Lime training

Recently did a couple of lime training events for Cumbria Green Build. I like to do this sort of thing spontaneously, partly because I seldom get time to prepare properly, but at the last moment I wrote up some handouts.

LIME MORTARS

Lime mortar is a mix of aggregate and lime putty. There are many mixes that are gauged with materials such as cement to give an hydraulic set. This adversely affects the plasticity and porosity and in my opinion should be avoided.

A typical mix will be one part of lime putty and 2.5 to 3.5 parts of aggregate. 1:3 is always a good starting point but you may require a stronger mix (1:2.5) for a sharp sand and weaker (1:3.5) for a soft sand. When mixing, there should be just enough lime to cover each particle of aggregate. If there is any less the mortar looses plasticity, if there is too much the risk of shrinkage is increased. Lime mortars must be mixed thoroughly and when properly mixed they become very plastic and will stick to a trowel even when held upside down. Proper mixing is essential to ensure the optimum vapour exchange. Lime mortars set by carbonation through absorbing carbon dioxide. If mortar dries too quickly it cannot carbonate and becomes friable, however if it remains wet it will not set at all. If it dries too rapidly it should lightly wetted or covered with dampened hessian.

Traditional mortars appear to use any available sand and I have occasionally seen mortars with no obvious aggregate, just fine silt. It is not unusual to find mixes that have large lumps of unslaked lime. In theory these mixes will always fail though they clearly work in the long term.

Avoiding failure in modern lime work is a difficult issue, especially as the specifier may have professional or contractual liabilities. A cement mortar or render can be thrown onto the wall and will usually look good for many years, despite the problems developing underneath. However a lime mix may show dampness or spall and crack in the first winter. This is not failure, it is the result of using a softer and more breathable material, which in the long term is beneficial for the building. The specifier should resist the temptation to design a gauged mix or a complicated application procedure as this may give an inappropriate finish. It is very important to educate the client so they understand the limitations of lime.

If one makes a mix with a sharp sand, with a maximum particle size of about 5mm then this can be used as a general purpose mix for pointing, bedding, render base coat and haired plaster base coat. If the pointing is to be in thin joints of ashlar stone then the particle size should be 2mm max. Hair is used in plaster base coat to reduce cracking when applied onto laths and if cracks do appear then the hair will help retain strength. For a plaster top coat you will need a fine and soft sand, for example silver sand and a strong mix, possibly as much as 1:1.

Natural hydraulic lime (NHL) is becoming more popular in the UK. This is available as a dry powder much like cement and can mixed in any site mixer. It does require 15to 20 minutes to give a thorough mix, this will ensure the optimum vapour exchange. NHL is usually available in three grades of strength, 5, 3.5 and 2. This describes the rate at which set occurs and eventual hardness. NHL 5 is rather hard and should be reserved for structural work only.

Vapour exchange is the rate at which moisture evaporates through the mortar and is measured in g air/m²/hour/mmHg. Soft mixes perform better and hard mixes, like 1:3 OPC, are in effect waterproof.

Matrix Ratio VEx
Lime putty 1:2.5 0.71 approximately
NHL 3.5 1:2.5 0.68
OPC/hydrated lime 1:1:6 0.23
OPC 1:3 0.15 approximately