Cement-Based vs Calcium Sulphate Liquid Screed: The Chemistry

Cementitious liquid screed uses a modified cement binder with plasticisers and often synthetic fibres, giving it good compressive and flexural performance and reasonable tolerance of residual dampness in the substrate. It's frequently specified in commercial builds, on grade slabs and anywhere splashing or occasional wetting is likely.

Calcium sulphate screed uses anhydrite or hemihydrate binders which react with water in a way that leaves minimal internal stress, so cracking and curling risk is generally lower across large bay sizes. It's a common choice for domestic underfloor heating installations because of its flow characteristics and thermal contact with pipework.

Because the chemistry differs, drying times, moisture testing (RH or CM) and priming requirements are not interchangeable between the two systems — a drying schedule written for one won't apply to the other. Your screeder should confirm which binder is specified and set expectations early rather than mid-project.

Where a project has mixed requirements — say, a wet room needing CT screed and adjoining living space on CA screed — junction detailing and movement joints need particular attention. We can talk through which binder suits each zone of your build.

Cementitious mixes commonly include superplasticisers to achieve a flowable consistency without excess mixing water, since too much water weakens the cured screed and slows drying. Retarders or accelerators may also be used depending on ambient temperature and pump run length on larger pours.

Calcium sulphate mixes are formulated by the manufacturer as a proprietary package, generally supplied ready-mixed from a dedicated screed lorry, with less scope for on-site adjustment than a traditional cement mix. This consistency is part of why CA screeds achieve such reliable flatness across a floor.

CA screed's laitance layer is a natural by-product of the calcium sulphate reaction and needs mechanical sanding to expose a sound, receptive surface before tiling, adhesive fixing or resin coatings are applied — skipping this step is a common cause of adhesion failure.

CT screed can usually accept a wider range of finishes with less mandatory preparation, though moisture testing before covering remains essential for both binder types, since trapping residual moisture beneath an impermeable floor covering is a leading cause of later flooring failures.