Fibre Reinforced Facade Encyclopedia: Composition
Alpha gypsum is calcium sulfate hemihydrate produced by calcining gypsum under steam pressure rather than in an open kettle, which yields dense, regular crystals with a low water demand. Mixed at a water to plaster ratio of around 0.25 rather than the 0.7 of ordinary building plaster, it sets to a hard, dense, high strength solid. It is the binder that makes glass fibre reinforced gypsum a moulded architectural material rather than a decorative plaster.
Beta plaster needs roughly three times as much gauging water as alpha gypsum. That water leaves pores behind, which is why beta plaster reaches 10 to 20 MPa in compression and alpha gypsum reaches 40 to 70 MPa.
At a glance
| Property | Typical value | Note |
|---|---|---|
| Composition | Calcium sulfate hemihydrate, CaSO₄·½H₂O | Same chemistry as beta plaster |
| Water to plaster ratio | 0.22 to 0.35 by weight | Beta plaster needs 0.6 to 0.8 |
| Compressive strength, dry | 40 to 70 MPa | Beta plaster reaches 10 to 20 MPa |
| Flexural strength, dry | 10 to 20 MPa | ShapeShell-RG published value 24 MPa as a composite |
| Set density | 1,700 to 1,900 kg/m³ | Beta plaster is 800 to 1,100 kg/m³ |
| Setting time | 15 to 35 minutes | Controlled by retarder and accelerator dose |
| Setting expansion | 0.05 to 0.3% linear | Presses the mix into fine mould detail |
| Chemically bound water | about 21% by weight | Released endothermically when heated |
| Exposure | Interior, dry conditions only | Gypsum is soluble; not a wet area material |
Alpha and beta are the same chemistry, made differently
Natural gypsum is calcium sulfate dihydrate. Calcining drives off three quarters of the chemically combined water and leaves hemihydrate, which recombines with water and recrystallises when gauged. Both alpha and beta hemihydrate are that same compound. The difference is entirely in the crystal produced.
Beta hemihydrate is made by heating gypsum in an open kettle or rotary calciner at atmospheric pressure. The water flashes off as steam, shattering the crystal into irregular, porous, high surface area particles. Those particles need a great deal of water to make a workable mix, and the excess water that is not chemically consumed eventually evaporates and leaves porosity behind. Alpha hemihydrate is made by calcining in an autoclave under steam pressure, usually in the presence of a salt or acid solution, so the water leaves as a liquid and the crystal grows as a dense, well formed prism. Those crystals pack efficiently and need far less gauging water.
Porosity is the whole story. Strength, hardness, density and surface quality in a set gypsum all track inversely with the amount of water used to gauge it.
Why glass fibre reinforced gypsum needs it
Glass fibre reinforced gypsum is cast as a thin shell, typically 3 to 6 mm in the ShapeShell-RG system, with chopped glass fibre distributed through it. A shell that thin has to be strong, stiff and hard enough to be demoulded, trimmed, handled, transported and fixed without damage, and it has to come off the mould with a surface good enough to prime and paint without skim coating. Beta plaster cannot do any of that at 3 to 6 mm; it is soft, porous, chalky at the surface and low in flexural strength.
Alpha gypsum also sets with a small positive expansion, typically 0.05 to 0.3 percent, which presses the mix outward against the mould face. That is what captures fine profile, sharp arrises and crisp detail in a mould, and it is why cast alpha gypsum reproduces ornament and curvature so faithfully. Modern systems contain no cellulose fibre, unlike traditional fibrous plaster, which removes both a moisture path and a source of variability.
Working with it
The mix is gauged by weight, not by eye, because the water to plaster ratio is the property that determines strength. Water is added to the plaster rather than the reverse, allowed to slake briefly, then mixed to a smooth slurry. Retarders extend working time and accelerators shorten it, and both change set expansion, so a production mix is a fixed recipe rather than something adjusted at the bench.
Set is exothermic and rapid. Elements can usually be demoulded within 30 to 60 minutes, which is the reason a gypsum mould turns around many times faster than a cement one and why glass fibre reinforced gypsum suits complex interior geometry produced to programme. Demoulded elements are then dried; a piece that is set but not dry still holds free water and will not reach its published strength until that water has gone. Threaded inserts, fixing lugs and steel frames are cast in during the pour where the geometry needs them.
Moisture and durability
Gypsum is slightly soluble in water and softens as it takes up moisture. Alpha gypsum is denser and far less absorbent than beta plaster, and polymer additions and water repellent admixtures improve it further, but this is a difference of degree. Glass fibre reinforced gypsum is an interior material for dry conditions. Bathrooms, external soffits, plant rooms and anywhere subject to condensation belong to glass fibre reinforced concrete instead, which uses a cement matrix and alkali resistant glass fibre and is unaffected by water.
Fire behaviour
Gypsum contains roughly 21 percent chemically bound water by weight. When heated, that water is driven off in an endothermic reaction that holds the material near 100 °C until calcination is complete, absorbing heat and shielding what is behind it. This is the mechanism behind the fire performance of gypsum linings generally, and it is a genuine property of the material rather than an additive effect.
Standards and references
- ASTM C1355 Standard Specification for Glass Fiber Reinforced Gypsum Composites
- ASTM C472 (physical testing of gypsum plasters: setting time, expansion and strength)
- ASTM C473 (physical testing of gypsum panel products) and ASTM C28 (gypsum plasters)
- EN 13279-1 and EN 13279-2 (gypsum binders: definitions, requirements and test methods)
- AS 2588 (gypsum plasterboard) for the related sheet product
- AS 5637.1 (group number for internal linings) and NCC 2022 Volume One Specification 7 (check the edition adopted in your jurisdiction)
Related terms
Questions specifiers ask
What is the difference between alpha gypsum and ordinary plaster of Paris?
The chemistry is identical; the manufacturing is not. Plaster of Paris is beta hemihydrate calcined at atmospheric pressure into porous particles needing three times the water. Alpha gypsum is autoclaved under steam pressure into dense crystals needing far less, and sets three to five times stronger as a result.
Why does the water to plaster ratio matter so much?
Only part of the gauging water is chemically consumed. The rest eventually evaporates and leaves pores. More water means more porosity, and porosity is what sets strength, hardness, density and surface quality in a cast gypsum.
Can alpha gypsum be used outside?
No. Gypsum is soluble in water and softens when damp. Alpha gypsum is far less absorbent than beta plaster but is still an interior, dry conditions material. External and wet area work belongs to glass fibre reinforced concrete.
How quickly can an alpha gypsum element be demoulded?
Usually within 30 to 60 minutes of pouring. It then has to be dried before it reaches its published strength, which is a separate and controlled step.
Does alpha gypsum need reinforcement?
In a thin cast shell, yes. Chopped glass fibre distributed through the mix carries tension and gives the shell the toughness to survive demoulding, handling and fixing. Alpha gypsum alone is strong in compression but brittle.
Specifying a sculpted interior surface? ShapeShell-RG is our glass fibre reinforced gypsum system, cast as a 3 to 6 mm alpha gypsum shell from CNC-milled moulds. See ShapeShell-RG specifications
Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.