Chromium behavior during cement-production processes: A clinkerization, hydration, and leaching study

Abstract

The behavior of chromium during the production of cement clinker, during the hydration of cement and during the leaching of cement mortars was investigated. The microstructures of clinker and mortar properties were investigated using free lime, XRD, SEM/EDS, and TG/DTA techniques. Chromium was found to be incorporated in the clinker phase. The formation of new chromium compounds such as Ca 6Al 4Cr 2O 15, Ca 5Cr 3O 12, Ca 5Cr 2SiO 12, and CaCr 2O 7, with chromium oxidation states of +3, +4.6, +5, and +6, respectively, was detected. After the hydration process, additional chromium compounds were identified in the mortar matrix, including Ca 5(CrO 4) 3OH, CaCrO 4·2H 2O, and Al 2(OH) 4CrO 4, with chromium oxidation states of +4.6, +6, and +6, respectively. Additionally, some species of chromium, such as Cr 3+ from Ca 6Al 4Cr 2O 15 and Cr 6+ from CaCr 2O 7, CaCrO 4·2H 2O, and Al 2(OH) 4CrO 4, were leached during leaching tests, whereas other species remained in the mortar. The concentrations of chromium that leached from the mortar following U.S. EPA Method 1311 and EA NEN 7375:2004 leaching tests were higher than limits set by the U.S. EPA and the Environment Agency of England and Wales related to hazardous waste disposal in landfills. Thus, waste containing chromium should not be allowed to mix with raw materials in the cement manufacturing process.