Two cases of massive sulphate attack to cement treated embankments and track bases in a high speed railway line are described. In the first case, two large access embankments to a railway viaduct were treated with jet–grouting columns. In the second case a compacted cement treated soil was placed over a rigid concrete caisson. The treated layer, 1.5 m thick, expanded at a continuous rate of 0.9-1.3 mm/month. In the two cases the soil was excavated from nearby cuts in gypsiferous Tertiary (Oligocene) claystones. Ettringite and thaumasite crystals were found within the expanding levels. The chemical evolution of an interface between a cement treated body and a compacted soil is presented. Solutions adopted to remediate the created problem are briefly described.