Decay of natural stones caused by fire damage

Abstract

AbstractAlmost every representative ancient building suffered from a fire during its history. Therefore, several limestones, sandstones, a gypsum, granites, tuffs, an orthogneiss and two marbles have been tested to analyse the effect of fire. Thermal expansion measurements up to 1000 °C reveal that every rock shows a specific expansion behaviour. Variations are caused by the single crystal thermal expansion properties of rock-forming minerals and by different damage processes. In silicate rocks, intragranular fracturing is the predominant damage phenomenon. Carbonate rocks show, at low temperatures, a behaviour mainly controlled by the anisotropic expansion of calcite. At higher temperatures, mineral reactions, such as decarbonatization, are directly evidenced by sudden jumps in thermal expansion curves. If water is present, a second stage of deterioration follows fire damage: the huge volume increase due to portlandite formation from decarbonized CaO causes severe scaling at the outermost surface of limestone when exposed to the environment. Small amounts of silicates in carbonate rocks may improve the stability of those rocks due to dicalciumsilicate formation. At high temperatures, an increase in the expansion coefficient may be explained by partial melting for some rock types. Phase changes (e.g. quartz) are monitored by a sudden increase in the expansion coefficient. Investigations on gypsum reveal that dehydration reactions reduce fire temperatures in the vicinity of gypsum rocks significantly. In general, all experiments show that samples are severely damaged after being subjected to fire. Real fire tests show that the penetration depth of heat and the associated damage types vary as a function of lithology. While for granites, cracks in feldspars predominate, the firing of limestone causes a scaling of the outermost layer. The investigations may lead to an improved assessment of natural building stones that have been damaged by fire. Implications can also be drawn for the recent use of facade panels made of natural building stones in case of a future fire.