Geochemical Effects of Groundwater Interaction with Steel Slag Aggregate Used in Road Construction

This study assessed the effects of groundwater interacting with aggregate produced from steel slag, used as a ballast material in a waterproofing system protecting the road surface under high groundwater table conditions. Field investigations were carried out, with hydrochemical analyses of groundwater and water draining through the structure, mineralogical characterisation of the slag, and geochemical modelling using the PHREEQC programme with inverse modelling. The results showed that water flow through the slag aggregate caused strong alkalisation of the solution and changes in the concentrations of Ca, Mg, Na, Cl, sulphates and carbonate components. Inverse modelling enabled the identification of eight acceptable mass balance models. The main primary phases involved in the transformations were larnite, merwinite, mayenite, halite and, locally, slag glass. Their dissolution contributed Ca, Mg, Al, Si, Na and Cl to the solution. The increases in Na and Cl concentrations were interpreted primarily as the result of an external influx of road salt, rather than as an intrinsic property of the slag. The secondary products were dominated by amorphous silica, calcite, ettringite and brucite, indicating silica removal, carbonation, sulphate fixation and the partial immobilisation of Mg. The results confirm that slag aggregate remains geochemically active in contact with groundwater; however, simultaneous carbonation and secondary mineralisation favour the gradual stabilisation of the water–slag system.