| Phosphogypsum?PG?is a waste by-product that is composed mainly of calcium sulfate dihydrate?CaSO4·2H2O,CSD?produced during the production of wet process phosphoric acid through dissolving phosphate ore with sulfuric acid.In China,PG has become a famous bulk industrial solid waste and the current production is estimated to be around 7080 Mt per year.It demands urgent consumption as a useful resource in large quantities from the point of environmental protection and utilization.Besides CSD,gypsum also exists in the phases of hemihydrate?CaSO4·0.5H2O,CSH,?-and?-form?and anhydrite?CaSO4,AH?.Contrary to the low value of CSD and AH,CSH owns superior performance as an important class of cementitious material and has been widely used in modern building materials,molding,bone remedy and other fields.Therefore,the preparation of?-CSH from CSD shows a great economic benefits and market prospect to improve the grade and value of the gypsum resource and promote the high-value-added utilization of chemical gypsum.Salt solution method is the most promising technology,which features mild conditions,excellent control of the phase-transition process and stable product quality.The preparation of?-CSH from CSD is commonly conducted in chloride solutions which lead to severe erosion of equipment.A novel method was invented to prepare?-CSH from PG in this paper.Simultaneously,effects of salt concentration,reaction temperature and phosphorus impurities on the on the transformation kinetics and crystal morphology were studied.Moreover,controlling the crystal morphology of?-CSH and tuning the corresponding phase transformation were deeply investigated.In an aqueous solution,water activity is the only determinant of the feasibility of the transformation from CSD into?-CSH,and decreasing water activity can promote the phase transformation.The electrolyte aqueous solution has a lower water activity and thus is thermodynamically required for the transformation.In this experiment,in the Ca?NO3?2aqueous solution with a concentration range of 3.33.8mol/L,?-CSH was successfully prepared from PG at 91102 oC under atmospheric pressure.Increasing both Ca?NO3?2concentration and temperature can accelerate the dehydration rate of PG.The phosphates including Ca?H2PO4?2·H2O,Ca?H2PO4?2·H2O and Ca3?PO4?2 show totally retarding effect on the phase transformation of CSD to?-CSH under experimental conditions.The presence of 0.21 wt.%H3PO4 accelerates the whole phase transformation.Further increase in H3PO4 content to 0.41 wt.%retards the phase transformation.The inhibitory effect of phosphorus impurities on the induction period of?-CSH is more significant than that on the crystal growth.Except for Ca3?PO4?2,all impurities studied were found to result in larger diameters of the as-formed?-CSH as well as bigger particle fineness.Furthermore,the presence of Ca?H2PO4?2·H2O or CaHPO4·2H2O at high concentrations induces partial phase transformation from?-CSH to AH.The mechanical strength of?-CSH paste is a crucial index of its application prospect,which is morphology-dependent.In the Ca?NO3?2 solutions,the morphology of?-CSH can be controlled effectively in the presence of succinic acid.As the concentration of succinic acid increases from 0 to 0.4%,the obtained?-CSH crystals change from long rods to plates and the corresponding aspect ratio range shrinks from 4.58.5 to 0.30.5.It is also found the the presence of succinic acid decreases the dehydration rate of PG,which can be alleviated or even reversed by adding Na2SO4.When the concentration of succinic acid increases from 0to 0.4%,the time required to complete the dehydration process of PG prolongs from 5.0 h to6.5 h.However,the time decreases to 3.75 h in the presence of 0.2%Na2SO4.From a comparison of mechanical strength,it is found that in the presence of 0.3%succinic acid,the obtained?-CSH with a aspect of1.0 and characteristic particle size of 21.3?m shows 42%water requirement for standard consistency and the corresponding paste exhibits the largest mechanical strength of 33 MPa. |
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