Ptychographic X-ray computed tomography data for three Portland cement pastes

Mortars and concretes are ubiquitous materials with very complex hierarchical microstructures. To fully understand their main properties and to decrease their CO₂ footprints, a sound description of their (spatially-resolved) mineralogy is compulsory. Developing this knowledge is very challenging as about half of the volume of hydrated cement is a nanocrystalline component, calcium-silicate-hydrate (C-S-H gel). Furthermore, other poorly crystalline phases (e.g. iron-siliceous hydrogarnet or silica oxide) may coexist which are even more difficult to characterise. Traditional spatially-resolved techniques like electron microscopies involve complex sample preparation steps that often lead to artefacts (e.g. dehydration and microstructural changes). Here, we have used synchrotron ptychographic tomography for obtaining spatially-resolved information on three unaltered representative samples: neat Portland paste, Portland-calcite and Portland-fly ash blend pastes with spatial resolution below 100 nm in samples of up to 5×10⁴ mm³ of volume. For the neat Portland paste, the ptychotomographic study gave densities of 2.11 and 2.52 gcm^-3 and contents of 41.1 and 6.4 vol% for nanocrystalline C-S-H gel and poorly crystalline iron-siliceous hydrogarnet, respectively. Furthermore, the spatially-resolved volumetric mass density information has allowed to characterise inner product and outer product C-S-H gels. The average density of inner product C-S-H is smaller than that of outer product and its variability larger. Full characterisation of the pastes, including segmentation of the different components, is reported and the contents are compared with the results obtained by thermodynamical modelling.

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, r=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.

PC sample:

tomo_beta_S02536_to_S03341_Hann_freqscl_0.35_0xxx

tomo_delta_S02536_to_S03341_Hann_freqscl_1.00_0xxx

PC-CC sample:

tomo_beta_S04692_to_S06001_Hann_freqscl_0.35_0xxx

tomo_delta_S04692_to_S06001_Hann_freqscl_1.00_0xxx

PC-FA sample:

tomo_beta_S03351_to_S04661_Hann_freqscl_0.35_0xxx

tomo_delta_S03351_to_S04661_Hann_freqscl_1.00_0xxx