Metagenomic and mineralogical insights into biomineralization of shell lime putty prepared with fermented Terminalia chebula fruit extract for built heritage applications

The study investigates an ancient Indian fermentation technique using 10 % w/v Terminalia chebula fruit extract (K10 %) and its interaction with shell lime. Fermentation ages of 1, 4, 7, and 14 days of K10 % were identified as key stages based on carbon dioxide evolution rates, with peaks at 4 and 7 days (8826.67 ppm/h and 6923.32 ppm/h, respectively). These stages were further investigated via 16S rRNA (V3–V4 region) metagenomic sequencing, identifying dominant microbial taxa including Acinetobacter guillouiae, Bacillus safensis, Enterobacter cowanii, Lactobacillus helveticus, Lactobacillus casei, and Sphingobacterium multivorum. These strains facilitated biomineralization through carbonic anhydrase activity, supported by exopolysaccharide production and hydrolytic enzymes, in putty variants prepared with a K10 %-to-binder mass ratio of 0.95. Gas chromatography–mass spectrometry profiling of the putty extracts revealed a progression of unsaturated fatty acids and an increase in ester content, particularly in the variant prepared with 4-day aged K10 %. XRD analysis at 90 days confirmed the highest levels of calcite (64.4 % and 60.17 %) and tobermorite (5.27 % and 5.14 %) in the lime putty variants prepared with 4- and 7-day aged K10 %. These variants also confirmed elevated carbonation levels (53.36 % and 51.95 %), as determined by thermogravimetric analysis. Mineralogical developments contributed to enhanced carbonation, matrix densification, and reduced water permeability, corroborated by superior mechanical and other durability parameters compared to the reference putty. Overall, the findings demonstrate that K10 % extract significantly enhances the functional and durability characteristics of lime binders, offering a bio-based, sustainable approach suitable for internal and external applications in heritage restoration and eco-friendly construction.