Modified landplaster as a wallboard filler

Abstract

A gypsum slurry includes calcium sulfate hemihydrate, water and calcium sulfate dihydrate is coated with a hydrophilic, dispersible coating. The coating is less soluble than the calcium sulfate hemihydrate to delay exposure of the landplaster to the remainder of the slurry, preventing premature crystallization and the early stiffening that accompanies it. Another embodiment is a gypsum slurry that includes calcium sulfate hemihydrate, a polycarboxylate dispersant, water and coated calcium sulfate dihydrate. In this case, the hydrophilic, dispersible coating is selected to serve as a modifier to enhance the ability of the dispersant to fluidize the gypsum slurry.

Description

CROSS REFERENCES TO RELATED APPLICATIONS [0001] This application is related to co-pending U.S. Ser. No. 11 / 152,317, entitled “Modifiers for Gypsum Slurries and Method of Using Them”, U.S. Ser. No. 11 / 152,418 entitled “Gypsum Products Utilizing a Two-Repeating Unit System and Process for Making Them”, and U.S. Ser. No. 11 / 152,661, entitled “Fast Drying Wallboard”, all filed Jun. 14, 2005, and herein incorporated by reference.FIELD OF THE INVENTION [0002] The present invention is directed to a composition utilizing a coated landplaster as a filler in gypsum slurries. More specifically, landplaster is coated with a coating that is less soluble than stucco to reduce or delay its ability to catalyze crystallization reactions. BACKGROUND OF THE INVENTION [0003] Gypsum-based building products are commonly used in construction. Wallboard made of gypsum is fire retardant and can be used in the construction of walls of almost any shape. It is used primarily as an interior wall and ceiling pro...

AI Technical Summary

Benefits of technology

The present invention is a coated landplaster that improves the performance of gypsum products. The coating prevents premature crystallization and stiffening of the slurry, allowing for better performance of the polycarboxylate dispersants. The coated landplaster can also serve as a delivery vehicle for modifiers that enhance the performance of the dispersant. The use of coated landplaster results in lower requirements for calcined gypsum, savings in fuel and power consumed by the calcining process, and an increase in capacity. The coating is less soluble than the landplaster, and its removal takes time to control the setting reactions. The coating is highly dispersible, allowing for easier dispersion in the slurry and reducing the water needed to fluidize the slurry. The use of coated landplaster results in higher strengths than other fillers, and the coating can also deliver modifiers to enhance the performance of the dispersant.

Problems solved by technology

These materials are relatively expensive.

When used in large doses, polycarboxylate dispersants can be one of the single, most expensive additives in making gypsum products.

Another disadvantage associated with polycarboxylate dispersants is the retardation of the setting reaction.

Damage can occur if the boards have not attained a minimum green strength by the time they are cut to length and handled during the manufacturing process.

If the board line has to be slowed down because the board is not sufficiently set to move on to the next step in the process, production costs are driven up, resulting in an economically uncompetitive product.

However, it has been found that the modifier does not work consistently, depending on how and when it is added to the slurry.

Many fillers also reduce the compressive strength and the nail pull strength of wallboard.

This leads to premature stiffening of the slurry.

The prior art has failed to adequately address the problem of improving the efficacy of a given polycarboxylate dispersant.

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