Making of anhydrite cement from waste gypsum

The paper presents the results of research on the utilization of phosphogyp sum produced as the waste of phosphoric acid manufacture. Phosphogypsum is a fine powder with high calcium sulfate content. The phosphatic and fluorid e impurities present in phosphogypsum cannot be removed completely either b y washing or chemical treatments. However, phosphogypsum, when heated at el evated temperature, produced an anhydrite and the impurities become inert. The formation of anhydrite cement was examined by microscopy and X-ray diff raction. Data showed that a stable anhydrite can be produced by heating pho sphogypsum at 1000 degrees C. The effects of different chemicals on setting and hardening of anhydrite cement and its hydration characteristics were s tudied. Results show that with the use of sodium sulfate and ferrous sulfat e activators, maximum attainment of strength can be achieved. A correlation was established between hydration and chemically combined water. Microscop ic studies revealed that formation of euhedral prismatic and rhombic shaped gypsum crystals govern high strength development in the anhydrite cement. Manufacture of anhydrite cement from phosphogypsum is recommended because o f its lower energy requirements than the traditional building materials. Pu blished by Elsevier Science Ltd.