Mortars and Renders
The Relative Merits of Adding Cement
For many years those specialising in historic building repairs have
known the dangers of using hard, cement-based mortars. But the specialist
world has been split between those who advocated the use of small amounts
of Portland cement as an additive to a lime mortar and those who rejected
all cement additives. New evidence sheds light on the controversy, with
some radical conclusions.
The addition of cement to lime mortars is a widespread, almost traditional practice, but few consider why it is done or the consequences. There is also confusion over the substances and chemistry involved.
Non hydraulic lime hardens by a slow process of carbonation, reacting with atmospheric carbon dioxide over a period of weeks. Hydraulic limes and cements set 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'. The additives include finely crushed brick powder, PFA, HTI, pozzolana, trass or cement (white or OPC). These all contain finely divided and therefore highly reactive silica and/or alumina, which are the constituents necessary to obtain a rapid chemical set by reaction with water. Of these, cement is by far the most widely used in this country, and the cheapest. Typical proportions, commonly in use, are 1:1:6 (cement: non hydraulic lime: aggregate) and 1:2:9.
There are, as one would expect, both advantages and disadvantages in gauging non hydraulic mortars with cement to make them hydraulic.
- it imparts a chemical set which occurs before full shrinkage occurs, thereby reducing the risk of cracking
- layers may be built up more rapidly, without the need to wait a long time for one to set fully before applying the next
- it hardens rapidly, thereby providing protection from rain before carbonation has been completed. This helps to beat the inclement British weather
- being an artificial substance manufactured under closely controlled conditions, it is reliable and predictable in use
- it is available in a choice of colours, useful when it is necessary to match the colour of an existing mortar or render.
- the rapid setting time limits the time available to the user in which to work with the gauged mortar
- some cements contain appreciable amounts of soluble salts, in particular potassium sulphate, which may become a source of salt damage to stonework
- the use of cement tends to lead to the user treating the gauged lime mortar as if it were a fully hydraulic lime or cement. Too much reliance on the initial chemical set leads to neglect of the importance of the longer term carbonation of the non hydraulic component present
- the danger that segregation occurs, whereby the cement separates from the lime as the mortar dries and hardens.
Segregation is a major hazard of gauging lime mortars with cement. As the mortar sets, the cement colloid tends to migrate into the pores of the lime mortar as they form, clogging them and leading to a greatly reduced porosity. If the proportion of cement is high enough, segregation is much less likely to occur, but the resulting mortar will be hard. If the cement proportion is low, the mortar will be less hard, but segregation is more likely to occur. The resulting mortar will be seriously weakened, with a poorly formed pore structure leaving it very susceptible to frost damage and deterioration, even after carbonation of the non hydraulic lime present has taken place.
The Smeaton Project, a research programme commenced by English Heritage indicates that a 1:1:6 mix, containing a 50 per cent cement binder, is unlikely to segregate, while a 1:2:9 mix, containing a 33 per cent cement binder, is almost certainly at risk. Until recently it was considered good practice to gauge lime mortars with as little as 5 per cent cement, just enough to impart a chemical set but not enough to make the mortar appreciably harder. However all of the Smeaton Project test samples containing less than 25 per cent failed.
Given the possible hazards of segregation, an un-gauged lime mortar relying solely on carbonation is likely to be more resilient in the long run than one gauged with a small amount of cement. This will require care in its application and careful nurturing to ensure that it carbonates properly. If a chemical set is required, a safer alternative would be to use an hydraulic lime. In these the hydraulic components are so closely associated with the non hydraulic that segregation does not occur. These tend to be hard and impermeable, but not usually as hard as a 1:1:6 mix. Brick dust is a cheap and highly effective pozzolanic additive, providing a useful alternative to cement.
Cement is not in itself harmful, but insensitive and indiscriminate use of it is. It can be used as a useful pozzolanic additive to non hydraulic mortars, but those specifying and using it should be clear why they are doing so, and what its effects are likely to be. Given that it is now widely accepted that mortar should be weaker and more porous than the material that it is jointing or repairing, it is probably better in most circumstances to rely on a good non hydraulic lime mortar using well-matured lime putty and sharp and well-graded aggregate, applied with care and subsequently well tended to ensure correct carbonation.
- J Ashurst and N Ashurst, Practical Building Conservation Volume 3: Mortars, Plasters and Renders, English Heritage Technical Handbook, Gower Technical Press, Aldershot, 1988
- W Czernin, Cement Chemistry and Physics for Civil Engineers, Bauverlag, Wiesbaden, 1980
- Mortars, Cements and Grouts used in the Conservation of Historic Buildings. ICCROM, Rome, 1982
- B Induni and L Induni, Using Lime, Lydeard St Lawrence, 1990
- JM Tutonico, I McCaig, C Burns, and J Ashurst, The Smeaton Project AC1: Phase 1 Report, English Heritage, London, 1994
HTI powder - High Temperature Insulation, a finely ground fireclay containing reactive silica and alumina.
PFA - Pulverized Fuel Ash, a waste product of coal fired power stations consisting of tiny granules of reactive silica. Effectively an artificial volcanic ash.
Pozzolana - natural volcanic ash from Italy containing reactive silica.
Trass - natural volcanic ash from Germany containing reactive silica.