Changes in the phase assemblage of concrete exposed to sea water

In the present study the phase changes in an ordinary concrete standing for 10 years in a tidal zone were investigated with a range of techniques. From the exposed surface and inwards different zones had formed. SEM–EDS analysis of a polished section of the surface near region, showed a Mg rich layer with a thickness of 10–20 μm, as well as the filling of cracks leading from the surface with a Mg rich phase, most likely brucite. In the outermost 2 mm, an increase in the calcium carbonate content was identified by XRD and TGA. In the same zone SEM–EDS analyses indicated enrichment in ettringite and thaumasite. In the first 20 mm calcium hydroxide leaching was observed using XRD and TGA. Chlorides appeared to have penetrated up to approx. 70 mm from the surface. Part of the chlorides were found to be bound in alumina containing phases and in the C–S–H by SEM–EDS. These experimentally observed phase changes generally agreed with the predictions of a thermodynamic model.