GB385611A - Improvements in or relating to calcium sulphate plaster mixes and to methods of applying the same - Google Patents

Abstract

A process for the production of porous plasters of the accelerated anhydrite type comprises the use of a gas producing agent selected so that one, or more, of the reactants or resultants in the gas formation functions as an accelerator for the hydration of anhydrite. Examples of such agents are calcium carbonate, water-aluminium sulphate, and metallic zinc-water-sulphuric acid. They are used in amount up to 4 per cent. No free acid should be left as this interferes with the setting. Additional neutral or alkaline accelerators, e.g. potassium sulphate, may be added. The solid materials may be either mixed dry and gauged in the usual way or the gas producing agent or the additional accelerator or both added to the gauging water. Greater strength when set is obtained by the addition of up to 20 per cent of calcium sulphate hemi-hydrate either to the dry mix, or gauged separately and added to the main mix after gauging. The rate of evolution of gas may be controlled, (1) by the addition of a small amount of glue, gelatine, or bentonite; a delay of 15 minutes in the evolution of gas may be so obtained; (2) by the use of gas producing agents in the form of compact particles, e.g. by the use of a ground mineral carbonate; or (3) by the use of ice cold water for gauging. Raw material for this invention comprises natural anhydrite having a carbonate content equivalent to 1 per cent by weight of carbon dioxide, to which is added 1 per cent of powdered hydrated aluminium sulphate. On mixing and setting, a volume increase of 50-75 per cent is obtained. If 2 per cent of aluminium sulphate is used an increase of 100 per cent and if 3 per cent an increase of 200-300 per cent is obtained. A larger amount of water is necessary in expanded plasters than in non-expanding plasters. Fillers such as sand, pumice, cork, clinker, brick dust, or fibrous material may be added. The material may be applied by trowelling or in the case of a mix of slow expansion, by spraying. In an example, 12 parts of anhydrite are mixed with 1 part of plaster of Paris, 1/2 part of aluminium sulphate, 1/5 part of potassium sulphate, 1/10 part of finely divided chalk, and 4 parts of water. In 15 minutes expansion of 2-2 1/2 times the original volume takes place and the mixture hardens. Specifications 236,695, 317,672, 337,926, [all in Class 22, Cements &c.], and 381,155 are referred to.