Gypsum Calcination Temperature: From Gypsum to Stucco

Industrial calciners turn gypsum into stucco (calcium sulfate hemihydrate) at roughly 120–150 °C, according to the US EPA, removing about 15.7% of the gypsum’s weight as water. Heating it hotter or longer forms soluble anhydrite; above about 300 °C, slow-setting insoluble anhydrite forms. That is why calcination temperature decides your plaster’s setting time, water demand and strength.

Where a figure below is our own calculation from molecular weights or published data, we say so.

What Happens When Gypsum Is Heated

Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. About 21% of its weight is water bound in the crystal. Heating drives that water off in two steps:

1. Dihydrate to hemihydrate. CaSO₄·2H₂O → CaSO₄·½H₂O + 1½ H₂O. About three-quarters of the bound water leaves. The product is hemihydrate, sold as stucco, calcined gypsum, building plaster or plaster of Paris. This is the step a gypsum powder plant is built for (US EPA AP-42, section 11.16).
2. Hemihydrate to anhydrite. CaSO₄·½H₂O → CaSO₄ + ½ H₂O. The last of the water leaves. The first anhydrite to form is soluble anhydrite (anhydrite III), which takes water back readily. At higher temperatures it changes into insoluble anhydrite (anhydrite II), which rehydrates only slowly.

Hemihydrate is useful because the reaction runs backwards: mix it with water and it rehydrates to gypsum, sets and hardens. Raw gypsum does not set. Dead-burned anhydrite sets so slowly that in a board or block it behaves almost like an inert filler.

These transitions have no single temperature. They depend on holding time, water vapor pressure, particle size and impurities; a 2024 study observed dehydration to hemihydrate starting well below 100 °C over long holding times (Krejsová et al., 2024). The figures below are the ranges industrial calciners work in, not sharp thresholds.

Gypsum Calcination Temperature by Phase

Combined-water and weight-loss figures are our calculations from molecular weights: CaSO₄·2H₂O 172.17, CaSO₄·½H₂O 145.15, CaSO₄ 136.14, H₂O 18.02.

Two practical points follow. The useful window for building plaster is a few tens of degrees wide. And the hemihydrate and soluble-anhydrite ranges overlap, so a calciner run at the hot end of the window always makes some anhydrite III. The question is how much, and whether it stays constant hour to hour.

Alpha and beta hemihydrate have the same formula and combined water. They differ in how they are made and in their crystals: beta comes from dry calcining at atmospheric pressure, alpha from pressurized steam or hot solutions. This article is about beta plaster. See alpha vs beta gypsum for the comparison.

Weight Loss and Combined Water: The Numbers a Plant Works With

How much gypsum per tonne of stucco

Converting pure gypsum to hemihydrate removes 15.7% of its weight. So 1 t of pure stucco needs about 1.19 t of pure gypsum. The US EPA gives a real-world equivalent: about 0.908 Mg (1 ton) of gypsum makes about 0.77 Mg (0.85 ton) of stucco (AP-42, section 11.16). That ratio, 0.85, is close to the theoretical 0.84.

Impurities change the tonnage. Clay, limestone and silica pass through the calciner unchanged. As our own illustrative arithmetic: 1 t of 90% gypsum yields about 0.86 t of stucco, of which about 0.76 t is hemihydrate and 0.10 t is impurity. For the effect of raw gypsum quality on the whole plant, see natural vs synthetic gypsum.

Combined water is the first check, but not the only one

Combined water is the water still bound in the stucco. Laboratories measure it by heating a sample to constant weight (ASTM C471M sets the chemical test methods for gypsum products). It shows how far calcination went:

• Pure hemihydrate: 6.2% combined water.
• Stucco from 90% gypsum, fully converted: about 5.5%, because the impurities carry no water. (Our calculation.)
• Stucco from 95% gypsum: about 5.9%. (Our calculation.)

Your target combined water therefore depends on the purity of your gypsum. Set it from your own raw material analysis, not from a textbook figure. A 1976 National Gypsum patent gives a theoretical combined water of 5.4% for the gypsum in its example (US3956456A).

Combined water is an average, though. A stucco with 2% residual gypsum and about 5% soluble anhydrite returns the same 6.2% as pure hemihydrate (our calculation), yet will not behave like it on the line. Plants therefore pair combined water with setting time and consistency tests.

How Much Heat Gypsum Calcination Needs

The heat for calcination goes mostly into evaporating the released water, plus the heat of the reaction itself:

1. Reaction heat. The US National Bureau of Standards measured the heat of hydration of hemihydrate to gypsum at about 4,100 cal per mole (Newman and Wells, 1938). Reversing it takes about 17 kJ per mole of gypsum.
2. Evaporation. Each mole of gypsum releases 1.5 moles (27 g) of water. At 2.26 MJ per kg, the latent heat of water, that is about 61 kJ.
3. Total. About 78 kJ per mole, or roughly 0.45 GJ per tonne of pure gypsum, about 0.54 GJ per tonne of stucco. (Our calculation.) This is a floor. It excludes warming the gypsum to calcining temperature and all equipment losses.

For comparison, a United States Gypsum patent states that prior-art kettle calciners needed roughly 750,000 BTU per ton of gypsum (US5743954A). That is about 0.87 GJ per tonne (our conversion). On that figure, roughly half of the heat went into the reaction and the rest into warming material and into losses (our calculation).

Free moisture is the other large heat demand. FGD gypsum delivered at 10% free moisture carries about 0.11 t of water per tonne of dry gypsum. Evaporating that adds about 0.25 GJ per tonne, which raises the theoretical floor by more than half. (Our calculation.) Whether that water is driven off in a separate dryer or in the calciner, the fuel has to be supplied.

Temperature also matters for fuel. Yoshino Gypsum reports that lowering the set calcination temperature by 6.5 °C in its calciner cut fuel consumption by about 5% (US10350564B2). Running hotter than the product needs costs fuel and makes more anhydrite.

Calcination Temperatures in Real Calciners

Each calciner type reaches stucco by a different route, so the temperatures quoted for them are not directly comparable. Gas temperature, wall temperature and product temperature can be hundreds of degrees apart.

The design, energy use and best raw material for each type are compared in gypsum calciners compared.

Under-Calcined vs Over-Calcined Gypsum: Effects on Setting

The two faults pull setting in opposite directions. In a stucco that has both, they can partly hide each other.

The last row is the reason for good calciner design. In kettles, hot spots near the firing surface or submerged combustion tubes can dead-burn part of the charge while the rest is calcined normally (US5743954A). Yoshino Gypsum describes uneven calcination as product containing “a relatively large amount of excessively calcined gypsum and/or insufficiently calcined gypsum” (US10350564B2). BPB reported that splitting fluidized-bed calcination into two stages gave stucco with less soluble anhydrite and less residual gypsum, and that this stucco needed less retarder to reach the right setting time (US4919613A).

For a plant manager, consistency matters more than any single temperature. A stucco slightly off target but the same every hour can be corrected with accelerator, retarder and water; a stucco that drifts cannot. The same chemistry explains soft board edges: overheated in the dryer, they calcine again above about 100 °C (see gypsum board drying process).

How to Control Gypsum Calcination on the Line

Calcination control comes down to keeping the heat input matched to the gypsum flow, and checking the result often enough to catch drift.

1. Control on product temperature. Measure the temperature of the stucco at the discharge and use it as the main control variable. In Yoshino Gypsum’s design, a sensor on the extraction path reads the temperature of the calcined product and a control system uses that reading to run the calciner (US10350564B2). National Gypsum’s flash calciner had thermostats at several points, including the product outlet (US3956456A). Find the setpoint during commissioning against laboratory results for your gypsum, then hold it.
2. Hold the feed steady. Calcination quality depends on the ratio of heat to gypsum. Surges in feed rate, or in the free moisture of FGD gypsum, change that ratio before the temperature loop can respond. Use a weigh feeder, and keep the moisture of the incoming gypsum steady through covered storage and a consistent dryer outlet.
3. Watch the raw material. A change of quarry face or FGD source changes purity, and with it the combined-water target. Re-run the target whenever the raw gypsum changes.
4. Test every shift. At minimum, measure combined water, free water, normal consistency and setting time. ASTM C472 (current edition C472-26) sets physical test methods for gypsum plasters, including normal consistency and setting time, and EN 13279-2 does the same in Europe. Combined water shows how far calcination went. Setting time and consistency show how the stucco will behave. See EN 13279 gypsum plaster standard for the European test regime.
5. Avoid over-temperature as a safety margin. Operators sometimes run hot to make sure no gypsum stays uncalcined. That trades a visible fault for a hidden one, AIII. It also costs fuel: about 5% for 6.5 °C in Yoshino’s data.
6. Record and trend. Log product temperature, feed rate, fuel flow and lab results together. A slow drift in fuel per tonne at constant temperature can point to a change in gypsum moisture or a fouled heat-transfer surface before quality suffers.

Cooling and Aging after Calcination

Stucco leaves the calciner hot, at roughly 130–160 °C. Knauf describes cooling it to below 110 °C, and preferably further (US10676396B2). Cooling limits further dehydration in the silo.

Aging then stabilizes the stucco. Freshly calcined beta hemihydrate has a “stressed and quite unstable” microstructure. Calcined plasters are aged to stabilize the crystal structure and reduce their final water demand. Re-adsorption of water is the main mechanism (US9751805B2). During aging, soluble anhydrite also picks up moisture and returns to hemihydrate. This is one reason a stucco tested straight from the calciner and the same stucco tested a day later can differ.

Plan cooling and aging capacity with the calciner, not after it. This is also why most gypsum board plants calcine on site and use the stucco fresh after aging.

FAQ

What temperature is gypsum calcined at?

For building plaster (beta hemihydrate), industrial calcination runs at roughly 120–150 °C, the range the US EPA gives in AP-42. Some processes run up to about 160–180 °C at the discharge, depending on design. Gas or wall temperatures are much higher, which is why the product temperature at the discharge, not the burner temperature, is what plants control.

What is calcined gypsum?

Calcined gypsum is gypsum that has been heated to drive off about three-quarters of its chemically bound water. The product is calcium sulfate hemihydrate, CaSO₄·½H₂O, also called stucco, building plaster or plaster of Paris. When mixed with water it rehydrates to gypsum and sets hard. That reaction is the basis of gypsum board, blocks and plasters.

How much weight does gypsum lose when calcined?

Pure gypsum loses 15.7% of its weight when converted to hemihydrate and 20.9% when converted fully to anhydrite. These figures are calculated from molecular weights. In practice, the US EPA gives about 0.77 Mg of stucco from 0.908 Mg of gypsum, a ratio of about 0.85. Impurities and free moisture change the real figures for your plant.

What happens if gypsum is over-calcined?

Moderate over-calcining produces soluble anhydrite (AIII), which sets very fast and needs more water than stucco. Severe over-calcining produces insoluble anhydrite (AII), which rehydrates very slowly and adds weight without strength. Both raise fuel use. Under-calcining leaves residual gypsum, which accelerates the set and weakens the product.

How do you check whether stucco is correctly calcined?

Measure combined water against a target set from your gypsum purity, about 6.2% for pure hemihydrate. Then check normal consistency and setting time, because a mix of residual gypsum and soluble anhydrite can show normal combined water. Plants test these every shift, to ASTM C472 and ASTM C471M or EN 13279-2.

Planning a Gypsum Powder Plant?

The right calcination temperature depends on your gypsum, your calciner and the products the stucco must make, so start with a raw material analysis. See the calciner options and plant scope on our gypsum powder production line page, and use the gypsum powder plant setup guide to plan raw material, heat source and laboratory before you compare suppliers.

Sources

• US EPA, AP-42 Fifth Edition, Volume I, Chapter 11.16: Gypsum Manufacturing — https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf
• Krejsová J. et al., “New insight into the phase changes of gypsum”, Materials and Structures 57, 128 (2024) — https://link.springer.com/article/10.1617/s11527-024-02404-z
• Newman E.S. and Wells L.S., “Heats of Hydration and Transition of Calcium Sulfate”, Journal of Research of the National Bureau of Standards 20 (1938), RP1107 — https://nvlpubs.nist.gov/nistpubs/jres/20/jresv20n6p825_A1b.pdf
• US10676396B2, Process and apparatus for calcination of gypsum, Knauf Gips KG (Google Patents) — https://patents.google.com/patent/US10676396
• US10350564B2, Apparatus and method for calcination of gypsum, Yoshino Gypsum (Google Patents) — https://patents.google.com/patent/US10350564
• US3956456A, Gypsum calcination, National Gypsum (Google Patents) — https://patents.google.com/patent/US3956456A
• US4919613A, Method and apparatus for calcination, BPB Industries (Google Patents) — https://patents.google.com/patent/US4919613
• US5743954A, Method for continuous refractoryless calcining of gypsum, United States Gypsum (Google Patents) — https://patents.google.com/patent/US5743954
• US4569831A, Process and apparatus for calcining gypsum, Fuller Co. (Google Patents) — https://patents.google.com/patent/US4569831
• US9751805B2, background on beta hemihydrate aging (Google Patents) — https://patents.google.com/patent/US9751805
• ASTM C471M-25, Standard Test Methods for Chemical Analysis of Gypsum and Gypsum Products (Metric) — https://store.astm.org/c0471m-25.html
• ASTM C472-26, Standard Test Methods for Physical Testing of Gypsum, Gypsum Plasters and Gypsum Concrete — https://store.astm.org/c0472-26.html