Crystal Structure Regulation And Hydration Of Calcium And Strontium Sulfoaluminate

Calcium sulfoaluminate or ye'elimite(C₄A₃$)and strontium sulfoaluminate((?)₄A₃$)are two similar compounds,which both belong to the sodalite family.The reaction process in the Ca O-Al₂O₃-SO₃ and Sr O-Al₂O₃-SO₃ ternary systems corresponding to C₄A₃$and(?)₄A₃$are complex,and there are many types of intermediate phases.These heterophases not only interfere with the formation process of both but also have a significant impact on their performance and hydration properties.Both compounds contain two crystal structures,orthogonal Pcc2(o-C₄A₃$)and cubic I-43m(c-C₄A₃$)for C₄A₃$,and orthogonal Pcc2(o-(?)₄A₃$)and cubic I23(c-(?)₄A₃$)for(?)₄A₃$.In addition,the structural difference in the crystal structures is responsible for the distinct hydration properties of the two compounds,and the different phase proportions in the crystal structures would directly affect the early strength and setting time as well as the performance modulation of sulfoaluminate cement and related phase-change materials.In this present Ph D thesis,pure orthogonal and cubic phases of C₄A₃$and(?)₄A₃$were synthesized by the high-temperature solid-state firing reaction via ion doping regulation method,and the firing process was optimized;based on the synthesis of two pure phases,the controllable adjustment of the content of the two phases was achieved by changing the ion doping concentration,and the mechanism of the two-phase crystal structure transformation was explained by the crystal structure by combining the Rietveld refinement and Eu³⁺ions fluorescent probes,and the different hydration properties of the two pure phases of both compounds were further explored.The main research conclusions of the present work are as follows:(1)Research on the crystal structure regulation and hydration of ye'elimiteThe cubic unstable phase of ye'elimite was successfully stabilized at room temperature by ion doping.The chemical formula used was Ca₄(Al_1-xFe_x)₆SO₁₆.The phase transition process from Pcc2 to I-43m can be accurately adjusted by Fe³⁺ions substitution.For the Fe³⁺-doped concentration(x)between 0 and 7 at.%,orthogonal and cubic ye'elimite coexist in as-obtained products.When the x content up to 10 at.%,the transformation from Pcc2 to I-43m was effective,according to the space group of I-43m with a=b=c=9.2147(?)and V=782.4313(?)³(Z=2).This effective transformation was reached without a significant change in the variable-temperature XRD analysis.For the hydration study of o-C₄A₃$and c-C₄A₃$,it has been found that the main hydration exothermic peak of c-C₄A₃$is 6 h earlier than that of o-C₄A₃$,but the heat release of the latter is more concentrated.The hydration products of both phases are identical,and they all produce AFm,AFt,and AH₃(gel).The Fe³⁺ions into the cubic Ca₄(Al_0.9Fe_0.1)₆SO₁₆ phase enter the three hydration products in the form of solid solution,and the solute solubility of AH₃ is higher than AFt and AFm,in which AFt is the weakest.Moreover,the introduction of Fe³⁺ions greatly enhances the crystallinity of AFm and AFt,and the decomposition temperature of both is slightly increased.(2)Research on the crystal structure regulation and hydration of strontium sulfoaluminateThe stability of the cubic metastable phase of strontium sulfoaluminate at room temperature was achieved by replacing the Al³⁺ions in the structure with Fe³⁺ions.In this part,the adopted formula was Sr₄(Al_1-xFe_x)₆SO₁₆.The phase transformation mechanism from o-(?)₄A₃$(Pcc2)to c-(?)₄A₃$(I23)can be effectively controlled by adjusting the proportion of Fe³⁺ions.When the Fe³⁺-doped concentration exceeds 20 at.%,o-(?)₄A₃$completely transforms to c-(?)₄A₃$,corresponding to the space group of I23,a=b=c=9.4359(?),V=838.7323(?)³.The no endothermic effect in the DSC curve for the sample at x=20 at.%further confirms that Sr₄(Al_0.8Fe_0.2)₆SO₁₆has been well transformed in a cubic phase.Through the hydration study of the Sr₄(Al_1-xFe_x)₆SO₁₆ sample,it has been demonstrated that the hydraulic reactivity of(?)₄A₃$was directly correlated with the changes in the parameters of the crystal structure doped with Fe³⁺ions.All in all,the early hydration maximum peak of c-(?)₄A₃$is much more intense than that of o-(?)₄A₃$,o-(?)₄A₃$tends to concentrate heat release,but the cumulative heat release of c-(?)₄A₃$is also more intense than that o-(?)₄A₃$.The(?)₄A₃$samples react with water to form the reaction products first,and then no crystalline phase hydration product is identified as the reaction progresses.In contrary to the XRD analysis,the amorphous AH₃ is detected in TGA analysis in the three hydrated samples,probably due to poor crystallinity which cannot be detected by XRD.(3)Research on the crystal structure control of strontium sulfoaluminate via other ionsSr₄(Al_1-xGa_x)₆SO₁₆was synthesized through the introduction of Ga³⁺ions into the crystal lattice of(?)₄A₃$,and the transformation process from Pcc2 to I23 can be easily adjusted by regulating the dopant content of Ga³⁺ions.According to the structural refinement,when the doped content reaches 20 at.%(Sr₄(Al_0.8Ga_0.2)₆SO₁₆),o-(?)₄A₃$is completely converted to c-(?)₄A₃$,and the space group is I23,a=b=c=9.4398(?),V=837.8938(?)³.Compared with c-(?)₄A₃$,o-(?)₄A₃$has low structural symmetry and more Sr site positions,but the Eu³⁺ions emission intensity of c-(?)₄A₃$phase in the PL spectra is much more intense than that of o-(?)₄A₃$with a complex spectral distribution.Therefore,the introduction of Eu³⁺ions mainly influences the crystal structures of the Pcc2 and I23 phases.The substitution of Sr²⁺by the Ca²⁺and Eu³⁺ions in the(?)₄A₃$structure(Eu:(Sr_1-xCax)4Al6SO16)facilitates the transformation from orthogonal phase to cubic phase.And when the doped content of Ca²⁺ions beyond 30 at.%,the pure cubic phase is obtained.Besides,the Eu³⁺ions emission intensity presents a gradually increasing with Ca²⁺ions addition.Moreover,the fluorescence intensity of the orthogonal phase(x=0 at.%)is much lower than that of the cubic phase(x=30 at.%),and the spectral distribution is more complex.