V and Cr substitution in dicalcium silicate under oxidizing and reducing conditions - Synthesis, reactivity, and...

Dicalcium silicate (C₂S) is known to incorporate potentially hazardous metals (Cr and V) in a belite-rich cementitious system. The effect of the electrovalence nature of V and Cr on C₂S polymorphs' (α´, β, γ) stability under oxidizing and reducing conditions as well as their reactivity are systematically investigated via analyzing oxidation states, phase composition, bonding system, and microstructure as well as oxide composition quantitively. It is shown that C₂S can incorporate Cr (VI) and V(V) consequently leading to stabilization of α´, β-C₂S. Instead, Cr (II, III) and V < (V) tend not to substitute in C₂S. Despite reactive polymorphs (α´, β-C₂S) stability due to Cr (VI) and V(V) incorporation, the early age (48-72 h) C₂S reactivity is drastically reduced due to Cr (VI) and V (V) incorporation. Moreover, one batch leaching test revealed that the V (V) leaching is inversely proportional to aqueous Ca²⁺ ion at pH > 12 while Cr leaching is sensitive to its oxidation state, and dissolution of C₂S. Even though C₂S can incorporate Cr (VI) and V (V) ions, the final leaching is governed by the immobilization potential of C-S-H gel, pH as well as types of calcium chromate and vanadate complexes.