Supersulphated Portland Cement (IS: 6909)
Supersulphated Portland Cement (IS: 6909) is manufactured by intergrinding or intimately blending a mixture of granulated blast furnace slag not less than 70 per cent, calcium sulphate and small quantity of 33 grade Portland cement. In this cement tricalcium aluminate which is susceptible to sulphates is limited to less than 3.5 per cent. Sulphate resisting cement may also be produced by the addition of extra iron oxide before firing; this combines with alumina which would otherwise form C3A, instead forming C4AF which is not affected by sulphates. It is used only in places with temperature below 40 o C.
Water resistance of concretes from supersulphate Portland cements is higher than that of common Portland cements because of the absence of free calcium oxide hydrate. In supersulphate Portland cements the latter is bound by slag into calcium hydroaluminates of low solubility and calcium hydrosilicates of low basicity, whereas concretes from Portland cement carry a large amount of free calcium oxide hydrate which may wash out and thus weaken them. Supersulphate Portland cement has satisfactory frost and air resistances, but it is less resistant than concrete from Portland cement due to the fact that hydrosilicates of low basicity show greater tendency to deformation from humidity fluctuations and resist the combined action of water and frost less effectively.
Properties: It has low heat of hydration and is resistant to chemical attacks and in particular to sulphates. Compressive strength should be as follows:
72 ± 1 hour 15 N/mm2
168 ± 2 hours 22 N/mm2
672 ± 4 hours 30 N/mm2
It should have a fineness of 400 m2/kg. The expansion of cement is limited to 5 mm. The initial setting time of the cement should not be less than 30 minutes, and the final setting time should not be more than 600 minutes.
Uses: Supersulphated Portland cement is used for similar purpose as common Portland cement. But owing to its higher water-resisting property, it should be preferred in hydraulic engineering installations and also in constructions intended for service in moist media. RCC pipes in ground water, concrete structures in sulphate bearing soils, sewers carrying industrial effluents, concrete exposed to concentrated sulphates of weak mineral acids are some of the examples of this cement. This cement should not be used in constructions exposed to frequent freezing-and-thawing or moistening-and-drying conditions.
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