Lime Mortar Aggregates
and their effect on the performance of a lime mortar
by Bob Bennett

Over 50 years ago, when the building industry started to repair the war torn architecture in Britain, Portland cement was king and the use of lime was fading fast. Because mortar made with Portland cement sets so rapidly, traditional lime mortars could not compete and for most new construction, lime mortars offered few advantages. For historic buildings originally constructed with lime mortars the change to cement proved disastrous. Had it not been for the Society for the Protection of Ancient Buildings and their assorted membership of architects, surveyors and lovers of old buildings, we might have lost much of what we revere today.

Until recently, lime was fairly difficult to obtain, but today there are a number of retail outlets offering lime putty and a range of hydraulic limes, together with good technical advice. So, if your builder says "you can't get that stuff now, mate!" ditch him, as others know better. Both hydraulic and non-hydraulic lime binders are readily available.

Let us now consider what is perhaps the most overlooked and possibly the most important part of a mortar; the aggregate.

For centuries, man has built with materials which were to hand. He did not buy bricks from the continent and hardwoods from the other side of the world. When preparing a mortar the aggregate was usually gathered from close to the site. In the majority of historic buildings the aggregate was not washed, or sieved, or made to conform to a standard administered by a government department: it was used 'as dug'. Where these crude mortars have helped structures to survive for hundreds of years, it must be worthwile finding out just what their component parts were. The use of a similar mortar for repairs may not only be preferable on aesthetic or historical grounds but may well be essential to the continued survival of the structure.