Reactions of some calcium silicates with metal cations

The reactions of Pb²⁺, Cd²⁺, Mn²⁺, Zn²⁺, Cu²⁺, Mg²⁺, Co²⁺, or Ni²⁺ with calcium silicates such as tobermorite, xonotlite and wollastonite were investigated. Among these cations, Pb²⁺, Cd²⁺, Mn²⁺, Zn²⁺ and Cu²⁺ appear to replace surface Ca²⁺ but it is difficult to delineate the extent of this reaction from that of precipitation as hydroxides, hydroxy carbonates or carbonates and double decomposition reaction with calcite which is present as an impurity. Two unsubstituted tobermorites exhibited an Mg²⁺ exchange of only 0.4 meq/g which is due to exchange with surface Ca²⁺. Almost all of the Ca²⁺ in tobermorites and xonotlite was irreversibly replaced by Co²⁺ or Ni²⁺ which led to their amorphization. The mechanism of Co²⁺ and Ni²⁺ replacement of Ca²⁺ is by the breakdown of structural CaO bonds and appears to take place from edge to core in these minerals. The replacement of Ca²⁺ by Co²⁺ and Ni²⁺ in these unsubstituted calcium silicate minerals is irreversible and lead to their amorphization and hence their reactions are not strictly analogous to cation exchange in clays and zeolites.