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Conventional drying of colloidal materials and gels (including cement) canlead to detrimental effects due to the buildup of internal stresses as waterevaporates from the nano/microscopic pores. However, the underlying nanoscopicalterations in these gel materials that are, in part, responsible formacroscopically-measured strain values, especially at low relative humidity,remain a topic of open debate in the literature. In this study, sodium-basedcalcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel, the major binding phase ofsilicate-activated blast furnace slag (one type of low-CO$_2$ cement), isinvestigated from a drying perspective, since it is known to suffer extensivelyfrom drying-induced microcracking. By employing in situ synchrotron X-ray totalscattering measurements and pair distribution function (PDF) analysis we showthat the significant contributing factor to the strain development in thismaterial at extremely low relative humidity (0%) is the local atomic structuralrearrangement of the C-(N)-A-S-H gel, including collapse of interlayer spacingand slight disintegration of the gel. Moreover, analysis of the medium range(1.0 - 2.2 nm) ordering in the PDF data reveals that the PDF-derived strainvalues are in much closer agreement (same order of magnitude) with themacroscopically measured strain data, compared to previous results based onreciprocal space X-ray diffraction data. From a mitigation standpoint, we showthat small amounts of ZrO$_2$ nanoparticles are able to actively reinforce thestructure of silicate-activated slag during drying, preventing atomic levelstrains from developing. Mechanistically, these nanoparticles induce growth ofa silica-rich gel during drying, which, via density functional theorycalculations, we show is attributed to the high surface reactivity oftetragonal ZrO$_2$.

physical chemistry chemical physics/pccp. physical chemistry chemical physics

Drying-Induced Atomic Structural Rearrangements in Sodium-Based  Calcium-Alumino-Silicate-Hydrate Gel and the Mitigating Effects of ZrO$_2$  Nanoparticles