Civil Engineering Reference
In-Depth Information
Figure 7.
Effect of expansive additives based on different calcium aluminate hydrates or
HAC on free expansion of cement paste.
[54]
The different calcium aluminate hydrates exhibit very different
behavior with respect to ettringite formation and expansion. The reaction
rate of CAH
10
and C
2
AH
8
to form ettringite appears to be much faster than
the other forms of calcium aluminates described above. The use of CAH
10
or C
2
AH
8
alone as the Al-bearing component in the formulation of expan-
sive cement may result in little expansion in concrete members. This is due
to the timing effect postulated by Aroni, et al.
[55]
The magnitude of
expansion is relatively small when the ettringite crystals grow too quickly
in the early stage, as the structure of cement paste is not well formed.
Ettringite formation from the reaction of C
3
AH
6
is apparently delayed and
reduced. This makes the Al-bearing material inefficient. Delayed ettringite
formation also causes stability problems. The expansion period of such
expansive cement can last up to 28 days. It is not desirable for the
application of shrinkage-compensating cement concrete. A good CAC-
based expansive additive may have an optimum composition comprising
anhydrous CAC, CAH
10
and/or C
2
AH
8
present in the surface layer of CAC
particles. This surface layer prevents quick setting. It will react with sulfate
to form ettringite and adjust the hydration rate of the inner anhydrous CAC.
This optimized structure of expansive additive particles provides expansive
cement with controllable expansion characteristics. The compounded-
expansive additive approach appears to be an example of this material-
design philosophy.
[55]
