Page 542 - Handbook of Thermal Analysis of Construction Materials

Section 3.0 - Applications 513

3.5 Weathering of Roofing Slates

Slates are related to clays. The DTA technique has been applied to
study the weathering quality as well as an aid to the identification of slate
of unknown origin. Figure 17 shows the differential thermal analysis of
[3]
samples of roofing slate of three different qualities. The first curve
represents a slate of excellent durability. The peaks at 610° and 850°C may
be due to some type of chlorite. The inflection at 575°C is caused by the
nd
presence of quartz. In the calcined material (2 curve), the presence of quartz
is more clear. The third curve is obtained with a slate that was found in
practice to delaminate slowly on roofs under conditions of low atmospheric
pollution. The poor durability may be caused by the slow oxidation of pyrite
in the slate and subsequent reaction between the oxidation product and calcite
to form calcium sulfate. The presence of pyrite and calcite is indicated by the
peaks at 450° and 770°C, respectively. The fourth curve of slate of pure
quality is dominated by the large calcite peak. The exothermic peaks at 930°
and 420°C suggest the presence of chlorite and a small amount of pyrite,
respectively.

Figure 17. DTA of roofing slates. [3]