Speaker
Description
Calcium aluminate cement is widely used as a binder in refractory castables because it enables shaping complex geometries and allows the material to withstand extreme temperatures. Their typical manufacturing process consists of mixing, curing, demolding, drying and firing. In modern dense and highly dispersed castables with very low permeability, water remains in the castable body during the drying up to 200°C and beyond. This leads to an increase in internal pressure and hydrothermal conditions occur. Consequently, problems such as cracking and explosive spalling can arise.
Different temperatures during hydration of CAC lead to the formation of distinct hydrate phases and consequently, different microstructures can develop in the hardened material. Hydrothermal conditions lead to the transformation of the original hydrate phases, while still preserving microstructural features inherited from the initial hydration stage. This study presents the influence of curing temperature (5, 23, and 40 °C) on phase development and porosity in a refractory castable model system subjected to hydrothermal conditions.
Quantitative X-ray diffraction measurements revealed that CAH10, C2AHx, C3AH6 and AH3 are formed during curing at different temperatures and that the degree of hydration also varies. When hydrothermal conditions occur, the remaining clinker phases react and the already formed hydrate phases first transform to AH3 and C3AH6. While C3AH6 remains stable with increasing temperature and pressure, AH3 converts to boehmite above 140 °C.
Although the final mineralogical phase composition is the same, microstructural analysis via SEM and mercury intrusion porosimetry revealed differences in the three initially differently cured materials. While the total porosity is comparable, the samples that were cured at 40 °C have a higher proportion of large pores and a different appearance of C3AH6. The study could show that the initial microstructure that is created during curing is preserved after hydrothermal conditions induce additional reactions and phase conversions.
Affiliations
Almatis GmbH, Giulinistr. 2, 67065, Ludwigshafen, Germany, andreas.koehler@almatis.com
| Keywords | Cement, Refractory Castable, Hydrothermal, Drying, Hydrate phases |
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