Micro-spectroscopic investigation of Al and S speciation in hardened cement paste

Synchrotron-based micro X-ray fluorescence (micro-XRF) and micro X-ray absorption near edge spectroscopy (micro-XANES) have been used to determine the spatial distribution of Al and S and to identify the Al- and S-bearing species in compact hardened cement paste hydrated at 50 °C. The contribution of the S-bearing cement phases to the composed S K-edge XANES spectra collected in ten S-rich regions was determined using least-squares fitting. Ettringite and calcium monosulfoaluminate were identified as the main S-bearing species in the selected regions. Factor analysis was employed to determine the contribution of the various Al-bearing cement minerals to the composed Al K-edge XANES collected in different Al-rich regions of the cement matrix. Principal component analysis revealed that all spectra could be fitted using three components. Target transformation further suggested that the two Al-bearing clinker phases (aluminate, ferrite) and secondary phases of the hydrate assemblage (ettringite, AFm phases, hydrotalcite) contributed to the set of components that made up the experimental spectra. Least-squares fitting allowed the relative contribution of each reference compound to be determined. Aluminate and/or ferrite were detected in all Al-rich regions. AFm phases were identified in six out of the ten regions studied, while ettringite was detected in only two regions. The study confirmed that AFm phases are important cement minerals in hardened cement paste hydrated at 50 °C.