How do you remove stains from concrete?

[#1]

To effectively remove staining from concrete, you'll first need to determine what's caused it and then treat it with an appropriate product. If it's a rust stain, use CLR Calcium Lime Rust Remover or for grease stains, use CLR Oil And Grease Remover. As a general-purpose cleaner, Chemtech 5L Concrete And Driveway Cleaner works amazingly well for most stains. - MitchellMc

I use and have been using oxalic acid for years to remove stains from concrete and timber. It is sold for use as a wood bleach, 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. I prefer the crystal form as against the liquid form as I can then mix it as strong as I like. Here's a link - https://www.bunnings.com.au/diggers-2kg-rust-stain-cleaner_p0960276 - Stephen-H

If memory serves me well, CLR or CLR Oil and Stain Remover will do the job well, just spray a tiny corner of the stain if you like to test it. But for my workshop it removed old oil stains, etc, excellently. Personally, I'm not a fan of high pressure water blasting concrete floors as it tends to damage the fine top surface over time. - Johnnynl

Many, many years ago when I was 17 (around 1980) I asked my father how we get car oil stains out of the driveway as my car leaked oil a few times. He said don't worry, go buy a 2 litre Coca-Cola and take a sip so you know it’s the real stuff and give it to me and I'll do my magic. He poured it over the patch of oil, waited 20 mins, scrubbed it with a hard brush, not a metal type one and hosed it off and all was cleaned. - icemanx

Easiest option. Degreaser just scrub it with a hard broom and hose off. - SmokeHound

In my situation, the concrete was a carport floor, already pretty rough and cracked! My gardener turned up with a high pressure water unit. Green staining came right off back to clean concrete again. - 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 Nappysan or ammonia 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.) If you are dealing with oil stains, you wouldn't want to use chlorine as it might react. You might want to look at investing in a pressure washer if it's a job you need to do often or you have other uses for one. They are very handy and use a surprisingly small amount of water. - MartyH

[Ready2go]

in the last year, I've became a total convert to CLR and CLR oil and grease remover. both products work as well as they advertise. I had a big oil spill on exposed aggregate concrete that had been there for months. Other concrete cleaners didn't do anything and my Karcher was a waste of time. the CLR did an impressive job.  

for algae then Glitz Concrete cleaner also works really well- spray on and hose off