Lime facts:
What is lime?
Lime comes from Limestone (Calcium carbonate) which is an organic sedimentary rock. Limestone is burnt in kilns and becomes Quicklime (Calcium Oxide). Quicklime is then added to water and slaked to produce Lime Putty (non-hydraulic lime) or commonly known as Fatty lime. The longer lime is slaked the better the quality of the putty - a minimum of 3 months is required for the slaking process, any shorter time and the mortar will be green and still releasing water. Should you purchase lime putty, it will have a skim of water on top which you need to drain off before using. Your putty should be reasonably solid and if you don't use all the lime putty from the bucket, just add the lime water back to keep the putty sealed and healthy for the next project.
To make lime mortar, a good quality sand is added and carefully mixed in with the Lime Putty. Sands and aggregates can become complicated, so your lime supplier should have already sourced good quality sand for your ready mixed mortar. Any sand which has been sourced from estuaries must be avoided, as they have too high salt content- these sands will cause a whole host of problems.
All mortars, putty's and lime washes must be kept out of the way of frost!
Why are there different types of Lime Mortar?
There are two main classes of lime used in building today. The first being Non- hydraulic lime (Fatty lime, Lime putty mixed with good quality sand), which dries and sets by carbonation when exposed to air- (absorbing CO2 from the atmosphere).
The second is Hydraulic lime (also mixed with quality sand to make a mortar) which uses water to set. NHL is hydraulic lime and comes in different strengths. Hydraulic lime has a higher element of impurities in the lime, and creates a harder set but also reduces the flexibility and the porosity of the lime mortar. The main aim of NHL is to be water/ moisture resistant. The type of work where it is used would be the construction of viaducts and bridges and exposed rendered or areas requiring pointing or rendering that are subject to severe weather.
There is another product which builders use to plasticise cement with, this is Hydrated Lime. Hydrated Lime is a feeble lime and is best either in cement to plasticise it or on the garden.
However always seek advice if you're not sure about which product is best for your property.
Why use Lime mortar?
Lime mortars (non-hydraulic) are flexible and soft. Properties undergo a certain amount of movement during their life and the flexibility of lime mortar will accommodate small amounts of natural movement, unlike its cement counterpart. Because lime mortar is breathable it also allows moisture/damp to flow in and out. This especially important on cob/stone walls which require an element of moisture to keep them in good condition. Cob keeps its own micro climate and repairs should be sympathetic, using cob and lime mortar. Cement rendered cob walls are not ideal and generally a life span of 15 - 20 years is all that is to be expected. Cement can cause structural problems to cob and stone.
How Green is Lime?
Lime mortar is a very efficient and an environmentally friendly product. It uses Co2 from the atmosphere to carbonate (set), producing 19kg per tonne of CO2, compared to Portland cement which produces 175kg per tonne. Another added bonus with lime mortar is you can recycle and re-use. Old mortars can be crushed down and re-used. (Hard work, but worth it if your conscientious about the environment).
Did you know?
Lime plasters are known as passive fireproof products. The finished plasters release water vapour when exposed to flames, acting to slow down the spread of fire. ( this varies on the thickness of the plaster). Lime is also naturally anti-bacterial and anti-fungal.