Building Materials, Third Edition

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Gypsum wall plasters gain one-half of their one-month strength in a day. Plaster and sand
mortars of 1:1 proportions may be expected to develop 80 per cent of the neat strength at
corresponding ages, while those of 1:2 proportion generally possess one-half to two-third of
the neat strength.
The gypsum to sand neat plaster in proportion of 1:3 should set in 2 to 32 hours and in 1.5
to 8 hours when mixed with wood fibres. The dry set density of gypsum wall plaster is
850 – 1040 kg/m^3 , and compressive strength of 1:2 gypsum wall plaster is 6 to 15 N/mm^2.
Gypsum wall plasters are divided into following four categories.

q2x —2€
— is 60.5 per cent or more of calcined gypsum (plaster of paris) with
material added to control workability, time of set and cohesiveness.

q2‡2p˜ 2€
— is 60.5 per cent or more of calcined gypsum and, wood fibre 1.0
per cent or more to increase cohesiveness, and the remaining material to control workability
and time of set.

g—
™ 2qis used for finishing coat. It may or may not carry a retardent. Calcined
gypsum may be white or grey.

q2‚ —2ƒ— 2€
—  consists of cementing material, predominantly calcined
gypsum, which has been mixed at the mill with the proper proportions of sand and other
desirable constituents. It is prepared for use simply by adding water. There are two grades of
Gypsum Ready Sanded Plaster, the scratch or first coat, and the ˜ or second coat.
The scratch coat contains 2 sand to 1 cementing material by weight. The browning coat
contains 3 sand to 1 cementing material by weight. The cementing material carries at least 60.5
per cent by weight of calcined gypsum and other ingredients to control set and workability.
Some of the properties of gypsum plasters are given in Table 19.1

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