What is efflorescence and is it a problem?

Cement-based materials contain soluble salts. Water passing through dissolves them, carries them to the surface, evaporates and leaves the salt behind as a white crystalline bloom. So the deposit is a symptom of water movement, and the interesting question is always where the water is coming from.

Where it appears and what it usually means

• On new brickwork in the first year, which is often just construction moisture drying out and settles down on its own.
• At the base of a wall, which suggests rising damp or ground water against the footing.
• In grout lines on a balcony or a shower, which suggests water sitting in or under the tile bed.
• Under a slab edge or on a retaining wall, which suggests drainage behind or beneath it is not working.

Why cleaning it is not the fix

Removing the deposit changes nothing about the water. It returns, because the mechanism is intact. Repeated efflorescence in the same spot is a reliable indicator of a persistent moisture path, and that is what needs investigating.

There is a more serious version. Where salt crystallises within the material rather than at the surface, it expands and breaks the material apart, which shows as crumbling brick faces or spalling render. That is worth acting on rather than watching.