The impact of salts onto the building fabric is far more important than the effects of dampness and humidity. Most physical damages related to old buildings - such as crumbling, peeling, detachment of plaster etc. - can be attributed to the effect of crystallizing salts.
Even in professional circles the true effect of salts and the damages they can create is to a large extent unknown or severely underestimated.
Why are Salts a Problem?
- Cumulative effect: they add up over time. The older the building, the saltier it becomes. A 100-year-old Victorian building: some salts. A 400-year-old medieval building, much more salts. Why this matters? – keep reading.
- Hygroscopic effect (wetting): salts are able to trap and pull in humidity from the environment - a property known as hygroscopicity. The bound moisture in salts keeps the building fabric damp. As the building ages and the overall salts content of the building increases, the fabric becomes damper and damper. In their wet phase salt also become mobile, being easily transported by the water flow into other areas of the building fabric.
- Crystallization and expansion (drying): when the weather becomes dryer, salts dry out and as a result they crystallize. During the crystallization phase salts expand about 10X in volume, creating strong intermolecular forces (crystallization pressure), which over time through repeated actions break down the building fabric. The more salts, the more crystallization pressure, the more crumbling and breakdown. The crystallization of salts can occur both on the surface (efflorescence) or in-depth under the surface (subflorescence).
Some additional observations:
- Once salts get into the building fabric, they stay there (often invisibly, hidden). They can be made active by changes in temperature and ambient humidity (wetting & drying cycles).
- It can be observed that in hot climate or during hot summers the damp patches tend to fade (the water evaporates, salts dry out). However during the wet season the problem returns. Over time the problem worsens, the crumbling of the wall fabric increases.
- Different building materials react to salts differently. Lime having larger pores ABSORBS (buffers) most salts, giving no indication that there is an ongoing problem until the lime pores fill up with salts and lime starts breaking down. Powdering of the paint or surface are one of the first signs that the plaster is affected by salts. Cement, having less and much smaller pores, BLOCKS the salts. It will resist them initially, but the increasing quantity of crystallized salts under the cement and growing crystallization pressure eventually cracks it or detaches it, resulting in a cracked or bossed plaster.
- The more salts are present in the building fabric, the effect of renovation tends to last less and less. For a Victorian building with less salts in the fabric renovation can last a decent amount of time. In case of older buildings with more porous bricks and higher salts content, the effect of renovation actions last much less, and such buildings need more frequent or ongoing replastering.
Where Salts Originate From?
Salts can originate from a variety of sources. In small quantities they can be found in the building fabric, as bricks are made of clay which contain a small amount of salts.
- Ground: is the most common source of salts. They are drawn upward into the building fabric by a variety of mechanisms, capillarity just being one of them.
- Air: in the close proximity of the beach, salts can also originate from the air: mist, fog etc.
- Chimneys: the soot in the chimney is very rich in a type of salts called sulphates. These over time permeate the building fabric and the plaster, resulting in hazy damp patches in the chimney breast area, which can appear on all floors.