Experimental Verification of Calcite Formation Potential by Ureolytic and Non-Ureolytic Bacterial Strains in Geopolymer Mortar

This study aimed to examine the calcite precipitation potential of non-ureolytic bacterial strains of two species, <i>Viridibacillus arenosi</i> (A₆) and <i>Bacillus zhangzhouensis</i> (D₂₅), as compared to the known ureolytic bacterial strain, <i>Sporosarcina pasteurii</i> (SP), within geopolymer mortar. Tests were carried out after 56 days of injection treatment to confirm the precipitation process, incorporating healing efficiency measured by ImageJ software, recovery of UPV, water permeability, capillary water absorption, and microstructural and mineralogical analysis SEM/EDS and XRD. The non-ureolytic isolates D₂₅ and A₆ showed the highest healing efficiencies, at 96.9% and 91.9%, respectively, followed by the ureolytic bacteria SP at 77.8%. A₆ exhibited the most substantial reduction in permeability at 97.3%, indicating extensive crack healing, followed by D₂₅ at 92.9% and SP at 82.1%. Furthermore, SEM and EDS analyses confirmed the formation of calcite crystals and calcium depositions in the bacteria-treated samples. Complementary evidence was provided by XRD, which revealed distinct calcium carbonate peaks in the treated specimens, peaks that were entirely absent in the control samples, thus strongly confirming the role of bacterial activity in the precipitation process. The results confirm that non-ureolytic bacteria can efficiently boost calcite precipitation in geopolymer mortars, offering superior healing performance and a more sustainable alternative to ureolytic strains.