Modulation Of Ye'elimite Crystalline Structure

Ye'elimite?C₄A₃$?is an dominant component in sulfoaluminate cements.It is easy to form aluminate impurities in its formation process,such as Ca₃Al₂O₆?C₃A?,Ca₁₂Al₁₄O₃₃(C₁₂A₇),CaAl₂O₄?CA?,CaAl₄O₇?CA₂?.Aluminate impurities not only interfere with the hydration process of C₄A₃$,but also have a significant impact on its hydration performance.In addition,C₄A₃$contains two crystal structures,orthogonal of Pcc2?o-C₄A₃$?and cubic of I-43m?c-C₄A₃$?.Due to the difference of crystal structures,two crystals have distinct hydration properties,and the different proportion of the two crystals would directly affect the early strength and setting time of sulphoaluminate cement.Therefore,the preparation of pure phase of different C₄A₃$crystals is of great significance to the study of hydration difference between two crystalline forms and the performance modulation of sulphoaluminate cement and related repairing materials.In this paper,pure orthogonal and cubic phases were synthesized by various preparation techniques,and combined with Rietveld refinement and Eu³⁺ions fluorescence probe,the sensitive identification of impurity phases in samples were detected.In addition,the modulation of two ye'elimite phases was realized via cation substitution.On this basis,the hydration behavior of two pure ye'elimite was analyzed.The main experimental conclusions are as follows.In the solid-state reaction of orthogonal ye'elimite,excessive 0-5 wt.%CaSO₄·2H₂O was used to compensate for the loss of sulphur.With the help of the highly sensitive characteristics of Eu³⁺ions fluorescence probe to the symmetry of structure sites and the temperature phase transition characteristics of ye'elimite,the associated impurity phase in sintered samples is C₃A.Moreover,with the decrease of CaSO₄·2H₂O,the content of C₃A impurities decreases.When the content of C₃A is less than 2 wt.%,the overlap of the diffraction crystal planes of C₄A₃$and C₃A increases the difficulty of pure phase identification.Combining with Eu³⁺ions fluorescence probe and structural refinement,it was found that the orthogonal ye'elimite with99.97 wt.%purity was prepared by heating at 1000 ? and 1300 ? for 2 hours,respectively.The space group of orthogonal ye'elimite is Pcc2,Z=4,and the lattice parameters a,b and c are 13.0332?,13.0371?and 9.1661?,respectively.The metastable phase of cubic ye'elimite was successfully retained at room temperature by ion doping.The design formula is Ca₄(Al_1-xGa_x)₆SO₁₆.The phase transformation process from Pcc2 to I-43m can be effectively controlled by adjusting the proportion of Ga³⁺ions.For the dopant concentration between 0 and 14 at.%,orthogonal and cubic ye'elimite coexist in the obtained products.When the doped content up to 14 at.%,the cubic pure phase was obtained at 1000 ? for 2 hours and at 1300 ? for 3 hours.The space group of cubic ye'elimite is I-43m,Z=2,a=b=c=9.2184?,V=783.3728?³.In addition,the obtained results indicate that when the Ga³⁺-doped concentration?x?is below and above 10 at.%,the introduction of Ga³⁺ions mainly influences the crystal structures of the Pcc2 and I-43m phases,respectively.The transformation of two phases is the result of the interaction of Al/GaO₄ tetrahedra and adjacent CaO_n polyhedra.Through the hydration study of pure orthogonal and cubic ye'elimite,it is found that under the same conditions,the hydration speed of cubic phase is about 8 hours faster than that of orthogonal phase,but the heat release of the latter is more concentrated.The hydration products of the two different structures are identical,and they all produce AFt,AFm and AH₃?gel?.The Ga³⁺ions in the Ca₄(Al_0.86Ga_0.14)₆SO₁₆ cubic phase enter the three hydration products in the form of solid solution,and the solute solubility of AH₃ is higher than AFm and AFt,of which AFt is the weakest.Futhermore,the addition of Ga³⁺ions greatly improves the crystallinity of AFt and AFm,and the decomposition temperature is increased by 5°C and10°C,respectively.