Cation Exchange Properties of Low Crystallized Calcium Silicate Hydrate with Alminium

Abstract

Al-containing low crystallized calcium silicate hydrate (C-S-H) was prepared by hydrothermal reaction at 95°C for 20 h, using water glass, calcium chloride and sodium aluminate as raw materials. The products were characterized by powder X-ray diffraction and NMR methods. and its ion exchange properties toward Ca²⁺, Mg²⁺, K⁺ and Li⁺ ions were studied.
C-S-H was predominantly formed and zeolites were hardly formed at the condition of H₂O/Na₂O =150. A type or X type zeolite was formed as by-products in the case of adding large amount of Al content at the condition of H₂O/Na₂O (molar ratio) = 50 and low Ca content. Cation exchange capacity (CEC) toward Ca²⁺ ion increased with increasing Al content of C-S-H. Ca²⁺ ion exchange was thought to correspond to the exchange reaction between Ca²⁺ and Na⁺ ion, some of which was introduced by Al substitution for Si and the other Na⁺ ion must be present connecting to broken bonds on C-S-H surface. Cation exchange reaction toward Mg²⁺ ion proceeded more slowly than that of Ca²⁺ ion. However its CEC was more than that of Ca²⁺ ion on many samples. CEC of Mg²⁺ increased with increasing Ca content of C-S-H. It was estimated that Mg²⁺ ion exchange reaction caused breaking of Ca-O bond in C-S-H structure. Amount of up-take of Mg²⁺ ion was roughly equal to the amount of release of Ca²⁺ and Na⁺ ions. Al containing C-S-H had CEC value more than 100 meq/100 g toward K⁺ ion but it was low value in the case of Li⁺ ion exchange.
It was suggested that Al containing C-S-H was composed of connecting and branching structure of silicate chain by NMR method.