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The possibility of obtaining β - anhydrite from nitrogypsum, which is waste from a nitrocellulose plant, was investigated. It was shown by means of qualitative IR analysis that the product obtained by heating nitrogypsum for 5 hours at 700 o C was â - anhydrite. When the β- anhydrite was mixed with water at a W/S (water/solid) ratio of 0.54 in presence of different accelerators (CaO, mixture CaO - ash, ash, Na2SO4 and K2SO4), pastes were formed which hardened on standing. The compressive strength of the hardened samples was measured after 7 and 28 days and their composition determined by qualitative IR analysis. On the basis of these results, it was observed that a relationship exists between the composition (depending on the used accelerator) and the compression strength of the samples. Namely, the formation of large cores of double salts: syngenite (K2SO4 ⋅CaSO4 ⋅H2O) and glauberite (Na2SO4 ⋅CaSO4), in the presence of the accelerators K2SO4 and Na2SO4, respectively, was due to the rapid and complete crystallization of the dihydrate (CaSO4 ⋅2H2O). This fast crystal growth of the dihydrate resulted in high compressive strengths of these samples. In the other samples (prepared in presence of the accelerators: CaO, mixture CaO - ash and ash), dihydrate did not form and, consequently, their compressive strength was low

Selenite was boiled in LiCl solutions of different concentrations (1 M, 2 M, 3 M, 4 M and 5M) at the respective boiling temperatures and atmospheric pressure. The products were subjected to X-ray diffraction analysis, qualitative infrared analysis, differential thermal analysis, thermogravimetric analysis and microscopic examination. The product obtained in the 1 M LiCl solution was the ?-form of calcium sulphate hemihydrate (?-CaSO4.0.5H2O). In more concentrated LiCl solutons, 2M and 3 M, the ?-form of calcium sulphate hemihydrate (?-CaSO4.0.5H2O) was formed. The product obtained in the 4MLiCl solution was also the ?-form of calcium sulphate hemihydrate, only mixed with the ?-form of calcium sulphate (?-CaSO4). Finally, in the 5 M LiCl solution the ?-form of calcium sulphate or ?-anhydrite was formed.

The utilization of nitrogypsum and bottom ash, wastes from the factory "Milan Blagojevic" (Lucani?Serbia), was investigated. For this purpose, mixtures composed of calcined nitrogypsum (CaSO4. 0,5H2O), ash (mass ratio 1:1) and water (water/solid mass ratio 0.5), as well as similar mixtures consisting of 1.0 mass%5.0 mass%10.0mass%of lime (CaO) with the same gypsum/ash and water/solid mass ratios, were prepared. These mixtures gave solidified products at room temperature and atmospheric pressure, which after 28 days had satisfactory compressive strengths for application in the civil industry. The product with 5.0mass%of added lime has the best mechanical characeristics. X-ray and qualitative IR analysis showed that this product mainly consisted of calcium sulphate dihydrate (CaSO4.2H2O) and carbonates, as well as a small quantity of ettringite (3CaO.Al2O3.3CaSO4.32H2O).

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