Determination of the elemental distribution and chemical speciation in highly heterogeneous cementitious materials using synchrotron-based micro-spectroscopic techniques

Synchrotron-based micro-X-ray fluorescence (XRF) combined with scanning electron microscopy-based energy dispersive micro-analysis (EDS) has been used to determine the elemental distribution of contaminants (e.g., Ni) and of chemical elements inherent to the cement matrix (e.g., Si, Ca, Al, S) in hardened cement paste. Detailed information on the cement microstructure was gained by using backscattered electron (BSE) imaging. The results obtained from the complementary use of micro-XRF, EDS and BSE reveal that Ni is primarily distributed around inner calcium silicate hydrates (inner-C-S-H) and that Ni is preferentially associated with Al. This suggests the formation of a Ni–Al phase and its direct association with inner-C-S-H. Further information on the chemical speciation of Ni in relation to Al and S was obtained at selected regions of interests in the cement matrix using synchrotron-based micro-X-ray absorption spectroscopy (XAS). Data analysis shows that Ni is predominantly immobilized in layered double hydroxides, while predominant formation of ettringite was indicated from the Al and S XAS data.The present study demonstrates that the combined use of micro-XRF, BSE, EDS and micro-XAS, opens up a powerful analytical approach to determine the distribution and the speciation of chemical elements in complex heterogeneous cementitious materials on the same region of interest with micro-scale resolution.