Post-isomorphic substitution of trivalent metal cations for Ca²⁺ in portlandite crystals

Isomorphic substitution, position replacement of one cation by another of similar size, leads to incorporation of a variety of cations into solid crystals without any significant changes to the primary crystal structures. To date, isomorphic substitution has been known to take place almost exclusively via co-crystallization of the cations during formation of the crystals. We report here the discovery of isomorphic substitution of trivalent metal cations for Ca²⁺ ions in portlandite crystals at room temperature as evidenced by the transient appearance of metastable phase, the formation of Ca-based layered double hydroxides at high pH, the distinct shift of suspension pH after phase transition, and the in situ topochemical reaction. This post-crystallization isomorphic substitution provides an innovative pathway for the synthesis of materials through chemical manipulation of crystals as well as a new insight into interpretation on their weathering and transformation processes.