Overview

	Figure 1: pourable water demand

Shortcomings of Current Processing Methods

Gypsum plasters have been used for thousands of years, based on the dehydration of gypsum rock to plaster of Paris or CaSO4.2H20 hemihydrate. This process, called calcination, normally results in very disordered and highly fractured hemihydrate crystals that spontaneously break into smaller particles on mixing with water. These small particles with high surface area need to be mixed with a lot of water to make a free flowing slurry, about 4-6 times more water than is needed to rehydrate the hemihydrate back to gypsum (CaSO4.2H2O). This water holding property is useful in many applications since it gives the plaster “body”, or what is called “plastic rheological properties”, so that the plaster will not flow unless under shear stress from something like a trowel. For many applications, however, this “plastic” behavior results in more water used than needed, and consequently more energy is required to evaporate this extra water. As such, with increasing fuel costs, drying operations have become a significant concern for many industries using calcined gypsum. These plasters are known as beta hemihydrate plasters and require about 70-90 ml of water per 100g of plaster to make a slurry that can flow freely (figure 1).

About 100 years ago it was discovered that if the calcination was done in an autoclave under high water vapor pressure, the hemihydrate crystals were less fractured and less subject to breakdown on mixing with water. This form of hemihydrate is called alpha hemihydrate to distinguish it from the beta form. The more stable crystals of alpha hemihydrate require less water to make a flowable slurry, only about 2-3 times what was needed for rehydration, giving a water demand of between 28 and 45 ml/100g of plaster. When less water is mixed with the setting plaster the overall density of the set gypsum is increased and the compressive strength rises dramatically even above that of high-strength concrete.

	Illustration - gypsum rock
	Illustration - Gypsum is a basic building material used throughout the home, including walls, floors, ceilings, and insulation.

Conventional Processing of Alpha-Hemihydrate

There are generally two ways to produce alpha plaster, by putting lumps of rock into a steam heated autoclave (dry method) or by mixing a finely divided gypsum into a slurry in water under steam pressure (wet method) while controlling pH and adding crystal habit modifiers.

The dry method requires the use of gypsum rocks, of a certain size, placed in the autoclave for calcination. The wet method, however, can use either ground gypsum rock or finely divided byproduct gypsum from various chemical processes like flue gas desulphurization (FGD). In general, the equipment to perform the wet process calcination is very specialized and expensive. It is also rather difficult to control the calcination and water removal steps to consistently produce a high quality product.

Most applications of gypsum require plaster properties that are part way between alpha and beta in order to add “body” to the slurry while avoiding the cost of the high water demand. The ideal situation is to have a low water demand plaster that remains a pourable slurry suitable for processing into products. For additional strength, lower water demand is a desirable property that cannot be achieved by an inexpensive beta plaster.

Several companies have tried to make an “alpha like” beta plaster, but they have only been able to reduce the water demand to about 60 ml/100g plaster. There are no commercial processes making gypsum plaster with a water demand of between 45 and 55 ml/100g, half way between alpha and beta. As a result, to achieve a plaster with this performance, a manufacturer is required to build both an alpha and a beta plant as well as a blending plant to mix the two. Such a choice represents a significant capital outlay and is difficult to justify particularly given today’s high energy costs.

GBT’s Hi-PSI-G alpha-hemihydrate binder technology is technically aimed at addressing a direct response and immediately available offering aimed at solving this industrial dilemma. In the course of development, GBT has developed a small-scale plant, conducted an extensive array of testing, and assembled necessary resources including facility space and expert personnel.

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