Nanometer-Resolved Spectroscopic Study Reveals the Conversion Mechanism of CaO·Al₂O₃·10H₂O to 2CaO·Al₂O₃·8H₂O and 3CaO·Al₂O₃·6H₂O at an Elevated Temperature

Abstract

The main binding phases of calcium aluminate cement (CAC) concrete, CaO•Al 2 O 3 •10H 2 O (CAH 10) and 2CaO•Al 2 O 3 •8H 2 O (C 2 AH 8), slowly convert to 3CaO•Al 2 O 3 • 6H 2 O (C 3 AH 6) and Al(OH) 3 (AH 3). 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 CAH 10 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, CAH 10 first converts to C 2 AH 8 by solid-state decomposition, followed by the through-solution formation of C 3 AH 6. The C 3 AH 6 crystallizes from both the relics of dissolved C 2 AH 8 and from the surface of existing C 3 AH 6 crystals. The solid-state decomposition of CAH 10 occurs in multiple sites inside the CAH 10 crystals; the spatial range of each decomposition site spans a few tens of nanometers, which overcomes the kinetics barrier of ion transportation in the solid-state. Our work provides the first nanoscale crystal−chemical evidence to explain the microstructure evolution of converted CAC concrete.

Hydration of calcium aluminates at 60°C – Development paths of C2AHx in dependence on the content of free water

“…In all of the investigated pastes (CCC, synthetic CA/CA 2 , synthetic CA 2 ) C 2 AH 8 formation and dehydration to C 2 AH 7.5 occurred prior to the formation of C 3 AH 6 . This sequence is in accordance with the works by Rashid et al and Geng et al . Whereas the two latter references concerned the conversion of CAH 10 , Rashid and Turrillas reported the intermediate formation of C 2 AH 8 prior to the formation of C 3 AH 6 also for the hydration of freshly prepared pastes of CA for temperatures up to 90°C.…”

“…This indicates that the C 2 AH x formation prior to the C 3 AH 6 formation would also occur in well‐sealed samples. Although the C 2 AH 8 is supposed to dissolve before C 3 AH 6 starts to precipitate the intermediate formation of C 2 AH 8 , nevertheless, seems to be essential for the C 3 AH 6 formation. Geng et al suggested that relics of dissolved C 2 AH 8 provide nucleation sites for C 3 AH 6 .…”

“…Although the C 2 AH 8 is supposed to dissolve before C 3 AH 6 starts to precipitate the intermediate formation of C 2 AH 8 , nevertheless, seems to be essential for the C 3 AH 6 formation. Geng et al suggested that relics of dissolved C 2 AH 8 provide nucleation sites for C 3 AH 6 . In the CCC sample (see Figure ) as well as in the CA/CA 2 mix with and without precuring (see Figures and ) the C 2 AH 7.5 content remains stable for periods of 7‐10 hours before dehydration to C 2 AH 5 occurs.…”

“…In order to avoid unpredictable damages more profound knowledge on the hydration of CAC at elevated temperatures would be essential. Two recently published articles concerning the phase transformation of CAH 10 and C 2 AH x at 60°C demonstrate the high actuality and significance of this topic. However, both investigations were performed far away from real‐life conditions, eg, during heating‐up of precast binders for the use in refractories.…”

“…Different mechanisms behind the transformation of CAH 10 or C 2 AH x to C 3 AH 6 were discussed in the literature among which the solid‐state and the through‐solution mechanisms are the most prominent . Independent on the mechanism, the conversion of initially formed metastable phases was observed to be accelerated with increasing temperature.…”