What is Efflorescence ? why is it a problem efflorescence is a crystalline, salty deposit that can occur on the surfaces of bricks, masonry or concrete. It is generally a white or off-white colour with a powdery appearance. While not a risk structurally, it affects the appearance of a building, particularly if you are painting the surface, or have painted.
Calcium hydroxide, which is formed due to a hydration reaction of cement / cementitious products, this is carried through the capillaries to the surface by moisture upon reaching the surface, it mixes with carbon dioxide from the air to produce which calcium carbonate and atmospheric moisture, which evaporates leaving the salt deposits.
The effect leave the unsightly white powdery salts behind, this process will cause issue with the painted surface.
These salts will basically push any sealant off the surface, causing paint to lift, flake or blister resulting in many cases unhappy customers whom regularly blame the paint for not keying / sticking to the substrate.
With regard to Newly plastered surfaces, freshly laid concrete floor or render we highly recommend a thorough drying period prior to applying any paint which will seal the surface.