Masonry Salt and Contaminate investigations with suggested remedial works
Based on studies of historic mortars, the three principal types of salt contamination are sulfates, chlorides, and nitrates. These soluble salts are a primary cause of decay, as they can migrate through the porous mortar and crystallize, exerting pressure that leads to crumbling and detachment.
Sulfates
Sulfates, most commonly appearing as gypsum (calcium sulfate), are a frequent contaminant in historic masonry . They can originate from atmospheric pollution, such as sulfur dioxide reacting with the mortar's surface, or from the materials themselves. When gypsum forms, it can reduce the mortar's porosity and harden the surface, while other sulfates like magnesium sulfate can cause significant crystallization damage .
Chlorides
Chlorides, typically found as halite (sodium chloride), often stem from rising damp that carries salts from the ground into the masonry . They can also be introduced through past restoration products or environmental pollution . Chlorides are highly soluble and mobile, allowing them to penetrate deep into the mortar and cause damage through repeated cycles of crystallization and dissolution as humidity fluctuates .
Nitrates
Nitrates are another common soluble salt found in historic mortars, frequently associated with groundwater and rising damp . Like chlorides, they are transported into the masonry by water. Research indicates that nitrates, along with chlorides, penetrate the render layers and cause mechanical damage by crystallizing in the subsurface, leading to the progressive destruction of decorative surfaces .
M Womersleys test for these three salts using semi-quantitive test srtips and suggest appropriate treatement and remedial action.
