Study On The Hydration-hardening Mechanism And Microstructure Regulation Of Hemihydrate Phosphgypsum Under Alkaline Condition

Phosphogysum is a widely produced waste byproduct from wet phosphoric acid process.In 2019,nearly 80 million tons of phosphogysum in China were discharged and the utilization rate is only about 40%.Phosphogysum contains mainly of Ca SO₄·2H₂O but also involves some impurities such as P₂O₅,fluorides,residual acids,and organic matters which make it more difficulty to reuse.The stockpiling of phosphogypsum takes up a large amount of land and causes serious pollution to the environment.Therefore,how to accelerate the utilization of phosphogypsum is the focus of current research in gypsum industry.Phosphgypsum can self harden with considerable strength after calcination,which makes it possible to become a gypsum resource.However,the application of hemihydrate phosphogypsum is limited due to the influence of impurities.Lime neutralization is the main pretreatment method of phosphogypsum,but the use of hemihydrate phosphogypsum in alkaline condition is seldom studied.In order to achieve the maximum utilization of phosphogypsum,it is important to learn about the physical and chemical properties of hemihydrate phosphogypsum and develop the utilization technology of hemihydrate phosphogypsum in alkaline condition,which is also in favour of improving its mildew resistance,admixture adaptability,composition effect with other materials and reducing corrosivity.Therefore,the key to improve the utilization of phosphogypsum is to master the physical and chemical properties of hemihydrate phosphogypsum and study the key generic techniques in application.In view of this point,my study focused on hemihydrate phosphogypsum and systematic studied its performance in alkaline condition.The main research contents and results are as follows:(1)The physical and chemical properties of phosphorus building gypsum,desulfurized building gypsum and natural building gypsum were studied in detail by using modern analytical testing techniques.The results indicated that the main components in them are hemihydrate calcium sulfate,and the particle morphology is loose and porous laminar,containing nano-level narrow slot pores,with high specific surface area and water requirement of standard consistency.The particle size of hemihydrate phosphogypsum is widely distributed,including several main particle size ranges,and the impurities in hemihydrate phosphogypsum are organic matters,eutectic phosphorus,quartz,clay-like minerals and residual acids.Importantly,grinding can not change the particle size distribution characteristics of phosphorus building gypsum,but washing can reduce the content of P,F and increase pH.Compared with desulfurized building gypsum,phosphorus building gypsum is selective to retarder.Protein and amino acid retarder have better effect on hemihydrate phosphogypsum.Besides,the adaptability between hemihydrate phosphogypsum and water reducer is better than that of desulfurized building gypsum.Furthermore,impurities in hemihydrate phosphogypsum have negative effect on water reducer,but increasing the particle fineness is helpful to improve the water reduction rate.Thus,optimizing the adaptability between hemihydrate phosphogypsum and additives can significantly improve its performance.(2)The hydration process of hemihydrate gypsum was studied in situ by ESEM,which confirmed its hydration mechanism is dissolution and precipitation.Futhermore,the dissolution model of particles was first proposed and the characteristic parameters of hydration process in acidic environment were characterized.The results revealed that particles of hemihydrate phosphogypsum are composed of single crystals or polycrystals with impurities absorbing on the surface.The particle dissolution is a process of non-uniform dissolution of surface,where the location of defects,impurities and grain boundaries are preferred to dissolve.In the process of dissolution,irregular particle boundaries tend to be arc-shaped,forming flocculent intermediates with holes,and further dissolution is accompanied by particle fragmentation.As a result,the small particles disappear first and the particle size shows an overall decreasing trend during hydration.Hemihydrate gypsum can hydrate to dihydrate gypsum spontaneously,its hydration process includes dissolution stage,dissolution-crystallization stage and equilibrium stage.During equilibrium stage,the small crystals dissolve and the large crystals grow persistently without releasing heat,the strength develop steadily.Increasing the impurity content and fineness of hemihydrate phosphogypsum can promote its dissolution and change the crystal morphology of dihydrate gypsum,while reducing the hydration space is beneficial for the formation of crystal block which can weaken the influence of crystal morphology on performance of the hydrated paste.(3)The hydration and harden of hemihydrate phosphogypsum in alkaline condition was studied by adjusting pH.The results shown that the dissolution of hemihydrate phosphogypsum particles was hindered,the nuclear precipitation and crystal growth were slow down,as well as the hydration heat release rate.In consequence,the retarding effect and strength degradation were observed in hemihydrate phosphogypsum during hydration in alkaline environment.Meantime,the phenomenon are more obvious when retarder is added.In addition,adding inorganic salts and different kinds of alkali can promote the hydration of hemihydrate phosphogypsum in alkaline condition and reduce setting time,but only Ca(OH)₂can significantly improve the strength of the hydrated paste.Considering the hydration process and crystal morphology evolution of hemihydrate phosphogypsum in alkaline condition,which is also compared with desulfurized building gypsum,the influence mechanism under alkaline condition is proposed:the formation of Ca₃(PO₄)₂resulting from eutectic phosphorus precipitate at the surface of hemihydrate phosphogypsum particles and selectively adsorb on the crystal surface of dihydrate gypsum,which hinder the particle dissolution,nucleation and crystal growth along the C axis and turn it to be independent thick block-shaped resulting loose crystal network,while excessive Ca(OH)₂particles tend to absorb Ca₃(PO₄)₂when the neutralization reaction is happening,thus the crystal surface at the C axis grows again contributing to the development of crystal network.(4)Based on the hydration mechanism of hemihydrate phosphogypsum in alkaline environment,the hydration process and performance of hemihydrate phosphogypsum compositing Portland cement were studied.The results illustrated that cement can improve the short term water resistance of the hydrated paste,but its alkaline environment would affect the hydration of hemihydrate phosphogypsum and alter the dihydrate gypsum crystal appearance since the Ca(OH)₂from cement hydration can not absorb Ca₃(PO₄)₂.Therefore,the crystal network need more hydration products from cement to wrap dihydrate gypsum crystal and form new network.Unfortunately,high Portland cement content in the over-sulfur system would easily lead to the formation of delayed ettringite resulting in crack of the hydrated paste under wet environment and wetting-drying cycle.