CHEMICAL, MINERALOGICAL AND PETROGRAPHIC ANALYSES ON EXPANSIBLE BLAST-FURNACE SLAG USED AS A FOUNDATION MATERIAL IN THE IV MODULE OF THE GENOVA-VOLTRI PORT (ITALY)

After the construction of the paving for container storage in the Genova-Voltri port, considerable ground swellings (up to 30 cm) took place, leading to dimensional damage of both crane runways and loading/unloading area, including the container ground slots. This is due to expansible chemical reactions occurred within blast-furnace slags employed as a foundation material. The aim of this study was that to determine the chemical and mineralogical characteristics of blast-furnace slag and to identify the phases that are expected to be responsible for expansible chemical reactions. For this purpose, materials from three geognostic drillings 20m deep were analyzed and their stratigraphic sequence reconstructed. The study of the drillings (from 0 to 3m deep) led to a full characterization of the foundation materials used for building the IV module in the port of Genoa-Voltri. The present study revealed the presence of a complex stratigraphy, with layers indicating the presence of metallurgical slag. The slag-rich layers are present at varying depths in the various surveys and their average thickness is about 90cm in the three examined ones. Thin section study, X-ray powder diffraction, scanning electron microscopy observations and microanalyses of some selected samples allowed to identify the presence of chrysotile and of the phases commonly described in the metallurgic slag. The chemical and mineralogical data showed the presence of anhydrous phases: merwinite (3CaO MgO 2SiO2), olivina 2(MgO, FeO) SiO2, alite C3S (3CaO SiO2), belite C2S (2CaO SiO2), ferrite phase C4AF (4CaO Al2O3 Fe2O3), lime CaO, periclase MgO, wuestite FeO and a phase with intermediate composition FeO-MnO-MgO. Hydrated phases such as brucite, portlandite, gibbsite, goethite, and products of hydration of the cement phases have been also identified by XRPD and TG/DSC analyses. The formation of these hydrated phases caused a significant volumetric instability, mainly due to the presence of free CaO and free MgO, which in the presence of water form portlandite, Ca(OH)2 and brucite, Mg(OH)2. According to literature studies, the hydration reaction of free lime can lead in few days to a volume increase up to 100%. Hydration reaction of free periclase is instead slower, leading to considerable volume increase even after months or years. Therefore, it is not possible to exclude that in the next future relevant swelling phenomena should occur again.