Page 397 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Portland Cement-Calcium Aluminate Cement 375

In the 20/80 paste, appreciable strength is developed at 4 hours
hydration (Table 1). SEM examination at a half hour hydration failed to
identify ettringite crystals. X-ray diffraction also failed to detect early
ettringite formation. There is apparently not enough sulfate available and
the hydration and strength development behavior of this system are similar
to those for pure CAC paste. [46]
The strength development of the 80/20 paste is quite unique. SEM
studies at 0.5 and 8 hours hydration provide evidence of a morphology
change during this period—small needle-like crystals observed at 0.5 hours
were transformed to larger plate-like crystals. It has been suggested that a
slow conversion process of ettringite to monosulfoaluminate hydrate may
take place during this period. The monosulfoaluminate hydrate is detect-
able by x-ray diffraction at 24 hours hydration.
The observation of a Ca(OH) peak at 48 hours by x-ray diffraction
2
analysis suggests that the slow strength development in the 80/20 paste may
be due to a delay in the hydration of C S. The observation that C S remains
3
3
unhydrated until the conversion is completed suggests an incompatibility
between the OPC and CAC hydration products.

5.5 CAC-Based Expansive Cement Reactions

The use of calcium alumina cement (CAC) as an alumina-bearing
component in expansive additives or cements has been reported by re-
searchers since the 1920s. [47]–[49] The CAC-based expansive additive is
generally a dry mixture comprising different types of particulates including
calcium alumina cement, calcium sulfate hemihydrate or gypsum, and lime
or hydrated lime. It can be simply obtained by mixing commercially available
materials. Its application is, however, limited due to the problems of quick
setting and unstable expansion. The use of hydrated calcium alumina
cement (H-CAC) instead of high alumina cement in the composition of
expansive cement was reported to be a potential solution to the quick-
setting problem. [50] The expansion characteristics of expansive cement
containing pre-hydrated alumina cement have also been reported. [51]–[53]
A recently developed chemically compounded-expansive additive
has shown promise. [54] The compounded-expansive additive comprises
mainly calcium aluminates, calcium sulfate, and calcium hydroxide. The
primary difference between the C-HAC system and the compounded-
expansive additive system is that the latter contains all the expansive
components in the compounded material. Particulate products are formed
during the process of compounding.