Understanding salts

Salt damage to internal walls, comes second to mould in the list of concerns expressed by buyers, owners and occupiers of residential property.

Scientific evidence

Yet there is very little scientific literature on salts and the movement of salts in the residential setting. A large number of damp proofers are convinced that salts come from rising damp, but provide no evidence. After Steve Hodgson, PCA CEO, claimed a 2.5M sloping band of nitrates was caused by rising damp, we asked him for scientific evidence, or working on joint scientific study to demonstrate the movement of salts in brick. He refused the study.

Rising damp is the absorption of groundwater (that is a high water table, rather than moisture in the soil). Groundwater is rarely found in contact with the footings of a building, so it follows that rising damp is rarely the root cause of dampness in a wall.

So what causes salts and salt bands on walls?

The salt band on this wall slopes up towards the left-hand corner of this building. The reason for the shape of the band, is the bricks are subject to different exposure to moisture (rainfall and its bounce off the ground), heat, and interstitial condensation (or evaporate – two opposing aspects of absorption). For example rainfall, splash and evaporation are affected by the tree, not by ground water.

The shape of a wall affects how the rain is absorbed into it, evaporates from it and percolates through it. Note the band of salt is rarely at ground level, it is normally about 1M up, whether the wall is in contact with the ground or not. Also note that more water comes from rain rather than ground water.

Calcium Sulphate

The most common salt seen on walls is calcium sulphate, also known as gypsum, the main ingredient in plaster and found in mortar. Salts become mobile when dissolved in water. They move towards and concentrate on the evaporative surface. Salts concentrate in salt bands.

Here is a new wall. Rain is causing salts to dissolve. Moisture is evaporating from the surface. There are two horizontal bands either side of the shelf and a vertical band along the edge of the wall.

Calcium sulphate salts are the white salts often seen on concrete, render or plaster. This concrete floor was flooded.

Hygroscopic salts

hygroscopic substance is one that readily attracts water from its surroundings, through either absorption or adsorption.

Hygroscopic salts cause deliquescence, that is they condense vapour at low relative humidity. For example pure sodium chloride (i.e. table salt), goes lumpy at a relative humidity over 75%RH, it is why rice is traditionally put in salt shakers. Similarly Silica gel is used to keep electronic equipment dry in transport.

By contrast calcium sulphate is not deliquescent, which is why it forms fluffy salts.

Calcium nitrate

Calcium nitrate, is a fertiliser. It is strongly hygroscopic that is it causes condensation or deliquescence a relative humidity above 50%RH.

Calcium nitrate salt forms dark bands. Here the band is about 1.3M above the base to the buildings, but rises above the doorway on the end of terrace house.

Here the pattern of nitrates is both horizontal, and vertical up the chimney breast. It may be the nitrates are coming from manure on the ground and the chimney breast has created its own salt band from those salts, within the chimney through a constantly changing equilibrium between solution and evaporation.

It is generally understood that calcium nitrate salts come from the historic burning of fossil fuel. The scientific literature is less clear. Dark salt bands are most commonly found about 1M above the ground and near or behind chimneys. While carbon or soot is almost always present on bricks around an old fireplace, calcium nitrates are not as common as might be expected.

Can calcium nitrates come from manure?

Hygroscopic salts on inside walls

The damp patches in the above photo are from calcium nitrate salts on and across a chimney breast. The above property is a basement flat, approximately 1M below ground. We used a measuring tape to show the ground level outside.

The solution chosen in the above property, is to make a feature of the nitrates by taking plaster back to the original brick, applying PVA glue, which is transparent when dry, to seal in the salts.

Converted barn in Birmingham

The critical relative humidity of calcium nitrate is 46.7%RH at 30˚C. Hygroscopic salts absorb more vapour the warmer it gets, until the the wall is so ladened with water that gravity causes it appear to weep in the height of summer.

In this case the plaster was removed by a damp proofing contractor and dry lined with a water absorbent dot and dab adhesive. The calcium nitrate dissolved and passed through adhesive onto the new surface.

In normal circumstances the dampness would be evaporating when relative humidity is less than 100%RH. Evaporation causes heat loss, which would show in the infra-red image. However, where the humidity is above the critical relative humidity, moisture is condensing, not evaporating. Infra-red is an aid to identify hygroscopic salts.

Calcium chloride

Calcium chloride is another hygroscopic salt, also found near chimney breasts, but rarer than calcium nitrate, at least in London property. Calcium chloride causes condensation or deliquescence at 33%RH.

Tips for hygroscopic salts

The hygroscopic salts are benign, don’t cause mould, but can cause decorative spoiling, and so would show up during a homebuyers survey. Salt neutraliser may hide the effect, but hygroscopic salts are virtually impossible to remove. There are essentially three effective ways of blocking them.

A1) Method for larger hygroscopic salt affected areas where heat loss is a consideration;

  1. Remove plaster up to at least 400mm from area of dampness.
  2. apply a salt neutraliser, allow to dry,
  3. apply PVA glue, allow to dry,
  4. install thermal insulating plaster board with a vapour barrier, using an acrylic or silicone adhesive.

A2) Substitute plasterboard for thermal insulating plasterboard see https://youtu.be/FITVHTmUmBE

B) Lower cost, less destructive method for larger hygroscopic salt affected areas;

  1. Sand whole section of wall, top to bottom, and at least 400mm from damp area, with coarse grainsandpaper,
  2. apply a salt neutraliser, allow to dry,
  3. apply PVA glue, allow to dry,
  4. cover the wall in plastic backed wall paper, plastic backed lining paper (for painting) or thermal liningpaper such as Sempatap (see directions https://www.youtube.com/watch?v=_R1B44fGttE).

C) method for small hygroscopic salt affected areas;

  1. Sand the wall with coarse grain sandpaper to 300 mm of small damp spots and 400mm of larger,
  2. apply a salt neutraliser, allow to dry,
  3. paint with a primer, allow to dry,
  4. paint with two coats of gloss paint, allow to dry,
  5. sand gloss coat with fine sandpaper and over paint a coat of primer and emulsion paint.

See related post on failed damp proofing treatment with damp proofers slurry.

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