Effect Of Ion Substitution On The Metastable Structure And Property Of Alite

Tricalcium silicate （C3S） solid solutions known as alites are the principle phaseof clinker, responsible for strength development. Therefore, it is important to study theeffects of substituents on the metastable structure and hydraulic properties of alite. Inthis dissertation, the effects of foreign ions doping such as Na⁺、Mg²⁺etc. especiallytheir combined doping on the crystal structure and property were studied throughsynthetic alites by XRD with the Rietveld method, petrographic analysis,thermoluminescence, isothermal calorimetry, FTIR, DSC, ICP-OES, and SAED.Based on ions’ chemical structure, a quantity called structure difference factor D(referred to Ca²⁺) was defined. The substitution position changed gradually from Ca toSi with the increase of D. The stabilization range of high temperature polymorphs ofC3S exhibited “几” font with D. Based on this result, it is believed that thestabilization of high temperature polymorphs of C3S was due to the distortion in thestructure caused by ionic substitution, which could block atomic displacement and goagainst phase transformation. The ion-stabilization law and mechanism were provedby the further experiments.The effects of single doping with Na⁺, Al³⁺etc. on the structure and property ofC3S were investigated. It was found that the lattice parameters of C3S were relatedlinearly to the amount of substituent ions. Mg²⁺and Ba²⁺favoured the stabilization ofM3and T3-type alite respectively at high concentration. Al³⁺replaced Ca²⁺as well asSi4+, but the substitution for Si4+increased with higher Al³⁺concentration and resultedin the stabilization of T3-type alite. The initial reaction of C3S with Al³⁺and alkaliswere dramatically increased compared with pure C3S, due to the vacancy defectcaused by heterovalent substitution.Furthermore, the effects of combined doping with seven typical foreign ionssuch as Na⁺, Mg²⁺etc. on the structure and property of alite were studied. Thecombined doping with typical concentrations of various ions mainly promotedM3-type alite stabilization. Mg²⁺and Al³⁺had the most significant influence on thestabilization of the higher-temperature forms of alite, and the absence of either Mg orAl resulted in the stabilization of T3-type alite. Alite with P2O5≥0.5%（mass fraction,the same below） mainly resulted in the stabilization of R-type alite. The latticeparameters of alites changed linearly with the amount of substituent ions. But forparticular concentration at the solid solubility limit, phase transformation boundary, orwhere substitution patterns changed, inflection point appeared in the curve, followingthe Vegard’s law. About one third of Al could replace Ca, and two thirds of Al couldreplace Si. A small amount of Fe2O3（less than0.4%） mainly replaced Ca, andreplaced Si as well in higher concentration. The valence bond structure of [SiO4]tetrahedron was basically not affected by ion substitution whether for Ca or Si. The structure character of [SiO4] tetrahedron depended on the structural symmetry of alite.As the structural symmetry of ion-stabilized alite increased （T1�'T2�'T3�'M3�'R），the higher symmetry of [SiO4] tetrahedron was attained.[SiO4] tetrahedron in T3polymorph got to symmetrical structure of regular tetrahedron. The structuralmetastability of alite directly correlated with the polymorphic form of alite and thekind and amount of incorporated foreign ions.The activity of alites with multiple foreign ions doping was increased comparedwith pure C3S, but the main reaction was retarded. Heterovalent substitutions had themost significant influence on the crystal defects due to their complex substitution,which resulted in significant effects on the hydration kinetics of alite. The hydrationbehavior of alite did not depend much on the concentrations of defects as the types ofdefects. Fe and P, especially Fe had the most significant influence on the hydrationkinetics. The initial reaction of alite with Fe doping was decreased, the inductionperiod was prolonged, and the main hydration was slowed down. There was an abruptchange in the original thermoluminescence（TL） intensity of alite around the phasetransformation boundary. The hydration reactivity of alites of the same polymorphicform was related with their intensity of the original TL, and this correlation wasmainly reflected in the chemical-controled stage. The metastable energy stored astraped electrons transforming into chemical energy has been believed to account forthis phenomenon.The alite polymorphsim and activity were directly affected by the calciningtemperature and cooling rate. T3form was stabilized for M3-type alites heat treated at700°C and800°C. The hydration activity of both pure C3S and typical alite wasincreased after heat treatment, especially for samples treated near the decompositiontemperature. Both pure C3S and alite treated at1350°C showed high activity andintense TL, most probably because1350°C was slightly higher than thedecomposition temperature, and very close to the Tammann temperature.The grindability of alite was decreased with MgO≥1%and resulted in asignificant decrease in the compressive strength. In the experimental research scope,the strengths of R polymorph had the highest strength, and the compressive strengthsof T2, T3and M3were similar regardless of substituent ion’s content.