Gypsum Powder Bulk Density: Values and Why They Matter for Plant Design

Gypsum powder bulk density, measured loose, is usually around 600–900 kg/m³ for building plaster. Published tests on commercial products give 594, 636, 693 and 899 kg/m³, and recycled plasters come out lower. The same powder packs denser when tapped and much lighter when aerated, so a plant designer needs more than one figure.

Bulk density is not a quality grade in most standards, but it sets the size of every silo, bin, conveyor and bag in a plaster plant. This article collects published values, explains the different “densities” that get mixed up, shows how each is measured, and works through what they mean for storage, conveying and packing. For the hardened product, see the section on set density near the end and our article on gypsum board weight and density.

Gypsum Powder Bulk Density: Published Values

The most consistent published data come from Brazil, where the national specification for building gypsum includes a unit-mass requirement and laboratories report it routinely. The values below are loose bulk density (unit mass) measured to Brazil’s NBR 12127 method, plus the Indian limits for premixed lightweight plasters.

All four commercial values come from Brazilian studies: two peer-reviewed journal papers, one workshop paper and one undergraduate research report, the source of the highest value. Brazil’s own unit-mass limit has also moved between editions of NBR 13207 (studies cite 700 kg/m³ for 1994 and 600 kg/m³ more recently), so check the current edition if you sell there.

Two readings for a plant designer. First, the spread between commercial products is wide: one study’s reference sample was 50% denser than another study’s product. Treat any single figure as an assumption to replace with your own measurement. Second, lightweight premixes sit at the bottom of the range. Mixing light perlite into plaster lowers the bulk density, which changes mixer loading, bin volume and bag size for the same weight.

Loose, Tapped and Particle Density: Three Different Numbers

“Gypsum powder density” can mean three different numbers, and confusing them causes most sizing errors.

Particle density is much higher than bulk density. The Brazilian university studies cited here report particle density for calcined gypsum between about 2.4 and 2.8 g/cm³ (2,400–2,800 kg/m³). For example, the Mauá undergraduate report measured 2.80 g/cm³ for a commercial plaster and 2.46–2.52 g/cm³ for recycled samples. As an estimate, a powder with a particle density of 2,600 kg/m³ and a loose bulk density of 600 kg/m³ is about three-quarters air.

That air is the reason the numbers move. Tapping, vibration and storage under load push particles together and squeeze air out. Conveying and fluidizing put air back in. The same tonne of plaster occupies a different volume at each stage of the plant.

How Bulk Density Is Measured

Several standard methods exist. They give different numbers for the same powder, so a result is meaningless without the method.

• ISO 3049:1974. The international method for gypsum plaster powders covered fineness and non-compacted bulk density. Plaster passes through a 2 mm sieve set in a conical vessel and falls, in 100 g portions, into a 1-liter measure fitted with an extension collar; the collar is removed, the surface struck level and the plaster weighed. Duplicate results must agree within 5%. ISO lists the standard as withdrawn, but national methods follow the same idea; check which one your market cites.
• IS 2542 (Part 1/Sec 12), India. Pour the plaster into a 0.02 m³ cube (internal side 271.5 mm) from no more than 50 mm above the rim, strike it level, do not tamp, and weigh to ±50 g. Bulk density in kg/m³ is the mass in kg multiplied by 50 (BIS, IS 2542 (Part 1):1978).
• NBR 12127, Brazil. The method behind the Brazilian values in the table above, used with the NBR 13207 specification.
• ASTM D7481. Loose (Method A) and tapped (Method B) bulk density using a graduated cylinder, for free-flowing and moderately cohesive powders up to 3.5 mm. ASTM states that loose values “cannot be used to estimate powder quantities of large vessels such as silos”. It also notes that results depend on how the powder was dispersed, its dryness and the container shape, and that larger vessels need another method such as ASTM D6683 (ASTM D7481-18).
• ASTM D6683. Bulk density as a function of compressive stress; ASTM notes that the data can be used to estimate bulk density in bins and hoppers (ASTM D6683-19).

For plaster, the lab practice that matters most is consistency. Test at a fixed time after production, at a stated temperature, on a sample that has not been shaken in transit. A sample taken hot from the cooler and one taken from a bag two weeks later are not the same powder.

What Changes Bulk Density in a Plaster Plant

These factors commonly move bulk density in a plaster plant:

1. Raw material and calcination. The studies above show a wide spread between commercial plasters, and recycled plasters calcined from waste came out lighter than commercial ones in both studies that tested them.
2. Fineness and particle shape. Grinding changes the particle size distribution and how well particles pack. See gypsum grinding mills for the mill side.
3. Aeration. Powder leaving a calciner, cooler, air slide or pneumatic line carries air and is lighter than settled powder. NIOSH notes that the fluidizing air in product must be driven out during bag filling (NIOSH, 2019).
4. Time and load in storage. Powder settles and consolidates under its own weight.
5. Formulation. Light aggregates such as perlite and vermiculite lower bulk density; dense fillers such as ground limestone raise it.
6. Moisture and lumps. Damp powder bridges and forms lumps, which change both density and flow.

The practical rule is to measure bulk density at each point where you need a volume: after the cooler, in the ageing silo, at the mixer discharge and in the bag.

Which Density to Use for Silos

Silo and bin design needs two densities, used in opposite directions. A common rule of thumb:

• For volume, use the lowest realistic bulk density, so the silo can still hold the required tonnage when the powder is aerated.
• For structural loads, use the highest realistic density, because consolidated powder at the bottom of a full silo is heavier than the loose value suggests. Ask the silo designer which density and which code they have used.
• For discharge, bulk density is not enough. Hopper angle and outlet size depend on how cohesive the powder is, which needs a shear test on your plaster, not a table value.

A worked example shows how much the choice matters. Our 3,000,000 m² a year reference board line uses 7.0 kg of plaster per m² of 12 mm board, about 21,000 t of plaster a year. Over 300 operating days that is about 70 t a day. As an illustrative calculation, one day’s plaster occupies about 117 m³ at 600 kg/m³ but only 78 m³ at 900 kg/m³. An ageing or buffer silo sized on the higher figure would be about a third too small for a lighter plaster. For why ageing silos are sized with the calciner, see the plaster of Paris manufacturing process.

Conveying and Dosing

Most transfers in a gypsum plant use screw conveyors or bucket elevators (US EPA, AP-42 section 11.16). Both are volumetric machines: they move a volume per turn or per bucket, and the tonnage follows from bulk density. Points that follow from this:

• Screw and bucket capacity should be checked at the lowest bulk density you expect, so the line still meets its tonnage with aerated powder.
• Volumetric feeders drift when bulk density drifts. For additives and binder, weigh rather than meter by volume where accuracy matters, as covered in gypsum plaster manufacturing after calcination.
• Pneumatic conveying adds air and lowers the effective density at the receiving bin. Size the receiving bin and its filter for that.
• Mixer loading for lightweight plaster is set by volume, not mass. A batch that fits by weight can overfill the mixer by volume.

Bag Size

Bag dimensions follow directly from bulk density. As an illustrative calculation, a 25 kg fill takes about 42 liters at 600 kg/m³ and about 28 liters at 900 kg/m³. In practice the bag has to hold the powder as it arrives from the packer, still carrying air, and the filled bag then settles.

This matters for three reasons:

• Packing dust. NIOSH reports that undersized bags cause more blowback and powder spray during filling (NIOSH, 2019).
• Pallets. Bags that are still full of air make loose pallets; NIOSH notes that bags must be free of trapped air before stretch or shrink wrapping, and that a pallet press can help.
• Product changes. A lightweight plaster needs a larger bag than a dense finish plaster of the same weight.

Our article on the gypsum powder packing machine covers packers and bag construction.

Gypsum Plaster Density After Setting

Set plaster density is a different property: it depends mainly on the mixing water, because more water leaves more pores. IS 2547 (Part 2):1976 caps dry set density at 850 kg/m³ for browning plaster and 1,040 kg/m³ for metal lathing and bonding plasters (check the current edition), and ASTM C472-26 includes a density test. For boards, see gypsum board weight and density.

FAQ

What is the typical bulk density of gypsum powder?

Loose bulk density of building plaster is usually around 600–900 kg/m³. Published tests on commercial plasters give 594, 636, 693 and 899 kg/m³. Recycled plasters calcined from waste measured 466–631 kg/m³, and Indian lightweight premixes are limited to 640–770 kg/m³. Measure your own powder, because method, aeration and formulation all change the figure.

What is the density of plaster of Paris?

It depends which density you mean. Loose powder is roughly 600–900 kg/m³, tapped powder is denser, and the solid particles themselves measure about 2,400–2,800 kg/m³ in the Brazilian university studies cited here. Set plaster is a separate property that depends on the mixing water; IS 2547 Part 2 caps dry set density at 850–1,040 kg/m³ for lightweight undercoats.

What is the difference between loose and tapped bulk density?

Loose bulk density is measured on powder poured gently into a container; tapped bulk density after tapping or vibrating it until it settles. Tapped values are higher because air is squeezed out. ASTM D7481 gives both methods with a graduated cylinder, and warns that loose values cannot be used to estimate the quantity held in large vessels such as silos.

Which bulk density should I use to size a gypsum silo?

Use the lowest realistic value for volume, so the silo holds its tonnage when the powder is aerated, and the highest realistic value for structural loads. Hopper angle and outlet size need a flow test on your own plaster. For tall silos, bulk density under compressive stress, as in ASTM D6683, is more relevant than a loose cup test.

Is bulk density a quality requirement for gypsum plaster?

In some standards it is. Brazil’s NBR 13207 includes a minimum unit mass; studies cite 700 kg/m³ for the 1994 edition and, more recently, 600 kg/m³ or more. India’s IS 2547 Part 2 sets maximum dry bulk density for premixed lightweight plasters. Elsewhere, check the current edition of the standard your market uses.

Planning Silos and Conveying for a Plaster Plant?

Measure the bulk density of your own plaster early, loose and settled, and give it to every equipment supplier so silos, conveyors and bags are sized on the same figures. See our gypsum powder production line for how the storage and packing sections fit, and the gypsum powder plant setup guide for the planning sequence.

Sources

• V. M. John, R. P. N. Antunes, “Argamassas de gesso” – Ambiente Construído 2(1), 29–37, ANTAC, 2002: https://seer.ufrgs.br/ambienteconstruido/article/download/3409/1823/11702
• C. A. R. Vasques, F. B. Birck, D. M. Arnold, C. S. Kazmierczak, “Análise das propriedades físicas e mecânicas de gesso reciclado a partir de resíduos da construção civil” – Revista Brasileira de Engenharia e Sustentabilidade 6(1), Universidade Federal de Pelotas, 2019: https://periodicos.ufpel.edu.br/index.php/RBES/article/view/14727/10729
• J. D. Souza, R. H. Geraldo, O. G. Teixeira, J. P. Gonçalves, G. Camarini, “Qualidade do gesso comercializado na região metropolitana de Campinas” – TECSIC 2017, ANTAC, 2017: https://eventos.antac.org.br/index.php/tecsic/article/download/2453/2288/7356
• R. B. Morgado, H. C. F. Cordon, “Avaliação do impacto da reciclagem sobre as propriedades físicas e mecânicas do gesso” – Escola de Engenharia Mauá, Instituto Mauá de Tecnologia (undergraduate research report), c. 2016: https://www.maua.br/files/122016/avaliacao-do-impacto-reciclagem-sobre-as-propriedades-fisicas-mecanicas-do-gesso-271017.pdf
• IS 2542 (Part 1/Sec 1 to 12):1978, Methods of Test for Gypsum Plaster, Concrete and Products, Part 1 Plaster and Concrete – Bureau of Indian Standards, 1978: https://law.resource.org/pub/in/bis/S03/is.iec.2542.1.1-12.1978.pdf
• IS 2547 (Part 2):1976, Specification for Gypsum Building Plaster, Part 2 Premixed Lightweight Plasters – Bureau of Indian Standards, 1976: https://law.resource.org/pub/in/bis/S03/is.2547.2.1976.pdf
• ASTM D6683-19, Standard Test Method for Measuring Bulk Density Values of Powders and Other Bulk Solids as Function of Compressive Stress – ASTM International, 2019: https://store.astm.org/d6683-19.html
• ISO 3049:1974, Gypsum Plasters – Determination of Physical Properties of Powder (withdrawn) – International Organization for Standardization, 1974: https://www.iso.org/standard/8156.html
• ASTM D7481-18, Standard Test Methods for Determining Loose and Tapped Bulk Densities of Powders using a Graduated Cylinder – ASTM International, 2018: https://store.astm.org/d7481-18.html
• ASTM C35-01(2019), Standard Specification for Inorganic Aggregates for Use in Gypsum Plaster – ASTM International, 2019: https://store.astm.org/c0035-01r19.html
• ASTM C472-26, Standard Test Methods for Physical Testing of Gypsum, Gypsum Plasters, and Gypsum Concrete – ASTM International, 2026: https://store.astm.org/c0472-26.html
• Dust Control Handbook for Industrial Minerals Mining and Processing, Second Edition – NIOSH, 2019: https://cdc.gov/niosh/publications/numbered/2019-124.html
• AP-42, Section 11.16 Gypsum Manufacturing – US EPA, 1993 (reformatted 1995): https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf