Formation and transition of calcium aluminate and calcium silicate compounds from pre-synthesized mullite in low-calcium system by solid-state reaction

Abstract

The formation and transition of calcium aluminate and calcium silicate compounds from pre-synthesized mullite in low-calcium system were systematically studied by solid-state reaction at 1350–1500 °C using XRF, XRD, FTIR, SEM and PSD methods. Ca₃Al₂O₆, Ca₁₂Al₁₄O₃₃, CaAl₂O₄, Ca₂SiO₄ and Ca₂Al₂SiO₇ can form via direct reaction of mullite with CaO at the beginning of the reactions, then Ca₁₂Al₁₄O₃₃ and Ca₃Al₂O₆ react with mullite to form CaAl₂O₄ and Ca₂SiO₄, and finally Ca₂Al₂SiO₇ reacts with CaO to generate Ca₂SiO₄ and calcium aluminate compounds as the sintering process proceeds. Elevating the sintering temperature is in favor of the formation of Ca₁₂Al₁₄O₃₃, Ca₃Al₂O₆ and Ca₂Al₂SiO₇ at the initial reaction stage, but Ca₂Al₂SiO₇ cannot totally transform to calcium silicate and calcium aluminate compounds in the low-calcium system, which deteriorates the pulverization performance of the final products. Increasing the calcium dosage accelerates the transformation of Ca₂Al₂SiO₇ to Ca₁₂Al₁₄O₃₃, CaAl₂O₄ and Ca₂SiO₄, which enhances the pulverization performance. If the CaO/Al₂O₃ molar ratio exceeds 1.4, Ca₃Al₂O₆ will generate by the reactions of pre-formed CaAl₂O₄ and Ca₁₂Al₁₄O₃₃ with excessive CaO.

Introduction

Calcium aluminate and calcium silicate compounds are quite important as components both in traditional and advanced materials, which are widely applied in various fields of ceramics, cements, refractories, metallurgical slags, etc. [[1], [2], [3], [4]] Based on the CaO–SiO₂–Al₂O₃ phase diagram, 6 binary compounds belong to the congruent melting compounds, i.e. Al₆Si₂O₁₃ (A₃S₂), CaAl₄O₇ (CA₂), Ca₁₂Al₁₄O₃₃ (C₁₂A₇), CaAl₂O₄ (CA), CaSiO₃ (CS) and Ca₂SiO₄ (C₂S); 4 binary compounds relate to the incongruent melting compounds, i.e. CaAl₁₂O₁₉ (CA₆), Ca₃Al₂O₆ (C₃A), Ca₃SiO₅ (C₃S) and Ca₃Si₂O₇ (C₃S₂); the ternary compounds of Ca₂Al₂SiO₇ (C₂AS) and CaAl₂Si₂O₈ (CAS₂) are involved in the congruent melting compounds, while Ca₃Al₂SiO₈ (C₃AS) and Ca₃Al₂Si₃O₁₂ (C₃AS₃) correspond to the incongruent melting compounds.

The formation process and transition mechanism of calcium aluminate and calcium silicate compounds have been widely investigated during the past decades [[5], [6], [7]]. For CaO–Al₂O₃ binary system, Williamson and Glasser found that the formation process of calcium aluminates is related to the local equilibriums, which occur in small inhomogeneous regions [8]; Tian et al. reported that during the formation process of various calcium aluminates, the distribution layer structure from inside to outside is as follows: Al₂O₃, CA₂, CA, C₃A and C₁₂A₇, where CaO gradually reacts with the inlayer Al₂O₃ by diffusion [9]. For CaO–SiO₂ binary system, Wang et al. discussed the electronic structures of various C₂S polymorphs, and found that the local state density of maximum valence band of β- and α′_L-C₂S localizes around the active O atoms, while it is homogeneously dispersed in γ-C₂S [10]; Ghose et al. found that α′- and β-C₂S can be stabilized by B₂O₃, P₂O₅ and BaO, which inhibits the transformation process of β-C₂S to γ-C₂S [11]. For CaO–Al₂O₃–SiO₂ ternary system, Fabrichnaya and Nerád reported that the experimental thermodynamic properties of CS-CAS₂-C₂AS mixture are considerably inconsistent with the corresponding equilibrium data [12]; the authors studied the transformation process of C₂AS with CaO, and found that CA, C₁₂A₇, C₂S and C₃S will form at the starting sintering stage, and the transition of C₂AS to C₂S and CA can be accelerated by elevating the reaction temperature [13].

Mullite was widely found to form at high temperature from the alumina and silica containing resources, such as in fly ash [14], ceramics [15,16] and refractories [17]. Meanwhile, the crystal structure, microstructure and physicochemical properties of mullite are highly dependent on the types and ratios of the starting materials as well as the processing routes [[18], [19], [20], [21]]. However, the formation and transformation process of calcium aluminates and calcium silicates from mullite have not reported until now. In the present work, the phase transition, microstructure as well as the pulverization performance from pre-synthesized mullite under different sintering conditions were systematically studied by solid-state reaction, which can provide theoretical supports for the effective utilization of mullite-containing resources. In order to reduce the calcium dosage, the goal phases of calcium aluminate compounds after sintering are CA and C₁₂A₇, and the low-calcium system to assure the molar ratio of CaO to Al₂O₃ (C/A) below 1.8 (especially when the C/A ratio is 1.0) was introduced in this study.