Leaching results in the assessement of slag and rock materials as construction material

Sammanfattning: Extraction of rock material and ore for construction and metal production involves large quantities of wastes and by-products. Iron- and steelmaking slag has durability qualities and latent cementitious properties which are positive in construction. The leaching of so-called hazardous metals is important in the evaluation of secondary materials for utilization, while knowledge of the leaching behaviour is not required for rock materials used in road making. Iron- and steelmaking slags have varying content of glass and puzzolanic minerals that hydrate in contact with water. The influence of the puzzolanic properties in leaching tests is not considered. Swedish rock materials have been investigated with the availability test, NT ENVIR 003, in order to form a basis for comparison of soluble trace elements. A long-term leaching test, NVN 7347, has been used to determine whether diffusion is the dominating leaching mechanism in a quenched, amorphous BF slag. Four different slag types, modified by rapid cooling, have been investigated with the compliance test, EN 12457-2. Overall, the solubility of metals from the rock materials was low but a substantial part of sulphide-bound elements were released under oxidizing conditions, and compared to metallurgical slags, the released amounts of some elements were larger. The major phase had an important influence on the solubility of trace elements. An amorphous slag without puzzolanic properties has low solubility and prevents leaching of enclosed trace elements. For a cementitious slag, both dissolution and stabilizing reactions take place during a leaching test and the matrix may be dissolved to a large degree. Quenching of a slag for increased glass content implies more phase transformations, and equilibrium will not be reached in a short- term test. The reactions of the puzzolanic minerals hamper the evaluation of leaching tests results. For a proper assessment, knowledge of material properties and leaching tests is essential. The solubility of the trace elements in the original and modified steelmaking slags was low and a reduction is difficult without better control of the major phases. Rapid cooling results in a more homogeneous slag with few phases and the control of the properties are thereby enhanced. Results from leaching tests of both rock materials and slags should be used in a wider context in order to give the test results a reasonable importance in the evaluation of the materials.