| 摘要 |
<p>A lightweight plaster composition comprises calcium sulphate hemihydrate or anhydrite, lightweight aggregate, a surface-active agent, and a water-soluble or water-dispersible polymeric material consisting of polyvinyl alcohol, polyvinyl pyrrolidone, polyethylene oxide or a water-soluble cellulose ether, the composition being free from any constituent which would precipitate the polymeric material from an aqueous solution thereof. The lightweight aggregate may be expanded perlite, exfoliated vermiculite, pumice or sawdust and may constitute 5-30% by weight of the composition; a heavy aggregate such as ground limestone may be added in amount not exceeding 70% by weight of the composition. The polymeric material usually constitutes 0.01-2.0% by weight of the composition, and the surface-active agent 0.001-0.2%. Suitable surface-active agents are sodium alkyl aryl sulphonates, alkali metal salts of alkyl sulphates, formaldehyde-sodium aryl sulphonate condensation products, anhydrous polyethylene glycol condensates, anhydrous cellulose sulphite liquor, saponified pine resin or potassium rosinate. Hydrated lime binder in an amount not exceeding 20% by weight of the composition may also be present. In examples the hemihydrate plaster is retarded with a proteinaceous material.</p> |
