A case study of thaumasite formation in an Austrian tunnel

Samples were taken from zones of the shotcrete lining in a tunnel where the shotcrete had completely lost its strength and could easily be removed by hand. In addition, drilling cores without obvious damage at the visible surface (Ø 50 mm and 200 mm) were taken. Furthermore a needle-like efflorescence from the inner shotcrete lining and ground water samples were collected. Interstitial solutions were pressed out from the completely damaged material. In addition, sections of 10 mm thickness were cut off from the 200 mm core, beginning from the back of the core in order to investigate possible chemical and mineralogical variations.
X-ray diffractometry (XRD) analyses indicated that thaumasite formation was the main reason for the damage and the needle-like efflorescence was identified as sodium sulphate (Na2SO4). Wet chemical and XRD analysis of non-altered shotcrete showed that the natural concrete aggregates consisted mainly of calcite, dolomite and quartz. Interstitial solutions were mainly dominated by Na+ and SO42- and values of up to 7400 and 17200 mg/l, respectively, were measured. In contrary, SO42- concentrations of the local ground waters were in the range of 450 to 550 mg/l, and the Na+ concentrations were as low as 0.5 - 4.0 mg/l. It is believed that Na+ and soluble SiO2 originate from water glass, which presumably had been used as setting accelerator for the shotcrete. Furthermore, the extremely high SO42-- concentrations in the expressed solutions have been caused by water evaporation.
Author: J. Tritthart, D. Klammer, F. Mittermayr, A. Brunnsteiner

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