Cement stains - Use of caustic soda for cement stains

[Bona]

I have cement stains on Indian sandstone paving. I know you can't use acid but I'm told that caustic soda will work. Your advice please. If negative, how else?

[Pablo]

how bad are the stains and what kind are they. If you can post a photo then it would be a great help.

[Bona]

Sorry, I don't have a camera. It's a film of grey cement from when the builders cut out the old grouting with a cutting tool. It just looks awful. The builder wants remove it with caustic soda. Will it damage the Indian sandstone flags and will it work?

[Pablo]

cement films can be really hit and miss to clean I'd recommend using hydrocloric acid rather than caustic soda. Make sure the paving is well wetted with a hose then apply the acid with a deck brush. Soup the acid into a bucket and dip the brush into it is the easiest way to do this. Scrub the hell out of it then once you see it stopping to fizz get the hose onto it and wash it off with lots of water. Do a few metres at a time and no more with this method I've never stained or adversely affected the finish of block and sandstone.

[Bona]

These flags have a high iron content and I see elsewhere on this site that it can cause brown staining. Would it be possible to do the job with an industrial jet washer? There must be a mechanical way of doing it rather than risk chemicals. At the moment the flags are a filthy grey colour instead of rich brown.

[London Stone Paving]

Generally you can use acid on Indian sandstone, its very hardy stuff. Using acid on the Indian limestone is the big no no.

What type of Indian Sandstone is it?

I understand you are a bit worried but pablo's advice is pretty good. why not test out on a small patch first?

Lithofin supply a good one called builders clean which we have used lots of times on sandstone with no adverse reaction

[lutonlagerlout]

^^
although using acids isnt nice ,i have never had a problem with indian sandstone and hydrochloric acid
LLL

[dig dug dan]

I know you can't use acid but I'm told that caustic soda will work

i sued acid on some and painted the stains with a paintbrush dipped in the stuff.
It worked a treat and got rid of the problem
Just steer clear of the pointing

[Bona]

What is the reason for wetting the slabs before applying the acid?

[local patios and driveway]

Sand blast?

[Pablo]

The water protects the paving from the acid which can do terrible dammge to it when dry it acts as a temporary sealer and dillutes the acid slighty also.