Study On Crystal Transformation And Hydration Mechanism Of P-Doped Calcium Sulphoaluminate Based On First Principles

Sulphoaluminate cement（CSA）is widely used in road and railway emergency repair,Marine structural repair and other fields due to its characteristics of fast setting speed,high early strength,corrosion resistance,adjustable expansion and so on.Calcium sulphoaluminate（C₄A₃$）is the key phase to determine the hydration characteristics and properties of sulphoaluminate cement.Phosphogypsum is a kind of solid waste discharged in the process of wet phosphoric acid production.At present,the storage of phosphogypsum in China is as high as 400 million tons,but its utilization rate is less than 35%.Preparation of CSA clinker with phosphogypsum is an effective means for its high value-added resource utilization,but the phosphorus impurities in phosphogypsum cannot be completely removed after pretreatment.In order to study the effect of P-doping on C₄A₃$,this thesis used first-principles simulation based on density functional theory,combined with XRD,BSEM/SEM-EDS and Rietveld quantitative experimental and analytical methods,and explored the formation process,crystal structure,clinker phase composition,hydration characteristics and other aspects of P-doped C₄A₃$.The main conclusions are as follows:（1）Three crystal structure models of P-doped C₄A₃$,C₂S and C₄AF were constructed to judge the substitution tendency of P atom in three main phases of CSA clinker.The defect energy（E_f）,bond order bond length（BO-BL）distribution and wave state density（PDOS）distribution under different substitution conditions were calculated by first principles.The results show that compared with p-doped C₂S and C₄AF,p-doped C₄A₃$has the lowest Ef entering the C₄A₃$process,and the distribution of BO-BL and PDOS of P-doped C₄A₃$crystal structure is the least different from the initial structure.Therefore,P atoms in CSA clinker are most inclined to enter the C₄A₃$lattice.（2）To determine the effect of P doping on the formation process and crystal transformation of C₄A₃$,orthogonal crystal system（ST）,cubic crystal system（SS）and P-doped C₄A₃$were prepared under different sintering conditions.The phase composition of C₄A₃$was qualitatively and quantitatively analyzed by XRD,SEM and Rietveld quantitative analysis.And the micromorphology was analyzed.The results show that the best sintering system of p doped C4A3$is holding at 1300℃for 4h.Under this condition,the content of C₄A₃$in P0.10 clinker reaches 97.8%.P doping promoted the transformation of C₄A₃$from orthogonal to cubic system,and the proportion of cubic C₄A₃$in P0.10 clinker reached 26.3%.（3）The E_f,BO-BL and PDOS distributions of P atoms in C₄A₃$lattice under different substitution conditions were calculated by BSEM-EDS,XRD and Rietveld.The results show that P atoms can enter the lattice of C₄A₃$and form the substituted solid solution.The doped P atoms preferentially replace S atoms,and P atoms also have a high probability to replace Al atoms.The binding energy（E_b）of P doped SS is less than that of P doped ST,and the crystal form of C₄A₃$changes from orthogonal to cubic.Based on the charge conservation condition,the change trend of E_f value and the rule of crystal transformation,the crystal structure model of P-doped C₄A₃$was proposed and its rationality was verified.（4）In order to determine the influence of P doping on hydration characteristics of C₄A₃$,the hydration process of C₄A₃$with four different P doping amounts was quantitatively analyzed by XRD and Rietveld,and the LCD projection and PDOS distribution of P-doped C₄A₃$were calculated using first principles.The results show that the P impurity can inhibit the dissolution and hydration rate of C₄A₃$in the early hydration of C₄A₃$,and then reduce the hydration rate of C₄A₃$regardless of the addition of C$·2H.Based on P doped C₄A₃$crystal structure model,using the active oxygen atoms to LCD projection and its electronic structure,such as the distribution of PDOS levels of primary principle analysis,first of all,by analyzing the different crystal structure model of LCD projection found that without P doped crystal structure of the active oxygen atoms around LCD projection intensity was significantly higher after doping;Further studies on LCD projection of different crystal structure models using PDOS distribution show that P doping can promote the PDOS peak of reactive oxygen atoms away from Fermi level,resulting in the decrease of hydration activity of C₄A₃$.