Speciation Distribution, Valence Transformation, and Immobilization Mechanism of Cr during Portland Cement Hydration

Abstract Chromium (Cr) immobilization pathways in Portland cement may vary with cement-matrix characteristics. In this study, representative 32.5- and 42.5-grade cements (C3-3 and C4-2) were compared using controlled Cr(III)/Cr(VI) addition and multiscale characterization. XPS showed partial reduction of Cr(VI) to Cr(III) during hydration, while XAFS and 29Si NMR indicated Cr(III) association/incorporation with C-(A)-S-H-related structures. In C3-3-Cr, TEM, and SEM-EDS further revealed localized CaCr2O4 nanocrystals, indicating a combined structural association/incorporation–precipitation pathway. In C4-2-Cr, Cr was more diffusely distributed, and no resolvable Cr-bearing nanocrystals were observed in the examined TEM regions, indicating predominantly structural association/incorporation. These differences were associated with the higher hydration reactivity and more homogeneous C-S-H-rich microstructure of C4-2. The results demonstrate that cement-matrix hydration characteristics regulate the relative contributions of established Cr immobilization pathways, providing a mechanistic basis for the selection of cementitious matrices for Cr stabilization.