Lime acts as a ‘plasticizer’ but there are other chemical additives (in liquid and powder forms) that can be used in the mixing water that do the same job. These include accelerators to make mixes set more quickly (useful in frosty conditions); ‘retarders’ to slow down the setting process, to give more time to work the mortar and give a harder set; and ‘pigments’ or ‘colourizers’, to give a range of colours to mortars.

As admixtures alter the properties and performance of mortar, they must always be used in accordance with manufacturer’s instructions. They must also be gauged very carefully at the mixing stage and from one batch to the next, to ensure that each successive batch contains exactly the same amount as the previous one.

Good quality mortar must:

• contain the correct mix proportions for the strength and durability required;

• be no stronger than the bricks or blocks being bedded in it;

• contain well-graded, dry, clean sand;

• be of consistent mix proportions from one batch to the next to ensure uniform colour and strength;

• contain consistent quantities of admixtures (where applicable) from one batch to the next;

• contain cement from the same source to ensure uniformity of colour;

• contain sand from the same source to ensure uniformity of colour;

• be properly and adequately mixed;

• not contain excessive water but have adequate workability