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  Sulfate resistance and phase composition of modern shotcrete

  In tunnel applications spraying of concrete is a well-established and economical alternative to con-ventional casting techniques. Shotcrete often is in direct contact with aggressive tunnel water i.e. sulfate-bearing groundwater. The special application technique and material composition with rela-tively small aggregate size, high paste volume as well as the addition of large amounts of aluminum-bearing set accelerators, lead to a signifcant difference from conventional concrete regarding phase composition and microstructure. Sulfate resistance of shotcrete has not been investigated extensively so far. In this study, the infu-ence of alkali-free set-accelerators on the sulfate resistance was examined and compared to shot-crete with an alkali-based reference accelerator. Shotcrete was produced on-site under full-scale conditions, and samples were taken for laboratory investigations. Testing procedures for the estima-tion of the sulfate resistance developed for conventional concrete were applied to shotcrete. The chemical phase composition before and after interaction with sulfates was studied by thermogravi-metric analysis (TGA) and X-ray diffraction (XRD). Furthermore, porosity and permeability data were acquired. The analysis of possible durability problems regarding sulfate ingress helps to avoid long-term dete-rioration problems caused by the application of non-suitable shotcrete mix-designs and hence leads to more sustainable and long-living underground structures.

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