Exchange of Co 2+ and Ni 2+ for Ca 2+ in xonotlite and tobermorite was investigated at 25°C.
We show here for the first time that all the Ca 2+ in xonotlite and tobermorite can be stoichiometricall~ exchanged with Co 2+ or Ni 2+.
For example, 92.8 and 92.6~ of the total Ca 2 +present in xonotlite and tobermorite respectively, were exchanged by Co 2 . This type of almost complete exchange of Ca 2+ by Co 2+ or Ni 2+ did not result in the destruction of the crystal morphology of xonotlite and tobermorite which suggested a pseudomorphism.
Studies of the kinetics of Co 2+ exchange by xonotlite and to~ermorites showed that the exchange is sloN and total exchange of Ca 2+ by Co z+ takes place in about 4-5 days.
The Co 2+ or Ni 2+ exchange for Ca 2+ is slow because of the necessity to break Ca-O bonds from the xonotlite and 2+ 2+ tobermorite structures.
Partial Ca 2+ exchange with Co or Ni did not lead to a loss of all crystallinity while total exchange led to amorphization as detected by XRD. oo © 3
*See Table 1 for method of synthesis and other details.
~ Shaken on an end-over-end shaker.
Maximum amounts of Ca 2+ that can be released from tobermorite and xonotlite are 13.71 16.81 meq/g respectively, based on their theoretical formulae. **Numbers in parentheses represent Ca + Na released to solution from this tobermorite contains Na in exchange positions.