What is 'laitance' and why remove it?

Laitance is a weak, milky layer of cement fines, water and the finest aggregate particles that rises to the surface of fresh concrete. It forms through bleeding — as heavier aggregate settles, mix water carries the lightest cement particles upward — and it is made worse by over-wet mixes, over-troweling, over-vibration and poor finishing. Once the concrete has hardened, that surface layer stays behind as a soft, dusty, porous skin with very little structural strength of its own, only loosely bonded to the sound concrete beneath. That is why it matters so much in repair and waterproofing: if you apply a coating, a resin floor, a tanking slurry or a repair mortar directly over laitance, the new material bonds beautifully to the laitance — and then the laitance peels away from the slab, taking your expensive system with it. It is one of the single most common causes of coating and screed failure. You can often detect laitance by scratching the surface (it powders and dusts), by a change in colour and sheen, or by a simple pull-off test that shears within the surface layer rather than in the substrate. Removing it means mechanical preparation to cut back through the weak skin and expose the sound aggregate beneath: diamond grinding for thin removal on smooth floors, shot blasting (captive abrasive blasting) for larger areas, or scabbling and grit blasting where more aggressive removal is needed. Acid etching is sometimes used on domestic work but is far less reliable and is generally avoided on commercial projects. The goal is a clean, open, sound surface at the right concrete surface profile (CSP) for the system being applied. MPS Concrete Solutions removes laitance by grinding and shot blasting as standard surface preparation before every flooring, coating and waterproofing installation, because the bond to the substrate — not the product itself — is what determines how long the finished system lasts.