Laitance is the weak, milky or powdery layer of cement dust, lime and sand fines that appear on the surface of concrete. These fines rise to the surface of concrete that was over-watered, or allowed to dry prematurely in the absence of curing membranes or other good curing practices.
Example of Laitance
Laitance is always present on new concrete, therefore preparation is required to ensure good adhesion with any subsequent coating.
Laitance can differ in thickness from barely measurable to extreme cases of 5mm+.