The history of the conservation of historic buildings could be written around our attitudes towards individual building materials. Stone, of course, takes pride of place as the most popular and revered natural construction component because of its prominence, beauty, antiquity and resilience through the ages. It is one of the most common building materials and it is the most studied: libraries for books and people bear witness to its appeal and to the study of its preservation.
But other building materials have not benefited to the same extent. Lime mortars, plasters and renders, for example, are currently enjoying a great revival in popularity and interest, the focus of attention here has been the binder, the lime, and not the sand and aggregates that have to be included to make up its composition. Mortars for pointing and bidding are very important for their conservation of historic stones. And pointing can have a significant effect not only on the character and appearance of old walls but also upon their welfare and longevity.
Yet, in conservation terms, humble sands and aggregates are very much underplayed in print. The shape, hardness, chemical and mineral composition, colour, porosity, grain size distribution and proportions in a mix can affect not only the workability but also the durability of the mortar, and particularly its flexibility and resistance to salts and frost, as well as the overall structural stability and aesthetic appearance of the wall itself.
The Role, Types & Properties of Building Sands & Aggregates
A fundamental starting point for good practice in masonry conservation and the repair and renewal of plasters and renders must be to get the mortar mixes right in terms of assessment, design, specification, procurement, site preparation and implementation. In addition to creating structural and other problems of physical and chemical incompatibility, the use of incorrect mortar mixes, by contrast, can be visually disturbing to the overall appearance of a building where its aesthetic integrity can be jeopardised if the rule of colour and texture is ignored. Poor choice of materials or their ill-advised blending can lead to rapid early failures and poor value for money.
Characteristics and behaviour of mortars are mostly governed by their constituents in preference to, though not excluding, the way that they are prepared, mixed and applied.