Building Materials, Third Edition

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Substances such as organic matters, clay, shale, coal, iron pyrites, etc. which are weak, soft, fine
or may have harmful physical or chemical effects on the aggregates are considered to be
deleterious. They affect the properties of concrete in green as well as in hardened state and are
undesirable. They may be classified as those interfering with the process of hydration, i.e.
organic matters, coatings such as clay, etc. affecting the development of bond between aggregate
and the cement paste, and, unsound particles which are weak or bring about chemical reaction
between aggregate and cement paste. The surface coated impurities in aggregate can be removed
by adequate washing. However, chemically-bonded stable coating which cannot be so removed
may increase shrinkage cracks. The salts present in the sea-shore sand should be washed out
otherwise efflorescence is caused afterwards. Mica, if present in sand, reduces the strength of
concrete. Iron pyrites and sulphides produce surface staining and pop-outs.

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Soundness is defined as the ability of aggregate to resist changes in volume as a result of
changes in physical conditions. The conditions affecting this property are freezing and thawing,
temperature changes, and alternate wetting and drying. Porous and weak aggregates containing
undesirable extraneous matter undergo excessive volume changes under favourable conditions.
The freeze-thaw resistance of aggregate is related to its porosity, absorption, and pore structure.
This may cause local scaling to surface cracking consequently leading to impaired appearance
and sometimes structural failure. Aggregates may also be chemically unstable. Some of the
aggregate with certain chemical constituents react with alkalis in cement which may cause
abnormal expansion and map cracking of concrete.

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The aggregates were considered to be inert material till 1940. Considerable trouble has been
experienced through extensive pop-outs and cracking in a fairly close pattern, of concrete work
probably a year or more after the concrete has been cast. Although this trouble may take a year
to become plainly visible the effects can often be observed in petrographic thin sections of the
concrete within a few months. The phenomenon is accompanied by extensive expansion and
may lead in bad cases to complete disruption and disintegration of the concrete and is known
as alkali- aggregate reaction or sometimes concrete cancer. The trouble is due to reaction between
silica in aggregate and alkalis in the cement. In some cases alkalis, mainly from the cement
supplemented by alkalis in the aggregate, react with carbonates in the aggregate to produce
similar result. The types of rocks which contain reactive constituents include traps, andesites,
rhyolites, siliceous limestone and certain types of sandstones. The reactive components may be
in the form of opals, cherts, chalcedony, volcanic glass (excepting basaltic glasses), zeolites, and
tridmite.

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The precise causes and behaviour of the phenomenon are, however, still a little obscure. A
reactive aggregate, if in finely ground state, will inhibit the action. The dividing line between
aggregate which will cause trouble and that which will not, appears to be retained on and