The geochemical processes underlying hardpan formation in an alkaline iron slag dump containing up to 30 cm thick hardpans were investigated. A modelling approach has been developed to simulate mineral precipitation sequences upon pore water evaporation with PHRQPITZ and PHREEQC. This modelling approach follows the thermodynamic reaction path in which reaction products, except for calcite, may react again with the aqueous phase and form new products as evaporation proceeds. Phases that were allowed to precipitate include am-SiO2, am-Al(OH)(3), ferrrihydrite, calcite, and salts that are known to occur in natural evaporites. The predicted results were compared with the observed mineralogy of evaporites and hardpans as analysed by XRD, FTIR-spectroscopy, and/or SEM-EDS. The evaporites precipitating from relatively unweathered iron slag were found to consist mainly of sodium salts (mirabilite, trona, glaserite) and to a smaller extent of calcite and gel-like phases of in particularly Si, but also Al and Fe. After 10 years of weathering, amorphous Sigels and calcite were found to be most abundant. These results are based on model predictions as well as on observations. This silica gel phase, which actually consists of heterogeneous Ca- and Fe-rich silica gels, covers particles and fills micropores and -cracks in the hardpan. Hardpans formed by gel-like phases may hold rain water by their swelling properties. (C) 2006 Elsevier Ltd. All rights reserved.