Synthesis and cation exchange capacity of Al-substituted tobermorite prepared from 100% <38 μm SiO2 and γ-alumina

Abstract

Naturally occurring tobermorite [Ca₅Si₆O₁₆(OH)₂·4H₂O] is a rare calcium silica hydrate (CSH) with crystal structure of a combination of layer and chain silicates. This has found many uses in structural industry as material for building blocks and autoclave lightweight concrete.
Several methods have been discovered to synthesize this industrially important mineral. Preparation of the materials for the synthesis, the particle size and choice of precursors, and their crystallinity greatly affect the kinetics of tobermorite formation and characteristics, usually requiring ultrafine (<10 μm) and high purity precursors, and <180°C for <8 hours hydrothermal treatment in an autoclave.
Synthesis is also possible with the addition of alumina (γ-Al₂O₃). Al³⁺ ions were found to substitute for the tetrahedral Si atoms in tobermorite and resulted to formation of cavities giving potential for ion exchange to occur. Tobermorite then has been discovered as a new family of cation exchangers and found many applications in removal of environmental pollutants, nuclear waste treatment, analytical and preparative chemistry.
Addition of Al was also found to increase crystallinity. Thus, synthesis at lower temperature (90-120°C) with coarse precursors (<45 μm) and at longer times was able to produce CSH. The cation exchange capacity of tobermorite formed at these condition, however, have not been determined.
The present paper experimented on the synthesis and cation exchange properties of tobermorite synthesized at 130°C for 8 hours autoclaving using 100% <38 μm precursors prepared by floatation, providing a less tedious and more doable synthesis.