Adhesive Failure on Anhydrite (Calcium Sulfate) Screed

Laitance-related adhesive failure typically shows a powdery or friable layer visible on the underside of the lifted covering or adhesive, whereas moisture-related failure tends to show damp patches or a change in colour on the screed surface. A moisture reading taken at the time of investigation helps separate the two, since a screed can be dry to the required threshold and still fail if it wasn't prepared correctly.

Checking whether the sanding and priming steps were carried out, and to what specification, against the screed manufacturer's technical guidance is the most direct way to confirm the cause.

The most common presentation is a floor covering, whether tile, vinyl or timber, that fails to key at all in isolated patches shortly after laying, rather than developing gradually as moisture-related debonding often does. Lifting a sample of the failed area usually reveals a fine chalky dust on both the screed surface and the underside of the adhesive, distinct from the damp appearance associated with a moisture problem.

Because laitance forms as a natural part of anhydrite curing rather than as a defect in the screed mix itself, the affected area is often widespread and fairly uniform across the floor, reflecting a preparation step that was missed across the whole job rather than a localised patch of poor workmanship.

A simple adhesion (pull-off) test on a small sample area, alongside a visual check for laitance residue, is usually sufficient to confirm the cause without needing more elaborate equipment. Where a wider claim is being considered, photographing the powdery residue at the time of investigation, alongside a note of the primer product used if known, strengthens the evidential record.

A moisture meter or RH probe reading taken at the same time rules out or confirms whether residual moisture is a contributing factor, since anhydrite screeds that were laitance-affected are sometimes also found to have been tiled or covered slightly early.

The remedy is to remove the failed covering and adhesive, mechanically abrade the screed surface to remove all laitance, vacuum thoroughly, and apply the correct primer before re-laying the floor covering. This work needs to follow the screed manufacturer's specified sequence rather than a generic approach, since calcium sulfate substrates behave differently to cementitious ones.

Where large areas are affected, it's worth checking whether the laitance issue is consistent across the floor, which would point to a preparation step being missed entirely rather than isolated poor workmanship.

Preventing a repeat of this fault means writing laitance removal and priming into the specification as an explicit, checkable step rather than assuming it will be picked up as part of general good practice, and confirming it has been done before adhesive is applied. A simple sign-off sheet noting the date of sanding, vacuuming and priming gives a clear record if a dispute arises later.

Because this step sits at the boundary between the screeding and flooring trades, agreeing at the outset who is responsible for laitance removal, and building it into the programme with enough time before floor covering starts, removes one of the most avoidable causes of adhesive failure on anhydrite screeds.