Advanced 3D nanoimaging of cement paste hydration using synchrotron Xray ptychographic tomography

Abstract

Hydration of Portland cements (PC) is a complex phenomenon caused by the presence of multiple components with multi-length scale variability. To investigate this complicated process, state-of-theart 3D nanoimaging techniques with excellent spatial resolution and the ability to scan large fields of view are required. In this work, synchrotron X-ray near-field ptychographic tomography is used to study four cement pastes within 0.2 mm thick capillaries. Synchrotron ptychotomography produces electron density and absorption coefficient tomograms that facilitate the characterisation of amorphous materials. By achieving a spatial resolution of around 220 nm with good contrast, nanofeatures such as the density and spatial distribution of amorphous constituents can be identified. Key findings include the revelation that a seeded C-S-H gel has a lower density than a CaCl2 accelerated PC paste. In addition, the density of the C-A-S-H gel in a PC blend containing metakaolin and limestone decreases when limestone is present. These density variations are critical in estimating the space filling in the paste. C-S-H nucleation seeding results in a reduced inner/outer C-S-H gel ratio, with different average sizes of high-density inner C-S-H gel found under different hydration conditions. Various additives, including nucleation seeding admixtures and CaCl2, alter the water/cement ratio and the overall density of the C-S-H gel. The addition of alkanolamine admixture increases C4AF hydration, resulting in the development of amorphous Al-rich iron-silicate hydrogarnet, which affects C4AF dissolution. The addition of limestone to the PC-MK blend changes the electron density of the C-A-S-H gel and increases the production of amorphous monocarboaluminate-like components, which influences porosity refinement.