1994

Sarvaranta, Leena; Kaasinen, Harri

Gypsum is useful as an industrial material because it readily loses its water of hydration when heated, producing partially or totally dehydrated calcined gypsum. When water is added, this calcined gypsum reverts to the original dihydrate - the set and hardened gypsum product. Gypsum building materials comprise machine applied plaster, plasterboard and flow screed plaster. A gypsum calcining plant may use either chemical or natural gypsum as raw material. Natural gypsum has traditionally been the most important, but industrial gypsum by products are gaining increased importance for mainly environmental reasons. Phosphogypsum is a by product of the manufacture of wet phosphoric acid, typically from a fertilizer manufacturing plant. Over 120 130 Mt/a of phosphogypsum are produced worldwide. This quantity exceeds by far the world's annual demand for natural gypsum and anhydrite. Roughly 1 Mt/a of phosphogypsum is produced in Finland. In coming years increasingly quantities of gypsum from flue gas desulphurization (FGD) will become available. In Finland the yearly amount of flue gas gypsum is estimated to reach up to 200 000 t/a by 1994, and in Germany up to 3 Mt/a by 1995. Other sources of gypsum raw material include anhydrite from the production of fluorspar, gypsum from the treatment of sulphate containing waste waters, and gypsum scrap from the ceramic industry and metal foundries. Comparative studies of flue gas gypsum and natural gypsum are reviewed. Flue gas gypsums contain very fine particles (< 100 µm), while crushed natural gypsums have broader particle size distributions. Flue gas gypsums require more water in the production stage than natural gypsums and undergo slower dehydration. The hydration behaviour of both types is similar. Flue gas gypsums are harder than natural gypsums. Different methods of gypsum modification are reviewed. Unmodified gypsum has poor moisture resistance and is suitable only for indoor use. Gypsum can be reinforced with glass, cellulose or polymer fibres. Polymer dispersions can be used to control the set time and fluidity of the plaster and to increase the moisture resistance. Light weight gypsum boards can be produced using light weight aggregates (perlite, polystyrene), fibres or bubbled surfactants. By using phase change materials (PCM) it might be possible to increase the heat storage capacity and moisture resistance of gypsum board. Possibilities of using calcium sulphite or calcium sulphate as admixture in hydraulic or polymeric matrices are not discussed in the present context.