Making of anhydrite cement from waste gypsum

The paper presents the results of research on the utilization of phosphogypsum produced as the waste of phosphoric acid manufacture. Phosphogypsum is a fine powder with high calcium sulfate content. The phosphatic and fluoride impurities present in phosphogypsum cannot be removed completely either by washing or chemical treatments. However, phosphogypsum, when heated at elevated temperature, produced an anhydrite and the impurities become inert. The formation of anhydrite cement was examined by microscopy and X-ray diffraction. Data showed that a stable anhydrite can be produced by heating phosphogypsum at 1000 degrees C. The effects of different chemicals on setting and hardening of anhydrite cement and its hydration characteristics were studied. Results show that with the use of sodium sulfate and ferrous sulfate activators, maximum attainment of strength can be achieved. A correlation was established between hydration and chemically combined water. Microscopic 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 of its lower energy requirements than the traditional building materials. Published by Elsevier Science Ltd.