Traditional mortars and renders have incorporated lime. There is now some clarity on the role it plays, and why owners of Victorian and Edwardian houses should insist on its use in restoring walls and avoid the indiscriminate use of cement.
Lime is produced from limestone through these steps:
Non-hydraulic lime (slaked lime) hardens by a slow process of drying and carbonation, reacting with atmospheric carbon dioxide to form calcium carbonate. This takes a period of some weeks.
Hydraulic lime, on the other hand, sets rapidly by reacting with water in a matter of hours.
A non-hydraulic lime can be made to set much more rapidly by the addition of an hydraulic or 'pozzolanic' additive. This practice is known as 'gauging'. Typical additives are finely crushed brick powder or cement. These contain highly reactive silica and/or alumina, which give a rapid chemical set by reaction with water. Of these, cement is by far the most widely used in the UK, and the cheapest. Typical proportions, commonly used, are 1:1:6 (cement:lime:sand) and 1:2:9. Use sharp sand with lime.
Portland cement was developed in 1824; it was used in the role of a hydraulic binder, which was very consistent and made the mix set rapidly. This rapid set was an advantage in cold or very wet conditions. Builders slowly abandoned lime mortar in favour of cement and sand mortars.
By the 1830's, the 1:1:6 mixture of cement, lime and sand ratio had been firmly established. Use soft sand with cement.
However, cement and sand mortars proved too strong for some applications and lacked some of the workability of lime mortar. By the late 1800's cement-lime mixes were again widely used where increased plasticity, workability and controlled strength was required.
Portland cement remained costly into the 20th century so a minimum was used. For example, the main brick mortar was lime and sand, but pointing included a little cement. Again, render generally used some cement.
Experts in the restoration of old buildings argue that mortar should be softer than the bricks they bind. Because most Victorian and Edwardian bricks are soft inside a thin, harder outer shell, they are vulnerable to damage to the surface layer, such as that caused by hard cement pointing resisting movement due to settlement or temperature changes.
In the case of renders and plasters, lime has benefits because of its flexibility and its porosity; it helps moisture that has penetrated a wall to evaporate.
Lime is also less prone to crack than cement. Lime is even self-healing; fine cracks allow carbon dioxide to penetrate. This reacts with free lime to harden and close the crack.
There is an ongoing debate on the use of cement with lime, or lime in a secondary role to cement.
When added to a normal cement and sand mix, lime makes the mortar more cohesive and adhesive, and it makes the mortar 'fatty' and workable so that it spreads well.
When it has hardened, lime allows mortar to achieve optimum strength because it increases the flexibility of the mortar. Lime will give improved bond strength with the brick.
Lime also reduces water penetration by 'autogenous healing'. This is because there will be some free lime in the set mortar. This will re-carbonate over time, sealing any hairline cracks. This is particularly important with renders.
Cement can be used as a 'pozzolanic' additive; you can gauge a non-hydraulic mortar with cement to make it hydraulic.
The advantages are:
The disadvantages are:
The Smeaton Project, a research programme commenced by English Heritage, concluded that a 1:1:6 (cement:lime:sand) mix is unlikely to segregate, while a 1:2:9 mix will almost certainly fail. As the cement proportion is reduced further, the mortar will certainly fail.
The conclusion is that the best option for mortar and pointing is an un-gauged non hydraulic lime mortar using well-matured lime putty and sharp and well-graded aggregate. This does not require special skills. Use non-hydraulic lime and sand in a 1:5 ratio.
The choices here are either a hydraulic lime, or non-hydraulic lime with some cement.
If you are patching render, you must use the same mix to avoid cracking.
There is more detailed guidance here.
Lime Plaster is the ideal finish for walls built using lime mortar; because it is flexible it will tolerate movement in the wall. Again, lime plaster is best painted with lime wash, rather than wallpaper or modern paints. Lime wash or distemper can be applied over lime plaster after a few days but any impermeable coating must not be used for several months as lime plaster needs air before it can harden.
As with other lime products, lime wash is porous and therefore excellent for surfaces where the evaporation of moisture is important.
Limewash is a paint made from lime putty which has been matured for several months and then thinned with water to make limewash. Limewash is naturally white, matt, and slightly chalky but can be coloured with pigments.
Lime wash can be used internally or externally. Because it works by absorbtion into the surface so it is best used on porous surfaces such as sandstone, lime plaster, lime render and soft bricks.
When lime wash is used externally, raw linseed oil or tallow mixed into the lime wash help shed rainwater.
On materials including cement or modern plaster, casein should be added to the lime wash to help it bond.
For companies specialising in this area, see the Products and Services Directory.