Besser block — concrete masonry block — turns up as retaining walls, boundary walls, garden walls and the structure behind a lot of rendered fencing across Sydney.
Left bare it is one of the more difficult surfaces to clean well, for reasons that come down to what it is made of.
Why it stains
Concrete block is highly porous and it is alkaline. Those two together drive most of its problems.
Porosity means it absorbs water, dirt and anything dissolved in them, rather than letting it sit on the surface. Staining goes in rather than on.
Alkalinity means free lime migrates to the surface with moisture and deposits as white bloom. New block walls effloresce heavily for the first year or two, and it is normal rather than a defect.
On top of that, the block face is a relatively weak cement-and-aggregate surface. It does not take abrasion well.
The acid mistake
The intuitive response to white bloom and general grubbiness on masonry is hydrochloric brick cleaner. On besser block that is a bad idea.
Acid attacks the cement matrix of the block itself, not just the deposit. It etches the face, exposes the aggregate unevenly, and leaves a blotchy appearance that is permanent.
It also leaves chloride salts behind in a porous, alkaline material — which feeds further efflorescence and, where there is steel reinforcement in the cores, starts a corrosion problem you cannot see. That is the mechanism behind concrete cancer.
If acid must be used on a stubborn mortar smear, it is heavily diluted, the wall is thoroughly pre-wet so it cannot penetrate, it is tested on an area nobody looks at, and it is neutralised and rinsed completely.
The right method
- Brush off efflorescence dry first. Wetting it redissolves the salts and carries them back into the block — the mechanism.
- Pre-wet the wall with plain water, which fills the pores and stops cleaner being drawn deep in.
- Dilute sodium hypochlorite for organic growth, which is the usual complaint on a shaded garden wall. Dwell, then rinse.
- Mildly alkaline detergent for general grime. Block tolerates alkaline far better than acid.
- Moderate pressure only, with a wide fan, kept moving. Enough to rinse, not enough to cut.
- Rinse thoroughly from the top down.
Pressure is the thing to be careful with. A concentrated jet erodes the block face and the mortar joints, and on an older wall it will rake out jointing that is holding the structure together.
Retaining walls specifically
A besser block retaining wall has soil and water behind it, which changes the picture.
Persistent efflorescence, damp patches or staining in a vertical streak usually means the drainage behind the wall has failed — a blocked agricultural drain, missing weep holes, or no membrane. Cleaning the face treats the symptom.
Weep holes along the base are structural. They must never be targeted with a lance and must never be blocked by garden beds or paving built up against the wall. Same argument as weep holes and subfloor vents.
A wall that is bulging, leaning or cracking in a stepped pattern is a structural matter, not a cleaning one.
Rust streaks
Orange staining running down a block wall usually means corroding reinforcement in the cores, or a corroding fixing.
Cleaning removes the stain and it will return, because the cause is inside the wall. Worth getting looked at rather than cleaned repeatedly — rust stains on concrete and stone covers the removal side.
Sealing or painting
Bare besser block benefits from sealing, because the porosity is the root of most of its problems.
Use a breathable penetrating sealer. A film on a wall with moisture behind it traps that moisture and causes the face to spall — the same argument as everywhere else on masonry, set out in sandstone sealer.
If painting, the wall must be fully cured, dry, and free of efflorescence, and it needs an alkali-resistant primer. Painting a new block wall too early is how you get peeling within a year.
For a professional clean, see concrete cleaning.
