Color changes in roofing slates exposed to high temperatures

According to EN 12326-1, roofing slate is a material which do not need additional tests regarding to fire performance, since it is obvious that it doesn’t burn. However, different types of roofing slates submitted to high temperatures showed important changes in color and also in water absorption. This might be important for some cases like when reusing a material coming from a house burn, or if one wants to estimate the temperature of a fire affecting roofing slates. For that, I submitted 6 different types of roofing slates to a thermal ramp ranging from 100 to 900 °C. The changes in aspect and water absorption are related to the slate bulk compositon. The slate samples used were:

BRA: Shale from Minas Gerais, Brazil

ITL: Carbonate slate (>20% CaCO3) from Liguria, Italy

ECA: Slate from Valdeorras, Spain

VXE: Phyllite from Lugo, Spain

ALT: Schist from Alta, Norway

BUR: Burlington slate, United Kingdom

Fire resistance

Color changes for the selected slates (Y axis) along the increasing temperature (X axis, in °C)

In the figure is clearly seen how the color is changing to red tones in all the slates with the increasing temperature. This is a normal effect in all the rocks, high temperatures favor the iron oxidation. However, for the carbonate slate ITL, color tends to white tones, due to the carbonate alteration. Respect to water absorption, all the slates increases their values due to the development of cracks and detachments because of the thermal stress. Again, the slate ITL results are different, reaching close to 20% of water absorption due to the disappearing of carbonate at 600 °C. Anyway, the conditions of this experiment are exceptional, and will never be reached under normal conditions of use of the slate, so these results are merely illustrative.

Water absorption evolution with increasing temperature

Water absorption evolution with increasing temperature