Assessing the dimensional stability of alkali-activated calcined clays in the fresh state: a time-lapse X-ray imaging approach

Alkali-activated calcined clays are promising candidates for playing a prominent role in the future construction industry. These binders may achieve excellent mechanical performance, but one issue deserving attention is the proneness to plastic shrinkage and surface cracking. Tackling this issue requires the deployment of laboratory techniques that allow shrinkage-inducing mechanisms to be quantitatively assessed. Here, we demonstrate that time-lapse X-ray imaging can be used to quantify shrinkage immediately after mixing, when the binder is still in its fresh state, with excellent time and space resolution. The numeric quantification of strain is complemented by the real time visual inspection of the displacing sample interface and of the bleed aqueous solution layer that may form. Implementation of this method to a set of alkali-activated cement pastes, prepared by combining calcined clays having different mineralogical composition with sodium silicate activating solutions having different SiO2/Na2O ratios, suggests that two main mechanisms control the early dimensional stability of alkali-activated calcined clays. These mechanisms are: (a) volumetric contraction occurring in response to capillary stress arising from water evaporation and (b) segregation by particle settling, favoured in the water-saturated regime.

Automated summary

Alkali-activated calcined clays show promise in the construction industry, but they are prone to issues such as plastic shrinkage and surface cracking. Laboratory techniques, like time-lapse X-ray imaging, can quantify shrinkage mechanisms and reveal that early dimensional stability is controlled by two main mechanisms: volumetric contraction due to water evaporation and particle segregation in the water-saturated regime.