Micro X-ray diffraction and elemental study on Al-tobermorite formation in aged modern concrete

In modern Portland cement, calcium aluminum silicate hydrates (C-A-S-H) do not generally form crystalline Al-tobermorite. In this study, to identify the mechanism of Al-tobermorite formation, synchrotron micro X-ray diffraction and field-emission scanning electron microscopy-based energy-dispersive X-ray spectroscopy (EDS) investigations were performed on a slice of a thick concrete wall in a nuclear power plant, where the presence of Al-tobermorite was previously confirmed. Rietveld analysis of the diffraction diagrams obtained from the 2D diffraction patterns showed that Al-tobermorite was formed almost everywhere in the cement paste near sandstone aggregate and that Al-tobermorite platelets were distributed across most of the cement paste area in a random orientation. The basal spacing d-value of Al-tobermorite was identical and equal to 11.39 angstrom. The ratio plots from the EDS maps indicated that the chemical composition of the mixture of Al-tobermorite and C-A-S-H was almost homogeneous all around the cement paste. The Al-tobermorite in this concrete is probably formed via a dissolution-precipitation process. From the random orientation even next to the surface of aggregate, we infer that the Al-tobermorite formation does not need the surface of aggregate to precipitate.

Automated summary

By studying a slice of a concrete wall in a nuclear power plant, the formation mechanism of Al-tobermorite was identified and it was found that Al-tobermorite forms almost everywhere in the cement paste with a uniform composition.