[Jo Denyer]

Hello,
I have white spots on a large concrete slab terrace. They have only just come after 16 years and are very unsightly. Please can you suggest how I can remove them? Thank you.
Jo

[joypulv]

Concrete goes through subtle changes for the first 20 years. It is about 18% air pockets, and does not stop water getting through. Your white spots are efflorescence (mineral salts and or crystals), possible from ground water below seeping upwards trying to escape and interacting with the concrete, or from rain settling into slightly sunken areas. You will need a strong bristle brush push broom, the scrubbing type. Try just water and a little detergent of any kind first. If that isn't enough, there are many products at your hardware store. Muriatic acid is one, but be careful. All those chemicals can be dangerous.