P N OJHA, BRIJESH SINGH, V V ARORA, PRAMOD NARAYAN, AMIT TRIVEDI & MANTU GUPTA


radiating needle form, were observed. The ettringite crystals were mainly developed in small size open-air voids. Few voids were filled with radiating needle-shaped ettringite microcrystals (Figure 3). The growth of both convergent- and divergent-type radiating microcrystals of ettringite were noticed within the voids. In a few instances the microcrystalline ettringite grains (Figure 4) spread outside the voids. The formation of more ettringite crystals was in progress within the voids. Twinned crystals of ettringite were also observed in a few clusters. The ettringite crystals were tightly packed. This ettringite may further grow into larger crystals. No alkali-silica reaction rims were observed between the coarse aggregates and mortar in these concrete cores.


Figure 3. Growth of complex structured ettringite crystals in cluster form in the air void


Sulfate, in the form of pyrite, was found in the aggregates from petrographic, SEM and XRD studies and this is the reason for crystalline ettringite formation. Ettringite percentage in open-air voids was maximum in the upstream samples (3-4% of open-air voids) and minimum in the gallery and downstream samples (1-2% of open-air voids). The percentage of crystalline ettringite present in open-air voids is about 2% of open-air voids in mass concrete. The percentage


12 DAM ENGINEERING Vol XXXII Issue 1


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