The essential criterion for mixing water is that the water should be clean, as any impurities will affect the strength of the finished mortar or result in staining. The term ‘drinkable’ (or ‘potable’) is often used to define the quality of clean and uncontaminated water fit for use in mortar.
Mortar sets as a result of the hydration process that takes place between the water and cement, which makes the cement set hard and binds the sand particles together. Only a small amount of water is actually required for the hydration process to take place, but enough water must be added to make the mortar workable. This will always exceed the amount needed for hydration of cement. In essence, water is added until the mix ‘feels right’ and meets the workability requirements of the bricklayer.
In reality, a typical cement-lime-sand mix of 1:1:6 will require approximately 7 litres of water for every 5kg of
cement. A vastly excessive amount of water produces an overly workable mix, can lighten the colour of the finished mortar and result in a weakened mortar mix.
Due to the amount of water that is present, mixed mortar will be usable for a few hours, depending on weather conditions. When it does start to ‘go off’ it can be ‘knocked up’ by adding more water, but this is not good practice as the mortar will have begun to set by hydration and will already have attained some of its initial strength. Re-used mortar and re-diluted cement paste will not achieve the original strength capability when they finally set.
Typical Cement-Lime-Sand Mortar Mixes
Engineering Bricks and Bricks and Lightweight
bricks and blocks, average blocks, average blocks in
dense concrete strength. Below strength. Above internal
blocks DPC DPC walls
1:%:3 1:1:6 1:2:8 1:3:10