Effect of Plaster-Water Ratio on Some Properties

Plaster-water ratio (by weight) 100/30

Setting time (min) 1.75
Compression strength (kg/sq.cm) 808
Dry Density (kg/cu metre) 1806

Plaster-water ratio (by weight) 100/40

Setting time (min) 3.25
Compression strength (kg/sq.cm)474
Dry Density (kg/cu metre) 1548

Plaster-water ratio (by weight) 100/50

Setting time (min) 5.25
Compression strength (kg/sq.cm)316
Dry Density (kg/cu metre) 1352

Plaster-water ratio (by weight) 100/60

Setting time (min) 7.24
Compression strength (kg/sq.cm)228
Dry Density (kg/cu metre) 1206

Plaster-water ratio (by weight) 100/70

Setting time (min) 8.25
Compression strength (kg/sq.cm)175
Dry Density (kg/cu metre) 1083

Plaster-water ratio (by weight) 100/80

Setting time (min) 10.50
Compression strength (kg/sq.cm)126
Dry Density (kg/cu metre) 990

Plaster-water ratio (by weight) 100/90

Setting time (min) 12.00
Compression strength (kg/sq.cm)98
Dry Density (kg/cu metre) 908

Plaster-water ratio (by weight) 100/100

Setting time (min) 13.75
Compression strength (kg/sq.cm) 70
Dry Density (kg/cu metre) 867


This table of relationships makes it clear that the less weight of water added to the plaster, the stronger the resulting mould will be. It also is clear that with less water, the setting time is reduced. So some compromise may be needed to be able to pour the mixture before it sets.