface Method

Micro-crack formation significantly compromises the long-term durability and performance of concrete structures. This study investigates the effect of Sporosarcina pasteurii on the strength properties of self-compacting concrete incorporating cement kiln dust (SCC–CKD), while also developing predictive models and optimizing mix performance using response surface methodology (RSM). SCC mixtures were prepared with 10% cement replacement by cement kiln dust, a water–cement ratio of 0.43, and 2% superplasticizer dosage. The bacterial suspension was introduced at densities of 0, 1.5 × 10⁸, 6.0 × 10⁸, 1.2 × 10⁹, and 2.4 × 10⁹ cells/mL, alongside a cementation reagent of 0.5 M concentration. The bacterial-to-cementation reagent ratio (B:C) was varied, and curing was conducted up to 56 days. Compressive, flexural, and split tensile strengths were evaluated, and microstructural changes were examined using scanning electron microscopy (SEM). The results indicate that strength properties improved with increasing bacterial density, reaching optimal performance at approximately 1.2 × 10⁹ cells/mL, beyond which a decline was observed. The developed RSM models demonstrated good predictive capability, with coefficients of determination (R²) of 94.70%, 79.46%, and 82.52% for compressive, flexural, and split tensile strengths, respectively. Optimization results revealed that a B:C ratio of 56:44, bacterial density of 1.54 × 10⁹ cells/mL and curing age of 52 days produced optimal strength values of 34.50 N/mm², 10.09 N/mm², and 5.07 N/mm² for compressive, flexural, and split tensile strengths, respectively. The findings demonstrate that the incorporation of Sporosarcina pasteurii enhances the mechanical performance of SCC–CKD through microbially induced calcite precipitation, and that RSM is an effective tool for predicting and optimizing its strength characteristics.