Pseudomorphism in xonotlite and tobermorite with Co²⁺ and Ni²⁺ exchange for Ca²⁺ at 25°C

Exchange of Co²⁺ and Ni²⁺ for Ca²⁺ in xonotlite and tobermorite was investigated at 25°C. We show here for the first time that all the Ca²⁺ in xonotlite and tobermorite can be stoichiometrically exchanged with Co²⁺ or Ni²⁺. For example, 92.8 and 92.6% of the total Ca²⁺ present in zonotlite and tobermorite respectively, were exchanged by Co²⁺. This type of almost complete exchange of Ca²⁺ by Co²⁺ or Ni²⁺ did not result in the destruction of the crystal morphology of xonotlite and tobermorite which suggested a pseudomorphism. Studies of the kinetics of Co²⁺ exchange by xonotlite and tobermorite showed that the exchange is slow and total exchange of Ca²⁺ by Co²⁺ takes place in about 4-5 days. The Co²⁺ or Ni²⁺ exchange for Ca²⁺ is slow because of the necessity to break Ca-O bonds from the xonotlite and tobermorite structures. Partial Ca²⁺ exchange with Co²⁺ did not lead to a loss of all crystallinity while total exchange led to amorphization as detected by XRD.