When building we use the term ‘sand’ a lot for mortars, but what do we mean by it and is sand what we really need for lime renders and plasters?
The picture above shows some ‘soft sand’ on the right and ‘coarse / sharp sand’ on the left. It is really important that any render is mixed with a coarse / sharp sand. This is often termed ‘Builders’ sand. The angular nature of the aggregate is important, however it is not the only important factor in choosing the best aggregate for the work.
Two other major factors are:
Grade of the mix. It is really, really important that any aggregate is well graded and this means that it has a wide range of particle sizes in it. You need a variety to ensure that it is well bonded with minimal air gaps. Dust is therefore key to a good render mix. Many mixes are just a combination of different uniform sized particles (e.g. 2mm mixed with 1mm and 0.5mm). These mixes do not give the variety needed.
Type of material. Many mixes are sands or crushed quartz. These rocks are not very porous in themselves, they rely on water moving around them to give the porosity needed for a lime render. Using a porous rock, like limestone, as part of the aggregate mix allows water to pass both through and around the aggregate.
We would therefore recommend that you use well graded limestone aggregates for renders.
Note that there are other issues as well like colour of aggregate, amount of pozzolans in the mix etc as this affects the colour of the render and also the amount of ‘free lime’ in the mix. Care is therefore also required on these fronts.