Abstract: In the manufacture of cementitious board by depositing a slurry of hydraulic cementitious material, commonly gypsum plaster, between two facing sheets and forming it to the desired width and thickness prior to setting, the formation of voids in the core, due to air trapped in the slurry is avoided by vibrating the lower sheet (10) after the slurry (15) has been deposited on it, usually before the upper sheet (14) contacts the slurry, and subsequently vibrating the upper sheet when it comes into contact with the mass of cementitious slurry. The vibration of the lower sheet, for example by rotating polygonal shaft (20), causes bubbles of unwanted trapped air to rise through the slurry towards the upper surface and the accumulated bubbles are then broken by the vibration of the upper sheet, which may be accomplished by a second rotating shaft (21), before they become incorporated into the formed board as voids.

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 building board comprises a core 31 of set cementitious material, such as gypsum, faced on at least one side by a fabric or web 33 of mineral fibres embedded in the face of the core and a continuous film 32 of the cementitious material having a higher density and lower porosity than the core extending over the outer face of the fabric. The film may have a smooth surface or a desired textured or figured surface. The preferred fabric or web 33 is a non-woven glass fibre tissue and it is preferred that a continuous film 32 of gypsum at the surface should not exceed 2 mm in thickness. The board is made by bringing the fabric or web into contact with the respective face of a layer of an aqueous slurry of gypsum plaster or other cementitious material, and vibrating the layer of slurry in contact with the fabric or web until slurry penetrates the web and the latter is completely embedded.

Abstract: Blocks or panels 10, more especially of porous or cellular structure, are faced with a fabric or web inorganic fibres, preferably in the form of glass fibre tissue, bonded to the block or panel by a composition 14 of thermosetting condensation polymer and gypsum, preferably in a ratio between 1:1.6 and 1:4.0. The amount of the composition is preferably such as to provide a film of minimal thickness over the surface of the fabric or web, corresponding to a glass: resin plus gypsum proportion of 8 to 25%. Such coatings when applied to foamed plastics, chip-board or glass fibre panels provide boards suitable for ceilings or partitions and having ample strength and flame resistance. The blocks or panels may be made continuously by applying fibrous tissues, impregnated with a fluid mixture of plaster and resin by roller spreaders, to the face of panel elements supported between release sheets, for example of polypropylene. Pressure may be applied and maintained, if desired, until the fluid mixture has set.

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: 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.