Potential use of phosphogypsum in concrete

An experimental study was conducted to investigate the potential use of pho sphogypsum (PG) in concrete. This was accomplished by preparing mortar mixe s at a water/cement ratio of 0.6 using two types of cement, ordinary Portla nd cement (OPC) and pozzolanic Portland cement (PPC), and two types of fine aggregate, natural river sand and limestone. These mixes were prepared at different replacements (by weight) of PG and purified PG ranging from 10-10 0%. The purified PG was obtained by calcining PG (washed in water and not w ashed) at temperatures of 170, 600, 750, 850, and 950 degrees C. The compre ssive, tensile, and flexural strengths of different hardened mortars were o btained following 3, 7, and 28 days of curing in air and in a moist room. I n addition, the setting time and the soundness of cement pastes, prepared u sing OPC and PPC at a water/cement ratio of 0.6 and at different replacemen ts of PG, were evaluated. At a particular PG percentage of replacement, the results indicated an increasing trend in strength gain over curing time. T he strengths of mortars, cured for a specific time, showed a decrease in th e strength with further replacement with PG. The purification process, by h eating PG up to 900 degrees C, has resulted in improving the strengths of t he mortar mixes. The highest percentage increase in strength was found to b e for PG calcined at temperatures of 850 and 900 degrees C. The incorporati on of PG in the cement paste has dramatically increased its initial and fin al setting times and has increased the percentage elongation of paste prism s used to evaluate the soundness. The improvement in strength upon calcinat ion is primarily due to the formation of anhydrate at higher temperatures, which subsequently hydrates to gypsum; the latter has some cementing value. (C) 1999 Elsevier Science Ltd. All rights reserved.