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TECHNICAL PAPER

6. INFLUENCE OF AGGREGATE MINERALOGY aggregate, or mineralogy does not significantly influence the
concrete strength in normal concrete up to 60 MPa . In normal
[1]
ON THE PROPERTIES OF CONCRETE
concrete, a weaker interfacial transition zone may be playing
This section discusses concrete as an artificial rock and how the a significant role in crack propagation, and the fracture will be
performance of the ‘man-made’ rock depends on aggregate intergranular. However, in high strength composite aggregate
mineralogy. itself contributes towards the strength of concrete due to the
existence of a strong ITZ, and the consequent fracture will be
transgranular.
6.1 Strength of concrete
Similar observations can be seen in some natural stones. In the
According to Alexander and Mindess , the mineralogical
[1]
aspects of aggregates that affect concrete strength are surface case of quartz rich sand stone, and quartzite, both look similar
texture, strength, stiffness and toughness. Moreover, the in appearance. However, under uniaxial compression; quartzite
geometric properties (that can be controlled as size, gradation fractures through the quartz grains; whereas, the sand stone
and surface area) directly affect the workability of concrete mixes breaks around the quartz grains [38] . This is due to the effects
in fresh state. These geometric characteristics also affect the of temperature and pressure exposed during the formation of
responses under stresses in the hardened stage attributed from these rocks.
their crack path tortuosity, interface or surface area with binder
and roughness . 6.2 Durability of concrete
[1]
Alexander and Davis [32] conducted studies on different types of Alexander and Mindess (2005) categorized the deterioration
[1]
aggregates with varied w/c ratios and found that the absolute mechanisms related to aggregates in concrete as intrinsic
compressive strengths of aggregates are not a significant and extrinsic. The physical intrinsic mechanisms are related
influencing factor in concrete strength. They tested concrete to dimensional incompatibility due to thermal and moisture
in three groups. The first group consist of andesite, dolerite, effects. The chemical intrinsic mechanisms can be referred to
quartzite, felsite, and siltstone as aggregates. The group 2 alkali aggregate reaction (AAR), deleterious sulphides, and
was with dolomite, and greywacke; whereas, the third phase sulphate attack. The physical extrinsic mechanisms can be
included quartzite, quartzite and granite. Upon freezing all listed as alternate wetting, and drying, abrasion, and freeze/
other material properties, the absolute variations in compressive thaw effects. External chemical attacks in the form of acids,
strength of concrete were ranged from 13 MPa to 17 MPa. From alkalis, or any other aggressive chemicals are categorized under
the study it was concluded that the variations might be resulted extrinsic chemical mechanisms. Apart from these categories,
from the different modes of ITZ (interfacial transition zone) concrete may experience a combination of these mechanisms.
formation with types of aggregates. However, these scenarios For a typical example, concrete in sodium cooled fast breeder
may be different in the case of high strength concrete. A study reactors [39] , it is exposed to temperature above 550ºC with hot
Aitcin [33] cited by Alexander and Mindess (2005) reported liquid sodium and the deterioration mechanisms are triggered
[1]
that aggregate type may affect the strength properties of high by both thermal, and chemical effects.
strength concretes. A study by Sengul et al., [34] reported that
aggregates performed better in normal concrete may not be The metastable, or deleterious mineral phases in aggregates
performing good in high strength concrete. If the aggregates play a significant role in the durability of concrete. They can
are from some specific origin with some typical geochemistry, cause various chemical, or physical alterations resulting in the
it may exhibit a wide range of variability in concrete strengths. deterioration of concrete. Table 5 compiles such deleterious
Qzturan and Cecen, [35] reported similar observations on crushed phases present in the aggregates with their crystalline nature,
basalt, limestone and gravel aggregates. In this case, the alteration phenomenon, and maximum or tolerable limiting
strengths with different types of aggregates varied from 40 to proportions in concrete [1,40] . Apart from this, some common, and
90 MPa. A study by Bush et al. [36] as cited by Alexander and specific durability issues are discussed in detail below.
Mindess (2005) reported high strengths for granite aggregate
[1]
concretes with respect to limestone and rhyolite aggregates. A 6.2.1 Alkali aggregate reactions (AAR)
recent study by Vishalakshi et al. [37] reports fracture energy and Alkali aggregate reaction (AAR) is referred to the reaction
compressive strengths of normal, and high strength concretes between alkalis present in the concrete with the reactive
with granite, anorthosite, charnockite, limestone, and gneiss [41,42]
as aggregates. They reported higher strength for granite components present in the aggregates . It was diagnosed
in the early decades of the 20 century in the USA, and Europe,
th
aggregate concrete, and lower strength for gneiss.
and after the 1970s it became a major research concern
On the whole, the response to unconfined compression worldwide [41] . The major AAR issues reported in concrete can
of concrete may be influenced by mismatch between the be broadly classified into Alkali silica reaction (ASR) and Alkali
elastic properties of aggregates, and binder phase. Type of carbonate reaction (ACR).

20 THE INDIAN CONCRETE JOURNAL | AUGUST 2022