It is now well accepted and demonstrated that calcium silicate, calcium aluminate and calcium sulfo aluminate (ettringite, AFm) phases exhibit a good capability to fix metals and metalloids. Unfortunately the role of minor phases and especially calcium-ferric aluminate phase, shorthand C4AF is not well defined. In other systems like in soils or sediments iron phases play a key role in the fixation of pollutant. In cement sorption isotherms, indicated that various metals can be retained by the C4AF hydrated products. Therefore the capabilities of those phase to retain heavy metal should not be neglected. Previous investigations have shown that the minerals formed during the hydration of C4AF are similar to those formed from C3A (pure tri-calcium aluminate) under comparable conditions. Nevertheless no investigation was conducted at the molecular level and there is still a controversy whether Fe substitutes for Al in the hydrated minerals in whole or in part, or if it forms FeOOH clusters scattered throughout the matrix. In this context we have conducted XAS experiments using synchrotron radiation. It was found that the hydration of C4AF forms C3AH6 (hydrogarnet) in which Fe randomly substitutes for Al as well as an amorphous FeOOH phase. Intermediate products like AFm (i.e., an ill organized lamellar phase) are also formed but rapidly evolve to C3AH6; iron does not seem to be incorporated in the AFm structure.

X-ray spectromicroscopy has been successfully applied to determine the evolution of the Cr oxidation state in Portland cement during leaching experiments. To our knowledge, this is the first study that demonstrates the possibility to study the chromium oxidoreduction phenomena in cement materials at natural Cr concentration (approximately 60 ppm) and at the micron scale. Line scans of Cr for Cr(VI) doped (2000 ppm) and undoped samples indicate that the altered layer (0-1000 microm from the surface) is characterized by a lower amount of Cr as compared to the core part, whereas an accumulation appears in the intermediate region (1000-1300 microm). This Cr-rich interface could correspond to an accumulation of ettringite (3CaO x Al2O3 x 3CaSO4 x 32H2O) as reported by previous works. This mineral exhibits the property to incorporate Cr(III) and Cr(VI) by replacement of aluminum and sulfate, respectively, in the structure. The most surprising result concerns the evolution of the Cr(VI)/Cr(tot) ratio along the line spectra, which is constant from the altered layer to the core (both for doped and undoped samples). This means thatthe same amounts of Cr(VI) and Cr(tot) are released during leaching. Even for the undoped sample, Cr(VI) was detected in the altered layer at 40 microm from the surface. This result is not in perfect agreement with literature, which usually states that Cr(VI) is mainly leached out. Although this result must be confirmed, it clearly indicates that Cr(VI) may be less mobile than predicted by models. An attempt is made to identify potential Cr(VI) fixation phases.

Steel production generates great amounts of by-products as steel slag. Unlike blast furnace slag, the use of Basic Oxygen Furnace slag (BOF slag) has been restrained due to insufficient volume stability and to the lack of environmental regulations. This study aimed at investigating the potential release and impact of pollutants, especially Cr and V that are present in rather high concentrations in slag, from a BOF slag used in a civil engineering structure (an industrial platform), using a multi-scale approach. The oneyear follow up of the experimental platform showed that concentrations of Cr and V were generally low in seepage waters, and in leachates from leaching test. Microanalyses carried out on slag allowed us to confirm the location of these metals in rather stable ferrous mineral phases, but V was also bound to more reactive silicates. No real toxicity effect of seepage waters has been revealed from eco-toxicological tests carried out with earthworms. La fabrication de l’acier s’accompagne d’une production d’importantes quantités de co-produits, les laitiers d’aciérie. Contrairement aux laitiers de haut-fourneau, l’utilisation des laitiers d’aciérie de conversion (laitiers LD) a été limitée en raison de leur instabilité volumique et de l’absence de réglementation environnementale. L’objectif de cette étude est d’étudier le relargage potentiel et l’impact des polluants, plus particulièrement de Cr et V qui sont présents à des concentrations assez élevées dans les laitiers, par un laitier LD utilisé dans une structure de génie civil (plateforme industrielle) à l’aide d’une approche multi-échelles.

Complex cementitious matrices undergo weathering with environmental exchange and can release metallic pollutants during alteration. The molecular mechanisms responsible for metal release are difficult to identify, though this is necessary if such processes are to be controlled.The present study determines and models the molecular mechanisms of Pb release during Portland cement leaching. Since Pb release is strongly related to its speciation (i.e. atomic environment and nature of bearing phases), the first objective of the present work is to investigate the evolution of Pb retention sites together with the evolution of the cement mineralogy during leaching. Complementary and efficient investigation tools were used, namely XRD, µ-XRF and XAFS. The second goal is to reproduce our results with a reactive transport code (CHESS/HYTEC) in order to test the proposed speciation model of Pb.Combined results indicate that both in the unaltered core and altered layer of the leached cement, Pb(II) would be retained through C-S-H "nano-structure", probably linked to a Q 1 or Q 2P silicate tetrahedra. Moreover in the altered layer, the presence of Fe atoms in the atomic environment of Pb is highly probable. Unfortunately little is known about Fe phases in cement, making the interpretation difficult. Can Fe-substituted hydrogranet (C 3 AH 6 ) be responsible for Pb retention? Modelling results are consistent with Pb retention through C-S-H in layers and also in an additional, possibly Fe-containing, Pb-retention phase in the altered layer.