Mineralogical and colour changes of quartz sandstones by heat

Seven German and three Hungarian monumental sandstones have been tested in laboratory conditions to analyse the effect of heat. The studied quartz sandstones have a wide-range of cements and grain-sizes including silica-, carbonate-, clay- and ferrous mineral-cemented varieties of fine-, medium- to coarse-grained types. Cylindrical specimens were heated up to 150, 300, 450, 600, 750 and 900degreesC in an oven. The mineralogical and textural changes were recorded and compared by using microscopy, XRD, DTA-DTG and SEM. Colours and colour differences (a*, b*, L* values) were also measured and evaluated. Colour changes are related to mineral transformations. The most intense colour change is caused by the oxidation of iron-bearing minerals to hematite that takes place up to 900degreesC. When temperature increases the green glauconite becomes brownish while the chlorite changes to yellowish at first. The colour of burnt sandstone is not a direct indicator of burning temperature, since there are sandstones in which the burnt specimens are lighter and less reddish than the natural ones. Porosity increase is related to micro-cracking at grain boundaries (above 600degreesC) and within the grains (at and above 750degreesC) and mineral transformations. The clay mineral structure collapses at different temperatures (kaolinite up to 600degreesC, chlorite above 600degreesC) and leads to a slight increase in porosity. The most drastic change is observed in calcite cemented sandstones where the carbonate structure collapses at 750degreesC and CaO appears at 900degreesC. Subsequently it is transformed to portlandite due to absorption of water vapour from the air. This leads to the disintegration of sandstone at room temperature a few days after the heat shock.