Comparative assessment of hexavalent chromium (Cr(VI)) immobilization in cement pastes with copper, steel, granulated blast-furnace, and ferronickel slags

Abstract

Hexavalent chromium (Cr(VI)) is a highly toxic and mobile contaminant that requires effective stabilization in cement-based waste treatment systems. This study comparatively evaluates the Cr(VI) immobilization behavior of cement pastes incorporating four metallurgical slags: copper slag (CS), steel slag (SS), ground granulated blast-furnace slag (GGBFS), and ferronickel slag (FNS), with focus on chemistry-controlled mechanisms. Cr(VI) was introduced in the form of K2CrO4, and immobilization characteristics were examined through phase analysis, spectroscopic techniques, and leaching tests. The results indicate that CaO-rich slags promote the formation of Cr-bearing phases, including CaCrO4, CrO4-substituted AFt/AFm phases, and calcium (alumino)silicate hydrates, which contribute to chromate retention and partial reduction to Cr(III). In contrast, MgO-rich FNS limits the development of key hydrate phases and shows lower stabilization performance despite comparable strength. Phase quantification and chromium valence analysis show consistent agreement with leaching behavior, suggesting that hydrate assemblage and calcium availability play a more decisive role than mechanical performance in governing immobilization efficiency. Toxicity characteristic leaching procedure (TCLP) results confirm higher Cr(VI) retention in CaO-rich slag (SS and GGBFS) systems compared with CaO-deficient slags (CS and FNS), as well as 100% cement paste. These findings provide mechanistic insight into chromium stabilization in cementitious matrices and support the use of selected industrial slags as alternative low-carbon binders for hazardous waste immobilization.