Gypsum Defoamer Applications: Board, Plaster, Self-Leveling

A gypsum product is judged by its surface, and air is what ruins it. A board with pinholes fails the finish check. A self-levelling floor with craters has to be patched or topped. A plaster that blisters under the trowel wastes a crew's afternoon. In each case the cause is the same, air trapped in the slurry, and the fix is the same, a gypsum defoamer applied at the right point in the process. This article looks at how the problem presents in the three main gypsum applications and how plants handle it.

How Air Gets into Gypsum Slurry

Air enters at four points. The stucco itself carries entrained air from calcination. High-shear mixing folds air into the slurry faster than it can escape. Pumping and transport keep the bubbles suspended. And the admixture package, retarders, superplasticisers, cellulose ethers and viscosity modifiers, can stabilise bubbles once they form by changing the surface chemistry of the slurry.

The bubbles that survive to the surface leave pinholes and craters. The ones trapped inside leave voids that cut section strength. Both are counted as defects, and both are preventable at the slurry stage, which is where a defoamer does its work.

Gypsum Board: Pinholes and Paper Bond

On a board line, the slurry is spread, covered with paper and formed in seconds. There is no time for bubbles to escape on their own. If the slurry carries foam, three things happen: pinholes appear on the face after set, voids weaken the core, and the bond between core and paper suffers where bubbles sit at the interface.

The fix on most lines is a liquid gypsum defoamer metered into the mix water, at a dose that holds the surface clean without destabilising the slurry. Polyether-modified siloxane types are common here because they break foam fast without hurting the wetting that the paper bond depends on. The trial target is simple: a clean face at the highest line speed the rest of the process allows.

Self-Leveling Compound: Craters and the Flow Window

Self-levelling compound has a different problem. The slurry is designed to flow and self-flatten, and the same low viscosity that lets it level lets air rise. Bubbles surface during the flow window and leave craters that set into the floor. On a large pour, hundreds of craters mean a topping coat or a callback.

The gypsum defoamer has to act within the flow window: break the bubbles as they rise, without killing the flow or altering the set time that the levelling layer must meet. Dosing runs at the lower end of the range, and the trial is judged on a small pour that simulates the real thickness and flow.

Plaster and Skim Coats: Blisters and Edge Lines

Hand and machine plaster carry less foam than board slurry, but what foam they carry is visible. Blisters form when air pockets under the surface expand or burst during set. Edge lines and feathering problems follow when the trowel works against a slurry with unstable air content.

For plaster, the defoamer dose is usually the smallest one that removes visible surface defects. Over-dosing shows up as a change in trowel feel, which plasterers notice immediately, so the setting is done in small steps rather than one jump.

The Dosage Logic That Applies Everywhere

The same rules hold across all three applications. Start at the low end, around 0.1 per cent of the dry binder, and step up until the surface is clean. Test on the actual slurry, because mixer type and raw material move the number more than the brochure suggests. Add the defoamer before the foam forms, to the water or the dry pre-blend, rather than into a foamed slurry. And watch the interaction with the rest of the package, because a retarder or viscosity modifier that stabilises foam will raise the defoamer requirement.

What the Trial Should Measure

A defoamer trial fails when it is judged by eye alone. The useful measurements are pinhole count per square metre on board faces, crater count on a reference self-levelling pour, wet density of the slurry, which rises when air is removed, and set time, which should stay inside the formulation window. When those four numbers hold, the defoamer is doing its job and the rest of the formulation is unchanged.

FAQ

Why does gypsum slurry foam?

Air enters with the raw material and mixing, and additives such as cellulose ethers and superplasticisers can stabilise the bubbles once they form.

What defects does foam cause in gypsum board?

Pinholes on the face, voids in the core and weakened bond between core and paper where bubbles sit at the interface.

How is a defoamer added on a board line?

Most lines meter a liquid defoamer into the mix water before the gypsum is added, so the bubbles break before they form.

Does a defoamer affect set time or strength?

At the correct dose, no. Over-dosing can destabilise the slurry and cause surface defects, which is why the smallest effective dose is the target.

Can I use the same defoamer for board and self-levelling?

Often yes, at different doses, but each application should be trialled separately because the flow and surface requirements differ.

How do I know the dose is right?

The trial targets are pinhole count, crater count, wet density and set time. When those four stay inside the specification, the dose is right.

Cleaning Up the Surface

Air in gypsum slurry is a process problem with a process answer. A defoamer added at the right point, at the right dose, removes the defect class before it reaches the surface. For grade selection, dosing guidance and a trial sample, contact [email protected] or call +86-537-3739818. Full product details are available at the company website: https://www.yuanlianchem.com