Exposed Aggregate Concrete: How to Clean and Seal It Without Losing the Stone

Why This Surface Needs Its Own Approach

Exposed aggregate is made by washing the cement paste off the top of a fresh pour to reveal the decorative stone underneath. It is common on driveways, pool decks and walkways across Macomb County — the textured, pebbled surface with visible river rock.

Structurally it is different from smooth concrete in one crucial way: the stones are held by a relatively thin matrix of cement paste between and beneath them. On a smooth slab, the surface is a continuous plane. Here it is an interlocking arrangement where each stone depends on the paste around it.

Erode that paste and the stones come out. They do not go back.

The Damage That Cannot Be Undone

Loose pebbles underfoot, or a collection of stones at the bottom of the driveway after rain, mean aggregate loss is already underway. Once a stone releases, it leaves a void, and the surrounding stones lose part of what was holding them. It accelerates.

Three things cause it:

• High-pressure washing. The most avoidable cause. A wand held close can visibly blow stones out.
• Freeze-thaw. Water sits in the voids between stones, freezes, expands, and levers the paste apart. Dozens of freeze-thaw cycles a winter do this relentlessly on unsealed aggregate.
• De-icing chemicals. Salt in wet concrete adds to freeze-thaw scaling, and the Concrete Masonry and Hardscapes Association warns that magnesium chloride and calcium magnesium acetate can degrade concrete chemically. Our ice melt guide covers which products do the least harm.

Unsealed exposed aggregate in Michigan is on a clock, and the texture that makes it attractive is exactly what makes it vulnerable — all those voids hold water.

Cleaning It Correctly

The textured surface traps far more than smooth concrete does — dirt, algae, moss, leaf tannin and organic debris all settle into the voids between stones. It needs cleaning, and it needs the right method.

1. Pre-treat with a cleaning solution and give it dwell time. On aggregate this matters more than anywhere else, because the chemistry has to do work that pressure is not allowed to do.
2. Use a surface cleaner at moderate pressure, never a wand held close. The rotating jets under a shroud distribute energy evenly at a fixed height, which is precisely what prevents localized erosion.
3. Treat efflorescence separately — the white mineral bloom common on aggregate needs a dedicated remover, not more pressure.
4. Rinse thoroughly, flushing debris out of the voids rather than pushing it deeper.

The difference between a surface cleaner and a wand on this material is not subtle. It is the difference between a clean driveway and a driveway with a permanent light patch where someone lingered. Our concrete cleaning crews use one on aggregate.

Sealing: This Surface Needs a Film

For most plain concrete we recommend a penetrating sealer. Exposed aggregate is usually the exception.

A film-forming sealer does three things here that a penetrating sealer does not:

• It helps hold the stones. The film bridges between aggregate and matrix, helping hold stones that are already slightly loose.
• It deepens the stone color noticeably. Dry aggregate looks washed out and gray; sealed aggregate shows the natural color of the river rock. This is the single biggest visual improvement available on the surface.
• It fills and bridges the voids where water would otherwise sit and freeze.

A penetrating silane or siloxane sealer is still a valid choice when an owner wants the natural, unsealed look and a longer reseal interval. It will not deepen the stone color or bind loose stone. Two practical notes on films. Exposed aggregate is thirsty — the texture means considerably more surface area than a smooth slab of the same dimensions, so it absorbs more sealer and coverage rates are lower than the label suggests. And a grit additive is worth considering on pool decks and walkways, since a sealed aggregate surface is less slip-resistant than the bare texture.

Maintenance Schedule

A freshly sealed aggregate drive often looks richer than owners expect, because the stone color that appears was always there under the dry gray surface.

What Sealing Will Not Fix

The limits: sealing helps hold stones that are loose but still seated. It does not replace stones that have already gone, and it will not rebuild eroded matrix. Small, isolated popouts can be patched with a polymer-modified repair compound and matching stones pressed into it, and widespread loss may call for an overlay once the cause is corrected. Matching an older surface exactly is difficult, so repaired areas can still read slightly differently.

That is the argument for doing it while the surface is intact rather than after a few winters of loss.

The Bottom Line

Exposed aggregate is one of the few surfaces where the wrong pressure washing does immediate, visible, permanent damage — you can watch the stones leave. It wants chemistry and a surface cleaner rather than force, and unlike plain concrete it usually benefits from a film-forming sealer that both protects the matrix and reveals the stone color underneath.