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 product. Gypsum has sound-deadening properties. It is relatively easily patched or replaced if it becomes damaged. There are a variety of decorative finishes that can be applied to the wallboard, including paint and wallpaper. Even with all of these advantages, it is still a relatively inexpensive building material.
Gypsum is also known as calcium sulfate dihydrate, terra alba or landplaster. Plaster of Paris is also known as calcined gypsum, stucco, calcium sulfate semihydrate, calcium sulfate half-hydrate or calcium sulfate hemihydrate. Synthetic gypsum, for example, that which is a byproduct of flue gas desulfurization processes from power plants, may also be used. When it is mined, raw gypsum is generally found in the dihydrate form. In this form, there are two water molecules associated with each molecule of calcium sulfate. To produce the hemihydrate form, the gypsum is calcined to drive off some of the water of hydration by the following equation:CaSO4.2H2O→CaSO4.1/2H2O+3/2H2O
A number of useful gypsum products can be made by mixing the stucco with water and permitting it to set by allowing the calcium sulfate hemihydrate to react with water to convert the hemihydrate into a matrix of interlocking calcium sulfate dihydrate crystals. As the matrix forms, the product slurry becomes firm and holds a desired shape. Excess water must then be removed from the product by drying.
Significant amounts of energy are expended in the process of making gypsum articles. Landplaster is calcined to make stucco by heating it to drive off water of hydration. Later the water is replaced as the gypsum sets by hydration of the hemihydrate to the dihydrate form. Excess water used to fluidize the slurry is then driven from the set article by drying it in an oven or a kiln. Thus, reducing the amount of water needed to fluidize the slurry turns into a monetary savings when fuel requirements are decreased. Additional fuel savings would result if the amount of material that required calcining were reduced.
Attempts have been made to reduce the amount of water used to make a fluid slurry using dispersants. Polycarboxylate superplasticizers are very effective in allowing water reduction and where water reduction results in increased density, a strength increase is achieved. 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. The high price of this component can overcome the narrow margins afforded these products in a highly competitive marketplace.
Another disadvantage associated with polycarboxylate dispersants is the retardation of the setting reaction. Gypsum board is made on high-speed production lines where the slurry is mixed, poured, shaped and dried in a matter of minutes. The board must be able to hold its shape to be moved from one conveyor line to another to put the board into the kiln. 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.
Modifiers have been found that increase the efficacy of the dispersant in fluidizing the slurry, allowing the modifier to replace a portion of the expensive dispersant while still reducing water demand. However, it has been found that the modifier does not work consistently, depending on how and when it is added to the slurry. Thus, there is a need for a delivery vehicle to carry the modifier to the slurry in a manner that allows it to perform consistently so that the amount of dispersant can be reduced.
The use of fillers that are easily fluidizable in water have been considered as another method of reducing fuel demand. However, one of the important properties of gypsum products, and especially gypsum panels or wallboard, is its fire resistance. Calcium sulfate dihydrate is approximately 20% water by weight. Replacing a portion of the calcined gypsum with fillers that are less fire retardant diminishes this property in the finished product. Many fillers also reduce the compressive strength and the nail pull strength of wallboard.
Landplaster has been used as a filler in gypsum products. It is also fire retardant, inexpensive, readily available and reduces the amount of calcined gypsum that is needed, but it also has disadvantages. Calcium sulfate dihydrate used in sufficient quantities to act as a filler also acts as a set accelerator for the hemihydrate by providing seed crystals that start the crystallization process more quickly. This leads to premature stiffening of the slurry.
Thus there is a need in the art for a filler for use in gypsum articles, particularly wallboard, that reduces fuel consumption by replacing calcined gypsum, by reducing the amount of water driven from the set product or both. The filler should have fire retardancy approximately equal to set gypsum and it should be inexpensive, readily available and should not decrease the strength of the finished product.
Thus, there is a need in the art to reduce the dosage of dispersants used in a gypsum slurry while maintaining flowability of the slurry. Reduction in dispersant use would result in saving of costs spent on the dispersant and would reduce adverse side effects, such as set retardation.