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TECHNICAL PAPER

Table 1: Characteristics of three phases in concrete as an artificial rock

PHASE CHARACTERISTICS
Aggregate phase Origin: Igneous/Sedimentary/ Metamorphic
Chemistry: Siliceous/ Calcareous/ Silico-calcareous
Shape: Rounded, angular, irregular, flaky or elongated
Texture: Glassy/ smooth/ granular rounded grain/ rough-medium grained, crystalline, honeycombed
Physical: Physical and mechanical properties are based on mineralogy which includes volume proportion and distribution
of minerals in the aggregate, rock origin, chemistry and texture.
Binder phase Materials: Ordinary Portland cement, water, SCMs such as fly ah, metakaolin, slag and silica fume, chemical admixtures
and fibers if any.
Properties: Hydrated binder phase includes calcium silicate hydrate (CSH), calcium hydroxide (CH), ettringite and
[2]
monosulphate. CSH imparts strengths, whereas the CH provides alkalinity to the system .
Interfacial transition This is the interface between binder phase and aggregates. Typified by band width of maximum 100 µm and more
zone (ITZ) significantly within 50 µm. There are two major hypotheses of formation of ITZ. One is the wall effect resulting in more
water content around aggregates and the second is the single sided growth effect in which cement paste matures only
from paste side [1,2,13] . In both the cases, the interface will be porous compared to the bulk binder phase, resulting in a
weaker zone. The ITZ is characterized by more porous CSH, large hexagonal prisms of portlandite (CH), ettringite and
monosulphate. In normal concrete, this zone is assumed to be the first to respond to stresses and fractures around the
aggregates through binder phase. On contrary, if the binder phase stiffer or stronger than the aggregates, it breaks
through aggregates and binder. The ITZ porosity can be improved by use of pozzoloans which refines the zone while
consuming more CH and producing denser CSH.

aggregates. The black phase is the binding cement paste. A in concrete are reduction in wearing resistance, improvement in
[1]
representative illustration of ITZ is presented in the schematic rigidity, restraining of paste creep and inertness .
in yellow colour around a single grain of coarse and fine
aggregates. Characteristics of these three phases are tabulated Types of aggregates used worldwide exhibit wide variations
[6-11]
in Table 1. in their geo-chemistry . Since the selection and use of
aggregate are localized practices, the history of geological
In addition to water to binder ratio, quantity and type of formulations and petrology of the region decides the properties
cementitious materials; the properties of concrete are of the parent rock. Comprehensive and conclusive scientific
considerably governed by an optimized skeleton of aggregates, interpretations are required before choosing a type of rock for
type and size of aggregates. Since aggregates occupy around aggregates in concrete. This paper analyzes the mineralogy of
70 % of concrete, mineralogy driven properties of aggregates aggregates with respect to their geo-chemistry and discusses
such as strength, durability, and volume stability are the key the relevance pertaining to engineering properties, chemical,
factors for assessing the performance of concrete. physical and thermal constancies.

3. AGGREGATES IN CONCRETE 4. MINERALOGY OF AGGREGATES

ASTM C125 [14] defines the aggregate as “a granular material Mineralogy of aggregates refers to the study of the
such as sand, gravel, crushed stone, or iron blast furnace slag, geochemistry of minerals present including their physical,
used with a cementing medium to form hydraulic cement structural and textural characterizations [8-10] . The engineering
concrete or mortar”. properties of aggregates are directly related to the mineralogy
of parent rocks. Table 2 provides a compilation of rocks likely
Aggregate plays a significant role in making concrete as an to be used as aggregate in concrete [1, 7-11, 15-18] . This compilation
engineering material. Apart from imparting volumetric stability, includes origin, colour, texture, major and accessory minerals
it also possesses a manageable thermal compatibility with present in the rocks. Table 3 represents the major rock forming
structural steel (which eventually the cement phases failed to minerals, and their properties. This compilation furnishes
[1]
do) . A study by Powers (1971), as cited by Addis, and Owens, information pertaining to the classification of minerals based
2001 [15] reports that the drying shrinkage of concrete can be on geo-chemistry, stable, and nearest compositions, colour,
reduced by order of 10-15 % by incorporating aggregate in their hardness on Mohs’ scale, relative reactivity and weathering
normal proportions. Other significant implications of aggregates potentials. Moreover, the diagnostic and characteristic

THE INDIAN CONCRETE JOURNAL | AUGUST 2022 7