Removing stains from concrete

[Prof]

What's the best thing to use to remove those green water/mould stains from concrete?

[Jason]

Hi @Prof,

I trust Workshop members will have some great suggestions for you.

In the meantime you might like to check out these previous discussions for ideas:

• How to remove stains from a crushed aggregate driveway

• Remove mould from external sandstone wall

Good luck,

Jason

[MartyH]

What have you tried so far @Prof? Sometimes just a pressure washer is enough. But if you don't have one, I'd start off with something fairly safe like diluted nappy san or amonia with a stiff broom and see how you go. If that's not working, then you could step up to liquid chlorine or pool acid. (Just use caution and protective gear.)

Let us know how you go as its a common problem, although typically it would be people complaining of oil stains, which you wouldn't want to use chlorine for as it might react. 

[Prof]

Thanks Marty,

I haven't tried anything yet as I didn't want to use something that didn't work and then have it react adversely with using a different chemical..

I don't have a pressure washer or have access to one..so it's going to have to be some chemical and a broom..

I think I might try a bleach solution first and see how that goes..I can always wash it off completely if it doesn't do the job..

[MartyH]

No worries @Prof. Let us know how you go. You might want to look at investing in a pressure washer if its a job you need to do often and/or you have other uses for one. They are very handy and use a surprisingly small amount of water. 

[Prof]

I looked at a pressure washer some time ago, but I never followed it up as other expenses got in the way..

When the driveway has dried, I'll try the bleach first and see how it goes..

I'll let you know when I've done it..

[Stephen-H]

Hi @Prof, I use and have been using Oxalic Acid for years to semove stains from concrete and timber. it is sold at some paint stores for use as a wood bleach. It is often sold in small bags. To use oxalic acid, mix one-half cup of the powder in four litres of warm water and mop it on the stain. Let it work for five to 10 minutes, keeping the stain wet, then scrub with a stiff-bristled brush and rinse. You can also get it from bunnings 2kg for $30. I prefer the crystal form as aginst the liquid form as I can then mix it as strong as I like. https://www.bunnings.com.au/diggers-2kg-rust-stain-cleaner_p0960276

[Jason]

Hi @Prof,

Just wondering if you have made any progress removing those stains?

Hope all is well.

Jason

[Prof]

Hi Jason..

I haven't removed the staining yet..My gardner is coming in a couple of weeks time and he says he might be able to remove them with a high pressure water cleaner..

I'll let all know if he's succesful..

[Jason]

Thanks for the update @Prof. I hope you have some luck with the pressure cleaner.

Jason