The effects of atmospheric multipollutants on modern concrete

Concrete samples were collected from the indoor walls of a tunnel in Milan (Italy), erected at the beginning of the 20th century for railway subway. Since the second half of the 20th century, during the construction of Stazione Centrale (Central Railway Station) of Milan, the tunnel has been turned into an automobile and railway crossing, thus increasing the deposition of aggressive pollutants on building materials. Weathering layers (commonly known as black crusts) caused by deposition of atmospheric pollutants on concrete surfaces were analysed in order to investigate the main mechanisms responsible for deterioration. A mineralogical and physical–chemical characterisation of the concrete and black crusts was performed by optical microscopy, atomic absorption spectroscopy, X-ray powder diffraction, scanning electron microscopy, Fourier transformed infrared spectroscopy and differential scanning calorimetry. The results attest that the main deterioration phenomenon affecting concrete is the sulphation process by dry deposition, leading to secondary salt crystallisation (gypsum formation) on the external surface of the samples. Moreover, concrete samples show widespread micro- and macro-cracking, high porosity, and corrosion phenomena at the aggregate–binder interface (AAR). Airborne particulate matter produced by fuel (oil-derived and coal) combustion was found embedded in the gypsum matrix of black crusts, suggesting its catalytic role in sulphation process.