Effect Of Calcination Pretreatment Process On Phosphogypsum-based Composite Binder

Phosphogysum （PG） is a by-product generated from the production of phosphoric acidand phosphorus chemical products by the extraction of phosphorus ore with sulfuric acid.PG mainly consists of CaSO₄2H₂O and some impurities such as Si, P, F, organic substancesand others. PG is highly acidic. It is an effective apprroach for gypsum building materials toreuse PG to produce phosphogypsum-based composite binder （PGCB）. But dihydrategypsum in PG is slightly soluble in water, with the solubility of2g/L at20°C. Its waterresistance is poor. Water-soluble phosphorus retards the setting time of PGCB and strengthdevelopment of PGCB mortar specimens, while water-soluble fluorides decrease the settingtime and reduce the density of PGCB and strength of PGCB mortar specimensPGCB was investigated in this study, and its ratio was proposed as: calcined PG（80%）,pulverised fuel ash（PFA,7%）, ordinary portland cement（OPC,10%）, hydrated lime （HL,3%）, and a small amount of admixtures. The main contents are as follows:1. The effect of calcining PG on removing impurities or consolidating impuritiesand on phase transformationPG was calcined in muffle furnace in laboratory. When the calcination temperature isup to150°C，calcination time2hours, the concentration of water-soluble fluoride was belowthe detection limit. When the calcination temperature is up to500°C，calcination time2hours, the concentration of water-soluble phosphorus was below the detection limit. Themixtures of dihydrate, hemihydrate, anhydrite（III） and slowly soluble anhydrite wereabtained by low temperatures （below500°C）. While the mixrures of slowly solubleanhydrite and insluble anhydrite were abttained over500°C. The results show that theimpurities in PG were significantly removed by calcining PG. And gypsum phase wasobviously changed by calcining PG.2. The properties of mechanical strength and water resistance of PGCBsThe results show that, with the caclination temperature increasing, the7days strengthdevelopment of PGCB mortar specimens increase slowly. The maximum value was obtainedat600°C. The28days strength development of PGCB mortar specimens increase as thecaclination temperature increasing. The maximum value was obtained at800°C. The flexural and compressive strength of PGCB mortar specimens （PG calcined at800°C） were5.9MPa and30.9MPa. The flexural and compressive strength of OPC mortar specimenswere7.5MPa and50.2MPa. The softening coefficient increased as the calciningtemperature increasing. The value of softening coefficient of PGCB mortar specimens （PGcalcined at800°C） reached to0.66. The others were blow0.4. Whether mortar specimens（PG calcined at800°C） cured for28days were in flowing water for10days or in static waterfor30days, The specimens were intact. Softening coefficient reaches to0.66. The resultsshow PGCBs have high strength.And they can be against water attack.3. The relationship between the strength and water resistance of PGCBs and phasetrasformationThe concentration of water-soluble phosphorus and fluorides of PG caclined over500°C were all close to zero.They affect the properties little. The properties of PGCBs werecontributed to the gypsum phase. Different gypsum phases were obtained at differenttemperatures. So that different properties of PGCBs were obtained. The results show that thelater strength and water resistance of PGCBs (PG mainly consists of hemihydrate andanhydrite（III） were poorer than the PGCB （PG mainly consists of slowly soluble anhydriteand insoluble anhydrite）.The results are beneficial for the calcination pretreatment process of PG and theapplication of PGCB.