Assessment of chemical sensitivity of Waelz slag

In the recycling industry, the recuperation of zinc from electric are furnace dust by the Waelz process generates important quantities of slag. This slag presents good mechanical properties, and for the most siliceous slag, a high stability that would enable its use by total or partial substitution of certain granulates in civil engineering. Our study (within the framework of a European program, co-funded by the European Commission - DGXII) concerns the physico-chemical and mineralogical characterization and leaching behaviour of several types of Waelz slag. The leaching tests used are regulatory tests and specific characterization tests of leaching behaviour. They take into account the influence of several main parameters of the valorization scenarios envisaged for the slag (e.g. pH, Redox potential, chemical nature of the leachant, type of contact - liquid/solid etc). The influence of the chemical nature of the alkaline leachant (lime or NaOH) is particularly important in the case of release of lead (strongly leached by a lime solution), of zinc and of arsenic. The agreement between ANC test results and controlled pH test results can be noted. Leachability is largely controlled by pH in solution. Alkaline slags are more sensitive to the influence of pH. The low buffer capacity of the silica-rich slags implies that application or disposal of untreated slag in acid environments (pH 4-6) should avoided. To optimize integration of slags in materials, it will be necessary to orient research towards less alkaline binders than classical OP cements. A concrete with low limestone content where the alkalinity in the porewater would have a less corrosive effect on the vitreous phases of slags would seem more appropriate. (C) 2000 Elsevier Science Ltd. All rights reserved.