The interaction between rainwater and polished building stones for flooring and cladding - Implications in architecture

Abstract

Stone polishing is a process executed to make the surface glossy, so that the material shines by reflecting the light. The polishing is mainly carried out in flooring and façade cladding to improve the aesthetic properties of the stones. In particular on calcareous stones, polishing is not a per- manent feature because various atmospheric agents in outdoor (e.g. rain, freeze and thawing cycles, UV solar radiation) are responsible for weathering with the loss of the mirror-like effect. In this paper, the effects of rainwater on polished limestones used for building cladding have been shown. In order to better reproduce the environmental conditions in building location, two samples horizontally and vertically oriented (simulating flooring and façade cladding respectively), were subjected to rainwater wetting. Other two samples with the same previous orientations were subjected to tap water wetting. After a month test it has been demonstrated how rainwater, differently to tap water, causes micro-corrosion on the samples surface by a modification of the colour and a decreasing of the gloss. This is attributable to a roughness increase. The corrosion intensity depends on the orientation of the samples. The reason of this corrosion is the capacity of the rainwater (with a low saline content) to easily dissolve air carbon dioxide by forming cor- rosive carbonic acid with respect to high saline content tap water.