Page 387 - Handbook of Thermal Analysis of Construction Materials

Section 4.0 - Calcium Aluminate Cements 365

gave autoclave treatment values of 2717, 1825, and 2717 MPa, respec-
tively. These results demonstrate that enhanced strengths occur by the
direct formation of the cubic phase on the original sites of C AF. This
4
results in a closely welded, continuous network with enhanced mechanical
strength.

4.3 Strength Development

CAC concrete can develop up to 90% of its ultimate strength within
the first 24 hours after mixing. [20] The CAC concrete develops substantially
higher strength than ordinary portland cement concrete or concrete made
with rapid hardening portland cement.
The high early strength characteristic facilitates emergency repair
work and is particularly useful in the production of precast concrete units.
The 28 day strengths decrease with temperature (above 25°C) and generally
with water-cement ratio. Strength is also reduced if CAC cured at ambient
temperature is suddenly exposed to a higher temperature environment.
The strength decreases cited are attributed to the conversion
reaction. It is influenced by several factors such as temperature, water-
cement ratio, stress, and the presence of alkalis. The precise mechanism
responsible for the deterioration of strength is not completely understood.

4.4 Strength and the Conversion Reaction

The conversion of hexagonal hydrate (CAH ) to the hydrogarnet
10
or cubic phase (C AH ) results in a solid volume decrease of about 50%
3 6
whereas that of C AH results in a decrease of about 65% of the original
8
2
volume of the reactants. This is due to the higher specific gravity of the
products compared with that of the reactants. The specific gravities of
CAH , C AH , C AH , and AH are 1.72–1.78, 1.95, 2.52, and 2.40,
10 2 8 3 6 3
respectively. In the CAC pastes, the specific gravity of the unconverted
material may be 2.11 and that of the converted material 2.64. In the
hardened concrete, this conversion may be accompanied by the formation
of pores resulting in lower strength. The strength loss has been explained
by the decrease in the combined water in the converted product and also by
the released water filling the small pores.
Lehman and Lecks [29] ascribe the higher strengths in CAC to
CAH having a high surface area, and the conversion, producing hydrogarnet
10
with a lower surface area, being responsible for the low strengths. It was