Gypsum Board Additives: What Each One Does in the Core

Gypsum board additives are the small ingredients, usually a few percent of the stucco weight or less, that control how the slurry flows, when it sets, how the paper bonds and what special properties the board has. The main ones are starch, a foaming agent, a set accelerator, a retarder, a dispersant, sodium trimetaphosphate, water repellents such as siloxane or wax emulsion, and glass fiber or vermiculite for fire-resistant board.

The basic recipe is simple: stucco, water and two sheets of paper. The additives turn that recipe into a board that can be made at line speed, dried without damage and sold to a standard. This guide explains what each additive does in the core, the dosage ranges found in published sources, and how to plan additive dosing for a new plant.

What Gypsum Board Is Made Of: Core, Paper and Additives

A finished gypsum board has three parts:

• The core is set gypsum, calcium sulfate dihydrate (CaSO₄·2H₂O). It is made by mixing stucco (calcium sulfate hemihydrate) with water, which turns back into interlocking gypsum crystals. Dihydrate gypsum holds about 21% of its weight as chemically combined water (from the formula). That water is why gypsum board resists fire.
• The paper is two recycled-fiber liners, a light face sheet and a gray back sheet. The paper’s specification and bond are covered in our article on gypsum board facing paper.
• The additives are dosed into the mixer as powders, liquids or foam. US EPA AP-42 lists dry additives “such as perlite, starch, fiberglass, or vermiculite,” mixed “with water, soap foam, accelerators and shredded paper, or pulpwood in a pin mixer” (US EPA AP-42, section 11.16).

On our reference line (12 mm board), each square meter of board uses 7.0 kg of stucco and 0.45 kg of facing paper. The additives are measured in grams per square meter, but they decide whether the line runs.

Gypsum Board Additives at a Glance

The ranges below are from published patents, one patent application (US 2019/0092689) and supplier literature. Patent ranges are broad because they are written to claim wide coverage. Working doses on a line are set by trial with the plant’s own stucco, water and paper. One lb/MSF (pounds per thousand square feet) is about 4.9 g/m².

Some boards also carry a biocide for mold resistance. The type and dose depend on the chemical rules in the market where the board is sold.

Starch and Foaming Agent: Bond, Strength and Weight

Starch and foam are the two additives that every board line uses, and they work as a pair.

Starch comes in two forms with two different jobs. A low-viscosity migrating starch travels with the evaporating water to the board faces and “causes a stronger bond to form between the gypsum core and facing material” (US 12,071,376). A larger-molecule core starch stays in the core and binds the crystals. Our article on starch for gypsum board covers types, dosage and feeding.

Foam lowers board weight. Typical ½-inch wallboard weighs about 1,600–1,800 lb/MSF (about 7.8–8.8 kg/m²). Lightweight formulations reach about 1,000–1,400 lb/MSF (about 4.9–6.8 kg/m²) (US 7,731,794). The trade-off is strength, and core starch, dispersant and STMP are what win it back. Bubble size control with stable and unstable soaps is explained in our article on gypsum board foaming agents.

Set and Flow Control: Accelerators, Retarders, Dispersants and STMP

Accelerators

Stucco on its own sets too slowly for a line that has to cut boards shortly after forming. The standard accelerator is finely ground gypsum. Grinding exposes fresh crystal surfaces that act as “seed” crystals, but humidity deactivates them quickly. So the ground gypsum is usually coated with sugar, dextrose or starch, which dissolves in the slurry and exposes the seeds when they are needed (US 10,737,979). One lightweight formulation uses 15 lb/MSF of such a “heat resistant accelerator,” made by grinding landplaster with about 5% sugar, dextrose, boric acid or starch (US 7,731,794). Sulfates also accelerate set. Potassium sulfate is one example. It was the accelerator in a published foaming agent plant trial (US 4,156,615).

Retarders

A retarder does the opposite. It keeps the slurry from stiffening too early in the mixer, the discharge or the edge-forming section. Published retarder types include protein-based retarders made from animal hair and hooves, cream of tartar, sodium citrate and DTPA (US 8,343,273). On a board line, accelerator and retarder are balanced together. The slurry has to stay fluid long enough to form, and then it has to set firmly by the time it reaches the knife.

Dispersants

Every kilogram of mixing water beyond what the stucco needs to set has to be evaporated in the dryer. Stucco needs only about 0.19 kg of water per kg to rehydrate (from the formulas). In one published lightweight formulation, the water-to-stucco ratio was 1.10–1.21 (US 7,731,794). So more than four-fifths of that water was there only to make the slurry flow. Dispersants reduce this. Naphthalene sulfonate is the long-standing choice. Conventional boards use “about 0.4% or less” by weight of stucco, and the US Gypsum lightweight patent goes much higher (US 7,731,794). Polycarboxylate ether (PCE) dispersants are marketed for “very high water reduction potential.” One supplier says its phosphate-based grade “has very limited impact on set time of stucco,” and offers a clay-tolerant grade for natural gypsum with high clay content (BASF). On our reference line, natural gas for calcination and drying runs at about 0.5–0.7 m³ per m² of board. So water reduction is one of the most direct ways to cut dryer fuel.

Sodium trimetaphosphate (STMP)

STMP gives set gypsum “increased strength, resistance to permanent deformation (e.g., sag resistance), and dimensional stability.” Unlike most additives, it “does not retard the rate of the formation of set gypsum” (US 6,342,284). It matters most for ceiling board, which must not sag between supports, and for lightweight board. STMP also strengthens the effect of naphthalene sulfonate dispersants (US 7,731,794).

Water-Repellent Additives: Siloxane and Wax Emulsion

Plain gypsum soaks up water. US Gypsum reports that a core with no water-resistant additives “can absorb as much as 40 to 50 wt. % water” when immersed for about two hours (US 7,892,472). Moisture-resistant boards bring this down with one of two additive families:

• Siloxane. A methyl hydrogen polysiloxane dosed at about 0.4–1.0% of the dry ingredients, with dead-burned magnesium oxide at about 0.1–0.5% as a catalyst. In the patent’s tests, a production board with 0.8% siloxane and 0.2% MgO absorbed 4.5% water (US 7,892,472).
• Wax emulsion. Paraffin-based emulsions, sometimes with montan wax, are the older alternative. Montan wax is “a lignite-wax” from a limited natural source, which makes it costly and harder to obtain (US 9,045,370). The same patent warns that some wetting agents in such systems are “powerful defoamers.” They can break up the foam and cause blisters, so foam and water repellent have to be tuned together.

Under EN 520, Type H boards must keep total water absorption to no more than 5% (H1), 10% (H2) or 25% (H3), with surface water absorption limits of 180, 220 and 300 g/m² (EN 520, as reproduced in DRS 632). The H1 limit needs a well-dispersed water repellent in the core and, often, a better-sized face paper.

Fire-Resistant Additives: Glass Fiber, Vermiculite and Clay

All gypsum board slows fire, because heat is used up driving off the chemically combined water in the gypsum. As the gypsum loses that water, it shrinks and cracks. Fire-resistant boards, Type X under ASTM C1396 and Type F under EN 520, add materials that hold the core together:

• Glass fiber helps “hold portions of the gypsum matrix together.” One fire-rated formulation uses 0.3–0.9% mineral or glass fiber by stucco weight (US 8,323,785).
• Vermiculite “is expected to expand under heated conditions to compensate for the shrinkage” of the core. The same patent uses high-expansion vermiculite at 5–10% of stucco weight. It notes that expansion without restraint can cause spalling (US 8,323,785).
• Clay, colloidal silica or alumina in fine particles have also been used, though one earlier formulation still “experienced significant shrinkage” (as cited in US 8,323,785).

The Gypsum Association describes Type X as “the baseline fire-resistive gypsum panel defined in ASTM C1396/C1396M.” A 5/8-inch board must give at least a one-hour rating in the specified wood-stud wall test. Under EN 520, Type F boards must pass a core cohesion test at high temperature in which none of the six specimens may break (EN 520, as reproduced in DRS 632). For how these boards are made on a standard line, see fire-resistant gypsum board (Type X / Type F).

Planning Additive Dosing for a New Gypsum Board Plant

The additive section is designed around the board types you will make. Plan it in this order:

1. List your board types and standards. For example, standard Type A, moisture-resistant H2 and fire-resistant F to EN 520, or regular and Type X to ASTM C1396. Each type adds additives.
2. Turn that list into an additive list. Mark each additive as powder, liquid or foam, and whether it is used on every board or only on special boards.
3. Size storage and dosing. Powders need silos or bag stations and screw or loss-in-weight feeders. Liquids need tanks, metering pumps and flow meters. Starch may need wet preparation, and foam needs a soap and foam-generation unit. Glass fiber needs its own feeder.
4. Plan changeovers. Moving from standard to moisture-resistant or fire-resistant board changes several doses at once. Recipe control in the mixing system reduces scrap at each changeover.
5. Check local supply. Some additives are easy to source locally, and some are usually imported. Supply distance and shelf life, especially for liquids and coated accelerators, set your stock levels.
6. Run laboratory trials with your own stucco and water before start-up, and line trials during commissioning, to fix the starting doses.
7. Set incoming checks for each additive. Examples are viscosity for starch, active content for foaming agents and solids for dispersants and emulsions.

FAQ

What additives are used in gypsum board?

The common ones are starch for paper bond and core strength, a foaming agent to cut weight, a set accelerator such as ground gypsum or potassium sulfate, a retarder, a dispersant such as naphthalene sulfonate or PCE to reduce water, and sodium trimetaphosphate for strength and sag resistance. Special boards add siloxane or wax for moisture resistance, and glass fiber and vermiculite for fire resistance.

What is gypsum board made of?

A gypsum board is a core of set gypsum (calcium sulfate dihydrate) between two sheets of recycled-fiber paper. The core is made by mixing stucco, which is calcined gypsum, with water, foam and small amounts of additives. On our reference line, a 12 mm board uses 7.0 kg of stucco and 0.45 kg of facing paper per square meter.

Why are accelerators and retarders both used in gypsum board?

They control two different moments. The retarder keeps the slurry fluid in the mixer and through forming, so it does not set inside the equipment. The accelerator makes sure the board has set firmly by the time it reaches the cutting knife. Plants balance the two to match line speed, stucco reactivity and water temperature.

What makes gypsum board moisture resistant?

Moisture-resistant board has a water repellent in the core, usually siloxane with a magnesium oxide catalyst or a wax emulsion. It often also has a more water-resistant face paper. Untreated gypsum can absorb 40–50% water by weight. A siloxane-treated board in published tests absorbed about 4.5%, under a 5% limit. That patent used an ASTM test; EN 520 Type H1 also sets 5%, measured by its own method.

What additives make gypsum board fire resistant?

Gypsum already resists fire because of its chemically combined water. Fire-rated boards, Type X or Type F, add glass fiber to hold the core together as it shrinks. Many also add vermiculite, which expands in heat to offset the shrinkage, or fine clay. One published fire-rated formulation uses 0.3–0.9% mineral or glass fiber and 5–10% high-expansion vermiculite by stucco weight.

Planning a Gypsum Board Line?

Additive storage, dosing and mixing are part of every line and are sized for the board types you plan to produce. See what a complete gypsum board production line includes, with consumption per square meter, and read our gypsum board plant setup guide for the full planning sequence.

Sources

• US EPA, AP-42 Section 11.16, Gypsum Manufacturing: https://gaftp.epa.gov/ap42/ch11/s16/final/c11s16_1995.pdf
• US Patent Application 2019/0092689, Migrating starch with high cold-water solubility for use in preparing gypsum board (United States Gypsum): https://patents.justia.com/patent/20190092689
• US Patent 12,071,376, Gypsum board with improved starch binder (Gold Bond Building Products): https://patents.google.com/patent/US12071376
• US Patent 7,731,794, High starch light weight gypsum wallboard (United States Gypsum): https://patents.google.com/patent/US7731794
• US Patent 11,773,020, Gypsum composition comprising uncooked starch having mid-range viscosity (United States Gypsum): https://patents.google.com/patent/US11773020B2/en
• US Patent 5,643,510, Producing foamed gypsum board using a foaming agent blend (United States Gypsum): https://patents.google.com/patent/US5643510
• US Patent 4,156,615, Foaming agents for gypsum board manufacture (Domtar; assigned to Stepan Canada): https://patents.google.com/patent/US4156615A/en
• US Patent 10,737,979, Gypsum set accelerator and method of preparing same (United States Gypsum): https://patents.google.com/patent/US10737979
• US Patent 8,343,273, Method of modifying beta stucco using diethylene-triamine-pentaacetic acid (United States Gypsum): https://patents.google.com/patent/US8343273
• US Patent 6,342,284, Gypsum-containing product having increased resistance to permanent deformation (United States Gypsum): https://patents.google.com/patent/US6342284
• US Patent 7,892,472, Method of making water-resistant gypsum-based article (United States Gypsum): https://patents.google.com/patent/US7892472B2/en
• US Patent 9,045,370, Aqueous wax emulsions having reduced solids content for use in gypsum compositions and building products (Henry Company): https://patents.google.com/patent/US9045370
• US Patent 8,323,785, Lightweight, reduced density fire rated gypsum panels (United States Gypsum): https://patents.google.com/patent/US8323785
• BASF, Gypsum Wallboard additives brochure (foaming agents and superplasticizers): https://www.basf.com/hk/documents/en/products-industries/construction/BASF_Gypsum%20Wallboard_Brochure.pdf.assetinline.pdf
• Gypsum Association, What is a Type X gypsum panel?: https://gypsum.org/what-is-a-type-x-gypsum-panel
• Rwanda Standards Board, DRS 632:2026 Gypsum plasterboards (draft, prepared with reference to BS EN 520): https://www.rsb.gov.rw/fileadmin/user_upload/DRS_632_2026.pdf
• ASTM C1396/C1396M, Standard Specification for Gypsum Board: https://store.astm.org/c1396_c1396m-24.html