Page 388 - Handbook of Thermal Analysis of Construction Materials

366 Chapter 9 - Non-Portland Cement Binders and Concrete

found that, during the conversion reaction, the surface area is actually
increased owing to the formation of alumina gel. [30] Mehta has affirmed that
a poor bonding capacity of the hydrogarnet and its low surface area are
important factors contributing to a loss of strength. It is also thought that the
conversion of the flat plates, with the overlapping or interlocking system, to
cubic morphology dislocates the intercrystalline bonds.
The fall in strength has also been attributed to the formation of
macrocrystalline alumina hydrate from the microcrystalline alumina
hydrate.
Although there is no unanimity on the details of the mechanism of
strength deterioration in CAC, success has been achieved in explaining the
strength loss in terms of the conversion of the hexagonal to the cubic phase.
The loss in strength, as the temperature is increased, can be explained by the
increase in the rate and amount of conversion. Similarly, the increase in the
conversion rate at higher w/c ratios may explain the low strengths at these
conditions.

4.5 Inhibition of C AH Formation
3
6
The formation of C AH (hydrogarnet) is often associated with
3 6
volume instability in CAC concretes. It has been demonstrated that an
additive comprised of a pozzolan and an inorganic sodium or potassium salt
can be effective in eliminating the conversion reaction with minimal effect
[3]
on early strength development. The anion can be selected from the group
consisting of sulfate, carbonate, nitrate, silicate, phosphate, chloride, and
bromide. The pozzolan is selected from the group consisting of: zeolites,
calcined zeolites, granulated blast-furnace slag, fly ash, silica fume, rice
hull ash, and metakaolin. The resulting calcium aluminate cement compo-
sition is such that at temperatures between 25–40°C strätlingite is formed
preferentially and formation of hydrogarnet is limited.
Strätlingite is preferentially formed from hexagonal calcium alumi-
nate hydrates and anhydrous calcium aluminate cement at high temperature
(e.g., 35–45°C). Conversion of those hydrates or anhydrates to hydrogarnet
is prevented. Some anions such as sulfate, silicate, phosphate, carbonate,
etc., also play an important role in preventing the conversion. This is a result
of two primary reactions: (1) reaction of these anions with calcium ions to
form insoluble calcium salts such as gypsum; the calcium concentration in
the hydration system is significantly reduced and delays calcium dependent
hydrogarnet formation; (2) substitution of sulfate in strätlingite-sulfate can