Etude de la valorisation des déblais de chantiers de tunnels riches en sulfates en granulats à béton
The construction of a 57 km length basis tunnel for the future railway link between Lyon (France) and Turin (Italy) promoted by Lyon-Turin Ferroviaire (LTF), subsidiary of Réseau Ferré de France (RFF) and Rete Ferroviaria Italiana (RFI), will result in the production of 16 millions of m3 of excavated materials. In order to promote efficient and sustainable management of natural resources and to reduce greenhouse gas emission and environmental impact, one wish of LTF is to use the most part of these materials as concrete aggregates. However, a part of the excavated materials from LTF tunnel has a sulphate content between 1.46 and 4.30% (expressed in mass of SO3) and cannot be used considering the European standards, which drastically limit the sulphate content in order to avoid structural damages resulting from internal sulphate attack. The sulphate content is limited to 0.2% for this type of construction. The first study focused on the behaviour of sulphates from the excavated materials, more particularly on their leaching in solution in order to determine the sulphate content that could be released in the concrete. The results showed that the sulphates are quickly available in the particles size lower than 4 mm. A possible treatment of materials by washing before use in concrete can be a solution to limit the internal sulphate attack. Secondly, to define a suitable cement to this type of aggregate, a comparative study of different cements supposed to be resistant to sulphates has been carried out. Three accelerated tests have been developed to assess the risk of degradation of cementitious materials by formation of ettringite and thaumasite. Only two cements showed a good behavior against the internal sulphate attack resulting from sulphated aggregates. These cements are a Portland cement with very low calcium aluminates and alkaline contents and an over sulphated cement. Finally, a final innovative solution has been proposed and concerns the using of excavated materials as a source of sulphates for regulating the hydration of cement. Sulfates from the excavated sand can regulate the cement hydration and then cancel any further damage due to sulphates.