Civil Engineering Reference
In-Depth Information
amount of ettringite. The estimated rate of hydration of alite in jet cement is
about twice that in portland cement. Hardened Jet Set cement paste has a
much denser microstructure than portland cement paste.
Observation of the hydration process indicated the following. In
the first few minutes of the dissolution of free lime, the hydration of
anhydrite and hemihydrate and the formation of calcium aluminate hydrate
and monosulfate hydrate occur. Ettringite is formed within one hour,
monosulfate hydrate within 2-6 hours and C-S-H gel within 1-16 hours,
with the maximum heat of hydration of C
3
S at about 10 hours. Measure-
ments of the non-evaporable water indicate that the amount of combined
water is 60-80% of the theoretically determined total amount of combined
water at complete hydration. (The total amount of combined water was
estimated to be 36%.)
[73]
The amount of ettringite in the paste is estimated
to be about 18-25% for the period one hour to seven days. The monosulfate
content increases from about 10% at six hours to about 15-25% in one day.
The alite in Reg Set cement paste is approximately 65-70% hydrated in one
day and 80-95% hydrated in seven days. It is suggested that fluoride is tied
up as Al(OH)
2
F. The possibility of fluoride-substituted ettringite and the
formation of halo-aluminate hydrates of the form C
3
A•CaX
2
•
n
H
2
O is
conceivable.
Uchikawa and Uchida have studied the hydration of the
haloaluminates, extensively.
[74][75]
Several kinds of calcium aluminate
hydrates form immediately after mixing. The hydrates C
2
AH
8
, C
4
AH
13
, and
cubic C
3
AH
6
form within one hour. The hexagonal calcium aluminate
hydrate converts to cubic C
3
AH
6
after one day. The reaction is 60%
complete after one hour. Addition of calcium hydroxide accelerates the
formation of calcium aluminate hydrates and the conversion of the hexago-
nal hydrate into C
3
AH
6
. The degree of hydration, however, is lower than for
the controls.
Addition of calcium sulfate anhydrate has little effect in the intial
stage of hydration. Calcium aluminate monosulfate hydrate, cubic hydrate,
and ettringite form and coexist for a long period. Hexagonal hydrate is
depleted with the formation of monosulfate hydrate and ettringite.
The addition of calcium sulfate dihydrate at a low SO
3
/Al
2
O
3
ratio
(0.5) results in the formation of monosulfoaluminate and cubic C
3
AH
6
. At
higher ratios (1.0), rectangular ettringite crystals and hexagonal plates of
monosulfate hydrate coexist. As the quantity of ettringite increases and that
of gypsum decreases, the amount of monosulfate hydrate increases.
