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TECHNICAL PAPER

Figure 20: Thin section image of river sand exposed to elevated temperatures
assessed by a petrographic methodology adopted by Haneefa or less similar strengths. Up to 300 C, the type of aggregates
o
o
et al. [21] . In this method, powder mineralogy of a thin layer was not influencing residual strengths of concrete. At 600 C,
of powdered samples are analyzed in crossed polarized the the residual strengths of calcareous aggregates were better
transmission mode for their optical properties. According to the compared to siliceous, and silco-calcareous aggregates. The
degree of staining or ferric oxidation, the interference colours percentage of residual strengths were 56.3, 20.0, and 10.8 %
may vary. These diagnostic interferences can be effectively used for calcareous, silico-calcareous, and siliceous aggregates,
to study the vulnerability of different types of aggregates to respectively. Moreover, the superior performance of calcareous
any destructive oxidation processes upon exposure to different aggregates in terms of residual Young’s modulus, and flexural
temperatures. The intensity of ferric oxidation, and combined strengths were corroborating the strength observations.
thermal effects are profiled based on powder mineralogy in However, the strength loss scenario was different in high
o
Figure 21. Limestone, granite and river sand were analyzed strength concrete. At 600 C, the performance of siliceous
before any exposure, exposure to heat at 550°C, exposure aggregates was 18 % better than the calcareous aggregates.
to 1N NaOH at 80°C for 14 days to have accelerated alkali At the same time performance silico-calcareous aggregates
interactions, and hot liquid sodium at 550°C. From the changes were inferior to calcareous aggregates as observed in normal
in interferences in colours, the degree of modifications was strengths concrete. The study revealed that; apart from
assessed [21] . mineralogy of aggregates; the strength of binding cementing
medium, ITZ characteristics and initial moisture contents of
Furthermore, a study by Xing et al. [73] provides differentiation aggregates also affect the thermal performance of concretes at
of type and nature of aggregates on performance of concrete elevated temperatures.
at elevated temperatures. Thermo-mechanical behavior of
concrete is studied with siliceous, calcareous, and silico- 7. METHODS TO CHARACTERIZE
calcareous aggregates. Figure 22 represents the porosity AGGREGATE MINERALOGY
evolution of different types of aggregates upon exposure to
different temperatures [73] . Porosity evolution was higher for Petrography and geochemical investigations are routine
procedures to characterize the mineralogy of aggregates. Some
calcareous aggregates compared to siliceous aggregates, acumens to these two methods are discussed in this section.
whereas the carbonate phases of silico-calcareous aggregates
exhibited an intermediate pore evolution with escalated 7.1 Petrography
temperatures. Siliceous phase flint in silico-calcareous
aggregates crumbled even at a moderately elevated Without a note on petrography, the discussion on the
temperature of 150°C. They tested normal strength concretes mineralogy of aggregates and concretes will be incomplete.
with these aggregates at ambient temperature and found more, Petrography is a standard characterization technique used in

28 THE INDIAN CONCRETE JOURNAL | AUGUST 2022