Before tiling to an anhydrite (or gypsum or calcium sulphate) screed, you must first ensure it is dry. They generally dry at a rate of 1 day per mm of screed thickness for depths up to 40mm, and an additional 2 days per mm for thicknesses thereafter. For example, a 40mm deep screed requires 40 days to dry and a 50mm deep screed requires 60 days. To be sure it is dry, test the screed with a hygrometer meter such as the TRAMEX Hygrohood; readings of 75% RH or lower will mean it’s ready for tiling.
After confirming it is dry, abrade the surface with suitable mechanised equipment to remove any laitance, loose debris and dust, and reveal the aggregate underneath.
The anhydrite screed should then be primed with a single coat of ARDEX P 51 diluted 1:3 with water, applied in one continuous coat using a brush or broom. Once the ARDEX P 51 is dry, tile as normal using an appropriate ARDEX Cement-Based Tile Adhesive.