Abstract
The main binding phases of calcium aluminate cement (CAC) concrete, CaO·Al2O3·10H2O (CAH10) and 2CaO·Al2O3·8H2O (C2AH8), slowly convert to 3CaO·Al2O3·6H2O (C3AH6) and Al(OH)3 (AH3). This reaction significantly speeds up at a temperature higher than ∼30 °C, and over time leads to significant strength loss in CAC concrete. Because of the lack of direct evidence that simultaneously probes morphological and chemical/crystallographic information, intense debate remains whether the conversion is generated by a solid-state or through-solution reaction. The conversion of CAH10 at an elevated temperature is studied herein using synchrotron-radiation-based X-ray spectromicroscopy capable of acquiring near edge X-ray absorption fine structure data and ptychographic images with a resolution of ∼15 nm. We show that, when stored at 60 °C, CAH10 first converts to C2AH8 by solid-state decomposition, followed by the through-solution formation of C3AH6. The C3AH6 crystallizes from both the relics of dissolved C2AH8 and fro...
Highlights
Our results provide nanoscale evidence of how CAH10 converts to C3AH6 under an elevated temperature, which can be directly used in modeling the microstructural evolution of calcium aluminate cement (CAC) concrete affected by the conversion reaction
The interpretation here follows the theory of crystal field splitting of d0 compounds such as Ca2+, which is briefly described in previous research of Cabearing minerals in cement-related systems;[22,25] detailed theoretical background can be found in the literature.[26,27]
The results are summarized as follows: (1) the atomic positions of CaO·Al2O3·10H2O (CAH10) and 2CaO·Al2O3·8H2O (C2AH8) are not completely solved, we provide spectroscopic evidence to prove that the local environment of Ca in the layer structure ([Ca2Al(OH)6]+) of C2AH8 highly resembles that of kuzelite
Summary
The unique hydration behavior of calcium aluminate cement (CAC), such as high reactivity (even at low-curing temperatures), leads to its broad application in emergency repair, fasttrack construction in cold weather, and infrastructures exposed to sulfate attack; it is widely used to make refractory linings for high-temperature furnaces.[1,2] Calcium aluminate cement contains CA (monocalcium aluminate) as the main cementing compound, with C12A7, CA2, and CA6 as the minor phases, where C CaO, A Al2O3, and H H2O in the cement chemistry notation.[3,4] The rapid hydration of CA produces. Coupled with the morphological observation that the short fibers agglomerate in the contour of the original CAH10 prism, the spectroscopic evidence leads to the hypothesis that CAH10 converts to C2AH8 fibers and AH3 gel through internal decomposition, i.e., C2AH8 + AH3 gel form within the region which was originally occupied by CAH10 prisms In such case, the conversion reaction happens in a solid-state route, and the external bulk water is not needed to transport dissolved ions. The formation of C3AH6 may be self-accelerated when nucleation is factored into the ratecontrol process.[14] It should be noticed that C2AH8 does not grow to macrocrystals throughout the heating duration in this study, and the number of C2AH8 crystals soon decreases after ∼1 h of thermal storage This explains why the maximum diffraction signal of C2AH8 remains low compared with that of CAH10, as shown by the SR-EDD study.[6] In real CAC concrete infrastructure, the rate of conversion may differ from the current study due to the difference of reaction conditions. The percentage and morphology of CAH10, C2AH8, and C3AH6 in a converted infrastructure are functions of the local temperature, local ion concentration, and spatial availability for crystal growth
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