When coloured concrete is being compacted and finished, excessive mixing water brings the finer grains of cement and sand to the surface to create laitance - a weak, unsound, low-strength, often dusty, light-coloured whitish ‘skin’ of cement ‘scum’. The material at the surface is lighter in colour than that below the surface - so much so that when the concrete has hardened most lay persons believe the pigment has faded. A surface with laitance is weak, is possibly crumbly and has poor abrasion resistance after the concrete hardens.
A fluid mixture of water, cement, and fine sand that appears at the top of concrete soon after pouring
a layer of weak nondurable material containing cement and fines from aggregates, brought by bleeding water to the top of overwet concrete. Laitance may be detected by scraping the concrete surface with a putty knife; if a quantity of loose powdery material is observed or easily removed, excessive laitance may be considered to be present.