In situ hydration imaging study of a ye'elimite paste by ptychographic x-ray computed tomography

Abstract

Eco-cements are a desirable alternative to ordinary Portland cements because of their lower CO2 footprints. For instance, the manufacture of Calcium SulfoAluminate (CSA) cements is more environmentally friendly than that of Portland cements as their production may decrease CO2 footprint by up to 40%. CSA cements contain ye'elimite, Ca4Al6O12SO4, as main phase. The hydration of ye'elimite leads to hydrated compounds such as crystalline ettringite (AFt), crystalline monosulfoaluminate (AFm) and amorphous aluminum hydroxide gel, Al(OH)3·nH2O. Here, we report the results of a ptychographic X-ray computed tomography (PXCT) study on the in situ hydration of ye'elimite with gypsum at different early ages. PXCT is a nondestructive X-ray imaging technique which provides 3D electron density and attenuation coefficient distributions of cement pastes with an isotropic resolution close to 100 nm allowing distinguishing between component phases with very similar contrast in more conventional absorption-based X-ray tomography. The sample was prepared by hydrating ye'elimite with gypsum. Four datasets were recorded at 48, 53, 58 and 63 hours of hydration. The main aim of this imaging study was to quantify the microstructure evolution, within this time interval, with submicrometer spatial resolution. The different component phases were identified and their mass densities determined. Furthermore, the tomograms were segmented and the volume percentage of each component were determined and compared at the four hydrating ages. The overall porosity content (air and pore solution) decreased from 11.5 to 8.8 vol% and the anhydrous material content (ye'elimite and gypsum) decreased from 14.7 to 7.5 vol% in the studied time interval. Correspondingly, the hydrated content (crystalline ettringite and aluminum hydroxide gel) increased from 73.7 to 83.7 vol%. The time evolution of several anhydrous particles was analyzed to determine the dissolution rate of the ye'elimite particles. Similarly, the pore filling process has also been investigated by quantifying their time evolution. These rates are reported and some insights about the mechanisms of these processes are discussed.