Ion Doping Effect And Reaction Kinetics Mechanism Of White Cement Clinker Calcination

With the rapid development of white Portland cement（WPC）clinker calcination technology in the world,many phenomena in the process of WPC clinker formation cannot be explained by the traditional chemical theory of clinker.It is of great practical significance for promoting the technology of Chinese WPC industry to reconstruct the theoretical system of WPC clinker based on pre-decomposition technology.Ion doping is an important method to optimize Portland cement clinker,which has a significant effect on the calcination process and physicochemical performance of clinker.In this paper,the effects of ion doping on mineral composition,chroma parameters and hydration properties of WPC clinker were studied by XRD-Rietveld method,SEM-EDS,spectrophotometer,calorimeter,Raman spectroscopy,XPS and other methods.High purity single minerals C3S,C2S,C3A and C4AF were prepared,and the chroma of each single mineral were analyzed by spectrophotometer.The effects of Zn²⁺,Ti⁴⁺and Na3Al F6 doping on mineral composition and chroma of WPC clinker were studied respectively.Jander model and Arrhenius model were used to study the effects of doping ions and the liquid phase content on the kinetic parameters（reaction rate constant k,the degree of reactionαand apparent activation energy Ea）of C3S formation.The results show that:（1）The color of WPC single mineral has the following characteristics.The whiteness of granular minerals from large to small was:C3S>C3A>C2S>C4AF.The whiteness of powdery minerals from large to small was:C3S>C2S>C3A>C4AF.Among them,C3S was yellow,C2S was green,C3A was yellow and green,and C4AF was black and red.C3A was the mineral with the highest greenness and yellowness in the clinker.The grain size and surface porosity of C3A have significant effects on the greenness of C3A.The increase of grain size increased the greenness of C3A.Porosity increased the reflectance of red light,thus reducing the greenness of samples.（2）Zn²⁺doping improved the overall visible reflectance of WPC clinker,and greatly increased the reflectance of blue wavelengths,thus increasing the L^*and whiteness of clinker and decreasing b^*.Zn²⁺doping promoted the C3A crystallization and reduced the ACn content.Zn O within 2%reduced the C4AF content.The substitutions of Zn²⁺in different minerals are different.In C₃A,Zn²⁺mainly replaced Ca²⁺.In C₄AF,Zn²⁺mainly replaced Fe³⁺and slightly replaced Al³⁺.The right amount of Zn O increased the cumulative heat of WPC hydration,but prolonged the induction period of cement hydration.（3）Ti⁴⁺doping improved the overall visible reflectance of WPC clinker,thus improving the lightness and whiteness of clinker.However,the reflectance of blue light waveband（0 nm～420 nm）decreased gradually,and that of yellow light waveband（577nm～592 nm）increased greatly,resulting in the increase of yellowness b^*.Ti⁴⁺doping promoted the transformation of C3S from poor symmetry to good symmetry.However,when the Ti O2 content exceeded 1.0%,Ca Ti O3 was formed and the C3S content significantly reduced.Ti O2 also stabilized the high temperatureα’H-C2S.With the increase of the Ti O2 content,theα’H-C2S content in clinker increased.In silicate minerals,Ti⁴⁺mainly replaced Si⁴⁺to form[Ti O4].The solution tendency of Ti element in mineral phase was:the interstitial phase>C2S>C3S.（4）Na₃AlF₆ increased the overall visible reflectance,thus increasing the L^*and whiteness of clinker.Because adding Na3Al F6 increased the reflectance of visible light in blue waveband（0 nm～420 nm）,the yellowness b^*decreased and the greenness a^*is increased.The addition of Na3Al F6 promoted the formation of C3S in clinker,and induced the transformation of R-C3S to M-C3S,and obviously increased the amorphous content.However,when the Na3Al F6 content was more than 1.0%,the C3S crystals in clinker decreased and turned into amorphous phase.The addition of Na3Al F6 reduced the C3A and C4AF content,and increased the C12A7 content.When the Na3Al F6content exceeded 2%,f-Ca O in the interstitial phase increased significantly.Therefore,from the perspective of mineral composition,the appropriate content of Na3Al F6 is 1%.Adding Na3Al F6 increased the hydration heat during WPC acceleration period,but prolonged the induction period of WPC hydration.（5）Zn²⁺,Ti⁴⁺and Na₃Al F₆ doping increased the reaction rate constant k of forming C3S and the degree of reactionα.The clinker with Na3Al F6 had the highest degree of reactionαand reaction rate constant k,whose effect on the formation of C3S was most significant.Doping Zn²⁺,Ti⁴⁺and Na₃Al F₆ reduced the E_a of solid phase reaction to varying degrees.The Ea of samples from large to small was:reference sample>0.5%Ti O2>0.5%Zn O>0.5%Na3Al F6.When the liquid content was 10%,the reaction rate constant of C3S was maximum.