Degradation of reinforced concrete from industrial buildings caused by combined sulphate-phosphate attack

A case study presenting considerable degradation of a reinforced concrete industrial building, constructed at Porto Marghera (Venice, North-Eastern Italy) in 1969, is here described. During the last fifty years the Porto Marghera area suffered from severe atmospheric pollution, with relevant emissions of sulphur and nitrogen pollutants, related to the industrial activities. Moreover, the site was affected by intense phosphate and ammonia soil pollution, related to the production of superphosphate and ammonium phosphate bearing fertilizers in the industrial plant. Concrete in the structure shows a remarkable lack of coherence with intense superficial exfoliation, up to several centimetres thick. Superficial black crusts are uniformly distributed, whereas in the lower portions of the structure in close contact with the soil the cement matrix below the cortical layer shows an anomalous yellowish colour. In these portions the conglomerate is highly incoherent and the binder is subjected to pulverization. The reinforcement bars are also deeply altered, showing severe oxidation and in some cases complete loss of shape. A selection of altered concrete samples was analyzed through a profile multi-analytical approach consisting of a combination of optical microscopy, X-ray powder diffraction, and scanning electron microscopy coupled with EDS analysis, with the aim of characterizing the secondary phases formed and modelling the alteration processes. Moreover, the dehydration behaviour of the secondary phases was investigated on synthesized and thermally treated specimens through Rietveld refinement. Furthermore, sequence of reactions, environmental conditions, and stability of secondary mineral phases were studied and modelled using PHREEQC2 software through a thermodynamic approach, taking into account several chemical-physical variables, such as pH and chemical composition. The results suggest severe decalcification of the cement matrix and formation of secondary sulphates and phosphates according to a marked mineralogical and textural zoning: I) gypsum, bassanite and anhydrite in the superficial efflorescence; II) monetite in the superficial crusts; III) gypsum-brushite solid solution and ardealite in the external strata; IV) octacalcium phosphate and struvite in the intermediate layers; V) apatite and struvite in the inner altered portions. The heterogeneous microstructural development of secondary phases was interpreted taking into account the different source of sulphate and phosphate pollutants, from atmospheric aerosols and through capillarity transport from the soil, respectively. Moreover, the permanence of apatite acidic precursors in the external strata suggests that these portions were exposed to high acidity conditions, related to sulphuric acid formation in the pore water after reaction with sulphur dioxide.