Nanoscale characterisation of limestone degradation using Scanning Force Microscopy and its correlation to optical appearance

In this work the weathering phenomenon of building stones has been studied on the nanoscale. Red Quipar limestone samples have been exposed to ultraviolet (UV) radiation and acid attack and have been analysed by Scanning Force Microscopy (SFM) as well as optical measurements. The morphology of the samples along each step of the process has been characterised using the Root Mean Square (RMS) roughness and the Power Spectral Density of surface roughness (PSD). The results show that sulphuric acid produces an important increase of the micro and nanoscale surface roughness while essentially no variations are observed when the samples are irradiated with UV light. SFM results are compared to the optical appearance using conventional photographic images. The increase of nanoscale surface roughness is directly related to the loss of brightness and the apparent colour variation observed in the optical images. Loss of brightness and colour variations in Red Quipar stones are explained by the increase of the surface roughness suffered from the acid attack. The SFM topographic images can be used to monitor at the nanoscale the evolution of the morphology of samples during different processes such as those used in these experiments. Finally, this work proposes a general scheme combining SFM with traditional optical measurements as a powerful tool to better understand the effect of physical properties (in particular surface roughness) with the perceived optical appearance. © 2014 Elsevier B.V.