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Calcium Sulfate Screeds

• Introduction
• Comparison with cementitious screeds

Introduction

What are calcium sulfate screeds?

These screed types use calcium sulfate, rather than cement, as the screed binder. Two forms of calcium sulfate are used, both the byproducts of industrial processes: Synthetic anhydrite is a byproduct of the production of hydrofluorocarbons or the desulfurisation of flue gases and requires the addition of accelerators to produce a set; Synthetic alpha hemihydrate is a byproduct of cleaning up power station emissions and requires the addition of retarders to produce a workable product.

Calcium sulfate screeds have been heavily promoted for use with underfloor heating systems as, compared with cementitious screeds, they allow large areas to be laid more quickly, at reduced depth, and with better encapsulation of the heating pipes. They do, however, have significant disadvantages which some consider outweigh the advantages.

Advantages and disadvantages of calcium sulfate screeds

Advantages

• Minimal drying shrinkage with a low risk of cracking or curling. Bonding to the substrate is not normally required.
• Can normally be laid significantly thinner than cementitious screeds.
• More suitable than cementitious screeds for pumped application meaning that large areas can be laid in a shorter time and encapsulation of underfloor heating pipes is improved.

Disadvantages

• Only suitable as a levelling screed for over-laying with the floor finish. Cannot be used as a wearing surface.
• Can rehydrate after curing meaning that they lose strength (temporarily) if allowed to get wet. Not suitable for wet areas, such as showers or kitchens unless the surface is sealed - which is difficult, see below.
• Drying time is significantly longer than for proprietary "quick-drying" cementitious screeds but can be accelerated by operation of the underfloor heating system.
• Require more preparation prior to floor laying than cementititious screeds. Surface DPMs cannot be applied and the screed must be completely dry before the finishes are applied. All laitence must be removed from the screed surface, by sanding or grinding, before the finishes are applied.

Comparison with cementitious screeds

	Quick-drying cementitious screed	Calcium sulfate screed
Products	Isocrete K-Screed Tarmac Truscreed Mapei Topcem and similar	Lafarge Gyvlon Tarmac Truflow Cemex Supaflo and similar
Thickness
Minimum thickness over insulation - commercial	75mm	55mm
Minimum thickness over insulation - domestic	65mm	55mm
Minimum cover over heating pipework	45mm	25mm
Drying times at 200C ambient
55mm thick	-	70 days (see Note below)
60mm thick	18 days	80 days (see Note below)
65mm thick	20 days	90 days (see Note below)
75mm thick	22 days	110 days (see Note below)
Movement control
Maximum bay size without movement joints	Approx 100 sq.m.	30m in either direction (900 sq.m.)
Movement joint aspect ratio	1:3	1:6
Characteristics
Suitable for wet areas, e.g. showers, kitchens	Yes	No
Re-hydration?	No	Screeds can re-hydrate after they have cured, when water is spilled on them for example, and the drying time then restarts from "zero".
Underfloor Heating	Careful compaction of screed is required to eliminate voids around the heating pipework and screed reinforcement.	Pipes are fully encapsulated by flowing screed and heat transfer from pipe to screed is quicker.
Preparation for applied finishes	Surface DPMs can be applied to allow the laying of floor finishes before the screed has fully dried-out.	Screeds require surface sanding or grinding, three to seven days after laying, to remove all laitence. Surface DPMs cannot be applied and moisture content must be checked before finishes are applied.

Note:

Drying times can be reduced significantly, to around 28 days, if the screed is part of an underfloor heating system. This must be carefully controlled and is subject to the approval of the screed and heating system suppliers.

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