Plaster of Paris Manufacturing Process and Plant

Plaster of Paris is made by crushing and grinding gypsum, then heating it to roughly 120–165 °C so that it loses about three-quarters of its chemically bound water and becomes calcium sulfate hemihydrate. The hot plaster is cooled, aged, ground or classified, blended with additives such as retarders, and packed or sent to a board or block line.

That is the whole process in one sentence. The details decide whether a plant makes plaster that sets the same way every day. This guide walks through each step with the published figures, explains the equipment a plaster of Paris plant needs, and shows how to size one. It does not quote prices; what drives the investment is the calciner, the raw gypsum and the products you will sell.

What Plaster of Paris Is

Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. Heated, it gives up part of its water:

CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O

The product, calcium sulfate hemihydrate, is commonly known as Plaster of Paris (British Geological Survey). Plant engineers call the same material stucco, calcined gypsum or building plaster. When it is mixed with water the reaction runs backwards, the hemihydrate dissolves, and gypsum crystals grow and interlock into a solid. That is why the same powder can be cast into moulds, trowelled onto a wall or formed into gypsum board.

Two forms of hemihydrate exist. Calcining ground gypsum under normal atmospheric conditions gives β-hemihydrate; calcining under hydrothermal conditions (pressurized steam) gives α-hemihydrate. The water demand is “much higher for the β-hemihydrate than for the alpha hemihydrate” because the two have different porosity and crystallinity (US 9,751,805). Ordinary plaster of Paris is β-hemihydrate, and this article covers β plants. For the differences in product and plant, see alpha vs beta gypsum.

The water balance in numbers

From the molecular weights (CaSO₄·2H₂O 172.17, CaSO₄·½H₂O 145.15), pure gypsum holds 20.9% water by weight and pure hemihydrate 6.2%. Calcination therefore removes about 15.7% of the weight of pure gypsum, and one tonne of plaster needs about 1.19 tonnes of pure gypsum. US EPA AP-42 gives the plant-floor version: about 0.908 Mg (1 ton) of gypsum calcines to about 0.77 Mg (0.85 ton) of stucco. Any clay, limestone or other impurity in your gypsum raises the raw material figure further.

Going the other way, the setting reaction needs only 18.3 parts of water per 100 parts of plaster, but “in practice 60–80 parts of water are used” so the plaster can be poured before it sets (NZIC, Plaster and Plaster Board Production). The excess water leaves as the cast dries, which is why set plaster is porous.

The Plaster of Paris Manufacturing Process, Step by Step

The sequence below follows the US EPA description of gypsum processing (AP-42, section 11.16) and adds the steps a plaster plant needs after calcination.

1. Receive and store raw gypsum. Quarried or mined rock, or by-product gypsum such as FGD gypsum. Rock is crushed near the quarry and stockpiled.
2. Crush and screen. At the plant, stockpiled rock is crushed again and screened to about 50 mm (AP-42).
3. Dry, if needed. If the moisture content of the ore is above about 0.5% by weight, it is dried in a rotary dryer or a heated roller mill (AP-42). FGD gypsum arrives fine but damp and always needs drying.
4. Grind. A roller mill grinds the dry rock until 90% passes 149 µm (100 mesh) (AP-42). This ground gypsum is called landplaster.
5. Calcine. The landplaster is heated in a kettle, flash calciner, rotary kiln or fluidized-bed calciner until it has lost about three-quarters of its combined water.
6. Cool. Hot plaster is cooled quickly so calcination stops. In kettle plants it drops into a hot pit; one New Zealand producer describes dropping it into pits “to cool it rapidly” to prevent any further calcination (NZIC).
7. Age. Fresh β-hemihydrate is held so its structure stabilizes and its water demand falls (see below).
8. Grind or classify again. For plaster products, stucco is ground further to the fineness the product needs (AP-42).
9. Blend. For bagged plaster, stucco is batch-mixed with retarders and stabilizers (AP-42), and for some products with lightweight aggregates or other additives.
10. Pack or convey. Bag packing and palletizing for sale, or conveying to silos that feed a gypsum board or gypsum block line.

Dust is collected at every transfer point. “Emissions sources in gypsum plants are most often controlled with fabric filters” (AP-42), and the dust they catch is product that goes back into the process.

Calcination: Temperature and What Goes Wrong

Published calcination temperatures sit in a narrow band. AP-42 gives approximately 120–150 °C (250–300 °F). The British Geological Survey gives 150–165 °C for making hemihydrate plaster. On the steam-heated rotary kilns we supply, the gypsum calcines slowly at a low 150–200 °C, which gives a stable phase composition. The exact figure depends on the calciner, the residence time and whether you are reading the temperature of the gypsum or of the heating medium. For the chemistry and the temperature curve in more detail, see gypsum calcination temperature.

Control matters more than the set point. Above the optimum range, more water is lost and anhydrite forms. The NZIC description lists soluble anhydrite forming at about 175 °C and insoluble anhydrite at about 425 °C, and notes that extreme local heat in hot spots can even split calcium sulfate and leave quicklime. With careful temperature control, that producer keeps both anhydrites “well below” permitted maximums of 5% and 2%. Under-calcined material leaves raw gypsum in the plaster; NZIC lists ground gypsum as a set accelerator because it provides nucleation sites, so the plaster sets too fast. Over-calcined material changes the water demand and the setting time. Either way, the problem shows up at the customer’s mixing bucket or at the board line.

That is why plaster plants test combined water every shift: at about 6.2% for pure hemihydrate, it is the first check on whether the calciner did its job.

Plaster of Paris Manufacturing Machines: The Calciner Options

The calciner is the core machine of a plaster of Paris plant. Four types are in use:

Choose from three inputs: the output you need, the heat source your site can supply reliably every day (natural gas, steam, thermal oil or another fuel), and the products the plaster must make. Our comparison of kettle, rotary kiln and fluidized-bed calciners goes through the trade-offs machine by machine.

The other plaster of Paris manufacturing machines are less contested but still need to be specified carefully: the crusher and roller mill (or dryer for FGD gypsum), the cooler, ageing and storage silos, the finish mill or classifier, the additive dosing and mixer, the bag packer and the dust collectors.

Ageing, Grinding and Blending After Calcination

Many first-time plants underestimate what happens after the calciner.

Ageing. Because of the rough thermal treatment, the microstructure of fresh β-hemihydrate is “stressed and quite unstable”, and in contact with water it partly disintegrates into very small particles. Plants therefore apply ageing to “stabilize the crystalline structure and reduce the final water demand”; re-adsorption of water is the main driver (US 9,751,805). In practice this means ageing silos sized with the calciner, not added later.

Grinding and classifying. Fineness affects water demand, setting and the surface of the cast. Plaster for casting moulds, for wall plastering and for board feed are ground to different targets.

Blending. Retarders and stabilizers set the working time of bagged plaster (AP-42). Lightweight plasters add aggregates such as perlite or vermiculite, and machine-applied plasters add their own set of additives. If you plan several products, size the additive dosing and the mixer for the product list you expect to have in year three.

Plaster of Paris Plant: Products and Standards

The products decide the plant. A plaster of Paris plant usually sells into one or more of these markets:

For what the European standard requires of each plaster type, see EN 13279 explained. For the tests, ASTM C472 covers free water, fineness, normal consistency, setting time, compressive strength and density of gypsum plasters (ASTM C472-26), and EN 13279-2 sets the European test methods. Build a small laboratory into the plant and test combined water, free water, fineness, consistency, setting time and strength every shift.

Raw Gypsum and Sizing a Plaster of Paris Plant

Raw gypsum sources

• Natural gypsum. World mine production was about 160 million tonnes in 2025. The largest producers were the United States (20 Mt), Iran (16 Mt), Oman (14 Mt), China (12 Mt), Spain (11 Mt) and Turkey (10 Mt) (USGS Mineral Commodity Summaries 2026). Natural gypsum is “especially suitable for the manufacture of building plasters” because its clay content improves workability (BGS).
• FGD gypsum. A by-product of flue-gas desulfurization at coal-fired power plants. In China it is now the main raw material for new lines. In the United States synthetic gypsum was about 39% of domestic gypsum supply in 2025 (USGS). It needs drying, and its supply depends on the power plant.
• Phosphogypsum. A by-product of phosphoric acid production. It carries impurities and, in some sources, naturally occurring radioactivity (US EPA), so test it and check the rules in your country before you plan on it.

Send a representative sample for analysis before the preparation and calcination sections are fixed. The gypsum powder plant setup guide covers raw material sourcing in more detail.

How to size the plant

Work back from what the plant will sell:

1. Set the plaster output. For a board line, work from board output. Our 3,000,000 m² a year reference board line uses 7.0 kg of plaster per m² of 12 mm board, which by rough calculation is about 21,000 t of plaster a year. For bagged plaster, start from the volume you can sell and allow for seasonal demand.
2. Convert to raw gypsum. Multiply by about 1.19 for pure gypsum, then add for impurities. As an illustrative calculation, 30,000 t a year of plaster from gypsum of 90% purity needs roughly 30,000 × 1.19 ÷ 0.90 ≈ 39,700 t of raw gypsum a year, before handling losses.
3. Allow for availability. Divide by the hours the plant will actually run, not the hours in a year, to get the hourly capacity of the calciner and mills.
4. Size storage. Raw gypsum storage follows your supplier’s delivery pattern; ageing silos follow the calciner; finished-product storage follows your sales and the bagging shift pattern.
5. Fix the heat source. Natural gas, steam, thermal oil or another fuel decides the calciner type and burner or heat exchanger design.

FAQ

What is the manufacturing process of plaster of Paris?

Gypsum is crushed, dried if its moisture is above about 0.5%, and ground until about 90% passes 149 µm. It is then calcined at about 120–165 °C to remove three-quarters of its combined water, cooled quickly, aged, ground or classified, blended with retarders and other additives, and bagged or sent to a board or block line.

At what temperature is gypsum converted to plaster of Paris?

Published figures are about 120–150 °C (US EPA AP-42) and 150–165 °C (British Geological Survey). The steam-heated rotary kilns we supply calcine slowly at about 150–200 °C. Above the optimum range, anhydrite forms: about 175 °C for soluble anhydrite and about 425 °C for insoluble anhydrite in one producer’s description. Careful temperature control keeps both low.

What machines are used to make plaster of Paris?

A crusher and roller mill (or a dryer for FGD gypsum), a calciner (kettle, flash calciner, rotary kiln or fluidized-bed calciner), a cooler, ageing and storage silos, a finish mill or classifier, additive dosing and a mixer, a bag packer, and dust collectors at each transfer point. The calciner is the machine that decides plaster quality.

How much gypsum is needed to make one tonne of plaster of Paris?

About 1.19 tonnes of pure gypsum, because calcination removes about 15.7% of its weight as water. US EPA AP-42 puts it as 1 ton of gypsum giving about 0.85 ton of stucco. As an illustrative calculation, gypsum of 90% purity needs about 1.32 tonnes of raw gypsum per tonne of plaster, before handling losses.

Is plaster of Paris the same as building plaster?

The binder is the same β-hemihydrate. “Plaster of Paris” usually means the plain calcined product; building plasters are formulated from it with retarders, lightweight aggregates or other additives and must meet a product standard such as EN 13279-1 or the national standard of the market. High-strength casting plasters are often α-hemihydrate, which is made in a different plant.

Planning a Plaster of Paris Plant?

Tell us your products, target output, gypsum source and heat source, and we will compare calciner configurations from different manufacturers against your brief. See our gypsum powder production line for the line itself, and the gypsum powder plant setup guide for the full planning sequence.

Sources

• US EPA, AP-42, Fifth Edition, Volume I, Section 11.16, Gypsum Manufacturing: https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf
• British Geological Survey, Gypsum Mineral Planning Factsheet (2006): https://nora.nerc.ac.uk/id/eprint/534434/1/mpf_gypsum.pdf
• New Zealand Institute of Chemistry, Plaster and Plaster Board Production (IX-Materials-D): https://nzic.org.nz/unsecure_files/book/9D.pdf
• US Geological Survey, Mineral Commodity Summaries 2026: Gypsum: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-gypsum.pdf
• US Patent 9,751,805 B2, Method for stabilizing beta-hemihydrate plaster: https://patents.google.com/patent/US9751805
• ASTM C472-26, Standard Test Methods for Physical Testing of Gypsum, Gypsum Plasters, and Gypsum Concrete (catalog listing): https://www.en-standard.eu/astm-c0472-26-standard-test-methods-for-physical-testing-of-gypsum-gypsum-plasters-and-gypsum-concrete/
• ASTM C59/C59M, Specification for Gypsum Casting and Molding Plaster (ICC catalog listing): https://shop.iccsafe.org/astm-c-59-c-59m-00-2006-specification-for-gypsum-casting-and-molding-plaster-pdf-download.html
• US EPA, TENORM: Fertilizer and Fertilizer Production Wastes (phosphogypsum): https://www.epa.gov/radiation/tenorm-fertilizer-and-fertilizer-production-wastes
• BSI, BS EN 13279-1:2008, Gypsum binders and gypsum plasters. Definitions and requirements: https://knowledge.bsigroup.com/products/gypsum-binders-and-gypsum-plasters-definitions-and-requirements