Bioreceptivity index for granitic rocks used as construction material

Bioreceptivity is a fundamental concept in the ornamental stone industry and in the fields of cultural heritage and civil engineering to understand the susceptibility of stone constructions to biological colonisation and subsequent biodeterioration. However, a bioreceptivity index (BI) has not yet been established for any construction material. The aim of the present study is developing a simple, robust and well-founded BI for granitic rocks. For this purpose, a standardised laboratory protocol was used to grow phototrophic biofilms on several varieties of granite. The colonisation was then assessed by chlorophyll fluorescence and colour measurements. Based on the results thus obtained, a BI including two components (BI_growth and BI_colour) is proposed. BI_growth quantifies the extent of the biological growth and BI_colour quantifies the colour change undergone by the stone due to the colonisation, which can be considered the bioreceptivity perceptible to the human eye. The values of BI, BI_growth and BI_colour were fitted to a scale of 0-10, thus enabling qualitative classification of the lithotypes according to their primary bioreceptivity. Eleven varieties of granite commonly used as construction material and with a honed surface finish (one variety with three additional surface finishes: polished, sawn and sanded) were thus assigned the corresponding BI, which represents a new quality factor for the stone industry. The index can therefore be used by end-users as a decision-making tool in the selection of appropriate lithotypes for building and/or ornamental purposes.