Surface consolidation of natural stone materials using microbial induced calcite precipitation

摘要

Purpose - Deterioration in natural stone is associated with many decay mechanisms and often the inherent composition of the materials themselves. Sandstone varies considerably but they all require a cementing matrix to bind amongst others, the silica (SiO_2) particles together (Reading, 1989). In calcareous sandstones and limestones this binding matrix is principally calcium carbonate based (Muir, 2006; Reading, 1989, McMillan et al, 1999) in the form of calcite (CaCO_3). Friable sandstone substrates and stones suffering from "surface dissolution" or disaggregation (Muir, 2006; Smith et al, 1992) have been traditionally consolidated utilising a host of chemical compounds that had, in many cases negative effects on their long-term performance (Muir, 2006). A principle issue amongst many was moisture entrapment and irreversibility of the consolidants adopted. The paper aims to discuss these issues. Design/methodology/approach - This paper investigates the effect of microbial induced calcite precipitation (MICP) as a natural treatment for the conservation of historic natural stone substrates. Sporosarcina pasteurii has been proven as a bacterium that can perform MICP effectively in extreme conditions making it the preferred bacterium for the MICP process within this study. Surface treatment experiments were analysed by measuring the mass increase and surface changes using scanning electron microscopy (SEM). Findings - The surface treatments showed a noticeable mass increase and observable deposition when viewed using a SEM microscope. Bio cementation of loose sand particles was observed and the degree of cementation was determined using a Moh's hardness test. Research limitations/implications - Recommendations for further work to improve this study are: use an increased Sporosardna pasteurii cell optical density which would provide a greater calcite output. Carry out a paired comparison initial surface absorption test (BS 1881: Part 208, 1996 or ASTM C 1585-04, 2004). To be carried out on untreated control and MICP samples which would determine the pore blocking effect and surface repair capability of the treated samples. Practical implications - A method for obtaining optimal results in terms of surface treatment would involve reducing the time between mixing and application, this would require having the two reaction constituents mixed only seconds before use. Using a late mix spray application system has the potential to allow the two mixtures to combine in the spray nozzle whilst exiting the apparatus. Originality/value - This paper investigates a safe, natural process for stone repair.