Conversion of calcium aluminate cement hydrates at 60°C with and without water

Abstract

There have been different hypotheses about the transformation mechanisms of calcium aluminate cement hydrates and this work aims to clarify the long‐running debate about the conversion approaches. In this work, CAH10 and C2AH8 were produced from the pastes of calcium aluminate cement (CAC) cured for 24 hours at 10 and 20°C separately. And the cured pastes were continually cured at 60°C for 3 days with water and without water, respectively. The hydration of the pastes was halted by freeze‐drying, and the phases and microstructure of hydrates were investigated by XRD and SEM, respectively. The results indicate that CAH10 and C2AH8 converted into C3AH6 and AH3 in water presence at 60°C, but did not transform into C3AH6 and AH3 without water. It is confirmed that the conversion of CAH10 and C2AH8 to C3AH6 and AH3 happens through preceding solution of CAH10 and C2AH8 and subsequent precipitation of C3AH6 and AH3.

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

“…In the second step the C 2 AH 8 is transformed into C 3 AH 6 trough solution. Zhang et al confirmed the through‐solution transformation of C 2 AH 8 to C 3 AH 6 but stated also CAH 10 to transform to C 3 AH 6 through solution since transformation was only observed in samples containing free water but not in freeze‐dried samples. Moreover, based on SEM images it was concluded that the small plate‐like C 2 AH 8 crystals or small fiber‐like CAH 10 crystals cannot be directly transformed via solid‐state reaction into the ten times bigger granular C 3 AH 6 crystals …”

“…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 .…”

“…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.…”

Strontium Retention of Calcium Zirconium Aluminate Cement Paste Studied by NMR, XRD and SEM-EDS

“…The resonances assigned to five-coordinated V Al and highly distorted tetrahedral Al environments have been observed in the region 20-50 ppm [18,21,22]. For example, the 27 Al MAS NMR spectrum of CA shows a peak maximum at ca. 80 ppm and a shoulder at 76 ppm due to six crystal ographically different aluminum tetrahedral sites [19,[23][24][25].…”