‘Bulking’ of Sand

Using sand that is not dry not only has implications for the water content of mortar mixes, which must be allowed for when mixing, but may also cause ‘bulking’, which is more serious.

Dry sand and saturated sand have about the same volume but a sample of damp sand will show a marked increase in volume. In damp sand, a film of water forms around each grain of sand, pushing it away from its neighbouring grains. This is known as ‘bulking’ and the volumetric increase can be as great as 30 per cent. When sand becomes saturated, the sheer volume of excess water present causes the surface tension round each grain to break down, preventing the bulking effect.

The use of damp sand can have an adverse effect on mortar mixes if they are being gauged by volume, because the mix will contain proportionately less sand in relation to cement than seems to be the case, resulting in a stronger mix than desired. Accordingly, it is best to use dry sand for mixing mortar.