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TECHNICAL PAPER

Moreover, SEM observations were proven or corroborated reactions . The acid attack in concrete can be controlled either
[1]
by the test method of ASTM C 289 [52] . In this regard, testing by designing an acid resisting system or by introducing acid
methods should be ideally chosen to characterize the soluble phases which neutralizes the acids. Additionally, the
aggregates for AAR. Considering the importance of structure mineralogical compatibility aggregates with binding phases
in which the aggregate intended to be used, the combination is significant. In another word, the resistance to acid attack of
of testing methods shall be employed to identify potentially binding phase and aggregates should be similar. The aggregate
reactive components of aggregates in concrete. Based on selection should be in such a way that; the concrete with acid
specific geochemistry or physical characteristics attributed to soluble binder phase should use acid soluble aggregates like
[1]
their mineralogy, the effectiveness of tests may vary. limestone or dolomite . On the contrary if acid soluble binder
phases are used with acid resistant siliceous aggregates, the
damage may be higher. The extend of acid attack may also
6.2.2 Freeze and thaw effects
be affected by the type of acid interacting and state of the
The freeze, and thaw effects on aggregates are based on their aggregate used in the concrete. Irrespective of mineralogy,
inherent properties such as permeability, pore structure (more strong, and hard looking aggregates with internal cavities may
[1]
importantly, distribution of pores) and absorption capacity. The be devastating under acid attack .
mechanism of damage is similar to cement paste, in which the
pores are filled with water, and freeze at lower temperatures. 6.2.3.2 Alkali attack
Since the volume of ice is greater than water, it exerts expansive Commonly used aggregates with desirable engineering
pressure on to the nearest boundary, and eventually damages properties are considered to be resistant to any sort of
or cracks the concrete [1,2,13] . The freeze -thaw soundness of strong alkali attacks . The problems with regards to reactive
[1]
aggregate may be influenced by the types of minerals present components in aggregates are already discussed in this paper as
in it. For example, the specific structure of micaceous mineral AAR. The performance of concretes in highly corrosive alkaline
chlorite can cause freeze thaw damage in concrete [60] . Chert exposures is governed by the response of binder phases. The
with high porosity, laminated limestone, shale and sandstone mineralogy of aggregate can only indirectly affect the alkali
with specific geochemistry may be destructive in concrete with attack, by influencing the ITZ characteristics attributed to shape
regard to freeze-thaw soundness [1,13] . or texture of parental rock.

Winslow, (1994) [61] observed that the imbibition of water into 6.2.3.3 Chemically corrosive and thermally
the aggregates was greatly influenced by a critical saturation
level. This level in concrete may be influenced by absorption triggered exposures
capacity, and pore size distribution of aggregates, and is usually Industrial applications can pose concrete in any sort of multiple
found to be around 91.7 %. Since, the governing factors are exposures; where, mineralogy of aggregates plays a significant
the permeability and length of flow path, the effect of freeze- role in governing the performances. The sodium cooled fast
thaw is minimal in smaller aggregates (or fine aggregates). If breeder reactor is a typical example. The accidental spillage
the aggregate possesses a relatively larger size, higher porosity, of hot sodium at 550°C simulates such an exposure for
and absorption with pore size distribution ranges 0.1 to 4.0 µm, concrete [21,39,63-68] . The effect will be both chemical, and thermal.
it may attain critical saturation levels more easily, eventually In these types of applications, thermally stable calcareous
causing damages. In freeze thaw soundness of aggregates, the aggregates are used. The use of siliceous aggregates granite,
pore size distribution is more significant than the individual small and river sand is ruled out for this application due to their
or big pore sizes. The range is 0.004 to 0.04 µm, and a maximum relative thermal instability [21,39,64,68] . Limestone is a potential
of 0.10 to 4.0 µm in extreme cases [62] . However, more studies candidate for this application. While studying these complex
may be required to unravel the effect of mineralogy on the reactions, Chawla, and Pedersen [67] observed a formation
freeze and thaw soundness of aggregates in concrete. of denser phases with higher specific gravities in calcareous
aggregates than siliceous aggregate. This layer in concrete
6.2.3 Deterioration in highly corrosive or with calcareous aggregate protects the unreacted portions
from further reactions with hot liquid sodium. Petrography
combined exposures
of limestone aggregate mortars exposed to fast breeder
6.2.3.1 Acid attack reactor environment studied by Haneefa et al. [64] is provided
in Figure 15. The surface erosion [Figure 15 (a)], modified
Type or mineralogy of aggregate play a major role in the aggregate interface (ITZ) with disrupted accessory minerals in
performance of concrete exposed to chemically erosive or acidic limestone [Figure 15 (b)], modification of grain boundaries of
environment like in sewer lines. In general, siliceous aggregates calcite with staining, and etching of mineral interfaces [Figures
are resistant to acids; whereas, calcareous ones are prone to 16 (c and d)], ferric oxidation of the periphery of accessory

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