Salt Crystallisation in Walls: How Salts Destroy Masonry (Causes, Damage & Solutions)

Salt crystallisation is one of the most underestimated causes of damage in masonry. While many people assume moisture is the main culprit, it is often salt that destroys walls from the inside out. This guide explains which salts occur, how crystal pressure develops, why salts keep returning and which repair methods actually work.

1. What Is Salt Crystallisation?

Salt crystallisation is the process in which dissolved salts form crystals when water evaporates. These crystals expand and exert pressure on:

• brick

• mortar

• plaster

This pressure can become so strong that the material literally breaks apart.

Crystal pressure = the force with which salt crystals push masonry apart.

2. Which Salts Occur in Masonry?

The most common salts are:

1. Nitrates

• often from groundwater

• typical in rising damp

• highly hygroscopic (absorb moisture from the air)

2. Sulphates

• present in brick, cement and gypsum

• cause strong crystal pressure

• responsible for crumbling plaster

3. Chlorides

• from road salt, sea spray or groundwater

• very aggressive

• accelerate corrosion of metals

4. Carbonates

• less harmful

• often visible as a white powder (efflorescence)

3. How Do Salts Enter a Wall?

Salts can enter masonry through:

1. Rising damp

Groundwater always contains salts → these are drawn upward.

2. Penetrating damp

Rainwater dissolves salts in the façade.

3. Building materials themselves

Brick, mortar and cement naturally contain salts.

4. Old leaks

Leftover salts remain active for decades.

5. Human sources

• road salt

• urine (old stables, cellars)

• fertilisers

4. Why Are Salts So Damaging?

Salts damage masonry in two main ways:

1. Crystal pressure (the most destructive mechanism)

When water evaporates, salts remain behind. These crystals expand and push:

• plaster off the wall

• brick faces off

• mortar joints open

Crystal pressure can reach hundreds of bars — enough to break stone.

2. Hygroscopic behaviour

Some salts absorb moisture directly from the air. Even when the wall is “dry”, the surface can appear damp.

This leads to:

• persistent damp patches

• mould

• musty odours

• peeling paint

5. How to Recognise Salt Damage

Typical symptoms:

• white powder (efflorescence)

• crumbling plaster

• flaking brick surfaces

• hollow‑sounding plaster

• damp spots that never dry

• salt rings on the wall

• peeling paint

Important distinction:

Efflorescence (salts on the surface) is harmless. Subflorescence (salts inside the wall) is destructive.

6. Why Do Salts Keep Coming Back?

Because:

• salts do not evaporate

• salts do not disappear

• salts sit deep inside the wall

• any new moisture reactivates them

Even if the wall looks dry, salts remain active.

7. How to Solve Salt Problems Effectively

1. Remove the source of moisture

Without this, every treatment fails.

Possible causes:

• rising damp

• leaks

• penetrating damp

• condensation

2. Remove salt‑contaminated plaster

Salts migrate into plaster and stay active.

Therefore:

• remove all affected plaster

• at least 1 metre above visible damage

• sometimes 1.5–2 metres for heavy contamination

3. Use a salt‑resistant renovation plaster

These plasters:

• are vapour‑open

• have large pores

• trap salts without cracking

Important: Renovation plaster does not remove salts — it manages them.

4. Restore the damp‑proof course (for rising damp)

Options include:

• chemical injection

• mechanical DPC

• improving drainage

5. Avoid vapour‑tight finishes

Do NOT use:

• latex paint

• cement render

• tiles on salt‑loaded walls

These trap moisture → more crystal pressure.

8. Common Mistakes in Treating Salt Damage

• only painting → salts push the paint off

• only replastering → salts return

• only injecting → salts remain active

• using vapour‑tight paint → wall cannot dry

• no salt analysis → wrong diagnosis

9. Summary

Salt crystallisation is one of the most destructive processes in masonry. It occurs when salts form crystals and expand, pushing plaster, brick and mortar apart.

Key insights:

• salts come from groundwater, rainwater, materials or old leaks

• crystal pressure can literally break walls

• hygroscopic salts keep walls damp

• efflorescence is harmless, subflorescence is dangerous

• moisture source must always be fixed first

• renovation plasters manage salts but do not remove them