What is sulphate attack, and what does it do to a chimney or render?
A slow chemical reaction between sulphates in brick or soil and cement mortar that makes the mortar expand — and why the classic symptom is a chimney leaning towards the south.
- Difficulty
- intermediate
- Time
- 12 min
- Read
- 3 min
- Safety
- warning
Short answer
Sulphates dissolved in persistently wet masonry react with a component of ordinary cement, forming a new compound that occupies more space than what it replaced. The mortar expands, the joints open, and the wall grows. Because it needs sustained wetness it appears where masonry stays wet: chimney stacks, parapets, retaining walls and render.
Sulphate attack is worth knowing about because its symptoms mimic structural movement and get misdiagnosed as subsidence. It has three requirements: a source of sulphates, ordinary Portland cement mortar, and sustained saturation. Take away any one and it stops, which is why the repair is always as much about keeping water out as about rebuilding.
Safety
Step by step
- Understand the reaction.Soluble sulphates in solution react with the tricalcium aluminate in ordinary Portland cement to form ettringite, which takes up considerably more volume than the material it replaced. The mortar therefore expands from within.
- Know where the sulphates come from.Some clay bricks contain them naturally, which is what the S rating on a brick describes. Flue gases deposit them inside a chimney. Some ground contains them, which is why footings in certain soils are specified in sulphate-resisting cement. Ash and some fill materials contain them too.
- Recognise the chimney symptom.The classic case. Rain drives on one face of a stack while flue condensate wets it from inside. The mortar on the wet face expands more than on the dry face, so the stack grows on one side and leans — traditionally towards the south or south-west, away from the prevailing wet wind. Horizontal cracking through the bed joints comes with it.
- Recognise it in render.Render on a sulphate-bearing background that stays wet shows horizontal cracking following the bed joints behind it, bulging, and eventually debonding in sheets. This is different from thermal or shrinkage cracking, which does not follow the joints.
- Recognise it in a wall.Horizontal cracking through mortar joints, joints that have visibly opened, and a wall that has grown taller or bowed. It looks like structural movement and is not.
- Distinguish it from the alternatives.Wall tie failure also causes horizontal cracking, but at a regular vertical rhythm matching the tie spacing. Foundation movement causes stepped diagonal cracking. Sulphate attack produces expansion, so the wall gets bigger rather than settling.
- Treat the wetness as the primary problem.Nothing happens without sustained saturation. A stack with a sound flaunching, a proper cap or terminal, working flashings and a lining is not saturated and does not suffer sulphate attack. A parapet with a sound coping is in the same position.
- Get a professional assessment for a stack.A leaning or horizontally cracked chimney stack is a structural and safety matter and it is at height. This is a roofer's or a structural engineer's job, not a ladder-and-a-look.
- Rebuild in the right materials.Where the affected masonry is rebuilt, it is rebuilt in sulphate-resisting cement or an appropriate lime mortar, with a flue liner, sound flaunching and correct flashings, so the conditions that caused it are removed as well as the damage.
- Specify for the ground where relevant.For foundations and below-ground work in sulphate-bearing soil, the mix is specified from a soil analysis. This is one of the few domestic situations where sulphate-resisting cement is genuinely called for.
Questions people ask
Why do chimneys lean one way?
The face taking driving rain stays wetter, so its mortar expands more than the sheltered face. Over years that differential growth tilts the stack, characteristically away from the prevailing wet wind — in most of the UK, towards the south or south-west.
Is horizontal cracking always sulphate attack?
No. Horizontal cracking at a regular vertical spacing is more likely to be corroding wall ties. Sulphate attack tends to produce cracking with visible expansion and is concentrated where masonry stays wet.
Can it be stopped?
The reaction stops when the masonry stops being saturated. The expansion that has already happened does not reverse, so the damaged masonry still has to be rebuilt, but stopping the water stops the progression.
Does it affect modern houses?
Much less. Modern bricks are made with controlled soluble salt content, chimneys are lined, and mortars and foundations are specified for the ground. It is largely a problem of older stacks, parapets and renders.