Salt analysis is an important diagnostic aid. The presence of chlorides and nitrates in plaster samples can confirm rising damp. The presence of salts can cause crystallisation and interaction with relative humidity and water vapour can lead to the cyclical deliquescence of salt crystallisation back to moisture. Hygroscopic salts are indicative of rising damp and are cited by manufacturers of treatment additives as one of the reasons for removal of plaster and reinstatement with sand and cement render and salt retardants. Salts within the pores of the masonry would still emerge, deliquesce and crystallise in response to RH and WV were it not for the presence of the render and additive.
Alternative views on rendering an internal wall following treatment for rising damp would advocate not applying render. Cement should not be used in repointing external walls. Cement used on internal walls results in locking salts within the wall. An alternative is to draw out the salts from the wall with sacrificial poultices and clays until the salt content is reduced to equilibrium. In heritage buildings this may be a preferable view.
However the need for clients to re occupy their homes in comfort mitigates against such long drawn out treatments often with no guarantee of success.
Here a third alternative can be considered such as removal of the original plaster and installation of cavity drain and membrane tanking. The downside with this is the wall may have to be lined floor to ceiling resulting in covering up a solid substrate into which fixings such as light switches and sockets need to be refitted and picture hooks which can then not support heavier paintings. The potential loss of decorative cornice mouldings to the ceiling above and not least the extra cost.
The ISSE are currently researching various alternatives to these approaches.
Geological maps are useful in pinpointing the presence of ground salts and sulphates.
Salts within masonry materials as a result of the geological formation of donor materials can lead to efflorescence. This can be a factor in blocking of pores through contamination and pollutant deposits which can prevent salt emergence caused by moisture ingress leading to internal damage to the structure of the masonry. Water repellent applications can produce a similar effect.
Cryptofloresence of previously and even long inactive salts in internal masonry pores can suddenly be triggered.
Frequent causes of this include pointing in cement adjoining softer stone or brick, cleaning of masonry with high pressure water and chemicals.
Sulphate attack of cement
Sulphates in the soil or imported via contaminated substrate used as floor foundations have included shale from former mining activities. This is now banned in such use. Often the shale would have caught fire hence known as red shale which is particularly prone to attack of cement.
Sulphates attack cement in mortar or concrete by expansion within the cement resulting in its destabilisation and erosion. This can result in mortar joints simply falling out having been turned to dust and expansion of concrete floors and resultant heave dislodging and weakening walls built off the floor slab. The process can take 20 years or more and is triggered by dampness within the substrate.
A damp proof membrane (DPM) will protect against such deterioration and thus the worst cases occur before widespread use of the DPM in the 1960’s.
Former mining areas are especially prone. Sulphate distribution maps are available to determine the risks of this being present in the soil.
Once sulphate has attacked the cement nothing can reverse it and the affected material has to be removed. This is costly and often when a property changes hands a shale or sulphate test will be undertaken by removing a section through the solid floor to be tested for sulphate.