Steam locomotive soot and the formation of thaumasite in shotcrete

Abstract

After approximately 40 years the sprayed concrete lining in the Koblenz railway tunnel started to break off from its stonework support. The old masonry showed relicts of a soot layer from the ages of steam locomotive services. Locally small amounts of low mineralised water were running through the tunnel lining. Due to the finding of ettringite at the rear surface of the shotcrete together with soot it was assumed that sulfur from the locally present soot layer was the cause for the shotcrete detachment. Detailed investigations showed the formation of thaumasite at the concrete interface to its support and up to 15 mm inside the concrete. Thaumasite could be detected filling microcracks and veins parallel to the shotcrete surface. The amount of secondary sulfate minerals and the extent of sulfate uptake are orders of magnitude larger to what could be expected if the sulfur of the soot layer would be present as sulfate. It is assumed that the finding of thaumasite in cracks and veins is caused by long term interaction with a low mineralised groundwater and is leading toward a complete alteration of the concrete into a mushy material.