How it forms
When concrete is placed, heavier aggregate settles and excess mix water migrates upward. This bleed water carries cement fines and very small particles to the top. Troweling, overworking the surface or adding water during finishing concentrates them further. Once the concrete hardens, the result is a thin, smooth skin with a higher water-to-cement ratio than the body of the slab, and therefore lower strength.
Why it matters for coatings
A coating is only as strong as the weakest layer beneath it. On concrete, that layer is often laitance. Typical consequences include:
- delamination in which the coating peels away carrying a thin, powdery film of concrete on its back;
- low and erratic results in a pull-off adhesion test, with failure located just below the coating;
- poor penetration of primers, since the dense skin limits absorption;
- loss of bond under traffic, thermal cycling or moisture, even where the coating itself is sound.
Laitance also hides the true state of the concrete. Contamination, weak zones and curing compounds may sit just under it.
How to recognize it
Field indicators are simple but should be confirmed on a test area:
- the surface is smooth, glossy or dusty and rubs off as a fine powder;
- a scratch with a hard tool leaves a soft, light-coloured trace;
- water beads or absorbs unevenly across the slab;
- a test patch of the specified coating shows weak adhesion before full production starts.
Removal methods
| Method | How it works | Typical use |
|---|---|---|
| Shot blasting | Steel abrasive is thrown at the slab and recovered | Large horizontal areas, opens the surface to a defined profile |
| Diamond grinding | Rotating abrasive heads remove the top layer | Edges, smaller areas, smooth profiles |
| Scarifying | Cutters chip the surface | Thick laitance, coatings or toppings to remove |
| Acid etching | Diluted acid dissolves the paste, followed by rinsing | Limited use, harder to control and to verify |
Mechanical methods are generally preferred because the result can be compared with the profile required by the coating. The target texture is expressed as a concrete surface profile (CSP) under ICRI Guideline No. 310.2R. Relevant references for preparation include SSPC-SP 13/NACE No. 6, ASTM D4259 (abrading concrete), ASTM D4260 (acid etching) and ASTM D4258 (cleaning concrete). The coating manufacturer's data sheet sets the profile and cleanliness required for each product.
Laitance removal is part of every floor and containment project we prepare; see our industrial epoxy flooring service.