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.
- 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.