It is essential to choose the right sand when making lime mortars

Sands used with lime mortars should be clean and uncontaminated by clay or silt. Particles of these unwanted substances occur in the 0.4mm and under range. The wet sieve analysis is the most effective way to detect their presence.

A normal dry analysis will not accurately reveal the presence of clay or silt as it does not examine particles passing through a 0.075mm sieve. Moreover, when the sand is dried before sieving, clay or silt particles can coat the sand particles. These coated particles will not pass through the sieve and will therefore be selected. When in contact with water during the mortar production phase, these particles will return to a colloidal state, retaining moisture and general instability. The result is a much longer drying period and, if winter is approaching, the mortar will not be sufficiently dried to withstand frost.

The fines in sand require more water. This is because they have a higher surface area that needs to be wetted. A high proportion of fines in sand and the consequent high water content in the mortar will have negative effects on its compressive and flexural strengths. High moisture will make shrinkage more likely and could lead to de-bonding especially in lime mortars that have been applied to low suction areas.

There is a tendency to choose sands because of their colour. The fact is the colour of lime mortars is determined by the fines contained in the sand and therefore fine sands are chosen for a number of jobs where they are not appropriate.

In renders for example, a good, well graded, coarse, sharp sand is needed for the backing coat. A finer sand can be chosen according to colour for the finishing coat. However if the colour of the sand is due to clay (earth) presence, the quantity of lime must be reduced to avoid producing a binder-rich mortar (as clay is also a binder).

Fine sands require more water. A high proportion of these sands in lime mortars will lead to longer setting time, the possibility of shrinkage, lime leaching and greater sensitivity to adverse weather conditions.

It is mainly to sand that lime mortars owe their void structure and therefore vapour permeability, so vital for its performance against the accumulation of condensation. It is for this reason that properly graded sands are important. If sharp, the void structure will be even more effective.

Mono-granular sands (particle size mainly between 1 or 2 grades) do not allow good vapour exchange; they also diminish lime mortar workability, increasing the danger of adding too much water in trying to achieve it. When making NHL mortars with good sand, it’s better to obtain workability by allowing a little more time for mixing rather than by adding more water. If time permits, it is also recommended to let lime mortars rest for a while: the water will settle between the particles, improving the hydration of the free lime content and resulting in a more homogeneous and workable mortar.