Acid wash exposed aggregate concrete by wetting the slab, applying diluted hydrochloric acid, scrubbing briefly, then rinsing thoroughly and neutralising. Two rules matter more than the method itself. Always add acid to water and never water to acid, because doing it the wrong way can cause the mixture to boil and spit concentrated acid back at you. And never acid wash coloured concrete, because the acid strips pigment and the damage is permanent. Beyond that, acid washing is a straightforward job that removes the cement haze left after the retarder wash-off and sharpens the appearance of the stones before sealing. It is also worth asking whether you need to do it at all, since a pressure wash or a dedicated efflorescence remover solves a good number of the problems people reach for acid to fix.
| Item | Detail |
|---|---|
| Acid | Hydrochloric acid, sold in Australia as brick cleaner, usually around 30 percent |
| Dilution | Start at 1 part acid to 15 or 20 parts water, strengthen only if needed |
| Slab age | 28 days minimum, never earlier |
| Contact time | 2 to 5 minutes maximum, never allow it to dry |
| PPE | Chemical resistant gloves, sealed eye protection, boots, long sleeves, respirator |
| Neutralise | Bicarbonate of soda or garden lime in water, then rinse |
| Never use on | Coloured concrete, sealed concrete, slabs under 28 days |
First, Decide Whether You Need To
Acid is the strongest option available and it is not always the right one, so it is worth ruling out the gentler fixes before you buy any. Plenty of slabs get acid washed unnecessarily.
Acid washing is genuinely useful for removing the thin cement haze that remains after the retarder has been washed off, which dulls the stones and makes a new slab look flat rather than crisp. It also clears stubborn efflorescence and some mineral staining. What it will not fix is oil, tyre rubber, organic staining or general grime, all of which respond better to a pressure wash and an appropriate cleaner. If your slab is simply dirty, wash it. If it has a white bloom, try a purpose-made efflorescence remover first, since these are milder and often sufficient. Reach for acid when the surface has a dull cement film that other methods have not shifted.
What You Need
Gather everything before you start, because once acid is on the slab you cannot stop to go looking for a hose fitting. The clock matters.
You will need hydrochloric acid, sold in Australian hardware stores as brick cleaner or concrete cleaner, along with a plastic watering can or acid-resistant sprayer, a plastic bucket, a stiff-bristled broom, and a hose with good pressure and flow already connected and tested. For neutralising, have bicarbonate of soda or garden lime and a second bucket ready. For protection you need chemical-resistant gloves, sealed eye protection or a face shield, rubber boots, long sleeves and long trousers, and a respirator if working in any enclosed area. Add plastic sheeting for adjacent surfaces and plenty of water for wetting down plants. Never use metal buckets, watering cans or tools, since acid attacks metal.
Safety Comes First
This is a hazardous chemical and it deserves respect rather than fear. Following a few rules makes it a manageable job.
Always add acid to water, never water to acid. Pouring water into concentrated acid generates heat rapidly and can cause the mixture to spit back at you. Fill your bucket with water first, then add acid slowly. Work outdoors with good airflow, since the fumes are irritating and unpleasant in still conditions or enclosed spaces such as a carport. Keep children and pets well away for the duration and until the final rinse is complete. Never mix acid with bleach, chlorine or any other cleaning product, because the reaction can produce toxic gas. Have clean running water available for immediate flushing if acid contacts skin or eyes, and read the product’s safety data sheet before you open it.
The Step by Step Method
Work in manageable sections rather than trying to do the whole slab at once, and keep moving. Two to three square metres at a time is a sensible working area.
Start by clearing the slab completely and sweeping it. Protect anything nearby, covering metal fixtures, downpipes and garage doors with plastic, and thoroughly soaking surrounding garden beds and plants with clean water so any splashes are diluted on contact. Next, wet the entire slab with water and keep it wet, because this is the step people skip and it matters most. A dry slab absorbs acid deeply and etches unevenly, while a saturated one lets the acid work on the surface film only. Mix your solution in a plastic bucket, water first then acid, starting weak at around 1 part acid to 20 parts water. Apply to your first section with a plastic watering can, working evenly. Agitate with a stiff broom for two to five minutes. You will see fizzing, which is the acid reacting with the cement. Do not let the solution dry on the surface at any point. Rinse that section thoroughly with plenty of water before moving on, and only strengthen the mix if a test section shows the weak solution is not working.
Neutralise and Rinse Properly
Rinsing alone is not enough, because acid residue left in the pores continues to work slowly and interferes with sealing later. Neutralising is a short step that prevents a long problem.
Mix bicarbonate of soda or garden lime into a bucket of water, roughly a cup to ten litres, and apply it across the whole slab once the acid work is finished. It will fizz where acid remains. Broom it around, leave it a few minutes, then rinse the entire slab thoroughly with clean water, working from the highest point downward so you are not pushing residue back over cleaned areas. Rinse for longer than feels necessary. Any acid left in the surface will cause a sealer to fail, and residual alkalinity from over-application of neutraliser is far less of a problem than residual acid.
When Not to Acid Wash
There are situations where acid washing will cause damage rather than improvement, and they are worth checking against before you buy anything. All of these are avoidable mistakes.
Never acid wash coloured concrete. Acid attacks the cement paste that holds the oxide pigment, bleaching and blotching the colour permanently with no way back short of resurfacing, and our post on coloured ready mix concrete explains why the pigment sits where it does. Never acid wash a sealed slab, since the acid attacks the sealer unevenly and leaves a patchy mess, so strip the sealer first if you need to treat the concrete beneath. Never acid wash concrete under 28 days old, because young concrete is still curing and acid will etch and weaken the surface. Be cautious on slabs adjoining sandstone, limestone, natural stone or unsealed pavers, since acid damages these readily.
Runoff Is Your Responsibility
Where the wash water goes is a legitimate legal and environmental question, not an afterthought, and it is the part most DIY guides ignore entirely.
Acidic runoff harms plants, soil life and aquatic ecosystems, and letting it enter a stormwater drain means putting it into a creek or waterway, which is an offence. Neutralise before the final rinse so what leaves the slab is close to neutral rather than acidic. Direct runoff onto lawn or a gravel area where it can disperse and be absorbed rather than toward a drain, and dilute heavily with clean water as you go. Where a slab drains directly to stormwater with no alternative, dam the outlet with sand or a sandbag, collect the wash water, neutralise it in a container until it stops fizzing, and dispose of it appropriately. If you are on tank water, keep runoff well away from the tank and disconnect downpipes before you start.
Let It Dry, Then Seal
Acid washing is nearly always a preparation step rather than an end in itself, and the finish depends on what you do next. Do not rush this part.
Let the slab dry completely before sealing, which takes several days in good weather and longer in cool or damp conditions, because sealing over trapped moisture produces a cloudy bloom under the coating. Check the surface is fully dry rather than just dry-looking, since the textured surface of exposed aggregate holds water in the gaps between stones. Once dry, seal it. A freshly acid washed slab is open, clean and porous, which is exactly the condition a sealer wants, and sealing locks in the improvement you have just made. Our guide on why sealing your concrete is essential covers product choice, and you can see our concrete sealers range. If any of this sounds beyond what you want to take on, a concreter or cleaning contractor will do it in a morning, and given acid is involved that is a reasonable decision rather than a defeat.
