IMPACT OF CARBON-DIOXIDE ON THE IMMOBILIZATION POTENTIAL OF CEMENTED WASTES - CHROMIUM

Portland cement and blended cements containing blast furnace slag affo rd both physical and chemical immobilization of chromium. Chromium occ urs in aqueous solutions in two oxidation states, Cr(III) and Cr(VI). Slag-containing cements are very effective at removing Cr(VI) from the internal pore fluid, probably by reducing Cr(VI) to less soluble Cr(I II). Carbon dioxide attack, or carbonation, is probably the most commo n form of concrete environmental attack and it promotes changes to the cement chemical composition and physical properties that can affect t he long-term retention of heavy metals. In the present paper the effec t of carbonation on the immobilization of Cr(III) and Cr(VI) has been studied in both Portland cements and blended cements containing blast furnace slag. The results show that although Portland cements matrices are more resistant to carbonation than slag-containing cement matrice s, the increase of chromium content in pore solution is more marked fo r Portland matrices. After 60 days the pore fluid of carbonated Portla nd cement spiked with 50000 ppm Cr(III) or Cr(VI) contains 20 or 40000 ppm respectively, whereas after carbonation of slag blends for the sa me time, the corresponding pore fluid Cr contents are 1 and 16000 ppm respectively. (C) 1997 Elsevier Science Ltd.