Company Announcement - The Concrete Institute is often asked how to remove stains from concrete. Bryan Perrie, MD of the Institute, here deals with removing fungal growth, efflorescence, and chewing gum from your concrete driveway.

Fungal growth:
Wet the concrete surface, apply chlorine bleach, and scrub vigorously, before rinsing thoroughly. “A deposit of dead fungal residue will usually be noticeable within a few days. This should then be brushed with a stiff bristle broom and the process repeated if necessary. Alternatively, use a high-pressure water cleaner to remove fungal growth and then broom chlorine bleach over the surface before rinsing thoroughly with clean water,” Perrie advises.

Efflorescence:
Remove excess salt deposits with a stiff bristle broom. If the result is not satisfactory, scrub with clean water and rinse the surface. To remove any remaining deposits, the concrete should be treated by acid cleaning. “However, handle acids with extreme care. Diluted hydrochloric acid, chlorine bleach or chemically based cleaning agents can be hazardous, particularly to your skin, if used incorrectly. Wear suitable protective clothing, especially gloves, at all times and work only in well-ventilated areas,” Perrie cautions. He says to remove efflorescence, only diluted acid should be used: one part hydrochloric acid to 20 parts of water is the recommended dosage. “When diluting the acid, always add the acid to the water, and never the reverse.”

The surface to be cleaned should then be saturated with water, ensuring that the surface is moist, but without any free water, before applying the acid solution. The diluted acid should be allowed to react on the concrete surface for about 15 minutes. The surface should then be thoroughly rinsed and scrubbed with clean water. “Repeat the process at least twice or until all traces of the acid solution have been removed.”

Chewing gum:
If your children have dropped chewing gum on a concrete surface, and car tyres or footprints have smeared it to the concrete surface, you should solidify the gum with ice cubes and scrape off as much as possible. Then apply a poultice (cat litter or a similar inert absorbent material) saturated with methylated spirits. Apply the poultice and allow to dry. “This should turn the residue gum brittle, making it possible to remove it with a stiff bristle brush. Finish the process by washing the affected area with hot soapy water, then rinse with clean water.” Perrie says another method is to scrape off as much of the chewing gum as possible and then remove the rest with a solvent such as amyl acetate.