Alpha vs Beta Gypsum (Hemihydrate): Production and Uses

Alpha and beta gypsum are the same compound, calcium sulfate hemihydrate, made in two different ways. Beta is calcined dry at atmospheric pressure and forms porous, irregular particles that need more than about 70 ml of water per 100 g. Alpha forms under pressurized steam or in hot solutions as compact crystals needing under about 50 ml, so it sets much stronger.

Beta hemihydrate is the building plaster behind gypsum board, blocks and wall plasters. Alpha is a specialty binder for high-strength casts, models, dental and industrial plasters. The two need different plants. This article explains the difference with published patents, papers and producer datasheets.

Alpha vs Beta Gypsum at a Glance

Same Chemistry, Different Crystals

Both forms have the same formula and the same 6.2% combined water. On mixing with water, both rehydrate to the same gypsum (dihydrate). The difference is physical: porosity and crystallinity (US9751805B2).

• Beta forms when gypsum loses its water as vapor in dry air. The water escapes from inside each gypsum particle and leaves a porous skeleton of small, poorly formed crystals. A 2005 study describes beta as “white, soft and porous irregularly shaped particles” (Kostic-Pulek et al., 2005).
• Alpha forms when gypsum dissolves and recrystallizes in liquid water or saturated steam. The hemihydrate grows as distinct crystals that the same study describes as “transparent and translucent, compact, smooth, nonporous”.

The large internal surface of beta particles is what costs water. More water is needed just to wet the particles and make a pourable slurry. The compact alpha crystals need much less (Kostic-Pulek et al., 2005).

Crystal shape matters within alpha too. In a 2022 study of alpha made from FGD gypsum, needle-like crystals with a high aspect ratio needed more water and gave lower strength, “even lower than β-HH”. Short columnar crystals with an aspect ratio near 1 gave much higher strength because they need less water (Li et al., 2022). Making good alpha means controlling the crystal habit, not only the phase.

How Beta Hemihydrate Is Made

Beta is made by dry calcination at atmospheric pressure. Ground gypsum is calcined at roughly 120–150 °C (US EPA AP-42, section 11.16) in one of several continuous calciners:

• Kettle calciners, heated indirectly through the kettle wall and flues. Georgia-Pacific describes beta as hemihydrate “formed by calcining in kettles at atmospheric pressure”.
• Fluidized-bed calciners, in which hot gas fluidizes fine gypsum powder.
• Rotary kilns, fired directly or heated by steam or thermal oil.
• Flash calciners and impact mills, in which gypsum meets hot gas for seconds.

The calciners are compared in gypsum calciners compared. The temperature window, and what over- and under-calcining do, are covered in gypsum calcination temperature.

After calcination, beta needs cooling and aging. Freshly calcined beta has a “stressed and quite unstable” microstructure. Aging stabilizes it and lowers its final water demand, mainly through re-adsorption of water (US9751805B2).

How Alpha Hemihydrate Is Made

Alpha needs liquid water or saturated steam around the gypsum while it dehydrates. Three routes are described in the literature.

1. Autoclave (steam) process

Lump gypsum is loaded into a pressure vessel and treated with saturated steam. For natural gypsum, one patent background describes steam at about 266–275 °F (130–135 °C), with pressure building to 4–5 bar over about four hours. Full conversion of dihydrate to alpha by this route “usually takes at least six hours” (US7335254B2). The product is then dried and ground. A variant presses moist gypsum into shaped bodies before autoclaving, with crystal growth controlled at 230–356 °F (110–180 °C), and runs four to seven hours per batch (US7335254B2). A 2022 review describes the autoclave route as simple but giving lower strength (Li et al., 2022).

2. Hydrothermal (slurry) process

Fine gypsum, for example FGD gypsum, is suspended in water and heated under pressure in a stirred reactor. Additives control pH and crystal shape, giving short, columnar crystals with low water demand. The 2022 study calls hydrothermal alpha well crystallized but more costly to produce (Li et al., 2022). The slurry must then be dewatered, and the alpha dried before it can rehydrate.

3. Atmospheric salt-solution process

Gypsum is converted in boiling or near-boiling solutions of acids or salts, without a pressure vessel (Kostic-Pulek et al., 2005). The conditions are mild, but industrial use is rarely reported, because of equipment corrosion, product instability and water treatment (Li et al., 2022).

What this means for the plant

Compared with a beta line, an alpha plant adds pressure vessels or reactors, crystal-modifier dosing, dewatering and a dryer for the wet product. Most routes also run in batches. One patent background calls the purification steps for autoclave alpha “expensive” and “troublesome” (US7335254B2). It is a different plant from a beta calcining line, not an option added to one.

Water Demand and Why It Decides Strength

Set gypsum gets its strength from interlocking dihydrate crystals. Every gram of water above what the reaction consumes stays in the set plaster as free water, and leaves pores when it dries.

The reaction itself needs only about 18.6 g of water per 100 g of hemihydrate (our calculation from molecular weights). Compare what each form needs to make a workable slurry:

• Beta at a water/plaster ratio of 0.70: about 51 g of the 70 g is surplus, which becomes pore space.
• High-strength alpha at 0.21 (the water/plaster ratio stated for Crystacal Alpha K): only about 2 g of the 21 g is surplus, so almost all of the water goes into the crystals (our calculation).

Producer datasheets show the effect on strength. These are commercial products tested at the manufacturer’s standard ratios. They are not a controlled comparison, but the trend is clear:

In laboratory work on columnar alpha made from FGD gypsum, oven-dry compressive strength reached about 60 MPa, with a 2-hour flexural strength of about 7.0 MPa (Li et al., 2022). That study tested to China’s JC/T 2038 standard for alpha high-strength gypsum plaster.

Water demand also matters for energy. In a board plant, every extra unit of mixing water must be evaporated in the dryer. One patent notes that the lower water/stucco ratio of alpha would cut board drying fuel and allow faster lines (US7815889B2). The trade-off is the cost and complexity of making alpha. Most board plants therefore stay with beta and manage water demand through aging, additives and foam. See gypsum board drying process for how water load drives the dryer.

Setting Time and Workability

Water demand is not the only difference.

• Setting. Alpha needs less water but has a longer setting time (US10676396B2, Knauf). Beta plasters are best suited for lightweight, fast-setting products (US9751805B2). In both cases, accelerators and retarders adjust setting to the process.
• Blends. Producers blend alpha and beta to make plasters with medium water demand (Georgia-Pacific).
• Expansion. Datasheets also state linear expansion, for example 0.19% for the beta plaster Almod. Plasters for dimensional work are specified on this property too.

Uses of Alpha and Beta Gypsum

Sources for the table: Georgia-Pacific lists alpha and beta formulations for sanitaryware, dinnerware, GRG (glass-reinforced gypsum), medical bandages, dental, pottery, metal casting and tooling plasters. The Almod datasheet lists dental, floor and wall uses for a beta plaster. The Crystacal D datasheet describes a high-strength hemihydrate used mainly in dental prosthetic and orthodontic work. Li et al. (2022) cite molding, special binders and precision casting.

For a building-materials investor, the volume market is beta: board, blocks and bagged plaster. Alpha serves smaller, higher-specification markets with their own customers and standards.

Which One Should Your Plant Make?

1. Feeding a gypsum board or block line? Make beta. It is what board and block lines are designed around. At 7.0 kg of stucco per m² of 12 mm board, our 3,000,000 m² a year reference line needs about 21,000 t of beta stucco a year (our calculation).
2. Selling bagged building plaster? Make beta to the product standard of your market: EN 13279-1, ASTM C28/C28M or a national standard. See EN 13279 gypsum plaster standard.
3. Targeting dental, model, mold or industrial plasters? You will need alpha or alpha–beta blends. Confirm the customers and their specifications first, then run process trials on your own gypsum. Alpha quality depends heavily on raw material and crystal control.
4. Using FGD gypsum? Both routes are possible. FGD is fine-grained and suits fluidized-bed beta calcination after drying, and published research has made alpha from FGD gypsum (Li et al., 2022).

FAQ

What is the difference between alpha and beta hemihydrate?

Both are calcium sulfate hemihydrate with 6.2% combined water. Beta is made by dry calcining at atmospheric pressure and forms porous, irregular particles. Alpha is made under steam pressure or in hot solutions and forms compact crystals. Alpha needs much less mixing water, under about 50 ml per 100 g against more than about 70 ml for beta, so it sets much stronger.

Is plaster of Paris alpha or beta?

Plaster of Paris is normally beta hemihydrate, made by heating gypsum dry at atmospheric pressure. The same material is called stucco or building plaster in board and plaster plants. High-strength casting and dental stones are usually alpha or alpha-rich blends. See plaster of Paris manufacturing process for how it is produced.

Which is stronger, alpha or beta gypsum?

Alpha. It needs far less water to make a workable slurry, so the set plaster has fewer pores. Producer datasheets show a beta plaster at 70% water reaching about 11 MPa dry compressive strength, and a high-strength hemihydrate plaster at 40% water about 32 MPa. Laboratory alpha made from FGD gypsum reached about 60 MPa oven-dry (Li et al., 2022). Poorly shaped, needle-like alpha crystals can, however, be weaker than beta.

Can alpha gypsum be used in gypsum board?

It can, and its lower water demand would cut dryer fuel. Most board plants still use beta, because it is made continuously at lower cost and its water demand can be managed with aging, additives and foam. Pressurized and alpha-rich stucco for board appears in patent literature (US7815889B2), but beta remains the standard board feed.

How is alpha gypsum produced?

By converting gypsum to hemihydrate in the presence of liquid water or saturated steam. Lump gypsum can be autoclaved in steam at about 130–135 °C and 4–5 bar for several hours, or fine gypsum heated as a slurry under pressure with crystal modifiers. The product is then dried and ground. Most processes run in batches.

Planning a Gypsum Powder Plant?

For board, block and bagged-plaster projects, the calcining plant is a beta-hemihydrate plant, and it is configured around your gypsum and heat source. See the scope and calciner options on our gypsum powder production line page, and use the gypsum powder plant setup guide to plan raw material, quality testing and sizing.

Sources

• Kostic-Pulek A. et al., “The treatment of gypsum as a product of the flue gas desulphurization process”, Ceramics–Silikáty 49 (2), 115–119 (2005) — https://www.irsm.cas.cz/materialy/cs_content/2005/Kostic-Pulek_CS_2005_0000.pdf
• Li Y., Ni W., Duan P., Zhang S., Wang J., “Experimental Study and Mechanism Analysis of Preparation of α-Calcium Sulfate Hemihydrate from FGD Gypsum with Dynamic Method”, Materials 15 (9), 3382 (2022) — https://www.mdpi.com/1996-1944/15/9/3382
• US7815889B2, Method and apparatus for pressurized calcination of gypsum (Google Patents) — https://patents.google.com/patent/US7815889
• US7335254B2, Alpha-type calcium sulfate hemihydrate compositions and methods of making same (Google Patents) — https://patents.google.com/patent/US7335254
• US9751805B2, background on beta hemihydrate structure and aging (Google Patents) — https://patents.google.com/patent/US9751805
• US10676396B2, Process and apparatus for calcination of gypsum, Knauf Gips KG (Google Patents) — https://patents.google.com/patent/US10676396
• US EPA, AP-42 Fifth Edition, Volume I, Chapter 11.16: Gypsum Manufacturing — https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf
• Georgia-Pacific Gypsum, Industrial Plasters — https://www.buildgp.com/industrial-plasters
• Saint-Gobain Formula, Almod product datasheet — https://www.saintgobainformula.com/fr/documents/product-datasheet/almod-wk-pds-en-20250527.pdf
• Saint-Gobain Formula, Herculite LX product datasheet — https://www.saintgobainformula.com/fr/documents/fiche-produit/herculite-lx-nk-pds-en-20221118.pdf
• Saint-Gobain Formula, Crystacal Alpha K product datasheet — https://www.saintgobainformula.com/en/documents/product-datasheet/crystacal-alpha-k-nk-pds-en-20251223-1.pdf
• Saint-Gobain Formula, Crystacal D product datasheet — https://www.saintgobainformula.com/zh-hans/media/4911/download