Building Materials, Third Edition

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strength. When used up to 0.2 per cent by weight of cement rapid setting with reduced 28 day
strength is resulted.

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X2Various kinds of oil are occasionally present in the mixing water. Mineral
oil (petroleum), not mixed with animal or vegetable oils, probably has less effect on development
of strength than other oils. However, mineral oil in concentrations greater than 2 per cent by
weight of cement may reduce the concrete strength by more than 20 per cent. The vegetable oils
have detrimental effect on concrete strength particularly at later ages.

e—X Algae, present in mixing water or on the surface of aggregate either reduces bond by
combining with the cement or reduces strength by entraining a large amount of air in the
concrete.

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Natural ground waters seldom contain more than 20 to 30 ppm of iron. However, acid mine
waters may carry rather large quantities of iron. Iron salts in concentrations up to 40,000 ppm
do not usually affect mortar strengths adversely.

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Sea water may be used if suitable fresh water is not available. The sea water generally contains
3.5 per cent of salts with about 75 per cent of sodium chloride, about 15 per cent of chloride and
sulphate of magnesium. It has been found to reduce the strength of concrete by 10-20 per cent
and slightly accelerate the setting time. Sea water may lead to corrosion of the reinforcement.
It has been found that the factors affecting corrosion are permeability of concrete and lack of
proper cover. If these are ensured and adequate amount of entrained air is there, the problem
of corrosion may be circumvented, or otherwise the reinforcement is likely to be corroded
whether it is pure water or sea water but with a difference in the rate of corrosion. Therefore,
sea water may be recommended for concrete without reinforcement. The chlorides in sea water
may cause efflorescence restricting it to be used in making mortars for plastering. The use of
sea water is not recommended for prestressed concrete because of stress corrosion and the
small diameter wires (if corroded may cause disaster).

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Most waters carrying industrial waste have less than 3,000 ppm of total solids. When such
water is used as mixing water in concrete, the reduction in compressive strength is generally
less than about 10 per cent. Waste waters from paint factories, coke plants, chemical and
galvanizing plants may contain harmful impurities. It is advisable to test any waste water that
contains even few hundred parts per million of unusual solids before using it for mixing
concrete.
One way of using sewage containing large organic matters (say 400 ppm) is to dilute it in a
good disposal system to reduce the concentration to about 20 ppm or less, an amount too low
to have any significant effect on concrete strength.