Acid resistance and self-healing crack-sealing performance of Shewanella oneidensis-modified low-calcium fly ash geopolymer concrete

Durability of cementitious materials in sewer infrastructure is greatly affected by sulphuric acid attack and cracking. A low-calcium fly ash-based geopolymer concrete (GPC) enhanced with Shewanella oneidensis, a dissimilatory metal-reducing bacterium (DMRB), was tested for its resistance to acid in simulated sewer conditions and its self-healing crack-sealing abilities. Specimens were submerged in 3% H₂SO₄ (pH 0.66) for 60 days; crack-healing was also observed over 32 days through bacterial surface coating. The properties were examined using mass-loss, electrical resistivity, chloride migration, capillary water absorption, and microscopic characterisation using SEM-EDS, XRD, FTIR, and XRF. Bio-modified specimens showed greater strength development (42.41 vs. 39.22 MPa at 60 days), better post-acid strength retention (26.20 vs. 18.92 MPa; 38.5% advantage), lower percentage mass loss (0.6% vs. 1.9%), lower chloride migration coefficient (1.05 × 10⁻ 1 ⁰ vs. 1.09 × 10⁻ 1 ⁰ m 2 /s), and higher residual electrical resistivity (3.6 vs. 3.0 kΩ·cm). During acid attack, the surface showed dissolution and formation of gypsum-carbonate reaction layers, typical of diffusion-driven deterioration in low-calcium systems; ICP-OES of the acid leachate confirmed all monitored elements were below detection limits, consistent with precipitation-dominated leaching. Microstructural analysis confirmed the retention of Fe and Ca phases and the formation of carbonate crystals in bio-treated specimens. Crack repair analysis indicated calcium carbonate crystals forming at crack interfaces in both bio-treated and coated samples, with crack-closing ratios reaching up to 0.08 in surface-coated specimens, where capillary absorption was also reduced compared to controls. Bio-modification enhances acid resistance under simulated sewer conditions and enables autonomous crack sealing under standard water-immersion conditions, with potential for sustainable sewer infrastructure applications.