Synthesis and Characteristics of Al³⁺-Substituted Tobermorite by Alkoxide Method

Abstract

The preparation of glasses, fibres and ultra-fine powders for fine-ceramic materials by the alkoxide method has recently attracted special interest. Tobermorite is generally synthesized by hydrothermal reaction under saturated steam pressure of 10kgf/cm² above 180°C in unhomogeneous suspension of CaO(solid)-SiO₂(solid)-H₂O (liquid) system. A study was made to investigate the synthesis of Al³⁺-substituted tobermorite at a low temperature (95°C) under atmospheric pressure by using homogeneous mixed solutions of the CaCl₂-Si(OC₂H₅)₄-AlCl₃-C₂H₅OH-H₂O system. The thermal behavior and cation exchange for the Al³⁺-substituted tobermorite were carried out by means of X-ray diffraction, thermal analysis (TG-DTA), infrared spectroscopy and chemical analysis. The procedure for the synthesis of Al³⁺-substituted tobermorite consisted of deposition of a CSH gel from homogeneous mixed solutions (Ca/Si atomic ratio: 1.0, Al/(Al+Si) atomic ratio: 0.03-0.20) by adding KOH solution and the crystallization of the gel by aging at 95°C under atmospheric pressure for 5 days. A basal spacing due to (002) plane in tobermorite was affected by the Al/(Al+Si) atomic ratio; 10.7Å below the atomic ratio of 0.03 and 11.3Å in the atomic ratio of 0.05-0.20. The crystallization of Al³⁺-substituted tobermorite was much accelerated in the range of the Al/(Al+Si) atomic ratio of 0.05-0.10 but converted to the CSH gel over the atomic ratio of 0.20. A basal spacing expanded with increasing Al³⁺ substituted for Si⁴⁺ in silicate ion chains of tobermorite dependent on the substitution of greater Al³⁺ than Si⁴⁺ in size and the limitation for this substitution were approximately 18% as Al³⁺. Consequently, Al³⁺-substituted tobermorite included much K⁺ which was supplied to neutralize the electric charge happened by the substitution. The capacity of cation exchange for K⁺→←Na⁺ of Al³⁺-substituted tobermorite increased with increasing amount of substituted Al³⁺. The maximum capacity was 80-90meq/100g. Even in the case of the CSH gel, the capacity indicated the same values but in the exchange capacity per unit-surface, the former showed higher value than the latter.