Fire resistance of roofing slates: Mechanical, mineralogical and aesthetic changes alongside temperature increase

Abstract

Fire is a major destructive agent for dimension stone. Practically all historic buildings have suffered one or more fires over the course of their lifespan. There are numerous studies on the effect of fire on stone. Generally speaking, these effects can be divided into aesthetic and structural. Aesthetic effects involve a color shift to reddish hues, due to the oxidation processes triggered by temperature increase. Moreover, structural effects involve the formation of cracks and delaminations that may cause the rock to disintegrate. However, even though roofing slates are a group of metamorphic rocks that are widely represented in the historical architectural heritage of many European countries, none of the scientific studies alluded to above have taken them into account. The present study examines 6 lithotypes, covering the metamorphic range of rocks used as roofing slates. The roofing slates were subjected to a heating test reaching 900 °C, and then analyzed using X-ray diffraction and fluorescence, Thermogravimetric and Differential Thermal analyses, and were also tested for bending strength and water absorption following the European standard for roofing slates, EN 12326. All samples studied exhibited a reduction in their bending strength, together with an increase in water absorption. Likewise, the oxidation processes triggered by the high temperatures induced a general oxidation of the iron, lending all of the samples a reddish hue. On the other hand, the roofing slate samples proved to be non-combustible and did not release any toxic or hazardous fumes.