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TECHNICAL PAPER

Figure 9: Fractured surface of limestone; (a) Pure limestone with rhombohedral cleavages fractures through its most favoured plane and exhibit blocky
or rectangular specks of coarse crystalline calcite; (b) Conchoidal or sub conchoidal fracture in microcrystalline calcite, and; (c) Inter-through mineral
fracture in limestone with accessory impurities like mica and hornblende, Haneefa et al. [21]

flint. Rocks composed of highly foliated structures are not worn smooth surface of chert, granular rounded grain texture of
suitable aggregates. Schist, slate, and phyllite are some typical sandstone, rough-medium grained texture of basalt, crystalline
examples of such types of rocks . These types rocks fractures fracture of granite, and honeycombed texture of pumice
[1]
through their most favorable cleavage planes, and make the are some typical examples of mineralogy driven textures
aggregate flakier, and more elongated. Flaky aggregate with a in aggregates. In rough textured aggregates, the bonding
relatively higher surface area may interfere with the rheology of between aggregate, and cement paste may be better compared
concrete, and the demand for more water content in concrete. to ones with smooth surfaces. However, more irregular, or
Some common global terminologies with regard to shapes honey-combed textures demand more water content to achieve
of aggregates are angularity number (indicates angularity of the desired workability of concrete in its fresh stage.
particles ranges 0 to 11), mean size (average particle size), Coral limestones possess high porosity, where the mineral grains
flakiness and elongation index (% of geometric dissimilarities of quartz are lines by magnesian calcite, as seen in Figure 7.
with respect to the cumulative mean size of aggregate), and Scanning electron microscopic images of coral limestone
sphericity (surface and volume relationship) [24-28] . shows floral shaped arrangement of magnesian calcite lining
around quartz crystals, as seen in Figure 10, which indeed
Similar to the shape, the surface texture of aggregates is also contributes towards the low density. Also, the occurrence
attributed to the mineralogical composition of the aggerates. of such high pores in the system can facilitate preferential
BS: 812 (1989) [24] discusses a classification of texture pertaining adsorption of precipitates (say salt depositions) over the large
to aggregates in concrete. The glassy texture of flint, water pores preventing material damage. This specific mineral
arrangement, and hence the property is important in choosing
coral limestones a s aggregates in construction depending upon
the exposure.

5.2 Hardness and density

The hardness, or wearing resistance of aggregates depends on
the hardness of major minerals present in the rock. In general,
the siliceous aggregates (i.e. granite) with feldspar, and quartz
(6, and 7 on Mohs’ scale, respectively) as essential minerals
are much harder than limestone with calcite (3 on Mohs’ scale)
(Table 2). Similarly, the high proportion of porous, or dense
minerals governs their density, water absorption, and strength.
The denser rock such as hematite possesses high specific gravity
(4.9 to 5.3) and strength due to the presence of high proportions
of heavier iron-based minerals [as presented in Table 3 and
Figure 10: Distribution of magnesian calcite (M) around quartz Figures 4j to 4l. This type of high-density aggregate is used
(Q) grains in coral limestone [23] in special concrete for radiation shielding purpose in nuclear

18 THE INDIAN CONCRETE JOURNAL | AUGUST 2022