These two get quoted against each other and they are not alternatives. One removes material sitting on the slab. The other removes the top of the slab itself.
Paying for grinding when you needed a clean is expensive and irreversible. Paying for a clean when the problem is in the concrete means paying twice.
The distinction
Pressure cleaning and chemical cleaning remove contaminants from the surface and from the upper pores — dirt, algae, moss, oil, tyre rubber, leaf tannin. The concrete itself is unchanged. Done properly, the slab looks like itself again.
Grinding uses diamond tooling to abrade away the surface layer of the concrete, typically a fraction of a millimetre to a few millimetres. Whatever was in that layer goes with it. The slab afterwards is a different surface — flatter, opened up, with the aggregate profile altered.
So the question is simply: is the problem on the concrete, or is it the concrete?
When cleaning is the answer
Most of the time, in our experience.
- Algae, moss, lichen and general greening
- Dirt, dust and traffic film
- Leaf and tannin staining
- Most oil stains, with the right degreaser and dwell time — see removing oil stains from a driveway
- Rust staining, which responds to chemistry rather than abrasion — covered here
- Efflorescence
- Surface mould on shaded paths
If the slab is structurally sound and the complaint is appearance, start here. It costs a fraction of grinding and it preserves the surface finish you have.
When grinding is the answer
- Coating removal. Failed epoxy, old paint, adhesive residue from removed tiles or carpet. Chemicals struggle with two-pack coatings — see getting paint off concrete.
- Levelling. Trip hazards at slab joints, high spots, lipping between pours.
- Surface preparation for a new coating. Epoxy and polyurethane systems specify a mechanically profiled surface. A wash does not produce one — see prepping a garage floor for epoxy.
- Laitance removal. The weak, dusty cement-rich layer on the surface of a poorly finished slab. It has to come off before anything is applied over it.
- Deeply penetrated staining that chemistry cannot reach.
- A polished concrete finish, where grinding is the finish, not the preparation.
The thing to understand about grinding outdoors
It changes the appearance permanently and it is not reversible.
On exposed aggregate, grinding cuts the tops off the stones. You lose the profile that made it exposed aggregate and end up with a flat, honed surface. Some people prefer the result. Many are surprised by it.
On coloured or oxide concrete, the colour is usually strongest at the surface. Grinding removes it, and the slab comes out lighter and patchier.
On stencilled or stamped concrete, the pattern is in the surface. Grinding removes the pattern.
And on any slab, grinding one area leaves it visibly different from the rest. If you grind, you generally grind the whole thing.
This is the same argument as pressure-washing sandstone at too high a setting — once the surface is gone, it is gone.
The middle ground
Two options sit between the two.
Soda blasting uses sodium bicarbonate media at pressure. More aggressive than washing, far gentler on the substrate than grinding, and the media is water soluble. Good for coating removal on a surface you want to preserve.
Resurfacing puts a new cementitious overlay on top rather than removing anything. The right answer when the slab is sound but cosmetically beyond help — see driveway resurfacing vs cleaning and sealing.
Dust and silica
Worth knowing if anyone is quoting you for dry grinding.
Grinding concrete generates respirable crystalline silica, and the workplace exposure standard in Australia was halved in 2020. Legitimate operators use either wet grinding or dust extraction rated for the job, with HEPA filtration.
A contractor dry grinding a slab with no extraction is creating a health hazard for themselves and for anyone nearby, and it is a reasonable signal about the rest of their standards.
The sequence if you need both
Clean first, then assess, then decide.
A surprising number of slabs that look like they need grinding look fine after a proper clean. Spending a few hundred dollars to find out before spending several thousand is the sensible order, and it is what we would suggest to anyone unsure.
See concrete cleaning. If after that the honest answer is that the slab needs grinding or resurfacing, we will say so.
