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TECHNICAL PAPER

Figure 15: Petrography of limestone aggregate mortars exposed to hot liquid sodium, Haneefa el al. (2014) [64]

mineral, and prominent cleavage cracks in calcite [Figure 6.3 Soundness of aggregates
15 (e)], widening, and etching of rhombohedral cleavage of
calcite [Figure 15 (f)], cubic fragmentation of calcite crystal The property of aggregate to resist volume changes due to the
through rhombohedral cleavages [Figure 15 (g)], unaffected physical changes of the surrounding environment is referred
[1]
calcite phase with intact ITZ and eroded binder phases as soundness. According to Alexander, and Mindess , the
[Figure 15 (h)] and cracked binder phases with washed out ITZ soundness of aggregates indicates a rough estimation of
are the mineralogical interpretation of these interactions. It ‘physical durability’ of aggregates in concrete. However, they
was found that the performance of aggregates in this exposure observed from the literature that the comparison of soundness
was also influenced by composition of binder phases and results of aggregates might hold good only within the rocks
w/c ratio. The disintegration was more at high w/c ratios and with similar mineralogy. Unsoundness of aggregate can cause
[1]
the performance of fly ash-based Portland pozzolana cement scaling of surfaces or cracks or pop-outs in the concrete .
compared to refractory high alumina, slag, and ordinary Portland ASTM C88 and BS 812: Part 121 [40,69] propose methods for
cements [21,39,64, 68] . Additionally, it was found that performance evaluating aggregate soundness. This test is a sort of
of geopolymers with limestone aggregates was much better weathering kind, in which sulphates of magnesium or sodium
compared to conventional cement-based systems [68] . Figure 16 are getting crystallized in aggregate pores during number of
provides SEM image in back scattered electron mode (BSE) wetting and drying cycles, and simulates various deterioration
of hot sodium exposed ordinary Portland concrete and mechanisms. The deterioration is measured in terms of mass
geopolymer concrete. The microstructure of normal concrete loss. ASTM C33 [70] provides limiting values for soundness for
was more disrupted; whereas, the geopolymer exhibited less coarse, and fine aggerates as 15 %, and 12 % (magnesium),
modified paste, and aggregate phases. 12 %, and 10 % (sodium) respectively.

24 THE INDIAN CONCRETE JOURNAL | AUGUST 2022