Prediction of the Long-Term Release of Hazardous Substances From Cement-Based Materials to Water
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ABSTRACT Leaching tests have been carried out on concrete with fly ash or blast furnace slag as additives. Different models for leaching processes are proposed and analyzed, and physical/chemical interactions that may occur during a leaching process are discussed. Furthermore, simulations with different conditions and with different mathematical solutions are shown. The results of the leaching tests are displayed as a time-dependent leaching whose behavior varies significantly for each metal. INTRODUCTION The chemical content in building materials, and the environmental risk these chemicals pose, represent a fairly new field of research. This risk can only be decided if the emission behavior of relevant substances is known. As diffusive emissions of hazardous substances in products often cause the major part of elevated concentration levels in the water environment, there is an increasing concern over building materials in contact with water as possible sources. In this study the leaching of hazardous elements present in concrete additives, such as fly ash and blast furnace slag, is discussed. Several studies have been carried out on leaching from concrete, mainly regarding leaching of heavy metals, originating from concrete additives [1][2]. A main topic has been harmonization of leaching tests, and the discussion about how to value the results has been scarce, as the concentrations of trace elements often have been below the detection limit. EXPERIMENTS Leaching tests have been carried out in compliance with two Dutch standards; NEN 7345 [4] which is a diffusion test, and NEN 7341 [5] which is an availability test. The methods and materials were reported earlier [3]. The amendments to the standard tests were a prolonged leaching period from 14 to 30 days and a pH of 4.5 in the leachant, as this is the pH of the soil water in Sweden. THEORY The term "availability" describes the quantity of a component that ultimately can leach from a material, under defined conditions. According to work carried out by a consortium to the European Commission in EUR 17869 EN [6] it gives information about the long-term leaching, as it indicates the mobile fraction of the total content of a substance. The available fraction has to be determined to calculate the diffusion coefficient, in the case where the element is not completely soluble [6]. The availability given by NEN 7341 attempts to simulate the total amount that is leached at a stage where the material is disintegrated and fully oxidized, and where the pH's buffer capacity is lost [6]. The release of substances in a porous monolithic cement-based material is not easily described. The phenomena involved are complex and coupled, at the surface as well as in the solid. For elements that are completely dissolved in the pore water of a porous material, pure diffusion can correctly describe the transport, caused by concentration gradients. 313 Mat. Res. Soc. Symp. Proc. Vol. 608 © 2000 Materials Research Society
A simDle diffusion model for leachinff rocesses The following expres
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