Long-term mechanical properties and durability of a biological coating mortar using Rhodobacter capsulatus PN7 immobilized in vermiculite

Abstract

This study evaluates the influence of vermiculite immobilizing the indigenous purple nonsulfur bacterium Rhodobacter capsulatus PN7 on the long-term mechanical performance and selected durability-related properties of a cementitious coating mortar intended for sewer concrete structures. PN7 produces extracellular polymeric substances (EPS)/glycocalyx, forms biofilms and oxidizes sulfide. Short-term tests confirmed that the strain remained viable after 72 h of mortar hardening and retained sulfide-oxidizing activity. Three mortar systems were investigated: vermiculite without bacteria (V), vermiculite immobilizing the Korean bacterial strain (V-HQ), and vermiculite immobilizing the Vietnamese PN7 strain (V-VN). Vermiculite replaced 0, 10, 20, and 30 vol.% of the fine aggregate. Flexural and compressive strengths were measured at 28, 360, and 720 days under three curing/exposure conditions. Capillary water absorption and the total electrical charge passed were determined as transport-related indicators of water uptake and resistance to ionic/chloride ingress. V-vermiculite progressively reduced mortar strength as its content increased. In contrast, PN7-immobilized
vermiculite mitigated part of the strength loss, with the effect becoming more evident at later ages. The relative performance of V-VN and V-HQ was not consistent across all mixtures and exposure conditions. The 30% V-VN mixture exhibited the lowest capillary water uptake, whereas 20% V-VN provided a more balanced combination of mechanical performance and durability. The results support the potential of PN7-immobilized vermiculite for biological coating applications; however, direct resistance to H2S-induced or biogenic sulfuricacid deterioration and long-term bacterial persistence require further verification.