抄録
Many samples of byproduct gypsum from salt making plants were collected and analysed. Only few samples obtained from (a) slow evaporation plants by hot spring or steam jacket heating, or (b) double decomposition reaction between brine or bittern and calcium chloride solution to exclude sulphate as calcium sulphate, were determined to be complete crystal gypsum CaSO4.2H2O (having crystal water 20-7-20-8% as loss on ignition), but other larger parts of samples were observed to be the mixed gypsum from anhydrous salt (CaS04) and hemihydrate (CaSO4.1/2H2O) or crystal gypsum (CaSO4.2H2O) and hemihydrate (CaSO4.1/2H2O).
These mixed gypsum,having the varying amount of loss on ignition from2-3% to 6-8%, 8-10% etc.,can not be used in any purposes, and must be treated to be converted to crystal gypsum CaSO4. 2H2O, and then to gypsum plaster of hemihydrate CaS04. 1/2H20.Powder of these byproduct mixed gypsum was treated with 20-30 times of 3% NaCl solution or sea water at room temperature for 3-4 weeks by stirring,and was easily obtained complete crystal gypsum (CaSat.2H2O), and then was calcined at about 150°C-180°C to obtain,gypsum plaster (CaSO4.1/2H2O)
Dolomite sample, which was obtained from Kuzuu in Tochigi Pref., was (a) calcined at 900-1000°C or (b) 700-800°C, hydrated with water and ground to dolomite plaster. These dolomite plasters were mixed with gypsum plaster above obtained in the proportions dolomite plaster 95-5: gypsum plaster 5-25, these gypsum-dolomite plaster samples were tested on their setting and hardening strengths and stability, and observed that very good gypsum-dolomite plaster was obtained by mixing gypsum plaster 15-25% to dolomite plaster 85-75%.
