Retention mechanisms in mortars of the trace metals contained in Portland cement clinkers

This work aimed to assess the retention in the mortars of the heavy metals fed in a cement kiln with natural raw materials, fossil fuels or waste derived fuels. Industrial cement has been studied, along with laboratory samples enriched during clinkering with lead, zinc and chromium. The relevant mortars were tested for metal release through leaching studies in various chemical contexts. Zinc appeared to be insoluble in the pH range 7–13. Lead is released only in alkaline medium above pH 12.5. Hexavalent chromium, whose salts are usually very soluble, is retained though in an hydrated phase, that is stable in the chemical environment ensured by the cement matrix. Lead and chromium releases are linked to the metal levels in the mortars. The solubility studies of a chromium+sulfate ettringite revealed that chromium release is linked to its proportion in the crystal. A selective dissolution of the chromium part of ettringite has been pointed out. This leads to a lowering of the chromium level in such a solid contacted with water. Theses results improved the understanding of the release mechanisms of chromium by the mortar blocks.