So, the purpose of this blog is becoming increasingly clear. I hope this might provide a forum in which I can share little bits of information regarding questions we get on a daily basis.
Just a few minutes ago I got a phone call from a homeowner who has recently repointed his home. However, he repointed 70% of the work without realizing that as he did not buy a pre-mixed mortar, he needed to add sand to the mix. The subsequent "dried lime" is cracking and not holding up.
Sand is a important component to mortar. It adds body, it increases durability, and it takes up shrinkage. It provides a vital service to the material. It's tough to make mortar without it.
The type of sand that can be used for mortars/plasters/renders in a perfect world should be clean, sharp, and well graded. Clay or silt will absorb water before the lime causing overdosing with water (hence clean). Sharp and well graded sands will reduce the lime content to "bind" the mix together. Most often these sands have a void ratio of about 33%.
How much sand is often the question most often asked. For more workable and creamy mortars (where you need to have clear and crisp joint profiles) two parts of sand to one part of lime is a often a good mix. For general work 2.5 parts sand is often utilized. For thin joints (under 1/4" or so where the exposed surface area is minimalized) one part of fine sand to one part of lime is often prescribed.
For more information check out the main website at http://www.virginialimeworks.com/
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