Page 389 - Handbook of Thermal Analysis of Construction Materials

Section 4.0 - Calcium Aluminate Cements 367

be found in the strätlingite plates in calcium aluminate cement paste
containing sodium sulfate and zeolite, indicating that sulfate plays a role in
the crystallization of strätlingite. This confirms the postulation cited by F. M.
Lea [20] that sulfate may be combined in strätlingite resulting in crystals
having the formula 9CaO•4SiO • Al O •7CaSO •80H O. The substitution
2
4
3
2
2
of sulfate may reduce the crystallization energy of strätlingite plates and
accelerate its formation. A large amount of preferentially formed strätlingite
in a calcium aluminate cement system makes hydrogarnet formation un-
likely. This process prevents strength reduction of the high alumina cement
products due to the conversion.
4.6 Durability

The hydration of CAC does not result in the formation of Ca(OH) .
2
This contributes to enhanced resistance in the presence of phenols, glycer-
ols, sugars, etc. The formation of aluminum hydroxide gel during hydration
provides resistance to dilute acids and sulfates. CAC has many industrial
applications. It is used in flues, sewers, effluent-tanks, coal hoppers, ash
sluices’ flues, and in industrial plants, e.g., oil refineries, breweries, dairies,
and tanneries.
CAC concretes are not resistant to alkali which readily attack the
protective gels (possibly also iron-containing gels). The alkalis may also
affect setting and strength development and accelerate the conversion
reactions.
CAC concretes are resistant to solutions containing dissolved CO 2
and sea water. The formation of chloroaluminates is likely to affect the
setting and hardening characteristics, and, therefore, the use of sea water as
mix water should be avoided.
CAC concretes can also be used to encapsulate amphoteric metals
such as Pb, Al, and Zn due to reduced reaction between the hydrates formed
and these materials.

4.7 Chemical Admixtures

The addition of fully hydrated CAC to hydrating CAC systems can
reduce setting times significantly. This behavior, also observed in portland
cement systems, can be attributed to a nucleation mechanism. [21] Set
acceleration can also occur with the addition of dilute solutions of sodium,
potassium and lithium salts, or organic bases, e.g., triethanolamine.