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Construction and Building Materials 186 (2018) 1072–1081
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Advances in understanding and analyzing the anti-diffusion behavior in
complete carbonation zone of MSWI bottom ash-based alkali-activated
concrete
Guodong Huang a,b,c, Yongsheng Ji a,c,⇑, Linglei Zhang a,c, Jun Li a,c, Zhihui Hou a,c
a
b
c
State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 21116, Jiangsu, China
School of Civil Engineering and Construction, Anhui University of Science and Technology, Huainan 232001, Anhui, China
School of Mechanics and Civil Engineering, China University of Mining and Technology, Xuzhou 21116, Jiangsu, China
h i g h l i g h t s
BA displayed a unique anti-diffusion behavior under natural conditions.
The anti-diffusion behavior was influenced by amount of sodium silicate used.
Anti-diffusion effect of BA significantly reinforced its carbonation resistance.
a r t i c l e
i n f o
Article history:
Received 17 January 2018
Received in revised form 7 July 2018
Accepted 7 August 2018
Keywords:
Carbonation
Municipal solid waste incineration bottom
ash
Alkali-activated
Anti-diffusion
Concrete
a b s t r a c t
This study investigated the carbonation resistance of municipal solid waste incinerator (MSWI) bottom
ash alkali-activated concrete (BA) and Portland cement concrete (CC) through the accelerated carbonation test. The results show that BA has lower resistance against carbonation compared to CC under the
conditions of the accelerated carbonation experiment. However, BA displayed a unique anti-diffusion
behavior under natural conditions which significantly reinforced its carbonation resistance. The antidiffusion behavior was closely related to the amount of liquid sodium silicate in the activator.
Moreover, the pH value in complete carbonation zone of BA was much higher compared to that of CC.
The main reason is that liquid sodium silicate turned into magadiite in BA and then the magadiite turned
back to liquid sodium silicate to release a large amount of OH under low concentrations of CO2. This
resulted in a rapid increase in pH value of the pore fluid and generation of the anti-diffusion behavior.
Ó 2018 Published by Elsevier Ltd.
1. Introduction
Alkali-activated materials are inorganic nonmetallic compounds which are prepared by the action of strong alkali on industrial solid wastes consisting of aluminosilicates [1]. Compared to
conventional Portland cement materials, alkali-activated materials
have good mechanical performance, high temperature resistance,
resistance against acids, alkali, and salts, and low permeability.
Moreover, the production of alkali-activated materials does not
require high temperature calcination or sintering processes unlike
the production of Portland cement. Alkali-activated materials can
also undergo complete polymerization at room temperature
⇑ Corresponding author at: State Key Laboratory for Geomechanics and Deep
Underground Engineering, China University of Mining and Technology, Xuzhou
21116, Jiangsu, China.
E-mail address: jysbh@126.com (Y. Ji).
https://doi.org/10.1016/j.conbuildmat.2018.08.038
0950-0618/Ó 2018 Published by Elsevier Ltd.
[2–4]. Furthermore, alkali-activated materials have lower energy
consumption, almost no pollution, and are easy to recycle and
reuse [4–6]. Therefore, they are environmentally friendly green
materials which may be used as a building material to replace Portland cement in the future [7–10].
However, studies on the durability of alkali-activated materials
have found that the carbonation resistance of alkali-activated concrete is weaker than that of Portland cement concrete, particularly
under the conditions of accelerated carbonation test. Laboratory
tests of alkali-activated concrete have shown relatively high carbonation rates and this problem is more serious as the concentration of CO2 increases [11]. The current research indicates that due
to a certain concentration of CO2 in the atmosphere, it enters into
reinforced Portland cement concrete via diffusion and dissolves in
the pore liquid, lowering its pH value. The steady decrease in pH
value of the pore fluid will cause the deactivated film of reinforcing
bar to become unstable and even damaged, resulting in accelerated
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G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
corrosion of the rebar and then a reduction in bearing capacity
[12]. Therefore, rebar corrosion due to carbonation is a common
problem for the ordinary Portland cement (OPC) system, particularly in warm and relatively humid environments. Moreover,
alkali-activated materials have higher carbonation rates than Portland cement concrete, which makes them more prone to rebar corrosion issues due to carbonation.
Several studies have investigated the carbonation of alkaliactivated slag (AAS) concrete and other related alkali-activated
materials [13]. These studies indicate that alkali-activated concrete
exhibits a higher carbonation rate than Portland cement concrete
at the same compressive strength and that the carbonation mechanism for the two materials is different [14]. Moreover, some studies have shown that the higher carbonation rate of alkali-activated
materials results in the degradation of its mechanical properties,
although there is some controversy about this [15–17]. However,
other studies have found results that are contrary to the reports
of carbonation problems in alkali-activated mortars and concretes,
as mentioned above [18]. These studies have experimentally
demonstrated that alkali-activated concretes removed from accelerated carbonation test after extended periods do not display carbonation issues [19]. This indicates that the outcome of the
accelerated test is not an accurate predictor of in-service performance of alkali-activated materials and the carbonation resistance
of alkali-activated materials in natural carbonation is equivalent to
that of Portland cement concrete [20]. The researchers pointed out
that the change in pore solution equilibria causes the formation of
sodium bicarbonates during accelerated carbonation, compared to
the hydrous sodium carbonates formed during natural carbonation
[21]. This shifts the carbonation mechanism to favor more rapid
reaction progress, and results in a higher apparent degree of acceleration (compared to natural conditions) than in Portland cements
[14].
Shi et al. reported that the natural carbonation rates for alkali
silicate-activated slag concretes are lower than 1 mm/year, while
laboratory studies have shown that carbonation depths in alkaliactivated concretes range from 13 mm to 25 mm after 240 h of
exposure to 7% CO2 [18]. Xu et al. studied aged concretes (up to
35 years) based on slag activated with carbonates or carbonate/
hydroxide blends, and observed an acceptable carbonation rate
for all samples (0.03–0.5 mm/year), verifying the high stability of
these AAS binders [22,23]. Bernal et al. believe that accelerated carbonation testing is unduly aggressive towards alkali-activated concretes, and test results must be cautiously interpreted
[14,15,19,20]. Therefore, there is a need for reasonable evaluation
of carbonation resistance for predicting the service life of alkaliactivated concrete under carbonating conditions. The main factors
affecting carbon resistance in alkali-activated materials need to be
analyzed by studying the carbonation process and the change in
the product before and after carbonation.
This study focuses on evaluating the effect of accelerated carbonation on alkali-activated concrete produced mainly by MSWI
bottom ash, and the improvement in carbonation resistance due
to anti-diffusion behavior in carbonation zone. The anti-diffusion
behavior refers to the automatic alkaline recovery in the complete
carbonation zone of alkali-activated concrete when removed from
the accelerated carbonation chamber. By macroscopically studying
the anti-diffusion behavior in carbonation zone and by microstructural analysis (Fourier Transform Infrared Spectroscopy (FT-IR) and
X-ray diffraction (XRD)) of the products before and after carbonation, we can elaborate on the mechanism of anti-diffusion behavior
occurring in alkali-activated materials and perform a reasonable
evaluation of their carbonation resistance.
2. Experimental materials and methods
2.1. Materials
(1) MSWI bottom ash
The MSWI bottom ash used in this experiment was supplied by GCL-Poly (Xuzhou, China) Renewable Energy Power Generation Co., Ltd. The municipal solid waste
was first separated via magnetic separation and then calcined at 800 °C. The bottom
ash was washed at the recycle station and its chemical composition is shown in
Table 1. In this experiment, the bottom ash was ground into a fine powder with surface area and specific gravity of 400 m2/kg and 2.47, respectively. The particles were
mainly spherical with a median particle size of 47 mm and 28% was retained on a 45
lm sieve.
(2) Granulated blast furnace slag (GBFS) and slaked lime
The GBFS used in the experiment is S95 granulated blast furnace slag powder,
which conforms to the Chinese standard GB/T 18046-2008 [24]. The surface area
and specific gravity of GBFS are 416 m2/kg and 2.89, respectively, and 35% was
retained on a 45 lm sieve. Its chemical composition is also shown in Table 1. Industrial grade slaked lime was used. Its purity is 95%, specific gravity is 2.24, and surface area is 400 m2/kg.
(3) Activator
Sodium hydroxide (NaOH) and sodium silicate solution (Na2SiO3) with 9.65%
Na2O, 25.22% SiO2, and 65.13% H2O were used as the activators. The sodium silicate
solution has a modulus (molar ratio of SiO2 and Na2O in liquid sodium silicate) of
2.6. A 4.8 M NaOH solution (NaOH dosage and water content are given in Table 2)
was prepared and this solution was used to obtain each set of concrete samples. The
NaOH solution was allowed to stand for 24 h before use.
(4) Others
The Portland cement used for the experiment was the PO 42.5 type, which conforms to the Chinese standard GB 175-2007 [25]. Commercially available limestone
with a maximum size of 30 mm and a specific gravity of 2.67 in saturated surface
Table 1
Chemical composition of raw material (%).
Raw material
SiO2
CaO
Al2O3
Fe2O3
MgO
K2O
Na2O
Zn, Pb, Ba, Sr, Cu
Loss
GBFS
Cement
MSWI bottom ash
31.35
26.55
43.82
34.65
62.9
24.44
18.65
7.77
14.18
0.57
3.62
6.18
9.31
2.68
3.26
–
1.5
2.52
–
0.31
2.24
–
–
0.61
0.7
3.2
1.62
Table 2
Mix proportion of the concrete kg/m3.
C-1
C-2
C-3
C-4
cement
MSWI bottom ash
GBFS
Ca(OH)2
water
sodium silicate solution
NaOH
sand
gravel
compressive strength (MPa 28 d)
330
0
0
0
0
165
165
165
0
132
132
132
0
33
33
33
165
92.4
133.3
0
0
88
0
254
0
17.6
31.7
0
651
651
651
651
1056
1056
1056
1056
45.2
49.6
46.7
18.5
Note: For example (C-2), Dissolve 17.6 kg of NaOH into 92.4 kg of water, which the molar solubility of NaOH solution is 4.8 M.
G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
dry conditions was used as the coarse aggregate. For the fine aggregate, river sand
with a specific gravity of 2.56 and a fineness modulus of 2.8 was used. Tap water
was used for the test.
2.2. Methods
2.2.1. Mix proportion of concrete
Table 2 shows the mix proportion of the CC and BA. The groups from C-1 to C-2
represent CC and BA, used for comparing the carbonation resistance of the two
kinds of concrete. The groups from C-2 to C-4 represent BA mixed with different
kinds of alkali-activators, which were used to determine the main factor causing
the anti-diffusion behavior in carbonation zone.
2.2.2. Casting and curing of concrete samples
The CC and BA samples were mixed in a temperature controlled room at 20 ± 2
°C. Before preparing BA, the bottom ash was mixed with NaOH solution to form a
slurry. This slurry was then aged for 4 h as a defoaming pre-treatment, which can
eliminate the effect of foaming and expansion on the performance of BA when it
comes into contact with alkali. The casting procedure was started by mixing the
slurry, GBFS and slaked lime for 1 min, and then sand and sodium silicate solution
were added and stirred for 30 s. Finally, coarse aggregates were added to the mixture and stirred for 1 min.
The casting procedure of CC was started by mixing the Portland cement, sand
and water for 1 min, then the coarse aggregates were added and stirred for 1.5
min. The casted sample dimensions were 100 mm 100 mm 400 mm and the
liquid-solid ratio of BA and CC was 0.5, where the liquid was the total mass of NaOH
solution and sodium silicate solution. All samples were placed in a curing room
after casting and demolded after 24 h. The curing was performed in a 20 ± 2 °C curing room (>95% room humidity) until testing.
2.2.3. Accelerated carbonation test
An accelerated carbonation test was performed according to the ‘‘standard for
test methods of long-term performance and durability of ordinary concrete” (GB/
T50082-2009) [26]. The environmental parameters of the carbonation chamber
were: CO2 mass fraction of 20%, temperature of 20 ± 2 °C, and relative humidity
of 70 ± 5%. Four days before the start of the test, the samples were dried at 60 °C
for 48 h and placed in a natural environment for 2 d. All surfaces were sealed with
molten paraffin, except for one. Treated samples were placed into a carbonation box
and carbonation was accelerated to specific durations (14 d, 28 d, and 60 d). To
measure carbonation depth, the cured samples were removed from the box and
cut into samples with a thickness of about 60 mm each. The remaining powder
on the section was removed, and then 1% phenolphthalein indicator was sprayed
to determine the depth of the fully carbonized zone based on the indicator color.
After 30 s, according to the original calibration points, the full carbonation depth
of every measuring point was measured using vernier calipers. The remaining part
of the sample was sealed with molten paraffin, and then placed into the carbonation
chamber to continue carbonation.
2.2.4. Sample collection of carbonized powder and determination of pH value
The samples from C-1 to C-4 were taken starting from the outer to inner regions
in the cross section of the concrete and the sample range was the surface to test
center of concrete (0–30 mm). The samples were taken every 5 mm and the stones
were removed leaving behind only the mortar. Then, the mortar was ground into
powder and poured into a 25-ml beaker. A small amount of distilled water was
added (water to solid ratio = 1:3) and mixed to form a sticky paste. The pH value
was measured using an acidometer and the value was read after 5 min.
2.2.5. Microstructural analysis of BA samples
(1) XRD analysis
The paste samples of CC and BA (C-1 and C-2), which were cured for 28 days,
were analyzed using an X-ray diffractometer (XRD, Bruker, D8 Advance, Germany)
with Cu K a 1 radiation and 2 h scanning range of 5°–70°.
(2) FT-IR analysis
The paste samples of CC and BA (C-1 and C-2), which were cured for 28 days,
were analyzed by Fourier Transform Infrared Spectroscopy (FT-IR, Bruker, VERTEX
80v, Germany). The wavenumber range was from 400 to 4000 cm1.
3. Results and discussion
properties and the compressive strength is up to 45.2 MPa, 49.6
MPa, and 46.7 MPa respectively, which they have the same level
of compressive strength. However, the compressive strength of
sample C-4 is only 18.5 MPa, which is obviously lower than other
samples and this will seriously affected the resistance of
carbonation.
The carbonation depth curves of CC and BA (error bars represent
one standard deviation) are shown in Fig. 1. The 14 d, 28 d, and 60
d carbonation depths of CC specimen (C-1) were 6 mm, 12 mm,
and 14 mm, respectively, while the corresponding carbonation
depths of BA specimen (C-2) were 16 mm, 20 mm, and 26 mm.
The results indicate that BA is far less resistant against carbonation
than CC at high carbon concentrations.
When only NaOH was used as an activator, the carbonation
resistance of BA specimen (C-3) showed a slight improvement
and the 14 d, 28 d, and 60 d carbonation depths reduced to 14
mm, 18 mm, and 22 mm, respectively. When only sodium silicate
solution was used as an activator, the carbonation resistance of
BA specimen (C-4) showed a significant decrease and the 14 d,
28 d, and 60 d carbonation depths increased to 17 mm, 21 mm,
and 27 mm, respectively.
3.2. The anti-diffusion behavior in carbonation zone of BA
After the carbonation of CC specimen (C-1) (Fig. 2, a-1, a-2, a-3),
the uncarbonized area showed red color and the fully carbonized
area showed no color after spraying phenolphthalein indicator,
and the uncarbonized and fully carbonized areas were clearly separated by a red line. As time went on, the dividing line (red line)
was still obvious and remained at the same position. Moreover,
the colors in uncarbonized area and fully carbonized area also
remained constant without any further changes.
The carbonation behavior of BA was completely different from
that of CC. An anti-diffusion behavior was observed in the complete carbonation zone of BA specimen (C-2) (Fig. 2). It can be seen
from the 14 d (b-1), 28 d (c-1) and 60 d (d-1)) carbonation test
results of BA specimen (C-2) that the uncarbonized area showed
a reddish color (below the red line), while the fully carbonized area
showed no color (above the red line) after the phenolphthalein
indicator was sprayed. This same behavior was observed for CC
too. However, the difference was that, about ten minutes later,
the color in fully carbonized area rapidly changed from colorless
Carbonation depth (mm)
1074
C-1
C-2
C-3
C-4
20
10
0
0
3.1. Mechanical properties and carbonation depth of CC and BA
Table 2 shows the compressive strength of the CC and BA after
28 days of curing. Samples C-1 to C-3 all have good mechanical
15
30
45
60
Time (day)
Fig. 1. Carbonation depth curves of CC and BA; Error bars represent one standard
deviation.
G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
1075
Fig. 2. Anti-diffusion behavior of BA samples; (a-1, a-2, a-3,) 14 d, 28 d, and 60 d carbonation depth of sample (C-1); (b-1, c-1,d-1) 14 d, 28 d, and 60 d carbonation depth of
sample (C-2); (b-2, c-2, d-2) in the anti-diffusion process of sample (C-2); (b-3, c-3, d-3) complete anti-diffusion behavior of sample (C-2).
to light red (above the red line) and the color in the uncarbonized
area (below the red line) changed from red to crimson (Fig. 2, b-2,
c-2 and d-2). Moreover, the dividing line between the uncarbonized area and fully carbonized area began to push back quickly
(red line). As the anti-diffusion behavior continued, the light red
color in fully carbonized area constantly deepened and the crimson
color in uncarbonized area continued to expand. This behavior
lasted for about three minutes. Ultimately, the anti-diffusion
behavior was completed after about fifteen minutes. It can be seen
from Fig. 2 (b-3, c-3, and d-3) that the fully carbonized area completely changed from light red to crimson and the color was consistent with the uncarbonized area. Moreover, the crimson color
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G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
completely covered the uncarbonized area and the dividing line
between carbonized and uncarbonized areas also disappeared
completely.
3.3. Influence of alkali-activator on anti-diffusion in carbonation zone
The anti-diffusion behavior of carbonized concrete is shown in
Table 3. It can be seen from Table 3 that the CC specimen (C-1)
and the BA specimen (C-3), which used only NaOH as the activator,
did not display the anti-diffusion behavior. However, the BA specimen (C-2) which was activated by sodium silicate solution and
NaOH as well as the BA specimen (C-4) which was activated only
by sodium silicate solution both showed the anti-diffusion behavior in carbonation zone. By comparing the compositions of BA
specimens (C-2, C-3, and C-4), it can be inferred that the occurrence of the anti-diffusion behavior in BA is closely related to the
sodium silicate solution. Moreover, by comparing the start time
and duration of anti-diffusion behavior in BA specimens (C-2 and
C-4), it can be seen that with the increase in content of sodium silicate solution, the start time was significantly advanced and the
anti-diffusion behavior was stronger. This suggests that the antidiffusion reaction rate and the degree of anti-diffusion are obviously enhanced with the increase in content of sodium silicate
solution.
Fig. 3. Distribution of pH value in CC and BA after 60d carbonation; Error bars
represent one standard deviation.
3.4. Determination of pH value in alkali-activated concrete
The pH value distribution of carbonized concrete (error bars
represent one standard deviation) is shown in Fig. 3. It can be seen
that the pH value of CC specimen (C-1) remained at 8.5 in the range
of 0–10 mm and this region was a fully carbonized region with a
relatively sufficient carbonation reaction. Then, the pH value
increased from 8.5 to 13.0 in the depth from 10 to 15 mm and this
region was a pH change zone of the carbonation reaction. Finally,
the pH value remained at 13 from 15 mm depth to inside and this
region was a stable uncarbonized zone. Similar to the pH distribution trend for CC specimen (C-1), the pH value of BA specimen
(C-3) without anti-diffusion characteristics remained at 8.5 in the
range of 0–20 mm. The pH value increased from 8.5 to 13.6 in
the depth from 20 to 25 mm and then remained at 13.6 in the
depth from 25 mm to inside.
The pH value distribution of BA specimen (C-2) was completely
different from that of the CC specimen (C-1). The pH value of BA
specimen (C-2) remained at 11.3 in the fully carbonized region
from 0 to 25 mm, even after carbonation. This is significantly
higher than the pH value in the fully carbonized area of CC specimen (C-1), suggesting that the carbonation reverse diffusion can
significantly improve the pH value in fully carbonized zone. Then,
the pH value increased from 11.3 to 13.9 in the depth from 25 to
30 mm and remained at 13.9 in the depth from 30 mm to the interior of the uncarbonized zone, which is higher compared to the CC
specimen (C-1) in the same zone. The pH distribution trend of BA
specimen (C-4) is similar to that of BA specimen (C-2) in the fully
carbonized region and pH change zone. Only in the uncarbonized
zone, the pH value of BA specimen (C-4) from 30 mm to the interior remained at 12.4, which is much lower than that of BA specimen (C-2). This is because NaOH was not used as the activator. The
above results indicate that even in the fully carbonized region of
BA specimens (C-2 and C-4), the pH can still be maintained at a
higher value. This is a very exciting result and it indicates that even
if the BA specimen has been completely carbonized, the passivation film of internal steel is still stable, which greatly reduces the
corrosion rate of the steel bar and improves its durability.
3.5. Microanalysis
3.5.1. XRD analysis
Fig. 4 shows the XRD patterns of the CC specimen (C-1) after 60
days of carbonation. It can be seen from Fig. 4(a) that the uncarbonized zone of CC specimen (C-1) had a very complex mineral
composition and its components included crystals such as quartz
(SiO2; PDF #01-070-2536), gehlenite (2CaO∙Al2O3∙SiO2; PDF #00035-0755), calcite (CaCO3; PDF #00-047-1743), tobermorite
(Ca5(Si6O16)(OH2); PDF #01-089-6458), hillebrandite (Ca2(SiO3)
(OH2); PDF #00-042-0538),
dicalcium silicate hydrate
(Ca2SiO43H2O; PDF #00-029-0374), and portlandite (Ca(OH2);
PDF #00-044-1481) as well as partially amorphous silicates and
aluminosilicates with amorphous chemical compositions [27].
The mineral composition of the complete carbonation zone in
CC specimen (C-1) changed significantly compared to the uncalcined zone. Fig. 4(b) shows that the characteristic peak of portlandite decreased dramatically after carbonation and the peak of
calcite increased markedly. This result indicates that carbonation
promoted the dissolution of portlandite (Ca(OH2), releasing a large
amount of OH involved in the carbonation reaction, and generated a large amount of calcite. Moreover, the characteristic peaks
of gehlenite, tobermorite, hillebrandite and dicalcium silicate
Table 3
Anti-diffusion behavior of carbonation.
C-1
C-2
C-3
C-4
Anti-diffusion behavior
start time
end time
NaOH
Ca(OH)2
water
sodium silicate solution
no
yes
no
yes
–
10 min
–
5 min
–
12 min
–
6 min
0
17.6
31.7
0
0
33
33
33
165
92.4
133.3
0
0
88
0
165
G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
(a)
XRD pattern of uncarbonized zone
(a)
1077
XRD pattern of uncarbonized zone
(b) XRD pattern of complete carbonation zone
(b) XRD pattern of complete carbonation zone
Fig. 5. XRD pattern of BA specimen (C-2).
Fig. 4. XRD pattern of CC specimen (C-1).
hydrate all decreased to different extents, which indicated that
these minerals are all involved in carbonation in varying degrees
and play a certain role in resisting carbonation [28].
Fig. 5 shows the XRD patterns of BA specimen (C-2) after 60
days of carbonation. It can be seen from Fig. 5(a) that the uncarbonized zone in BA specimen (C-2) had a significant peak of
magadiite that was formed by self-coagulated liquid sodium silicate which did not participate in the polymerization reaction
[14]. At the same time, the presence of portlandite was due to
the addition of Ca(OH)2 during preparation of the BA [29,30]. The
characteristic peaks of quartz, calcite, gehlenite and other crystals
also appeared in the XRD pattern of CC specimen (C-1).
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G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
The complete carbonation zone in BA specimen (C-2) showed a
certain degree of change compared to the uncalcined zone. From
Fig. 5(b), it can be seen that the characteristic peak of portlandite
reduced significantly due to carbonation. More importantly, the
characteristic peak of magadiite decreased dramatically compared
to the uncalcined zone. This indicates that magadiite turns back to
liquid sodium silicate when the pH value in pore fluid and the concentration of CO2 decrease to a certain level, and then it releases a
large amount of OH to increase the pH value in pore fluid [19,20].
3.5.2. FT-IR analysis
Fig. 6 shows the FT-IR spectra of the CC specimen (C-1). It can be
seen from Fig. 6(a) that the uncarbonized zone in CC specimen (C1) exhibited infrared vibration modes at 954 cm1 corresponding
to the stretching vibration of Si-O-Si bonds. This specific frequency
is characteristic of silicon tetrahedra (SiO4) in the chain structure of
calcium silicate hydrate (C-S-H). The shoulders at 1424 cm1 are
related to O-C-O bonds and this specific frequency is characteristic
of calcite. The mode at 1643 cm1 is assigned to the asymmetric
stretching vibration of H-OH bonds, and this specific frequency is
characteristic of water. The band at 3460 cm1 is associated with
bending vibrations of –OH bonds, and is related to bound Ca
(OH)2 [27,28]. These results are consistent with XRD results of
the uncarbonized zone in CC specimen (C-1).
Fig. 6(b) shows the FT-IR spectrum of complete carbonation
zone in CC specimen (C-1). The characteristic peak of –OH bond
at 3445 cm1 significantly decreased while the characteristic peaks
of O-C-O bond at 875 cm1 and 1413 cm1 significantly increased,
indicating that the carbonation consumed a large amount of Ca
(OH)2 and caused the formation of a large amount of calcium carbonate, which is consistent with the XRD results.
Fig. 7 shows the FT-IR spectra of the BA specimen (C-2). It can
be seen from Fig. 7(a) that the uncarbonized zone in BA specimen
(C-2) exhibited infrared vibration modes at 1033 cm1 associated
with bending vibrations of O-Si-O bonds, related to bound magadiite. A characteristic –OH peak appeared at 3459 cm1 due to –OH
mainly from the sodium silicate solution and the addition of a
small amount of Ca(OH)2. The characteristic peaks of quartz, calcite, and other crystals also appeared in the spectrum of BA specimen (C-2), which is consistent with the XRD results [14,15,20].
Fig. 7(b) shows the FT-IR spectrum of the complete carbonation
zone in BA specimen (C-2). It can be seen that the characteristic
peaks of O-C-O bonds at 874 cm1 and 1453 cm1 increased significantly, indicating that Ca(OH)2 was consumed and a large amount
of calcite was produced. More importantly, the characteristic peak
of –OH at 3459 cm1 showed no obvious reduction after carbonation, while the characteristic peaks of O-Si-O bonds at 1033 cm1,
related to bound magadiite, decreased significantly. This result
indicates that magadiite turned from solid back to liquid and
released a large amount of OH. Thus, the characteristic peaks of
magadiite significantly decreased and there was no change in the
characteristic peak of –OH. This is consistent with the XRD results
[19,20,31].
(a) FT-IR pattern of uncarbonized zone
3.6. Discussion
3.6.1. Reaction and carbonation mechanism of CC and BA
During the hydration process of CC (Fig. 8(a)), a large amount of
Ca(OH)2 was formed by hydration reaction of 3CaOSiO2 as well as
2CaOSiO2 [27,28]. The apparent characteristic peaks of C-S-H and
portlandite in Fig. 4(a) and the obvious characteristic peaks of Si-OSi and –OH bonds in Fig. 6(a) confirmed the aforementioned
mechanism.
As the carbonation reaction progressed (Fig. 8(b)), the OH in
the pore solution of CC was continuously consumed. However,
the crystalline Ca(OH)2 continued to dissolve, supplementing the
(b) FT-IR pattern of complete carbonation zone
Fig. 6. FT-IR pattern of CC specimen (C-1).
consumption of OH. In addition, the Ca(OH)2 crystals formed by
the hydration of Portland cement accounted for 20% to 30% of
the total mass in binding material, and the pH value of CC
remained at a high level until the solid-state Ca(OH)2 was depleted.
1079
G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
(a) FT-IR pattern of uncarbonized zone
to BA since it contains many substances that can participate in carbonation [7,10,28], e.g. Ca(OH)2.
The polymerization process of the BA specimen (C-2) is shown
in Fig. 8(c) (reactions (1)–(3)). Due to the strongly alkaline environment provided by the activator, active calcium as well as active
aluminum in the slag (GBFS, MSWI bottom ash) dissolved rapidly
and polymerized with the active silicon provided by the liquid
sodium silicate, thus producing C-S-H gel and C-A-S-H gel
[7,32,33]. Furthermore, insufficient reaction led to excessive crystallization of sodium silicate in the strong alkaline environment,
which then precipitated into the BA. The apparent characteristic
peaks of C-S-H, C-A-S-H, and magadiite in Fig. 5(a) and the obvious
characteristic peaks of the Si-O-Si and O-Si-O bonds in Fig. 7(a)
confirmed the aforementioned mechanism.
The carbonation process of the BA specimen (C-2) is shown in
Fig. 8(d) (reaction (4)). During the carbonation reaction, large
amounts of OH in the pore liquid were consumed, resulting in a
weakly alkaline environment [34]. Such an environment promoted
the dissolution of portlandite as well as magadiite and supplemented the consumed OH in the pore liquid. However, during
the preparation of BA, the majority of portlandite and sodium silicate were consumed in the polymerization reaction (reactions (3)
and (5)); therefore, the remaining content decreased [35]. Moreover, the anti-diffusion effect produced by magadiite was applicable only at low concentration of CO2, ultimately leading to poorer
resistance against carbonation in the accelerated carbonation test.
The significantly decreased characteristic peaks for portlandite and
magadiite in Figs. 5(b) and 7(b) confirmed the described mechanism. In summary, during the polymerization process of the BA
specimen (C-2), the portlandite mineral could not be generated
to resist carbonation. Also, the polymerization process consumed
large amounts of Ca(OH)2 and liquid sodium silicate which were
added during the preparation of BA. Therefore, the carbonation
resistance of BA was much weaker than that of CC [36,37,38].
Ca2þ þ SiO3 2 þ nH2 O ! C S H # ðstrong alkali environmentÞ
3þ
Ca2þ þ Al
ð1Þ
þ SiO3 2 þ nH2 O ! C A S H
# ðstrong alkali environmentÞ
Naþ þ nSiO3 2 þ nH2 O ! Na2 O nSiO2 nH2 O # ðSurplusÞ
ð2Þ
ð3Þ
Na2 O nSiO2 nH2 O Naþ þ nOH þ nSiO3 2
Fig. 7. FT-IR pattern of BA specimen (C-2).
The significant decrease in the characteristic peak of portlandite in
Fig. 4(b) and the obvious decrease in the characteristic peak of –OH
bond in Fig. 6(b) confirmed the aforementioned mechanism. In
summary, CC has good resistance against carbonation compared
ðlowCO2 concentrationandweak - alkali environmentÞ
ð4Þ
CaðOHÞ2 ðsÞ þ SiO3 2 þ nH2 O ! C S H ðIn the solid phaseÞ
ð5Þ
3.6.2. The mechanism of anti-diffusion in carbonation zone
The carbonation process of BA involves the penetration of CO2
and blocking by OH (Fig. 9). Specifically, CO2 dissolves into the
pore fluid and continues to push inward, while OH released from
the dissolution of magadiite as well as a small amount of Ca(OH)2
blocks the entry of CO2. The advancing speed of the carbonized surface (pH change zone in Fig. 9) is determined by the concentrations
and diffusion rates of CO2 and OH on both sides.
In the accelerated carbonation test, the concentration of CO2 is
set at 20%±3% and the high concentration of CO2 gas rapidly
intrudes into the surface of BA. The diffusion rate of CO2 (VCO2) is
very high and the consumed CO2 can be quickly replenished by
the high concentration of CO2. However, the amount of substances
in BA that can resist carbonation is very limited. Furthermore, the
conversion of magadiite into sodium silicate solution to release
OH is strongly inhibited in high concentrations of CO2. The small
amount of residual Ca(OH)2 in BA is unable to completely consume
1080
G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
Fig. 8. The process of reaction and carbonation in CC and BA; (a) The reaction process of CC specimen (C-1); (b) The carbonation process of CC specimen (C-1); (c) The reaction
process of BA specimen (C-2); (d) The carbonation process of BA specimen (C-2).
Y
pH
PH change zone
CO2
13.9
CO2
-
OH
After anti-diffusion
Concrete
surface
Complete
carbonation
zone
Uncarbonized
zone
8.5
Before anti-diffusion
0
11.3
25
30
X
Depth (mm)
Fig. 9. The mechanism of anti-diffusion in BA specimen (C-2).
progress of the carbonized surface (pH change zone in Fig. 9) in
BA is very rapid and the pH value in complete carbonation zone
is very low, as shown by the blue line in Fig. 9.
The concentration of CO2 dropped from 20% to 0.03% when BA
was removed from the carbonation box to the natural environment. The decrease in CO2 concentration led to the reduction in
diffusion rate of CO2. Also, the magadiite rapidly decomposed
and converted into liquid sodium silicate to release a large amount
of OH. As a result, the diffusion rate of CO2 was much lower than
the compensation rate of OH (VOH- VCO2). This conclusion was
confirmed by the decrease in the characteristic peak of magadiite
in Figs. 5(b) and 7(b). Due to the anti-diffusion effect, the pH value
in the pore fluid of BA increased rapidly and the carbonized surface
(pH change zone in Fig. 9) pushed back quickly. Ultimately, the
completely carbonized region turned from colorless to red in
Fig. 2 and the pH value in completely carbonized region increased
from 8.5 to 11.3 (red line in Fig. 9) after anti-diffusion effect. Therefore, the anti-diffusion behavior can significantly enhance the carbonation resistance of BA and also alleviate the carbonization
process under natural conditions. As a result, the rebar corrosion
resistance and durability of BA reinforced concrete can both be
improved.
4. Conclusions
the CO2 in the pores in time, which causes the diffusion rate of CO2
to be much faster than the compensation rate of OH (VCO2 V
OH).
This conclusion was confirmed by the decrease in the characteristic
peak of magadiite in Figs. 5(a) and 7(a). Therefore, the inward
Accelerated carbonation testing of CC and BA binders shows
that the carbonation resistance of BA is much lower than that of
CC, due to the absence of an anti-carbonization component such
G. Huang et al. / Construction and Building Materials 186 (2018) 1072–1081
as Ca(OH)2. BA displays a unique anti-diffusion behavior in the
complete carbonation zone. However, the anti-diffusion behavior
effect of BA is clearly reflected only at low concentrations of CO2
(0.03%) and this effect is inhibited under the experimental conditions of the accelerated carbonation test (CO2 concentration of
20%). The anti-diffusion behavior causes a significant rise in pH
value of pore fluid in the complete carbonation zone of BA, which
markedly improves the anti-carbonation properties of BA. The corresponding pH value for BA is significantly higher than that for CC.
The rapid decomposition of magadiite releasing a large amount of
OH is the main reason for the anti-diffusion behavior in BA specimens. With the increase in content of liquid sodium silicate, the
anti-diffusion behavior of BA in the carbonation zone is more evident. The findings of this work can help in the further development
of corrosion resistant building materials.
Conflict of interest
The authors declare that they have no conflict of interest.
Acknowledgement
The authors are grateful to the financial support from the Central Universities (2018XKQYMS02). The research works belongs to
one part of the projects which are financially supported by FRFCU.
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