Mechanical and self-healing behaviour of denitrifying bacteria-based mortar with nano-graphene oxide and fly ash

This study investigates an optimized denitrifying bacteria-based self-healing mortar incorporating nano-graphene oxide (nGO) and fly ash (FA). In this system, denitrifying bacteria induce CaCO₃ precipitation to seal cracks, nGO promotes nucleation and improves matrix densification, and FA contributes to pozzolanic reactions that refine pore structure and enhance later-age strength. Mechanical, durability, microstructural, and crack-healing properties were evaluated. The optimum combination of 10⁸ cells·mL⁻¹ bacterial concentration, 0.06 wt% nGO, and 15 wt% FA increased compressive strength by 52% at 28 days compared with the control mortar, while the combined system showed an overall improvement of 85%. With prolonged curing, the strength gain reached 120% at 180 days. Crack-healing efficiency was 100% for 0.1-0.2 mm cracks and remained ≥ 97% for cracks up to 1.0 mm after 28 days of healing. Ultrasonic pulse velocity recovery exceeded 99.5%, indicating restoration of internal continuity. Water absorption decreased from 2.95% to 0.79%, corresponding to a 73% reduction in permeability-related uptake. SEM-EDX analysis confirmed dense CaCO₃ deposition within healed cracks and the interfacial transition zone. These results demonstrate the potential of denitrification-based bio-nanomodified mortar as a durable and low-permeability construction material.