Building Materials, Third Edition

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IS:10262 has classified the OPC grade-wise from A to F based on 28 day compressive
strength as follows.
Category Strength (MPa)
A 32.5 – 37.5
B 37.5 – 42.5
C 42.5 – 47.5
D 47.5 – 52.5
E 52.5 – 57.5
F 57.5 – 62.5
Accordingly the 33, 43 and 53 grades of cement correspond to categories A, C and E,
respectively. However, most of the 43-grade cements available in the market fall in category D
and that 53-grade cements in category F.

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The three constituents of hydraulic cements are lime, silica and alumina. In addition, most
cements contain small proportions of iron oxide, magnesia, sulphur trioxide and alkalis. There
has been a change in the composition of Portland cement over the years, mainly reflected in the
increase in lime content and in a slight decrease in silica content. An increase in lime content
beyond a certain value makes it difficult to combine completely with other compounds.
Consequently, free lime will exist in the clinker and will result in an unsound cement. An
increase in silica content at the expense of alumina and ferric oxide makes the cement difficult
to fuse and form clinker. The approximate limits of chemical composition in cement are given
in Table 5.1.

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