laitance

Laitance is a thin, weak, cementitious film that forms on the surface of freshly placed concrete. The term comes from French laitance, meaning milk, referring to its milky appearance. It is typically a film of cement paste with fine particles that sits on the surface rather than within the bulk.

It forms when water in the concrete bleeds to the surface during curing, transporting cement fines to the top. Excessive water in the mix, finishing while bleed water is present, or inadequate curing can promote laitance. The resulting layer is usually pale or whitish and can be easily rubbed off.

The presence of laitance can impair the bond of subsequent coatings, toppings, or overlays and may seal the surface, reducing adhesion and durability. It can also contribute to premature failure of finishes if not removed, and may affect surface texture and slip resistance.

Removal methods are primarily mechanical. Scabbling, grinding, shot blasting, scarifying, or hydrodemolition can remove laitance and produce an appropriate surface profile for bonding. After removal, the surface should be cleaned and checked for readiness before applying coatings or toppings.

Prevention focuses on proper concrete design and finishing practices. Use an appropriate water-to-cement ratio and well-graded aggregates to minimize bleeding. Allow bleed water to evaporate before finishing, and employ proper curing to reduce laitance formation. If finishing must occur while bleed water is present, a surface retarder or delayed finishing method can help. Regular surface testing for bond readiness is recommended before applying coatings or overlays.