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.
All Science Journal Classification (ASJC) codes
- Building and Construction
- Materials Science(all)