d with the raising
hydrothermal temperature to 300˚C. When the foamed
cement was modified with 0.5% AP, the cement layer
adhering to CS had a relatively high Al2O3 content of
20.5% coexisting with 3.5% CaO and 18.2% SiO2. The
content of these oxides gradually rose with an increasing
AP content, contrarily, the Fe2O3 content declined. With
2.0 wt% AP, a 44.9% Fe2O3 detected corresponded to
lowering of ~22% from that of 0.5% AP, while the in-
crease of ~47%, ~10%, and ~37% was observed for all
cement-related oxides, Al2O3, CaO, and SiO2, respec-
tively, implying that although the temperature was ele-
vated to 300˚C, the AP was as effective in improving the
adherence of foamed cement as was the case at 200˚C.
Hence, the M-complexed AP compounds formed in
300˚C-autoclaved foamed cement withstood the hydro-
thermal temperature of 300˚C; they played an essential
role in enhancing bonding of foamed cement to CS, the-
reby resulting in the cohesive failure mode wherein in-
terfacial bonding failure took place in the cement layer
near the interface regions between CS and cement.
4. Conclusion
Air bubble-foamed cement (slurry density of 1.3 g/cm3)
consisting of refractory calcium aluminate cement (CAC),
Class F fly ash, sodium silicate activator, and cocamido-
AP content, %
0.0 0.51.0 1.52.0
Oxide content, %
Figure 16. Changes in the content of oxides present at in-
terfacial CS surface as a function of AP content for 300˚C-
autoclaved cement/CS samples.
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propyl dimethylamine oxide-based foaming agent, was
modified with acrylic polymer (AP) employed as a high-
temperature cathodic corrosion inhibitor of carbon steel
(CS) after exposure to the hydrothermal environment at
200˚C or 300˚C. Under these conditions, the functional
acrylic acid and alkyl ester groups within AP reacted
with the metal cations (M), such as Ca2+, Al3+, and Na+,
liberated from sodium silicate-activated CAC and Class
F fly ash, leading to the formation of M-complexed car-
boxylate group-containing AP in the cement body at
85˚C. Such in-situ transformation of these functional
groups into complexed groups progressively occurred as
the hydrothermal temperature rose. This transformation
improved the thermal stability of AP. Correspondingly,
the complexed carboxylate-rich AP withstood a hydro-
thermal temperature at 300˚C, ensuring that AP as the
corrosion inhibitor was capable of protecting the CS
against corrosion at this temperature. Addition of AP
delayed the onset of cement set while increasing the in-
tegrated heat released during cement hydration in calo-
rimetric experiments at 85˚C.
At 200˚C, AP did not cause any significant changes of
crystalline hydrate composition assembled in the AP-free
foamed cement; namely, its composition comprised four
hydrothermal hydration reaction products, hydroxyso-
dalite [Na4Al3Si3O12(OH)], intermediate hydrogrossular
[Ca3Al2Si2O8(OH)4], boehmite (γ-AlOOH), and Si-free
katoite [Ca5Al2(OH)12] phases that were responsible for
strengthening the 200˚C-autoclaved foamed cement. The
hydroxysodalite phase was formed by hydrothermal in-
teractions between the sodium silicate activator and mul-
lite phase in Class F fly ash, while the quartz in Class F
fly ash reacted with CAC to form hydrogrossular. On the
other hand, hydration of CAC engendered two other
phases, boehmite and Si-free katoite. Although, similar
crystalline phases were formed in cement with and with-
out AP, the compressive strength rose with an increasing
AP content. At the hydrothermal temperature of 300˚C,
the crystalline phases and their quantities differed from
those observed at 200˚C. For AP-free cement, three
phases, hydroxysodalite, katoite, and hydrogrossular,
were well formed and crystallized; in particular, a sub-
stantial amount of quartz in Class F fly ash hydrother-
mally reacted with CAC to form more hydrogrossular.
Consequently, these well-formed crystalline compounds
aided in improving further the cement’s compressive
strength, compared with that at 200˚C. Adding AP re-
strained the formation of these three crystalline hydrate
phases, while Na-P type zeolite was formed as additional
crystalline phase. Like the findings at 200˚C, the devel-
opment of compressive strength depended on AP content;
namely, strength rose with an increasing AP content.
The microstructure developed in the autoclaved
foamed cements was characterized by a honeycomb-like
porous structure constituted of numerous defected mi-
cro-size craters. Adding AP conferred two beneficial
alterations in this microstructure: First, the defected cra-
ters were transformed to defect-free discrete voids; and,
second, the size of craters became much smaller. Thus,
creating defect-free, small craters was one reason why
the AP-modified foamed cements developed a good
compressive strength. The other benefit from the pres-
ence of such advanced microstructure was the reduction
of infiltration and transportation of corrosive electrolytes
through the foamed cement layer, thereby mitigation of
the corrosion of underlying CS.
We believe that the AP addition to the foamed cement
significantly reduced corrosion rate of CS at the hydro-
thermal temperature of 300˚C because of the following
two key factors: the formation of 300˚C-withstanding
barrier layers constituted of complexed carboxylate-rich
AP and the improved adherence of the cement to CS
surfaces. For the latter, incorporating more AP yielded a
better cement adherence. Additionally, among the Ca-,
Si- and Al-oxides in hydraulic cement, Ca oxide prefer-
entially coupled to the Fe2O3 layer arrayed at the surface
of CS at 200˚C. The coverage of CS surface by Ca oxide
extended with an increasing temperature, resulting in
better adherence of 300˚C-autoclaved cement to CS,
compared with that of 200˚C-autoclaved one. The fol-
lowing three important factors governed the mitigation of
CS’s corrosion: 1) Minimized conductivity of corrosive
ionic electrolytes through the foamed cement layer; 2)
inhibited cathodic reactions at corrosion site of CS; and 3)
increased coverage of CS surface by a foamed-cement
layer at the interfacial boundary regions between the ce-
ment and CS.
For AP-free foamed cements, the corrosion rate, 175
milli-inch per year (mpy), of CS after autoclaving at
200˚C, reduced by 4 times when the autoclaving tem-
perature increased to 300˚C. For AP-modified foamed
cements, the corrosion rate of CS coated with AP-free
cement at 200˚C fell 2.6 times with 2 wt% AP. At 300˚C,
2 wt% AP lowered conspicuously the CS’s corrosion rate
to only 6.8 mpy from 43 mpy for AP-free one.
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