Formation behavior of tricalcium aluminate hexahydrate in synthetic sodium aluminate solution with high alkali concentration and caustic ratio

Abstract

The formation kinetics and mechanism of tricalcium aluminate hexahydrate (TCA) in synthetic sodium aluminate solution with high alkali concentration (caustic Na2O from 235 to 295 g·L−1) and caustic ratio (from 3 to 7) were studied by XRD, FTIR, SEM-EDS and PSD methods. Over high caustic alkali concentration, caustic ratio, lime addition and reaction temperature inhibit the formation of TCA in high-alkali aluminate solution. The optimum synthesis conditions of TCA were established, and the corresponding formation efficiency of TCA and the caustic ratio of solution after reaction are 77% and 21 respectively. The formation process of TCA is controlled by the internal diffusion, and the apparent activation energy is 6.03 kJ·mol−1. The formation kinetic equation of TCA is: 1 − 2α/3 − (1 − α)2/3 = 1.11 × 10−3 × e−6030/RT×t. The synthesis product at equilibrium is mainly comprised of TCA as well as minor Ca(OH)2 and CaCO3, and the morphology of TCA is agglomerates of cube particles, which is quite different from those in low and medium alkali solution.