Potential use of phosphogypsum in concrete

An experimental study was conducted to investigate the potential use of phosphogypsum (PG) in concrete. This was accomplished by preparing mortar mixes at a water/cement ratio of 0.6 using two types of cement, ordinary Portland 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 10–100%. The purified PG was obtained by calcining PG (washed in water and not washed) at temperatures of 170, 600, 750, 850, and 950°C. The compressive, tensile, and flexural strengths of different hardened mortars were obtained following 3, 7, and 28 days of curing in air and in a moist room. In addition, the setting time and the soundness of cement pastes, prepared using OPC and PPC at a water/cement ratio of 0.6 and at different replacements of PG, were evaluated. At a particular PG percentage of replacement, the results indicated an increasing trend in strength gain over curing time. The strengths of mortars, cured for a specific time, showed a decrease in the strength with further replacement with PG. The purification process, by heating PG up to 900°C, has resulted in improving the strengths of the mortar mixes. The highest percentage increase in strength was found to be for PG calcined at temperatures of 850 and 900°C. The incorporation of PG in the cement paste has dramatically increased its initial and final setting times and has increased the percentage elongation of paste prisms used to evaluate the soundness. The improvement in strength upon calcination is primarily due to the formation of anhydrate at higher temperatures, which subsequently hydrates to gypsum; the latter has some cementing value.