Page 5 - openaccess

TECHNICAL PAPER

Figure 1: Petrographic images; (a) concrete, (b) limestone rock (marine based)

Concrete can be referred as an artificial rock, because of its a mineral behavior between Jennite, and Tobermorite [12] .
evolution, existence, and response to the environment are Portlandite [Ca(OH) 2 with hexagonal crystals], ettringite with
similar to the petrology of rocks. In short, rocks are formed prismatic crystals, and monosulphate with plane hexagonal
in three forms, igneous rocks as a result of the cooling of crystals are the other major phases in hydrated cement
[2]
molten lava, sedimentary rocks as an effect of environmental paste . Properties of concrete are governed by the composition
actions, and metamorphic rocks by the effect of pressure, and of cement phase, water to cement ratio, and the nature of
temperature with time on igneous or sedimentary rocks [7-10] . parent rock of aggregates used. On the whole, after required
curing, concrete is said to be serviceable. From the period when
The evolution of a rock follows some typified duration, and raw materials are mixed with water, through its hardening phase,
chemical composition, which characterize the crystalline, and in-service period, concrete continuous to evolve with time .
[2]
and mineralogical properties of the rock in a later stage. The
formation of a rock may take thousands to millions of years. The The major phases in concrete can be classified into three
performance of rock in its existing environment is governed phases. The first phase is aggregates with their coarse, and fine
by the mode of formation and mineralogical compositions. distributions. The fractions below 4.75 mm is referred as fine
Minerals are the building units for rocks, which possess definite aggregates. The second phase is the cementing phase, which
chemical compositions, and distinct crystal or nearly amorphous binds these aggregate particles, and eventually makes the
structures. Feldspar, calcite, and quartz are some typical matrix an engineering material. The third phase is the interface
examples for major minerals in rocks [7-10] . between aggregates, and binding phase, hypothetically defined
as interfacial transition zone (ITZ) in concrete literature .
[2]
On the other hand concrete is a ‘man-made rock’ produced Figure 2 provides a schematic of a concrete section. The brown
based on a ‘pot-life’ of about one hour in its fresh state, which phases are coarse aggregates and the gray phases indicate fine
helps one to cast the ‘artificial rock’ to desired shape and size.
Consequent hardening of cement matrix phase in the system
results in gaining of strength, and improves the bond between
conglomerates of natural rock grains with hydrated cement
paste. Concrete exhibits interfacial transition zone [ITZ] between
the aggregate, and cement phase , likewise a rock exhibits inter
[2]
mineralogical boundaries referred as “relief’ of minerals in thin
sections under polarized light. The term ‘relief’ in petrography is
referred to the changes in interference in colour due to greater
differences in refractive indexes of mounting medium, and
adjacent minerals in the geo-chemical assemblage [11] .
The main strength imparting component in the cement paste
phase is Calcium Silicate Hydrate (CSH), which possesses Figure 2: Schematic of different phases in concrete

6 THE INDIAN CONCRETE JOURNAL | AUGUST 2022