Page 391 - Handbook of Thermal Analysis of Construction Materials

Section 4.0 - Calcium Aluminate Cements 369

retardation. Other additives that have a plasticizing effect on CAC concrete
include: methyl cellulose, soya bean flour, bentonite, finely divided fly ash,
and granulated slag. Alkalis present in the additives can also affect the
physical properties of CAC.
The use of superplasticizers, e.g., polynaphthalene formaldehyde
(PNS) and polymelamine formaldehyde (PMS), appear to improve
flowability of CAC concrete; loss in workability appears to be too rapid.
Normal range water reducers, e.g., hydrocarboxylic acids, function in a
manner similar to their role in portland cement concrete.
Ma and Brown reported that sodium phosphate modified calcium
aluminate cement (at addition levels of 10 and 20 mass%) can enhance
strength. [33][34] The phosphates included (NaPO ) , (NaPO ) •Na O,
3 r 3 n 2
Na P O and (NaPO ) . They concluded that the phosphate-based addi-
3 3
5 3
10
tions do not promote the crystallization of calcium aluminate hydrates
during the CAC hydration. Morphologically distinct hydration products
can form. An amorphous C-A-P-H phase serves to interlink the other
constituents forming between crystalline calcium aluminate hydrate grains.
This phenomenon appears to enhance the structural integrity of the calcium
aluminate cement and reduce the strength retrogression which occurs on
conversion.

4.8 Refractory Applications

Strength of CAC concrete is generally superior to portland cement
concrete at temperatures exceeding 1000°C. [21] The CAC systems are
usually stable at temperatures up to 1300°C. CAC materials containing
alumina or corundum are resistant to temperatures approaching 1600°C.
CAC concrete when fired loses most of its combined water at about 700°C.
Strength losses and porosity increases reach a maximum between 900 and
1100°C. Increased strength and new bond formation occur above about
900°C. Alumina gel passes through several crystal modifications to finally
form α-Al O . CAH and other hexagonal phases form CaO and C A
2 3 10 12 7
between 600 and 1000°C.
CAC concrete containing fire brick aggregate produces anorthite
(CAS ) and gehlemite (C AS). [35] The results are similar for aluminosili-
2
2
cate aggregate. CA and CA form when white alumina cement is fired with
2 6
fused alumina aggregate. Spiral (MA) and forsterite form when the aggre-
gate used is dead burnt magnesite. Applications for CAC concrete include
brick and tunnel kilns, kiln and chimney linings, kiln doors and coke ovens,
and furnace foundations.