There are two stages to removing cementitious stains from paving...

STAGE 1

Mechanical action - remove as much of the cementitious material as possible by using a hammer and bolster to chisel off the offending spots of material, fondly referred to as 'snots' in the building trade. Take care not to chisel into the paving, just chisel sufficiently to remove any 'snots'.

Scrubbing with a wire brush can remove most of the remaining material without inflicting long-term damage to the paving. The chiseling and scrubbing process may be all that is required to remove the worst of the stain. Natural weathering can eliminate, or render inconspicuous, any remaining slight staining.

STAGE 2

Chemical action - this should only be undertaken if the efforts of Stage 1 have left an undesirable or unsightly stain. Be very careful, chemical cement stain removers sold by builders' merchants contain an acid that reacts with the cementitious material, breaking it down and enabling total removal by repeated chemical treatment and scrubbing with a wire brush. If you are attempting to clean concrete flags or concrete blocks, the acid will also attack the surface of the paver, eating away a thin surface of the unit. The same holds true for natural limestone (as in some tarmac) or marble used for paving. Test the action of the acid on a small, inconspicuous area of the pavement. Some pavers can drastically, and irreversibly, change in appearance when treated with acid. If in doubt, leave it! It may be easier to replace the stained paving, assuming it is constructed from small elements, such as blocks, bricks or flags.

You MUST wear protective gloves, clothing and most importantly, eye protection when using these acid based cleaners. Read the manufacturer's instructions thoroughly. We dilute most acid cleaners to a strength not exceeding 5% acid to prevent serious discoloration or 'burning' of the paving. Some cleaners may already be diluted to this concentration - read the packaging.

Apply the acid cleaner as directed in the manufacturer's instructions, and leave it to work for 5-10 minutes. We use an old paintbrush dipped in the acid cleaner to treat small areas. For larger areas, choose a dull or cloudy day to prevent the area being dried too quickly by the sun and pre-wet the area with water to prevent the acid being absorbed by thirsty paving. Scrub with a wire brush and you should see the cementitious material coming free. Wash down with clean water and allow drying to inspect the results. Repeat if necessary, but no more than is absolutely essential.

Do NOT use acid or acid-based cleaners with imported sandstone