THERMAL PURIFICATION AND UTILIZATION OF PHOSPHOGYPSUM WASTE FOR PORTLAND CEMENT PRODUCTION

Abstract

Phosphogypsum (PG), the by-product of phosphoric acid industry, comprises of calcium sulfate dihydrate, and other contaminants which restricts the industrial use of phosphogypsum in cement industry as a set retarder. Remedial thermal treatment of phosphogypsum as a set retarder for Portland cement production is investigated in this research work. The SEM-EDS analysis of the raw phosphogypsum sample showed the presence of agglomerates in needle, rhombic and hexagonal shape. The treated PG samples fill and intertwine with each other to form a more compact structure. The XRD pattern of the treated phosphogypsum shows new silica and alumina rich products such as amorphous calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H), including crystalline phases of phosphogypsum dihydrate crystals. The sulphite content of the heated phosphogypsum was 43.64%, which translates to a purity of 93.83 %. The phosphorous content reduced appreciably from 1.05 mg/L to 0.38 mg/L and also the fluoride content reduced from 2.22 mg/L to 1.25 mg/L respectively for the raw and treated PG as shown in table 3.2. The compressive strength of the Portland cement produced was also observed to be higher in the treated condition than the untreated condition, due to the removal of the binding obstruction admixtures by thermal treatment of the PG. The heat of hydration value of 294 kJ/kg obtained is in accordance with ASTM C-150 (2015) and meets the requirement for ordinary Portland cement. In conclusion, the laboratory tests performed in this study indicated that phosphogypsum could serve as a replacement of natural gypsum as a set retarder because thermal treatment of phosphogypsum significantly removed the impurities present successfully.