ABSTRACT
Mortar is generally the least durable of the concrete-like materials because it contains binders, usually has a relatively high connected-porosity that allows water to percolate through it, and usually has only a modest hardness and abrasion resistance. Water percolating into masonry is always a potential source of damage and, where possible, the structure should be designed to throw falling rain away from façades and to channel absorbed water away, or at least to allow it to escape via weepholes then drip away from the face. The common sulphates found in masonry are the freely soluble sodium, potassium and magnesium salts and calcium sulphate, which is less soluble but will diffuse in persistently wet conditions. Chlorides can have a weakening effect on calcium silicate units but have little effect on mortars, clay units or concrete masonry units. Ightham Moat, a National Trust property, is typical of the problems that might arise when choosing replacements for the complex cocktail of materials present.
