Abstract: Possible emission of hydrogen sulphide from building materials incorporating a sulphide-containing constituent, for example ground blast furnace slag, can be suppressed or prevented by the application of a surface covering composition containing gypsum plaster and a metal compound capable of reacting with hydrogen sulphide to form a stable sulphide. It is preferred that the metal compound should have no substantial effect on the setting rate of the gypsum plaster, and may for example be an insoluble compound, while it is also preferred that the sulphide should be colorless and insoluble. Preferred metal compounds are zinc compounds and especially zinc oxide. Surface covering compositions for the purposes of the invention may contain from 0.01 to 1% by weight of the metal compound. The sulphide-containing constituent may be in a building structure or may itself be contained in a surface covering composition of rendering.

Abstract: A gypsum set accelerator for use in the production of gypsum wallboard comprises a tube or ball milled mixture of gypsum and up to 20 percent by weight of the gypsum of a synthetic water-soluble solid surfactant. A process for the manufacture of gypsum wallboard is also described comprising spreading between paper sheets an aqueous slurry of calcium sulphate hemihydrate defined above at a rate of between 0.5 and 15 kilograms per 100 square meters (1 and 30 pounds per 1000 square feet) of gypsum wallboard, and allowing the slurry to set. The accelerator has high potency and appears to provide wallboard of increased compressive strength.

Abstract: The present invention is directed to the treatment of gypsum plaster prepared from gypsum rock having a substantial content of soluble salts wherein the gypsum plaster is rendered more suitable for manufacturing processes such as the production of gypsum board by rapid and intimate mixing with a flow of cold water followed by rapid separation of the bulk of the water on a continuous filtration apparatus. A preferred form of an apparatus is a suction belt filter and it is preferred that the residence time of the wet plaster on the filter does not exceed 15 seconds. The total time elapsing between initial contact with water and recovery from the continuous filter preferably does not exceed 30 seconds. The salt content of the plaster is considerably reduced by this treatment without significant hydration occurring and the treatment is economic because cold water (up to 40.degree. C.) and no retarder or other additives are preferably employed.

Abstract: The present invention is directed to the preparation of gypsum plaster prepared from gypsum rock having a substantial salt content to render it more suitable for manufacturing processes such as the manufacture of gypsum board by rapid and intimate mixing with a flow of water followed by rapid separation of the bulk of the water, the recovered purified wet or damp plaster being used at once in the manufacturing process before an unacceptable degree of hydration has occurred. It is preferred to use for the washing cold water, say up to 40.degree. C. and to add no retarder or other additive at the washing stage in order to minimize cost and avoid interference with the subsequent manufacturing process. The time elapsing between contacting the plaster with water and recovery of wet purified plaster preferably does not exceed 30 seconds. A rotary or vortical mixer may be used and the preferred means for rapid separation is a continuous decanting centrifuge having a cone angle not exceeding 7.degree..

Abstract: A method and apparatus for calcining calcium sulphate dihydrate in a calcination vessel in which the dihydrate is heated in the vessel to calcination temperature not only by heat applied to the exterior of the vessel but also by hot gas, especially hot gaseous combustion products, supplied into the interior of the mass of calcining material within the vessel. The supply of hot gas according to the invention can improve the production rate in both batch and continuous calcination operations without adversely affecting the product quality, by permitting increase in the heat input to the vessel without entailing the risk of vessel bottom burn-out.

Abstract: Porous gypsum granules are produced by adding powdered plaster to a stable aqueous foam and agitating the foam until granules of desired size are formed. Agitation is terminated before the foam collapses, but when the foam has subsequently collapsed the damp granules are recovered and dried. The product granules have a generally spheroidal shape mainly in the size range 0.25 to 2 mm, with a fine compact outer surface layer and a coarser interior with voids which confer high absorbency. These can be used as carriers for active materials such as herbicides or insecticides for agricultural or horticultural purposes or as fillers for plastics. Apparatus for continuously mixing fluid materials and suitable for use with foam and gypsum consists of a trough-shaped conveyor with a series of rotary agitators spaced along it.

Abstract: Satisfactory or enhanced dry bond between the gypsum core of plasterboard and paper facing sheets is achieved by treatment of at least the inner faces of the paper sheets with a water soluble zirconium salt before contacting the sheets with a core slurry containing gypsum plaster and starch as a dry bond assistant. The preferred zirconium salt is zirconium acetate although other salts such as oxychloride, nitrate and sulphate can also be used. The preferred level of zirconium is from 1 to 2 g. per square meter of paper, expressed as zirconium dioxide, and excellent dry bonds are formed with as little as 0.2% starch by weight of core solids, which represents an important economy in the use of starch.

Abstract: Porous gypsum granules are produced by adding powdered plaster to a stable aqueous foam and agitating the foam until granules of desired size are formed. Agitation is terminated before the foam collapses, but when the foam has subsequently collapsed the damp granules are recovered and dried. The product granules have a generally spheroidal shape mainly in the size range 0.25 to 2 mm, with a fine compact outer surface layer and a coarser interior with voids which confer high absorbency. These can be used as carriers for active materials such as herbicides or insecticides for agricultural or horticultural purposes or as fillers for plastics. Apparatus for continuously mixing fluid materials and suitable for use with foam and gypsum consists of a trough-shaped conveyor with a series of rotary agitators spaced along it.

Abstract: The invention relates to an improved accelerator for gypsum board manufacture, the accelerator containing a ground mixture of gypsum and a long chain fatty carboxylic acid, e.g. stearic acid, or a salt thereof, to the manufacture of the accelerator, and to its use in gypsum board production. The accelerator is less liable to deterioration on storage than conventional gypsum accelerators, and its use yields gypsum board of improved properties.