What is actually on the slab
“Dirty concrete” is four different problems wearing the same grey. Identify which you have before you decide anything else, because the four want different answers and three of them are not solved by pressure.
| Soil | Looks like | Answer |
|---|---|---|
| Biological growth | Green or black, worse in shade and under trees | Dilute hypochlorite; kill it, do not just remove it |
| Petroleum | Dark patches, sharp edges, often under a parking spot | Alkaline degreaser and dwell; heat if you have it |
| Mineral | White bloom or rust-orange runs | Efflorescence remover or an oxalic product |
| Embedded grime | Uniform grey, no pattern | Mechanical — the surface cleaner |
Matching chemistry to soil
The reason experienced operators finish faster is not a bigger machine. It is that they put the right product down first and let it work while they set up. Fifteen minutes of dwell removes more than fifteen minutes of extra passes, and it does not wear the slab.
- Alkaline degreaser for anything petroleum. Apply to dry concrete, let it dwell, agitate if it is heavy.
- Sodium hypochlorite, dilute, for organic growth — roughly 0.75% at the surface for a post-treat, stronger only where growth is thick.
- Efflorescence remover for the white bloom, which is salts migrating out of the concrete and will return if the moisture path is not addressed.
- Oxalic acid for rust. Small quantities, rinsed thoroughly, never near planting.
The method, in order
- Sweep. Loose grit becomes shot under a disc.
- Wet the surroundings. Beds, lawn edge, anything that will meet runoff.
- Pre-treat the specific stains. Degreaser on petroleum, hypochlorite on growth.
- Dwell. Set up the machine while it works.
- Surface-clean in one direction, overlapping a quarter of the disc.
- Edge and detail with a wand and a 25-degree tip, same direction.
- Post-treat to kill what is left.
- Rinse the surroundings again.
Direction is a finish decision
Acid, and when not to
Muriatic acid gets recommended for concrete online constantly, and it is almost always the wrong call on a maintenance clean. It etches the surface — that is what it is for — leaving a slab that is brighter today and holds dirt far more readily afterwards, because you have opened the pores.
There are legitimate uses: preparing a slab for a coating, dealing with heavy mineral deposit that nothing else touches. On a customer’s driveway, on a clean-and-go job, it is a shortcut that costs the surface.
Surface damage risk
PSI & nozzle quick reference
- 0 degRed
- Spot jet. Etches most surfaces.4000 PSI
- 15 degYellow
- Stripping bare concrete.4000 PSI
- 25 degGreen
- General concrete and walkways.3500 PSI
- 40 degWhite
- Siding, wood, vehicles.1500 PSI
- ~65 degBlack
- Detergent draw, low pressure.500 PSI
Keeping it clean
A slab cleaned in March and green again in June was not cleaned properly; the growth was removed and not killed. The post-treat is the whole difference, and it costs five minutes and a few dollars of chemical.
Beyond that, the honest advice to a customer is about water: a downspout discharging onto the drive, a hedge that never lets the north side dry, a slope that puddles. Those are what feed the growth. Fixing the downspout does more for the concrete than any sealer.
Sealing concrete is a separate trade decision