|Choosing the correct sand when making a mortar is of extreme importance.
Sands should be clean and uncontaminated by clay/silt. These occur in the range from #300 and below and the most effective method to establish their presence is the wet sieve analysis.
Normal dry sieve analysis does not accurately reveal the presence of clay or silt as they do not examine particles passing #200 sieve. Furthermore when the sand is dried before sieving, clay or silt particles can coat some particles and these will not pass the #200 sieve. In contact with water, however, these particles will return to colloidal state, retaining moisture and general instability. The result is a much longer drying period which, if winter is approaching, would not allow the mortar to be sufficiently dried to withstand frost.
The fines in a sand will demand more water. This is due to much higher surface area to be wetted. A high proportion of fines in sand and the consequent high water content in the mortar will have negative effects in compressive and flexural strength. High moisture will promote shrinkage and could lead to de-bonding especially in mortars applied to low suction areas.
There is a tendency to choose sands because of their color. The fact is that the color of a mortar will be given 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 coats. A finer sand can be chosen for the finishing coat based on its color. If, however, the color of the sand is due to clay (earth) presence, as clay is a binder, the quantity of lime will be reduced to avoid producing a binder rich mortar.
Fine sands require more water. A high proportion of these sands lead to longer setting time, possibility of shrinkage, lime leaching and more sensitivity to adverse weather conditions.
Sands are mostly responsible for the void structure of a mortar and, therefore, for its vapor permeability, so vital for the performance against accumulation of condensation. It is for this reason that well graded sands are recommended. If sharp, the void structure will be even more efficient.
Monogranular sands (particle size mainly between 1 or 2 grades) will not allow good vapor exchange, they will also diminish workability and therefore increase the danger of too much water addition in order to achieve it. In making NHL mortars with good sand, workability should not be achieved by adding more water but by allowing a little more time for mixing. It is also advantageous, if time permits, to let the mortar rest for a while: the water will settle between the particles and allow better hydration of the free lime content resulting in a fatter, more homogeneous and workable mortar.
|A practical guide to choosing sands:
In rendering, sharp and well graded sands should be used for all coats.
In all cases NHL binder quantities should be carefully considered and this should be done in relation to the performance required and the quality of the sand.