15 th International Congress on the Chemistry of Cement Prague, Czech Republic, September 16–20, 2019 Ptychographic X-ray computed tomography analysis of cement pastes MIGUEL ARANDA 1,2,a , Ana Cuesta 1,b , Angeles De la Torre 1,c , Pavel Trtik 3,d , Ana Diaz 3,e 1 Departamento de Química Inorgánica, Cristalografía y Mineralogía, University of Malaga and ALBA synchrotron, Málaga, Spain 2 ALBA SYNCHROTRON, Barcelona, Spain 3 Paul Scherrer Institut, Villigen, Switzerland a g_aranda@uma.es b acuesta@cells.es c mgd@uma.es d pavel.trtik@psi.ch e ana.diaz@psi.ch ABSTRACT Ptychographic X-ray computed tomography provides 3D electron mass density and attenuation coefficient distributions of unaltered cement pastes with an isotropic resolution below 100 nm. This imaging technique allows quantitatively distinguishing between different components with very similar absorption contrast. Samples were measured at the cSAXS beamline: i) a neat Portland Cement (PC); ii) a PC-CC blend: 80 wt% of PC and 20 wt% of CaCO3, and iii) a PC-FA blend: 70 wt% of PC and 30 wt% of fly ash. The main aim of this study is to have a better insight of the microstructure of the amorphous/nanocrystalline gels with submicrometer spatial resolution. It is worth noting that it is possible to determine the gel mass density and water content within the attained 3D resolution (about 100 nm). Here, we focused on the spatial distribution of the different components and in the variation of the electron density values which are very related to the mass density values. Special attention is paid to the density values of the amorphous (or nanocrystalline) components. The electron and mass density values of the C-S-H gel for three pastes are thoroughly analyzed. The density values range from 2.05- 2.10 g·cm-3 for high density C-S-H gel for neat PC and PC-CC pastes to 1.80 g.cm-3 for low density C-S-H gel in PC-FA paste. The density value of poorly crystalline iron-siliceous hydrogarnet component, d=2.52 g·cm-3, has also been determined. A summary of our ongoing research focused on the analyses of cement pastes by synchrotron PXCT is reported and discussed