Page 537 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Clays and Accessory Minerals 507

DTA and dilatometric curves for a clay containing about 20%
dolomite and used in the manufacture of facing brick are shown in Fig.
12. [22] Above 700°C, the clay begins to shrink following the completion of
dehydroxylation and commencement of dolomite dissociation. The residue
from the dolomite reacts with the alumina and silica from the clay resulting
in a large expansion at about 850–1000°C. It is a requirement that bricks
made from this clay must be heated slowly to prevent cracking.
A vitrification range during which liquid phases are produced from
the crystalline components exists in most clay materials. A long vitrifica-
tion range is commercially desirable. Results from equilibrium systems of
pure oxides may be significantly different from those produced from
natural materials. It is, therefore, important to characterize raw materials at
each plant location for most industrial applications.

Figure 12. DTA (A) and dilatomeric, (B) on heating, (C) on cooling, curves for a clay
containing about 20% dolomite. [5]