Leaching behaviour of rock material and slag used in road construction - amineralogical interpretation

Rock materials used in road construction contain heavy metal elements bound in minerals that are more or less soluble. There are no requirements for i nvestigations of leaching behaviour before use of rock materials in Sweden, which is the case regarding other materials as, e.g., stags. This implies that there is a lack of data to be used when other materials are evaluated. Seven rock materials and two gravels representing nan-weathered material f or use in base or sub-base course from three counties in Sweden have been i nvestigated regarding mineral composition in order to explain the leaching behaviour. Microscopic studies of the mineral composition, acid-base-accoun ting and pH-measurements have been used to explain the leaching results ach ieved with the availability test. The identified transparent minerals were the expected silicate minerals for the sampled rock-forming materials. Over all, the content of identified opaque minerals was low. How an element is b ound in the mineral is decisive for the dissolution of the heavy metal elem ents. Sulphide bound elements have a notably high fraction that is soluble, especially under oxidising conditions. Chromium and vanadium present as su bstituted ions in the crystal lattice of oxides are not dissolved. The diss olution of the buffering rock forming silicates is much slower than the dis solution of the acid-producing sulphides. The results have been compared to similar leaching tests of metallurgical slag used in road construction. Th e dissolution of the major phase, the solubility of the heavy metal mineral and secondary reactions are factors influencing the dissolved amounts of h eavy metal elements. Compared to the crystalline rock materials, the amorph ous fuming slag from a copper smelter has very low solubility, while blast furnace slag is easily dissolved due to hydrolysis. The soluble amounts of sulphide bound elements in rock material is higher compared to blast furnac e slag. The kinetics of the acid-producing and acid-consuming reactions of the rock materials needs to be further investigated. The blast furnace slag and the fuming slag can be used in road construction without any risk of h armful environmental impact due to heavy metal leaching.