Additives for gypsum plaster, mortar and cast gypsum
Gypsum plasters, gypsum mortars, joint compounds and cast gypsum products all start from calcined gypsum that sets within minutes once mixed with water. Additives control how long the mix stays workable, how much water it needs and how well it holds that water on the wall. LigninCorp supplies set retarders, water reducers and water-retention agents for these products, with TDS, samples and dosage guidance.
Products covered
- Gypsum hand and machine plaster
- Gypsum mortar and self-levelling
- Joint compound
- Cast and moulded gypsum, ceiling tiles
Functions
- Set retardation and open time
- Water reduction and flow
- Water retention and workability
- Strength of the set gypsum
Recommended Products
Typical Dosage / Technical Notes
Citric acid 0.01–0.10% of dry gypsum; water reducers 0.1–0.8% of dry gypsum. See dosage.
Which additive does what?
Most gypsum formulations combine one additive from each group below. The balance between retarder and water reducer matters, because lignosulfonates and other water reducers also slow setting.
| Function | Products | Where it is used |
|---|---|---|
| Set retarder | Sodium lignosulfonate retarder, citric acid, sodium gluconate | Gypsum plaster, joint compound, gypsum mortar: longer working time |
| Water reducer / fluidizer | Sodium lignosulfonate, SNF, Dispersant NNO | Cast and moulded gypsum, ceiling tiles, flowable gypsum mortar: lower water demand, higher strength |
| Water retention / thickener | HPMC | Hand and machine plaster, joint compound: workability, open time, adhesion |
How are gypsum plaster additives added, and at what dosage?
In dry-mix gypsum products, all additives are blended into the gypsum powder in the plant mixer; very small retarder doses are often premixed with a little filler first so they spread evenly. Construction-grade citric acid typically starts at 0.01–0.10% of the dry gypsum, and water reducers at 0.1–0.8%. Ranges are typical starting points from our technical data sheets; the final dose is set by setting-time and flow tests on your gypsum.
| Product | Typical starting dosage | Notes |
|---|---|---|
| Citric acid, construction grade | 0.01–0.10% of dry gypsum | Strong retarder; overdosing can stop setting permanently. Trial mixes are essential. |
| Sodium lignosulfonate, retarder powder grade | Trial dosage | Optimize by gypsum type and setting profile; check with accelerators and superplasticizers. |
| Plasterboard binder (GAC-PB) | 0.1–0.3% of dry stucco | Water reducer for cast and moulded gypsum, ceiling tiles and joint compound. |
| Dispersant NNO | 0.3–0.8% of dry gypsum | Stronger water reduction for flowable and high-strength products. |
| Sodium gluconate, HPMC | Set by trial | HPMC viscosity grade is chosen for the application method. |
What happens if the dose is too high?
Too much retarder can leave the plaster soft or stop it setting altogether, which is why citric acid is dosed in hundredths of a percent. Too much water reducer can also extend setting time and make lignosulfonate-based products darker.
Trial checklist
- Setting time (initial and final) at the site temperature
- Water-to-plaster ratio and flow or consistency
- Water retention and open time on the wall
- Compressive and flexural strength after drying
- Colour of the set product
FAQ
What additives are used in gypsum plaster?
Most gypsum plasters combine a set retarder (such as citric acid, sodium gluconate or a lignosulfonate retarder) to control working time, a water-retention agent such as HPMC for workability and adhesion, and in flowable or cast products a water reducer such as sodium lignosulfonate, SNF or NNO.
How much citric acid retarder is used in gypsum plaster?
Construction-grade citric acid typically starts at 0.01–0.10% of the dry gypsum. The window is narrow: overdosing can leave the plaster permanently unset, so the dose is fixed by setting-time tests on the actual gypsum.
Can lignosulfonate be used in gypsum plaster?
Yes. Sodium lignosulfonate acts as a mild water reducer and retarder in gypsum systems. It slows setting and is brown, so it suits products where a slight colour is acceptable, and the accelerator or retarder balance is checked in the same trial.
What is the difference between this page and plasterboard additives?
This page covers gypsum plasters, gypsum mortars, joint compounds and cast gypsum. Additives for continuous wallboard production lines are covered on the plasterboard additives page.
What should buyers send for a recommendation?
Gypsum type (beta or alpha hemihydrate, anhydrite), product type, target setting time and water-to-plaster ratio, current additive package and the main problem to solve: setting, flow, water demand or workability.