Efflorescence: the white marks on new block paving

What efflorescence actually is

Concrete block paving contains lime as part of the manufacturing process. When the blocks are damp, dissolved lime is drawn to the surface by evaporation and left behind as a fine white deposit of calcium carbonate once the water evaporates.

It's essentially the same chemistry as limescale, and it's most noticeable on darker coloured blocks, where the pale bloom shows up in strong contrast.

Why it happens most on new paving

New blocks have the highest reserve of soluble lime near the surface, and the laying and jointing process introduces a lot of moisture into the paving. As that moisture works its way out over the following weeks and months, especially through wetter periods, efflorescence is most visible.

It typically fades naturally over the first six to twelve months as the readily available lime near the surface is used up, though it can reappear in patches during any particularly wet spell in that period.

How to tell it apart from an actual fault

Efflorescence sits on the surface as a dry, powdery film that can be brushed or rubbed off, and it typically appears in patches rather than as a uniform stain, often more visible after the paving dries out following rain.

Genuine staining — from oil, rust, or a poor-quality block with inconsistent pigment — doesn't rub off as a dry powder, tends to be more localised to a specific source, and doesn't fade with time the way efflorescence does.

• Pale, dusty bloom that brushes off dry: efflorescence, will fade with time
• Dark, oily patch that won't brush off: a stain, needs specific treatment
• Orange-brown patch: likely rust from metal contact, not efflorescence
• Colour that looks washed out all over, permanently: possibly a poor-quality block batch

What to do about it

In most cases, the honest answer is to leave it — sweeping or light brushing when dry removes the visible deposit, and it naturally reduces over time regardless of treatment.

There are dedicated efflorescence removers on the market, but they need to be used correctly and are rarely necessary. Avoid acid-based patio cleaners not specifically designed for the purpose, as they can etch or discolour the block surface.

When to actually be concerned

If a white bloom is still heavy after a year or more, or is accompanied by other issues like blocks crumbling at the edges, it's worth having it looked at, since that combination can point to a low-quality block product rather than ordinary efflorescence.