Hydration of calcium aluminates at 60°C – Development paths of C₂AH_x in dependence on the content of free water

Abstract

The influence of water loss during the hydration of calcium aluminates on the phase development is investigated at 60°C. This is relevant for applications in which calcium aluminate cement (CAC) based formulations are exposed to quick drying during hydration. The presented results provide new insights into the well‐known conversion processes occurring in CAC pastes. Using in situ XRD two different routes of the development of initially formed C2AH8 are determined: (a) transformation to C3AH6 + AH3 in the presence of sufficient free water and (b) dehydration to C2AH5 at a lack of free water. Moreover, the influence of precuring of the pastes at 23°C before heating to 60°C is investigated. The increasing loss of free water with increasing precuring time resulting from both, precipitation of hydrate phases and evaporation, causes incomplete hydration of CA or CA2 as well as dehydration of C2AH8 instead of conversion into C3AH6. Comparative investigations of sealed samples always revealed complete hydration of CA and CA2 as well as complete conversion of C2AH8.

In-situ laboratory X-ray diffraction applied to assess cement hydration

“…This fact is disregarded in most reports in the literature. However, Hüller et al [155] and Goergens et al [77] recently demonstrated that a significant water loss might occur during in-situ XRD tests (illustrated in Figure 18), which is more marked as the temperature increases. For instance, at 23 °C (the testing temperature usually adopted), a mixing water loss of 10 wt% (around 3 wt% of the sample) was observed after 32 hours.…”

“…Water loss over time for in-situ XRD measurements at different temperatures. (a) from [155], reproduced with permission from Wiley; (b) from [77], reproduced with permission from Elsevier.…”