In the building process or refurbishment of a floor, it's common to choose a screed that contains calcium sulfate (anhydrite). These screed types could be in a more traditional semi-dry form (CA type) or self-levelling (CAF).
The advantage of choose anhydrite is the low shrinkage of the product so that fewer fraction joints are needed on the surface. On the other side, they are usually less mechanical resistant than the cementitious counterpart. Moreover, with particular reference to self-levelling screeds (CAF), they often produce the effect of bleeding, that is a segregation of the mix that brings the finer parts on the surface, on top of the screed. The result is a surface even more weak and crumbly.
It is essential to chemically separate anhydrite from cement to avoid gypsum formation and the
swelling of the screed.
The procedure to safely applicate microcement products over calcium sulfate screeds, both CA and CAF type, is the following:
- do a mechanical preparation of the surface by sandpapering, grinding or shot blasting; this operation is crucial to remove the laitance, that is the weak fine part brought on the surface of the screed by bleeding;
- impregnate the surface with a specific primer. If the screed producer does not indicate a primer, Ecobeton products suitable for this application are Bonding Agent or Cerbero Base. Even if you can find a similar situation on gypsum drywall, where a primer is needed before painting, don't trust primer products that are not specific to anhydrite screeds: we must be entirely sure to separate the screed and the overlay chemically;
- proceed with the application of the microcement overlay as indicated on the respective technical datasheet.
Be aware that mechanical resistance is always an issue on screeds, and particularly on anhydrite screeds. Even if the microcement overlay could be very strong as Ercole, it would be laid in a few millimetres. All the loads, both static or impact loads, will be transferred to the underlayment. The minimum recommended compressive strength for the underlayment is 25 or better 30 MPa. You should carefully choose a suitable product on the market. If in doubt, feel free to contact us for a suggestion.