New insights into the environmental safety of incorporating Cr(III) into cement matrix: Cr(VI) formation driven by low-intensity fire

Cement kiln co-processing has become a popular strategy for dealing with solid waste containing Cr(III) because hazardous Cr(III) can be incorporated into the cement matrix to reduce its leaching risk. However, whether Cr(III) can be transformed and leached from the cement matrix under extreme conditions remains unclear. This study demonstrates that even when Cr(III) is solidified and encapsulated by the cement matrix, it can undergo substantial oxidation into soluble Cr(VI) at temperatures 100–250°C. Approximately 43.8 % of Cr(III) was oxidized into Cr(VI) when the cement matrix was fired at 250°C for 1 h, and CrO3 was identified as the predominant phase of Cr(VI). When the firing time was increased over 10 h, part of CrO3 was reduced into Cr(III), and the combination of CrO3 and Ca(OH)2 occurred accompanied with CaCrO4 formation. Toxicity characteristic leaching procedure (TCLP) indicated that Cr(VI) concentrations leached from cement matrix samples fired at 200 and 250°C for 5 h exceeded the regulatory limit. The cumulative leaching amounts of Cr(VI) from the cement matrix samples fired at 200 and 250°C by EA NEN 7375:2004 tank tests were 1381 and 1369 mg/m2, far higher than the limit of 25 mg/m2. Cement matrices after firing process exhibited significant degradation in mechanical properties. The high temperature process in the event of a fire for cement matrix not only poses critical environmental hazards but also severely impacts the structural stability of buildings.