In a recent dispatch, now accessible on the West Virginia Coal Association's web site via the link:

"United States Patent 5,362,471 - Producing Gypsum Flake from Flue Gas Desulfurization; 1994; Assignee: Air Products and Chemicals, Incorporated, Allentown (PA); Abstract: This invention relates to an improved process for producing gypsum calcium sulfate dihydrate flakes from powdered gypsum calcium sulfate dihydrate obtained from the desulfurization of flue gas by the wet limestone process. Gypsum, which is calcium sulfate, has wide application in the manufacture of construction products and particularly wallboard";

we documented one of the ways in which "waste" sludge generated by Coal-fired power plant flue gas desulfurization scrubbers can be utilized in the making of synthetic Gypsum, for use in the manufacture of what is known variously in the building trades as "wallboard", "gypsum board" and "drywall"; that is, the wall covering now used on the interiors of most homes, as a more efficient and modern-day replacement for the old lath and plaster type of construction.

In that report, we included a separate link to a document provided by one of few major suppliers of the mineral gypsum to the construction materials manufacturing industries, concerning that application for Flue Gas Desulfurization, FGD, Gypsum, i.e.:

"FGD Gypsum in Wallboard and Other Products; United States Gypsum Company; 2007";
wherein it's explained that "63% of all FGD Gypsum produced" in the US is utilized in the manufacture of wallboard, and, that, not only does a very significant portion of the wallboard made in the US utilize FGD Gypsum, expansion of that usage is planned.
Herein, we learn that the above "United States Gypsum Company", which, as can be learned via:
USG Corporation - Wikipedia, the free encyclopedia; is "the largest distributor of wallboard in the United States and the largest manufacturer of gypsum products in North America"; also says that plain old Coal Fly Ash can be mixed with that FGD Gypsum in the manufacture of wallboard, a practice which actually makes the wallboard product itself stronger and better.
As seen in excerpts from the initial link in this dispatch to:
"United States Patent 4,403,006 - Sag-resistant Gypsum Board Containing Fly Ash
Date: September, 1983
Inventors: Robert Bruce and Richard Kuntze, Canada
Assignee: United States Gypsum Company, Chicago
Abstract: A gypsum board consisting essentially of a monolithic cellular core of set gypsum and a fiberous cover sheet encasement provided with improved properties by the gypsum core having incorporated therein coal fly ash in an amount of about 1-20% by weight of stucco in the gypsum slurry used in forming the board and method of producing the board are disclosed.
(Don't be distracted by use of the term "stucco" if it makes you think of exterior stucco finishes. In this case, it's industry jargon for the "goop", for want of a better label, that goes into the making of wallboard.)
Claims: A sag resistant gypsum board consisting essentially of a monolithic cellular core of set gypsum solids from an aqueous stucco slurry and having a fibrous cover sheet encasement, with the gypsum core having incorporated therein a small amount of coal fly ash included in the slurry, the fly ash having a particle size (and other physical dimensions as specified).
The gypsum board ... in which the coal fly ash is present in an amount by weight from about 0.8% to about 17% based on the weight of gypsum. 

A method of producing fiber covered gypsum board having a set core of cellular gypsum which exhibits improved sag resistance, which comprises the steps of: preparing a foamed aqueous slurry of stucco and a small amount of coal fly ash having a particle size (specified); depositing the resultant slurry between fiber cover sheets; forming the slurry and cover sheets into boards of the desired dimension; and: drying the boards.

The process ... in which the slurry contains about 1-20% of coal fly ash by weight ... .
Background and Field: Gypsum board products comprising a monolithic core of set, interlaced gypsum crystals disposed between fiberous, especially paper, liners are well known in the art. Gypsum board is normally produced by feeding a slurry of calcium sulfate hemihydrate (stucco), water, foam, and any other desired additive such as setting time modifiers, between two sheets of paper and the like fiberous material. After conversion of the stucco into gypsum (calcium sulfate dihydrate) the set but still wet board is conveyed through a drying chamber to remove excess water. Such gypsum core boards, including lath and sheathing boards, have been employed in building construction for a long time in the construction of interior walls and ceilings, and are commonly referred to as gypsum wallboard, plasterboard and so forth. Such boards are presently commercially available in various thicknesses, generally 1/4th inch to 5/8th inch. Paper covered gypsum wallboard currently being marketed in a 1/2 inch thickness generally weighs between 1650 and 2150 pounds. per thousand square feet.
At the low end of this range, the gypsum core has a density of about 40-42 lbs. per cubic foot and a compressive strength of about 450 psi. 

As is well known in the building industry such gypsum boards may tend to exhibit a visible sagging after installation, particularly in ceiling constructions under humid conditions. This may become quite evident especially in the middle of ceiling mounted board nailed to 16 inch to 24 inch spaced apart supporting members. Certain surface decorations increase the tendency to sag.
Recently, coal fly ash has been finding increased practical uses in industry either as inert lightweight filler or in combinations with small amounts of gypsum and a lime source to activate its pozzolanic activity. Thus coal fly ash has been suggested along with other fillers for use in compositions such as dense gypsum metal casting molding compositions (and) thermal and sound insulating gypsum construction material.
For example further, fly ash has been added to the Portland cement used in making concrete. Here large quantities of fly ash, in proportions of up to about half of the Portland cement in the cement-aggregate mixture, are added to react with lime generated during the setting of the Portland cement to produce additional cementitious compounds. Utilization of fly ash in this manner often results in substantial cost savings. A saving of over 1 million dollars in the construction costs, for example, of the Hungry Horse dam has been reported.
It has been stated that concrete made with Portland cement and coal fly ash has greater strength and less heat of hydration, is more resistant to chemical action by acids, alkalis and sulfates.
(See, for just one example of the above, our report:

"United States Patent 5,772,752 - Sulfate and Acid Resistant Concrete and Mortar; 1998; Assignee: New Jersey Institute of Technology; The present invention relates to concrete, mortar and other hardenable mixtures comprising cement and fly ash for use in construction and other applications, which hardenable mixtures demonstrate significant levels of acid and sulfate resistance while maintaining acceptable compressive strength properties".)

Summary: It is a well known phenomenon of the art that as the thickness of gypsum board increases, its resistance to sagging under humid conditions increases. Paper covered gypsum wallboard of 5/8 inch thickness generally will exhibit only about half the sag encountered with paper covered gypsum wallboard of 1/2 inch thickness. 

It is one object and principle advantage of the present invention to provide gypsum board having improved properties for use in humid areas such that the resistance to sag of 1/2 inch board is improved to the point that it behaves similar to 5/8 inch thick gypsum board. 

Another object is the provision of a process for the production of fibrous cover sheet encased gypsum board provided with improved sag properties. 

It was surprisingly found in the present invention that the addition of coal fly ash of widely varying chemical and physical characteristics in a small amount such as 5% and more generally about 1-20% by weight of the stucco used in making the gypsum slurry for forming a gypsum board, of the type comprising a monolithic cellular core of said gypsum and a fibrous cover sheet encasement, greatly improved the sag resistance properties of the gypsum board."

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So, both Coal-fired power plant Flue Gas Desulfurization sludge and Coal-fired power plant Fly Ash, added to a level of up to 20% of the final mix, can be consumed and utilized together in the making of an improved version of the commodity construction product "gypsum board", just as they can be used together, as in:

Pittsburgh Converts Coal Ash and Flue Gas into Cement | Research & Development; concerning: "United States Patent 5,766,339 - Producing Cement from a Flue Gas Desulfurization Waste; 1998; Assignee: Dravo Lime Company, Pittsburgh; Cement is produced by forming a moist mixture of a flue gas desulfurization process waste product ... and ... aluminum, iron, silica and carbon (and) wherein said source of aluminum and iron comprises fly ash";
to make another commodity construction product, Portland-type cement; which practice, as seen for just one example in our above citation of "United States Patent 5,772,752 - Sulfate and Acid Resistant Concrete and Mortar", also leads to the production of an improved, higher-performance end product.
As we've documented in previous reports, and as we will further document in the future, the established potential market for Coal Ash and FGD wastes in the making of superior Portland-type cement and concrete far exceeds our current production and supply of those materials.
Their employment, as described herein by the process of our subject, "US Patent 4,403,006 - Sag-resistant Gypsum Board Containing Fly Ash", and by the report "FGD Gypsum in Wallboard and Other Products; United States Gypsum Company", in the making of a superior wall board, though minor relative to the potential demand in the making of cement, is a competing application.
We don't make nearly enough Coal Ash or FGD wastes to fully supply both of those markets; and, thus, we could and should start to view and treat the solid residua resulting from our essential use of Coal in the generation of economical electric power for what they truly are:
Valuable minerals; raw materials in limited supply relative to their potential markets, that, if those market potentials were realized, and if we applied ourselves to the tasks of harvesting and selling those valuable mineral products, could bring more badly-needed jobs, and revenue, to United States Coal Country.

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