Calcium Sulphate (Anhydrite)

Calcium Sulphate Anhydrite (CaSO₄) is the anhydrous form of calcium sulphate, occurring naturally as the mineral anhydrite and also produced synthetically as a by-product of chemical manufacturing, including the production of hydrofluoric acid and phosphoric acid. It is chemically distinct from gypsum (CaSO₄·2H₂O) and hemihydrate (CaSO₄·0.5H₂O) in that it contains no chemically bound water, which gives it different reactivity, solubility, and performance characteristics in construction applications.

Natural anhydrite is quarried from sedimentary deposits and processed by crushing and grinding to the required particle size. Synthetic anhydrite is produced at higher temperatures and may exhibit different reactivity profiles depending on its production route. The distinction between natural and synthetic grades matters for end-use specification, as reactivity and setting behaviour can vary significantly between sources.

In cement manufacture, calcium sulphate is interground with Portland cement clinker to regulate setting time. Anhydrite dissolves more slowly than gypsum in the cement pore solution, which influences early hydration kinetics and requires careful dosing. Its use as a set regulator is well established in cement plants where gypsum availability is limited or specific setting time profiles are required.

In the floor screed sector, anhydrite is the primary binder in calcium sulphate self-levelling screeds, widely used across continental Europe. These screeds offer high thermal conductivity and good dimensional stability with low shrinkage on drying, typically applied at 30 to 50 mm over insulation boards.

In cement production, calcium sulphate anhydrite is used as a set retarder, interground with Portland cement clinker to control the hydration of tricalcium aluminate and prevent flash setting. Its slower dissolution rate compared with gypsum makes it suitable for high-temperature grinding mills where gypsum dehydration can cause false set. In the floor screed market, anhydrite is the principal binder in calcium sulphate self-levelling screeds, pump-applied over insulation layers in residential and commercial construction. These screeds are particularly valued in underfloor heating installations for their high thermal conductivity, typically 1.0 to 2.0 W/m·K, which ensures efficient heat transfer from embedded heating elements to the floor surface. Anhydrite screeds also exhibit low drying shrinkage and good dimensional stability, reducing the risk of cracking and curling compared with cement-based alternatives. In speciality construction products including self-levelling compounds and tile adhesives, calcium sulphate anhydrite provides a stable calcium and sulphate source.