Civil Engineering Reference
In-Depth Information
substance is heated at a uniform rate and the evolved gas is quantitatively
measured, thus permitting the estimation of the components. For example,
characteristic peaks have been obtained when sulfoaluminate hydrates,
gypsum, Ca(OH)
2
, etc., are subjected to EGA.
[48]
In this method, water
produced by disassociation reactions is passed through P
2
O
5
layer and
adsorbed water is electrolyzed and measured coulometrically. The EGA
traces of calcium monosulfate hydrate, ettringite, and gypsum are shown in
Fig. 27.
[48]
The disassociation pattern of ettringite is similar to that obtained
in DTA. The first sharp peak and the second broad peak are related to the
loss of 24 and 6 mols of water respectively from ettringite. Four peaks are
registered by monosulfate hydrate. Gypsum exhibits typical dual peaks for
a stepwise dehydration.
Figure 27.
Typical EGA traces:
(a)
calcium aluminum monosulfate hydrate,
(b)
calcium
aluminum trisulfate hydrate,
(c)
gypsum.
Tetracalcium Aluminoferrite Plus Gypsum.
In the hydration of
C
4
AF + gypsum mixtures, depending on the initial properties, temperature
and time of hydration, several types of compounds are detected. The
compounds may consist of unhydrated C
4
AF, CaSO
4
•2H
2
O and the hydra-
tion products, C
4
(AF)H
13
(hexagonal phase), C
3
(AF)H
6
(cubic phase),
C
3
(AF)•3CaSO
4
•H
32
(ettringite), C
3
A(AF)•CaSO
4
•H
12
(low sulfo-
aluminate), and a solid solution of sulfoaluminate with C
4
(AF)H
13
.
