Influence Of Pretreatment Process For Phosphogypsum On Water Resistance Of Phosphogypsum Composite Cement

Phosphogypsum (PG) is a by-product in the production of fertilizer from phosphate ore with sulfuric acid. So the PG is an alternative gypsum resource and it can be used as a construction material. In this paper, phosphogypsum composite cement (PCC) is made from phosphogypsum, fly ash, lime, Portland cement, and some other additives. This study experimentally investigated the influence of pretreatment process for phosphogypsum on water resistance properties of phosphogypsum composite cement. Four types of pretreatment processes for phosphogypsum were investigated. (1) Dry the raw PG in an oven at 45℃, then calcining at 110℃-700℃for 2 hours in a muffle furnace. (2) Autoclave the raw phosphogypsum with steam under a pressure of 0.12 MPa (120℃) for 15 hours, then calcining at 250℃-800℃for 2 hours in a muffle furnace. (3) Mix the raw PG and lime and aging 24 hours, then calcining at different temperatures. (4) Autoclave the raw phosphogypsum with steam under a pressure of 0.1 MPa-0.8 MPa (120℃-180℃) for 2 hours to 6 hours.The content of impurities, pH, crystalline phase of pretreated phosphogypsum are analysed. The results show that phosphogypsum is effectively purified by treating it with autoclavation and/or calcination. Especially PG samples were calcined at over 700℃or autoclaved at over 180℃, levels of total fluoride, water soluble fluoride and water soluble P₂O₅ decreased significantly in these pretreated PG samples. In the mean time, the crystalline phase of the pretreated phosphogypsum samples mostly transformed into hemihydrates and/or anhydrite.PCC motar was produced in accordance with PCC to sand ration 1:3. The softening coefficient, weight loss after static water attack (WLSW), and weight loss after flowing water attack (WLFW) of the PCC were determined to evaluate their water resistance properties. The results show that water resistance properties of the PCC mortars with pretreated phosphogypsum improved significantly when compared to those of the PCC mortars made from original phosphogypsum. The strength increased with an increase in calcination and autoclavation temperature. The softening coefficient decreased, the WLSW and WLFW decreased initially then increased, when the PCC was made from 33% (by wt.) phosphogypsum which was pretreated by drying the raw phosphogypsum in an oven at 45℃or autoclaving at 120℃, then calcining the samples at 700℃for 2 hours. The softening coefficient is from 0.50 to 0.60, when the PCC is made from 52% (by wt.) phosphogypsum which was pretreated by autoclaving at 120℃-180℃for 2 hours to 6 hours. The results show that the optimum pretreatment processes are: drying at 45℃or autoclaving at 120℃, then calcining the samples at 700℃for 2 hours, and autoclaving at 180℃for 4 hours. In the mean time, the influence of the content of the three optimum pretreated PG on the properties of PCC was investigated. The results show that the strength of PCC mortars increased initially then decreased, with the increase of the content of different pretreated phosphogypsum. When the PCC contains 80% or 90% phosphogypsum, the strength of PCC mortars reached the highest. When the PCC was made from phosphogypsum which pretreated by autoclaving at 180℃for 4 hours, the setting time was the shortest, and the strength and the softening coefficient was the highest. With the increase of the content of different pretreated phosphogypsum, the WLFW increased significantly. The properties of the optimum PCC were investigated, which was made from PG after autoclaving at 180℃for 4 hours. The results show that the standard consistency water was 38.0%, the initial setting time was 2 h: 10 min, the final setting time was 3 h: 20 min, the flexural and compressive strengths of curing 7 days were 4.0 MPa and 20.9 MPa, respectively; the flexural strengths of curing 28 days was over 11.7 MPa, the compressive strengths of curing 28 days was 47.3 MPa, the softening coefficient was 0.50, the WLFW was 1.06%, and the WLSW was 0.27%.XRD patterns and SEM images show that the strength of the PCC mortars is related to hydration of pretreated phosphogypsum, fly ash and Portland cement, as well as ettringite and C-S-H formed in hydration reaction. The results also indicate that ettringite and C-S-H gel, coated on the surfaces of fly ash and phosphogypsum, are the major contributors for the strength of the PCC mortars in both the early and later stages, which improve the strength and water resistance of the PCC mortars.