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TECHNICAL PAPER

impurities usually present in the natural sources from river beds. increase in petrographic porosity [Figure 18 (a)]. However,
The TG/DTA indicates that the river sand is of an igneous origin modifications were found in the cases of accessory impurity
with possible minerals of weathered quartz and feldspar. A minerals present in the limestone. The disintegrated phase
noticeable change in river sand thermal decomposition from of mica [Figure 18 (b)] with impure quartz, ferric oxidation of
granite was the mass loss of 1.8 %, which is referred in the mica, amphibole, and pyroxene minerals [Figures 18 (c), (d)
literature as the polymorphic alteration of quartz (change of low and (e)] are some salient features of fire exposed limestone. If
quartz to β-quartz with changes in crystal symmetry) [7-9, 21,39] . River the calcite in the limestone are layered as a result of different
sand also exhibited mass gain above 1000°C, may be due to the formation kinetics, the differences in stiffnesses may impart
formation of high temperature polymorphs of siliceous minerals. cracks in the limestone [Figure 18 (f)]. The scenario is entirely
On the whole, the thermal stability of phases in the aggregates different in siliceous aggregates with tiled, interlocked, and
are significantly affected by the mineral phases and their stability hypidiomorphic texture exemplifying mineral assemblage.
at elevated temperatures. Thin section petrography of granite aggregate (before thermal
exposure) is presented in Figures (3g to 3l, 4, 5a to 5g). The
Apart from the thermal stability of minerals in aggregates, petrography of granite after thermal exposure is provided in
microstructure also plays a major role in controlling the thermal Figure 19. Extensive mineral, and cleavage staining due to ferric
performance of aggregates. Studies by Haneefa et al. [21,39,63,64] oxidation, and intergranular, and transgranular cracks are visible
provides scientific insight into the effect of the mineral in these images (20a to 20f). Staining due to ferric oxidation is
microstructure of aggregate on the thermal performance a phenomenon that occurs in granite, or siliceous rocks upon
of concrete. Above the mineral composition, the mineral heating. Dehydroxylation of iron bearing mafic minerals release
assemblage of aggregates is important in their thermal Fe ions upon heating, and get oxidized in the atmosphere [71] .
2+
performance. If the aggregate has a mono-mineral composition This is a very common phenomena during concrete fire, which
with no, or less cleavages; during the fire, there is only a less changes the colour of aggregate to dark. This may occur in any
possibility for mineral incompatibility due to alterations in the type of aggregate with dispersed iron oxide in it. The resulted
aggregate assemblage. A typical example is limestone. Thin product may appear on the aggregate surface as dust or pop
section images of limestone before thermal exposure can be outs, and may adversely affect the desirable properties of
referred from the Figure 5 [i, j and k). Figure 18 dictates the concrete [21,39,63,64] .
petrography of limestone after thermal exposure. The limestone
studied is the purest form with accessory impurity minerals The formation of cracks in aggregate upon heating may be
less than 2 %. Even after a thermal exposure of 550°C (normal attributed to their typical texture, and mineral assemblage. For
operating conditions of sodium cooled fast breeder reactors), example; in general, granite aggregate constitutes feldspar,
the calcite phase was intact without any cracks with a slight quartz, hornblende, and magnetite. These mineral assemblages

Figure 18: Limestone exposed to elevated temperature, Haneefa et al., 2013 [21]

26 THE INDIAN CONCRETE JOURNAL | AUGUST 2022