laitance
The weak top layer of cement and aggregate fines on a concrete surface that stands between a topical coating and the underlying substrate. This laitance layer prevents a bond forming between the topical coating and substrate, whereby it causes delamination of the topical coating as the laitance layer fails first.
laitance: concrete's weak surface skin that kills coatings
Laitance is a thin, weak layer of cement paste and fine particles that rises to the surface of fresh concrete as it cures. It forms naturally during the setting process when water migrates upward through the concrete, carrying with it cement fines and aggregate particles smaller than about 75 micrometers. The result is a chalky, sometimes glossy film, typically 0.25 to 2 millimeters thick, that sits on top of sound concrete underneath.
This layer causes serious problems for anyone applying coatings, sealers, or adhesive materials to concrete. Because laitance is weak and unbonded to the substrate below, topical coatings cannot grip properly. They bond to the laitance instead of the concrete itself, and when the laitance fails under load, moisture, or UV exposure, the coating delaminates and peels away in sheets. Epoxy paints, polyurethane coatings, and self-leveling floor systems are all vulnerable. A coating may appear sound for weeks or months before catastrophic failure.
Removing laitance before coating is non-negotiable in professional work. The standard methods are mechanical: abrasive blasting, grinding, or scarifying the surface until sound concrete is exposed. Some specifications call for removal to a depth of 0.5 to 1 millimeter. Chemical methods exist, such as acid etching or specialized laitance removers, but these are less reliable and carry corrosion and disposal risks. Water jetting alone does not work; it may clean away loose material but leaves laitance intact.
Laitance formation is worst in poorly finished concrete, work done in damp conditions, or slabs that were over-watered during curing. High water-cement ratio concrete produces thicker laitance. Concrete that has been exposed to rain, groundwater seepage, or high humidity before coating is applied will have a pronounced laitance layer. Even air-entrained concrete intended for durability can develop problematic laitance on its top surface.
The term comes from French, where it originally referred to milk-like sediment. In concrete work, the word is now standard across English-speaking trades, though some older texts use skim or cream to describe the same phenomenon. Quality control specifications for concrete substrates always include laitance assessment, often using a scratch test or visual inspection under raking light. Contractors who skip surface preparation in the name of schedule savings typically end up replacing failed coatings within a year.