Accelerated ageing tests on carbonate stones and assessment of their recession rate

Considering that recession rate estimates derived from available equations are not sufficiently reliable when applied to limestones, an experimental investigation was conducted to determine the possible correlations between stone recession of different types of carbonate rocks and petrographic and textural features by a series of aging tests under controlled environmental conditions. In this study samples of carbonate stones differing in their textural features and mineral composition have been subjected to accelerated ageing tests in an environmental test chamber simulating the wetting effect of rainwater using two different water compositions corresponding to those of the Italian cities of Bologna (pH ~ 7, Panettiere et al. 2000) and Stresa (pH ~ 6, Rogora et al. 2004). Bulk stone recession was evaluated considering sample weight loss as a function of the number of wetting cycles. Direct measurements of recession were performed by Confocal Microscopy (CM) and Structure from Motion (SfM) micro-photogrammetry, which allowed 3D surface reconstruction of the stone surface and evaluation of differential recession as a function of calcite grain size and of. Despite the different recession rate determined by differences in water composition, the influence of grain size tend to provide similar correlation coefficients. In addition, certain rock types may contain significant amounts of clay minerals which enhance material loss during water runoff. Linear recession measurements allowed the definition of stone-specific phenomenological coefficients (N) to be applied to recession rate estimates obtained from the currently available recession equations for carbonate rocks (Honeyborne & Price 1977, Weber 1985). The effect of grain size on stone recession have been also discussed and quantified. Indeed, there is a constant effect of grain size on recession rate, independently from the contribution of environmental parameters. The analytical setup here presented represents a pilot study in the identification of a rapid and efficient methodological approach able to determine the recession rate of a specific carbonate rock type in a given environment, in order to provide reliable estimates of future stone deterioration under specific environmental conditions from expected climate scenarios.