Assessment of the impact of high temperature on the facing stone from hard rocks

Abstract

Samples from two deposits of natural stone, which are located in Ukraine, were experimentally studied. Tests of granite samples were carried out at high temperatures of 300, 500, 700, 900 °C. All presented samples show the change in surface colour at a temperature of 300 °C and above. The behaviour of heated granites depends on their mineral composition, structure and texture. The surface of all the prototypes are lighter, some samples of granites have lost saturation of colors. When the samples were heated to 900 °C of natural stone, the largest growth of the component L (images of the samples of the stone brightens) of the CIELab colour system occurred in the samples of Buka gabbro by 44.4 %. The smallest growth of the component L occurs in the granodiorite by 28.6% when the samples are heated to 900 °C. In gray granites, redness appears when heated, which is mainly concentrated around mica and other minerals that are rich in Fe. The greatest change in colour was due to the transition phase of dark-coloured minerals (biotite and pyroxene) into polymorphic minerals. This gave the samples of granodiorites lighter colour because minerals change colour from black to gray or white. Shades of white were given by quartz, where white microcracks appeared when it was heated. Noticeable aesthetic damage to the surface of natural stone samples begins at temperatures from 300 to 500 °C. Thus, a fire with temperatures below this limit can be considered «safe» in terms of aesthetic damage, if we consider only the coefficient of heating of the fire and exclude ash and gases.