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

📝 Abstract

Conventional drying of colloidal materials and gels (including cement) can lead to detrimental effects due to the buildup of internal stresses as water evaporates from the nano/microscopic pores. However, the underlying nanoscopic alterations in these gel materials that are, in part, responsible for macroscopically-measured strain values, especially at low relative humidity, remain a topic of open debate in the literature. In this study, sodium-based calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel, the major binding phase of silicate-activated blast furnace slag (one type of low-CO $_2$ cement), is investigated from a drying perspective, since it is known to suffer extensively from drying-induced microcracking. By employing in situ synchrotron X-ray total scattering measurements and pair distribution function (PDF) analysis we show that the significant contributing factor to the strain development in this material at extremely low relative humidity (0%) is the local atomic structural rearrangement of the C-(N)-A-S-H gel, including collapse of interlayer spacing and 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 strain values are in much closer agreement (same order of magnitude) with the macroscopically measured strain data, compared to previous results based on reciprocal space X-ray diffraction data. From a mitigation standpoint, we show that small amounts of ZrO $_2$ nanoparticles are able to actively reinforce the structure of silicate-activated slag during drying, preventing atomic level strains from developing. Mechanistically, these nanoparticles induce growth of a silica-rich gel during drying, which, via density functional theory calculations, we show is attributed to the high surface reactivity of tetragonal ZrO $_2 $.

💡 Analysis

Conventional drying of colloidal materials and gels (including cement) can lead to detrimental effects due to the buildup of internal stresses as water evaporates from the nano/microscopic pores. However, the underlying nanoscopic alterations in these gel materials that are, in part, responsible for macroscopically-measured strain values, especially at low relative humidity, remain a topic of open debate in the literature. In this study, sodium-based calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel, the major binding phase of silicate-activated blast furnace slag (one type of low-CO $_2$ cement), is investigated from a drying perspective, since it is known to suffer extensively from drying-induced microcracking. By employing in situ synchrotron X-ray total scattering measurements and pair distribution function (PDF) analysis we show that the significant contributing factor to the strain development in this material at extremely low relative humidity (0%) is the local atomic structural rearrangement of the C-(N)-A-S-H gel, including collapse of interlayer spacing and 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 strain values are in much closer agreement (same order of magnitude) with the macroscopically measured strain data, compared to previous results based on reciprocal space X-ray diffraction data. From a mitigation standpoint, we show that small amounts of ZrO $_2$ nanoparticles are able to actively reinforce the structure of silicate-activated slag during drying, preventing atomic level strains from developing. Mechanistically, these nanoparticles induce growth of a silica-rich gel during drying, which, via density functional theory calculations, we show is attributed to the high surface reactivity of tetragonal ZrO $_2 $.

📄 Content

Drying-Induced Atomic Structural Rearrangements in Sodium-Based Calcium-Alumino-Silicate-Hydrate Gel and the Mitigating Effects of ZrO2 Nanoparticles Kengran Yang, V. Ongun Özçelik, Nishant Garg, Kai Gong and Claire E. White* Department of Civil & Environmental Engineering and Andlinger Center for Energy and the Environment, Princeton University, Princeton, USA Abstract Conventional drying of colloidal materials and gels (including cement) can lead to detrimental effects due to the buildup of internal stresses as water evaporates from the nano/microscopic pores. However, the underlying nanoscopic alterations in these gel materials that are, in part, responsible for macroscopically-measured strain values, especially at low relative humidity, remain a topic of open debate in the literature. In this study, sodium-based calcium-alumino- silicate-hydrate (C-(N)-A-S-H) gel, the major binding phase of silicate-activated blast furnace slag (one type of low-CO2 cement), is investigated from a drying perspective, since it is known to suffer extensively from drying-induced microcracking. By employing in situ synchrotron X-ray total scattering measurements and pair distribution function (PDF) analysis we show that the significant contributing factor to the strain development in this material at extremely low relative humidity (0%) is the local atomic structural rearrangement of the C-(N)-A-S-H gel, including collapse of interlayer spacing and 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 strain values are in much closer agreement (same order of magnitude) with the macroscopically measured strain data, compared to previous results based on reciprocal space X-ray diffraction data. From a mitigation standpoint, we show that small amounts of ZrO2 nanoparticles are able to actively reinforce the structure of silicate-activated slag during drying, preventing atomic level strains from developing. Mechanistically, these nanoparticles induce growth of a silica-rich gel during drying, which, via density functional theory calculations, we show is attributed to the high surface reactivity of tetragonal ZrO2. 1 Introduction Drying is a common (and potentially detrimental) processing technique used in a variety of industries, including sol-gel synthesis and ceramics. The removal of fluid (usually water) from a porous material can lead to a buildup of internal stresses.1 Typically the body will not dry uniformly, and therefore a drying front will develop in the material where the dried outer component wants to shrink, whereas the moist interior restrains shrinkage. This mismatch in strain leads to tensile stresses developing on the surface of the material, and if these stresses exceed the tensile strength, the material will crack. Mitigation strategies include careful control of material thickness,2 use of surface coatings to prevent cracking during evaporation3 and controlled drying.4 One class of material that is used in significant quantities around the world that is prone to suffer from drying, and therefore cracking, is concrete. Although there are several viable mediating technologies available for Portland cement-based concrete to prevent drying- induced microcracking, the underlying mechanisms responsible for the macroscopically- measured strains remain somewhat unclear, especially those occurring at low relative humidity (RH, < 40%). Moreover, the development of low-CO2 cement alternatives, such as alkali- activated materials (AAMs), has led to a reexamination of cement degradation phenomena, including drying-induced microcracking. Given that ordinary Portland cement (OPC) production accounts for 5 – 8% of anthropogenic CO2 emissions,5 there is a pressing need to develop and implement sustainable alternatives. AAMs are one of the most competitive alternatives, and have been shown to emit less CO2 (~ 40 – 80%) compared with OPC.6-8 AAMs utilize aluminosilicate-rich precursor materials, including industrial by-products such as ground granulated blast-furnace slag, fly ash from coal-fired power plants and calcined clays (e.g., metakaolin), which form mechanically hard binders (gels) when activated by alkaline solutions (or solids, as is the case for 1-part mixes9,10).6,11 Furthermore, given correct mix designs, AAMs have comparable mechanical performance12 and cost7 to OPC, and can be tuned to have superior properties via specific chemical compositions, such as high thermal performance13 and low permeability.14 Nevertheless, questions remain regarding the long-term durability of AAMs, with the underlying degradation mechanisms often founded at the atomic/nanoscale, such as carbonation-induced chemical reactions15-18 (from atmospheric and accelerated CO2 conditions) and sulfate attack of the binder gel.19,20 Progress is being made to elucidate the mechanisms responsible for chemical degradation of different types of

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