Biologically induced calcium oxalate mineralization on 15th century lime mortar, Murud Sea fort, India

Abstract

Abstract The historic lime mortar of Murud Seafort, Western India has endured the strong oceanic current in contrast to the basaltic stone blocks used in construction that have extensively weathered. The reason for the survival of the historic lime mortar was predominantely researched. The mortar samples were therefore characterized for their composition and microstructure, using granulometric analysis, thin section analysis, X-ray Fluorescence (XRF), X-ray powder diffraction (XRD), Fourier transform infrared (FTIR), Nuclear Magnetic Resonance (NMR), and Scanning Electron Microscopy with Energy Dispersive X-Ray Analysis (SEM-EDX). The analytical data confirmed the presence of water-insoluble calcium oxalate being formed at the surface crust, superficially on the mortar due to bacterial biomineralization. This biomineralization was responsible for its survival in saline conditions. The mercury intrusion porosimetry (MIP) revealed reduced porosity of the outer mortar surface by 9–14% besides a considerable reduction in mean pore diameter and void ratio to that of the same mortar’s inner core. The bacterial cell count and microbial identification of bacterial isolates method based on 16 S rRNA sequencing identified Bacillus spp. to be mainly responsible for biomineralization. The bacterial action has contributed to the hardening of the superficial material on the outer mortar that saved the historic mortar from the striking sea waves. The granulometric analysis shows the mixing of fine to coarse sized sand grains of basaltic origin sourced from the nearby river channel. With the aid of NMR analysis, it was concluded that calcium phosphate was not as evident as the calcium oxalate on the mortar surface in any of the samples analyzed.