Building Materials, Third Edition

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not less than 30 minutes is supposed to be acceptable. The 7 day and 28 day compressive
strengths of the cube/cylinder specimens prepared with impure water should not differ by 10
per cent from that of cubes/cylinders prepared with pure water. The tolerable concentrations
of some of the impurities in water are given in Table 7.1.

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™X2Water containing less than 2000 ppm (parts per million) of dissolved
solids can generally be used satisfactorily for making concrete. A higher concentration affects
certain cements adversely.

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™2ƒ— X2The presence of salts of zinc, manganese, tin, copper and lead considerably
reduce the concrete strength. Sodium phosphate, sodium borate and sodium iodate act as
retarders and cause a marked reduction in the strength of concrete. Sodium sulphide is
detrimental to concrete and even a sulphide content of 100 ppm needs testing. Zinc chloride
retards the set of concrete and the 3-day strength test cannot be performed. The presence of
calcium chloride accelerates setting and hardening of cement. Carbonates of sodium and
potassium cause a rapid setting and may reduce the concrete strength. Bicarbonates may
accelerate or retard the setting of cement.

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X2Water containing acids or alkalis (industrial waste water) is supposed to be
unsuitable for making concrete. Water with pH value between 6-8 should only be used. The
limit of acidity is guided by the requirement that the amount of 0.1 N NaOH required to
neutralise 100 ml sample of water using phenophthalein as indicator should not be more than
1 ml. This acidity is equivalent to 49 ppm of H 2 SO 4 or 36 ppm of HCI. The limit of alkalinity is
guided by the requirement that the amount of 0.1 N HCI required to neutralize 100 ml of
sample should be less than 5 ml. This alkalinity is equivalent to 265 and 420 ppm of carbonates
and bicarbonates, respectively.

ƒ—X2Sugar up to 0.05 per cent by weight of water is harmless. Sugar up to 0.15 per cent by
weight of cement retard the setting time, reduce the early strength and increase the 28 day