Construction and Building Materials 213 (2019) 51–60
Contents lists available at ScienceDirect
Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Maximizing CO2 sequestration in cement-bonded fiberboards through
carbonation curing
Zhen He a,b, Yaodong Jia c, Sam Wang d, Mehrdad Mahoutian e, Yixin Shao e,⇑
a
Shaanxi Key Laboratory of Safety and Durability of Concrete Structures, Xijing University, Xian, China
State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan, China
c
Callaghan Innovation, Advanced Materials, New Zealand
d
AMEC Foster Wheeler, Oakville, Ontario, Canada
e
Department of Civil Engineering, McGill University, Montreal, Quebec, Canada
b
h i g h l i g h t s
The maximum possible CO2 uptake by Portland cement during carbonation curing is determined.
Carbonation curing of cement-bonded fiberboards promotes CO2 uptake to 85% efficiency.
The flexural strength by carbonation curing can reach 70 to 100% of ultimate strength within 24 h.
Carbonated fiberboards are more resistant to freeze-thaw cycling and wet-dry cycling.
Cement fiberboards can serve as CO2 sink for carbon capture and storage.
a r t i c l e
i n f o
Article history:
Received 14 September 2018
Received in revised form 1 April 2019
Accepted 8 April 2019
Available online 12 April 2019
Keywords:
CO2 sequestration
Carbonation curing
Cement-bonded fiberboard
Cellulose fiber
Strength
Durability
a b s t r a c t
CO2 sequestration in cement-bonded cellulose fiberboards through carbonation curing was studied. The
maximum possible carbon dioxide uptake by ordinary Portland cement within a curing process was
determined by a cement solution carbonation. It was 28.5% based on cement mass after 18 h initial
hydration and 24 h carbonation. The best combination of material and process parameters in fiberboard
carbonation had facilitated a curing process to allow 24.4% carbon uptake in 8 h carbonation and 20% carbon uptake in 0.5 h carbonation, a 70–85% reaction efficiency within 24 h. It was found that the CO2
sequestration was dependent on the water content in fiberboards after preconditioning. An optimal
water content was ranged from 40 to 60%. Carbonated fiberboards had shown a flexural strength at least
70% of ultimate strength within 24 h and a much improved freeze-thaw and wet-dry durability performance. If all cement-bonded cellulose fiberboards in United States adopt carbonation production, the
annual CO2 consumption by fiberboards can reach 0.36 Mt.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Cement-bonded cellulosic fiberboards are widely used in construction for exterior sidings, roofings and backboard applications.
In the last 15 years, extensive research had been carried out on the
process, performance and applications of cement-bonded fiber
products [1]. Cellulose fiber cement composites demonstrated
higher impact resistance, toughness, ductility and crack resistance
compared with those cement-based products without fibers [1].
Carbonation curing was introduced to improve the durability of
fiberboards. It was found that carbonation curing reduced the capillary porosity of composites, the microcracking and the moisture
⇑ Corresponding author.
E-mail address: yixin.shao@mcgill.ca (Y. Shao).
https://doi.org/10.1016/j.conbuildmat.2019.04.042
0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
content in cellulose fiber cement composites, leading to a stronger
bond between cellulose fiber and cement matrix [2]. Carbonation
was applied to cellulose fiber reinforced roofing tile production
[3]. Water absorption and apparent porosity were decreased with
carbonation while bulk density increased. The improvement on
the mechanical performance suggests that the fibres retained their
tensile strength in the inorganic matrix. The accelerated carbonation was effective in mitigating the degradation suffered by the cellulose fibers in an alkali medium [3]. The use of cement-bonded
cellulose fiberboards is increasing. It is estimated that the demand
for cement fiberboards in US market alone will reach 250 million
m2 per year in 2022 [4].
The traditional method of making cement fiberboards is the
Hatcheck process. In this method, cement, fiber, silica sand and
water are mixed in a slurry form with high water content. After
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
thoroughly mixed, the slurry is dewatered through filtering and
cast into thin sheets which are then stacked to form laminates.
The fiberboard laminates are treated by preconditioning drying
to reduce the moisture content and then sent for autoclave curing.
The autoclave temperature and pressure for curing the cellulose
fiberboard are usually set at 170–185 °C and 9–10 bar pressure
[5]. Since autoclave curing is an energy intensive process, it is challenging and rewarding if a less energy intensive process can be
developed to replace autoclaving. In this regard, early age carbonation curing can be an alternative [6].
As a steam replacement, early age carbonation technology had
been applied to concrete masonry blocks production [7] and concrete bricks production [8]. Because of the enhanced compatibility
between cement binder and wood particles, carbonation curing
was also successfully applied to the production of cementbonded wood particleboard with low water to cement ratio. It
was reported that particleboards made by Portland cement bonded
recycled wood and carbonation activation could achieve CO2
absorption by 9% based on cement mass and satisfy high performance requirement [9]. The recycled wood based particleboard
could also be made using carbonation activated magnesium oxide
cement as binder [10]. Balsa wood based particleboards were successfully fabricated using Portland cement and carbonation curing
[11]. Without carbonation, particleboards could be made with fastsetting magnesium phosphate cement [12].
The mechanism of rapid carbonation of cement-based materials
was studied by Young et al. [13]. It is the reactions of di-calcium
silicate (C2S) and tri-calcium silicate (C3S) with CO2 that generate
strength-contributing phases, such as calcium silicate hydrates
and calcium carbonates, as described by Eqs. (1) and (2):
2ð3CaO SiO2 Þ þ 3CO2 þ 3H2 O ! 3CaO 2SiO2 3H2 O þ 3CaCO3
ð1Þ
2ð2CaO SiO2 Þ þ CO2 þ 3H2 O ! 3CaO 2SiO2 3H2 O þ CaCO3
ð2Þ
If initial hydration as pre-condition is introduced before carbonation, hydration products such as calcium hydroxide (CH) and
calcium-silicate-hydrate (C-S-H) can also be carbonated [14]. The
reactions can be described by Eqs. (3) and (4):
CaðOHÞ2 þ CO2 ! CaCO3 þ H2 O
ð3Þ
3CaO 2SiO2 3H2 O þ 3CO2 ! 3CaCO3 þ 2SiO2 þ 3H2 O
ð4Þ
As indicated by Eqs (1)–(4), carbonation curing is a carbon dioxide uptake process. Gaseous carbon dioxide is converted into solid
calcium carbonates during carbonation curing. Cement-bonded
cellulose fiberboards can serve as CO2 sink to permanently sequester carbon dioxide. In the near future, plenty of high purity of CO2
will be available from carbon capture and recovery at the industrial
point sources due to possible emission regulation. Instead of being
injected in geologic formation for storage, the carbon dioxide gas
can be utilized in the production of cellulous fiberboards to gain
economical, technical and environmental benefits. The question
to be addressed is how much CO2 can be sequestered in cement
fiberboards. The theoretical maximum CO2 uptake by ordinary
Portland cement (OPC) was about 50% of cement mass [15]. Since
not all calcium compounds can react with CO2 in curing period,
it is essential to know the maximum possible CO2 uptake by fiberboards within a feasible industry process window.
The purpose of this study is to determine the carbon dioxide
uptake capacity by cement-bonded fiberboards and to maximize
the carbon dioxide utilization by optimizing the material and process parameters. Those parameters include mixture proportions,
compact pressure, preconditioning time, and carbonation duration.
A fundamental study is carried out first to determine the maxi-
mum possible CO2 uptake in a cement powder solution carbonation, an ideal condition for carbonation. The carbonation behavior
of fiberboards is then evaluated by carbon dioxide uptake, immediate strength gain, long term strength gain, the microstructure
changes as well as the freeze-thaw and wet-dry resistance. A
scale-up production is successfully implemented and the carbon
dioxide sequestration capacity by cement fiberboard carbonation
as well as its energy consumption is estimated.
2. Experimental program
2.1. Setup for cement solution carbonation
To determine the maximum possible carbon uptake by carbonation curing, cement solution carbonation was carried out. The test
was designed to simulate the best reaction condition with suspended cement particles in a saturated CO2 solution with continue
CO2 supply. Experiments were conducted using general use ordinary Portland cement (OPC). Its Blaine fineness number was
3730 cm2/g, specific gravity 3.14 g/cm3 and carbon dioxide content
0.54%. The setup of cement solution carbonation is shown in Fig. 1.
10 g cement was mixed with 400 g water with a magnetic stirrer in
a flask. CO2 of 99% purity was injected into the solution at 5 Standard litres per minute (SLPM). To avoid pressure, the flask had an
open exit to allow flow-through. The flask was placed on a digital
balance to monitor the mass gain.
Four tests (A1–A4) were performed: carbonation of 2 h or 24 h
with or without 18 h initial hydration. For immediate carbonation
without initial hydration, CO2 gas was injected into flask immediately after cement powder was mixed with water. This was to simulate fresh concrete carbonation. For carbonation after 18 h
hydration, cement powder was mixed with water for 18 h by a stirrer and then CO2 gas was injected. After solution test, the cement
powder was filtrated by a medium speed filter paper and oven
dried at 100 °C. Then the powder was pulverized and analyzed
using thermogravimetry (TG) and X-ray diffraction (XRD) techniques. CO2 uptake was quantified by three methods: mass gain
method, Coulometric titration method and TG analysis.
The mass gain method uses digital balance to record the mass
increase during carbonation (Fig. 1). Since gas flow may cause
evaporation, the system is calibrated by running a parallel test
without cement. The actual CO2 uptake is calculated by summing
up the mass gain due to cement carbonation and the water evaporation caused by gas flow. The test is repeated at least twice for
each batch for average. The percent CO2 uptake is calculated taking
dry cement as reference.
Coulometric titration method is also used to measure CO2 content in carbonated cement powder. Coulometer made by UIC Inc. is
Fig. 1. Cement solution carbonation setup.
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
used in this research. The system consists of an acidification module (Module CM5130 Acidification) and a CO2 analyzer (Model
CM5014 CO2 Coulometer). The powder sample reacts firstly with
sulfuric acid in the acidification module to release CO2 which is
then detected by CO2 analyzer. Three samples are tested for each
batch and the average results are presented. CO2 content in asreceived cement is also determined. The difference of the two
CO2 contents represents carbon dioxide uptake by carbonation.
The percent CO2 uptake is expressed in terms of dry cement.
Thermogravimetry analysis is performed on the same filtered
powders using thermal analyzer (NETZSCH, TG 449 F3 Jupiter, resolution of 0.01 mg) between 20 °C and 1000 °C at the heating rate
of 10 °C/min. The mass loss between 450 °C to 1000 °C is considered as CO2 loss and is used to determine CO2 content due to carbonation. The temperature ranges are determined by differential
thermogravimetry (DTG) analysis. Again the percent uptake is calculated based on dry cement ratio. The difference in CO2 contents
between carbonated and hydrated cement represents the CO2
uptake by carbonation.
2.2. Setup for cement fiberboard carbonation
Both bleached cellulose fibers and unbleached cellulose fibers
were used with a diameter of 30 lm, length of 3 mm and a specific
gravity of 1.5 g/cm3. Table 1 summarizes the mixture proportions
of the cement-bonded cellulous fiberboards produced in this study.
Smaller samples (76 127 12 mm) were used in preliminary
study to determine the optimal combinations of material and process parameters in batches CBF 0 to CFB 7. CFB 8 was made from a
scale-up production based on the preliminary study and had a
dimension of 305 610 12 mm. The proportion in Table 2 was
designed for one board sample with a fixed surface area
(76 127 mm or 305 610 mm) and a thickness between 11
and 12 mm. The final thickness was determined by the compact
pressure to justify the water content for carbonation. Silica flour,
a required ingredient for autoclaving, was eliminated from mixture
proportion because it was not necessary for carbonation. Bleached
cellulose fibers were employed in all batches (CFB 0 to CFB 5, CFB 7
Table 1
Mixture proportion of cement fiberboard (per sample).
Batch
Cement (g)
Fiber (g)
Fiber/cement
ratio (%)
Water (g)
CFB 0-4, 7, control
CFB 5-6
CFB 8
121
135
2335
14.9
10.8
287.6
12
8
12
67.4
65.6
1297.3
Note: CFB 0 to CFB 5, CFB 7, CFB control: 76 127 12 mm, bleached cellulose
fibers;
CFB 6: 76 127 12 mm, unbleached cellulose fibers;
CFB 8: 305 610 12 mm, bleached cellulose fibers.
Water: water in saturated green body after compact forming.
to 8, CFB control) except CFB 6 in which unbleached cellulose fibers
were used. This was to determine if the less expensive unbleached
cellulose fiber could also be used as reinforcement in fiberboards.
Three identical board samples were prepared for each batch.
For batches CFB 0 – CFB 7, the fibers were soaked in tap water
over night. After fiber slurry was beaten to a uniform suspension,
cement was slowly added. After mixing for 20 min, the excessive
mixing water in the slurry was filtered out by a paper filter so that
a workable fiber-cement mixture was formed. The mixture was
then partitioned into three equal amounts in three steel molds of
76 mm by 127 mm each. Compression molding was used in forming the fiberboards with compact pressure between 0.7 and
1.4 MPa and compression duration of 10 min to have a thickness
between 11 mm and 12 mm. The compression also helped squeeze
out extra water to form a saturated green body. The remaining
water in each saturated green body is shown in Table 1. It was calculated by subtracting the volume of solid cement and the volume
of fiber from the total volume of compact specimen. The saturated
green boards were then demolded and subject to preconditioning
drying using an oven or a fan. This procedure is critical for carbonation since the precondition removes percentage of free water,
making space for CO2 gas to penetrate and carbonates to precipitate. Batch CFB 8 followed same procedure in a larger steel mold
to make a fiberboard of 305 610 12 mm.
Table 2 summarizes the results of preconditioning of seven
batches (CFB 0 – CFB 6) by oven drying and two batches (CFB 7
and CFB 8) by fan drying. The water content after precondition
was calculated by the ratio of mass of remaining water over mass
of initial water in saturated green body, i.e. Water content (%)
= Mass of remaining water/mass of initial water. The mass of initial
water = mass of saturated body – mass of cement –mass of fiber.
The mass of remaining water = mass of initial water – mass of
evaporated water by precondition. CFB 0 was the reference batch
without preconditioning drying. The sample was saturated with a
100% water content. CFB 1, 2 and 3 were used to study the combined effect of compact pressure and oven drying time on water
content. CFB 4 was conditioned at lower temperature to have
higher water content. CFB 5 and 6 were fiberboards with less fibers
(8%) in comparison to 12% in CFB 0 to CFB4. CFB 6 employed
unbleached fibers as replacement of bleached counterpart. CFB 7
was identical to CFB 3 and 4 except the drying process. Instead
of using an oven, fan drying was investigated in CFB 7. It was more
feasible to use fan drying in full-scale production although it took
longer time. Fan drying was carried out at ambient temperature
(25 °C) and 50% relative humidity (RH) for 18 h to achieve an
equivalent water content obtained in oven drying. Based on CFB
7, CFB 8 was a scale-up production to make a fiberboard of
305 610 12 mm conditioned by the same fan drying. The
hydration control batch was prepared using fiber-cement ratio of
12% and compact pressure of 0.7 MPa to form a board sample of
76 127 12 mm. The control samples were hydration cured for
eight hours (short term strength test) and 28 days (long term
Table 2
Preconditioning parameters for carbonation.
Batch
Compact pressure (MPa)
Thickness (mm)
Drying time
Temperature (oC)
Water content (%)
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
CFB
0.7
1.4
1.0
0.7
0.7
0.7
0.7
0.7
0.7
11.7
10.9
11.2
11.8
11.8
11.8
11.9
12.0
12.0
0
40 min
60 min
150 min
150 min
150 min
150 min
18 h fan
18 h fan
25
60
60
60
50
60
60
25
25
100
55
62
42
59
46
46
40
45
0
1
2
3
4
5
6
7
8
Note: CFB = cement fiberboard; min = minute; h = hour.
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
strength test) in a chamber at ambient temperature and a relative
humidity of 95%.
Fig. 2 shows the setup of carbonation curing for fiberboards.
Immediately after preconditioning, the board samples were placed
in the pressure chamber for carbonation curing. All batches were
carbonated with pure gas (purity of 99.8%) at a pressure of
0.5 MPa for a designated carbonation time (0.5 h, 2 h or 8 h) as
shown in Table 3. For batches CFB 0 to CFB 7, the carbonation
chamber was placed on an electric balance to measure the mass
curve of the system due to carbon dioxide uptake. Measurement
Group System 5000 was used for data acquisition. For CFB 8, larger
carbonation chamber was used. The mass curve was not recorded.
2.3. CO2 uptake measurement
CO2 uptake by fiberboards is estimated by two methods: mass
gain method and mass curve method. Mass gain method calculates
CO2 uptake in fiberboard by comparing mass of samples before and
after carbonation (Eq. (5)), in which m1 and m2 represents sample
mass before and after carbonation reaction. Carbonation-induced
water loss (mwater) was collected by absorbent paper and added
to final mass (m2 ). By treating the system as a closed system, it
was imperative to include the evaporated water, which was initially inside the samples prior to carbonation. Percent CO2 uptake
is expressed with reference to the dry cement mass (mcement). Mass
gain method was applied to all batches.
CO2 uptake ð%Þ ¼
m2 þ mwater m1
100%
mcement
ð5Þ
Mass curve method estimates the CO2 uptake through digitally
recorded mass of the system for Batches CFB 0 – CFB 7. It was difficult to obtain the mass curve for CFB 8 because of the size of the
boards and the chamber. After fiberboard samples were placed in
the chamber, the balance was zeroed. The CO2 gas was then injected
to a specified pressure and the valve was kept open so that there
was a continuous gas supply to maintain the constant pressure.
Since the pressure was a constant, the increase in mass of the system was due to the carbon dioxide uptake by fiberboard. At the
end of carbonation at which time gas was released to atmospheric
Fig. 2. Carbonation curing setup for cement-bonded fiberboards.
Table 3
Carbonation and hydration parameters.
CO2 uptake ð%Þ ¼
Mm
100%
mass of cement
ð6Þ
2.4. Performance tests
For each batch, immediately after carbonation, flexural
strengths (fs) and compressive strengths (fc), and microstructure
characteristics were determined. Long term strengths were also
tested after 28 days subsequent hydration. Flexural strength (fs)
was determined by a three-point bending test of a fiberboard
(76 127 12 mm) over a span of 101 mm. Scanning electron
microscopic (SEM) analysis was performed to study the fracture
surface morphology.
Durability tests of carbonated fiberboards were performed to
examine the board resistance to freeze-thaw cycling and wet-dry
cycling. It is important for outdoor applications such as sidings
and roofings. The commercial fiberboards made from Hatcheck
process and cured by autoclave were also tested as reference.
Fiberboard CFB 8 made from scale-up production was used in
durability tests. After 28 days subsequent hydration, CFB 8 boards
were cut into small samples of 76 127 12 mm for freeze-thaw
and wet-dry tests.
In freeze-thaw tests, two standard tests were followed: ASTM
C666 and CSA A231.2. For ASTM C666, samples were subjected
to freeze-thaw cycles from 17 °C to 4 °C with 4–6 h per cycle in
water. After 200 cycles, the samples were oven dried for 24 h at
60 °C for thickness measurement and residue flexural strength
tests. For CSA A231.2, samples were immersed in a 3% NaCl solution and subjected to freeze-thaw cycles from 15 °C for 18 h to
20 °C for 6 h. After 20 cycles, the thickness was measured and residue flexural strengths were tested.
In wet-dry tests, samples were exposed to a wet/dry cycle following procedure developed in [16]. One cycle included 24 h drying (23.5 h drying in an oven at 65 °C and 20% RH, with 0.5 h air
cooling at 22 °C and 60% RH), and 24 h wetting (23.5 h soaking in
water at 20 °C, and 0.5 h air drying at 22 °C and 60% RH). After 5
and 10 cycles, residue flexural strengths were tested.
3. Results and discussion
Batch
Gas pressure (MPa)
Curing time
CFB
CFB
CFB
CFB
0.5
0.5
0.5
–
Carbonation 2 h
Carbonation 8 h
Carbonation 0.5 h
Hydration 8 h
0, 1, 2, 3, 5, 6
4
7, 8
control
pressure and the residual mass of the system, M, was recorded. The
residue mass, M, is the sum of CO2 uptake by fiberboards and residual CO2 gas left inside chamber, m. It was noted that the chamber
initially contained one volume of air before CO2 injection, when
the balance was zeroed. After the gas was released at the end of carbonation, the gas inside chamber was a mix of air and residual CO2.
Therefore the residual CO2, m, should be measured and subtracted
from the residual mass of the system, M, to calculate CO2 uptake by
fiberboards. To measure the residual CO2 in chamber, the test was
repeated using CO2-insensitive expanded polystyrene (EPS) foam
samples of the same volume. Since EPS foam did not absorb CO2,
the residual mass of the system recorded by balance represented
the residual CO2 mass, m, left inside chamber after releasing. The
difference between M and m represents the CO2 uptake by fiberboard (Eq. (6)). Data collected by mass gain and mass curve methods are two different measurements from the same process and
therefore should be comparable. They are also independent from
any carbon content existing before carbonation. The thermal analysis of carbonate content was not performed for fiberboards
because of the influence of carbon-rich cellulose fibers.
3.1. CO2 uptake in cement solution carbonation
Table 4 summarizes CO2 uptake in cement solution carbonation.
For 2 h carbonation in solution, it was 19.0% without hydration
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
Table 4
CO2 uptake in cement solution carbonation (%).
ID
Hydration (h)
Carbonation (h)
Theoretical maximum (%)
A1
A2
A3
A4
0
0
18
18
2
24
2
24
50
50
50
50
3
1-C3S+C2S
2-Ettringite
3-Ca(OH)2
4-CaCO3
4
18H+24C
4
18H+2C
4
4
4
1
18H hydraon only
1
2
4
4
4 4
4
4
4 4
3
3
1
1
As-received
1
1
5
10
15
20
25
30
35
1
1
1
1
40
45
50
55
Fig. 3. XRD patterns of cement after hydration and carbonation in solution.
Mass gain
Titration
TG
Average
19.3 ± 0.2
24.7 ± 1.5
23.7 ± 0.8
28.8 ± 1.6
20.1 ± 0.1
25.6 ± 0.6
24.5 ± 0.2
29.6 ± 0.2
17.5
20.6
21.1
27.0
19.0 ± 0.3
23.6 ± 2.1
23.1 ± 1.0
28.5 ± 1.8
0.4
CaCO3
Ref-As received
0.35
18H hydraon only
0.3
DTG, %/°C
(A1) and 23.1% with 18 h hydration (A3). In prolonged carbonation
of 24 h, CO2 uptake was increased to 23.6% without hydration (A2)
and 28.5% with 18 h hydration (A4). Three independent measurements by mass gain, chemical titration and thermogravemitry
analysis were in good agreement. It was interesting to note that
the increase in carbon uptake due to initial hydration was in the
range of 4.1 to 4.9% in solution carbonation.
XRD analysis was performed on four cement samples: asreceived cement, 18 h hydrated cement, 2 h carbonated cement
after 18 h hydration and 24 h carbonated cement after 18 h hydration. Their XRD patterns are given in Fig. 3. It was clear that hydration products such as calcium hydroxide were formed in 18 h
hydration and consumed in the subsequent 2 h or 24 h carbonation. After 2 h carbonation, there were still some C3S and C2S
detected. After 24 h carbonation, all calcium silicates were consumed. On the other hand, significant amount of calcium carbonates were formed after 2 h or 24 h carbonation. The carbonated
cement powder was dominated by carbonates. It was evident that,
with 18 h initial hydration, both calcium silicates (Eqs. (1) and (2))
and hydration products (Eqs. (3) and (4)) were carbonated, leading
to a maximized degree of carbonation. Solution carbonation tests
provided the maximum possible CO2 uptake of cement within
the given process window.
Fig. 4 shows DTG curves of the four cement samples. While the
hydrated cement exhibited strong calcium hydroxide, the carbonated cement had only shown calcium carbonates. Calcium hydroxide had been completely consumed by carbonation no matter what
carbonation duration was used. Apparently, hydration products
also participated in the carbonation reaction if initial hydration
was introduced. A strong peak at 745 °C was indicative of formation of calcium carbonate which was the dominant phase in carbonated cement. Although DTG curves were not enough to
discuss the formation of calcium silicate hydrate, especially at
early age, the peak shift might suggest different microstructures.
For 18 h hydration reference, a peak was detected at 145 °C, representing hydration products (either calcium silicate hydrate or calcium aluminate hydrate). After 2 h or 18 h carbonation, the peak of
Experimental
18H+2C
18H+24C
0.25
0.2
Ca(OH)2
0.15
0.1
Carbonaon
0.05
0
Hydraon
0
200
400
600
800
1000
Temperature,°C
Fig. 4. DTG curves of cement after hydration and carbonation in solution.
hydration products was shifted to 108 °C and was independent
from carbonation duration. The final hydration products after carbonation were therefore different from conventional hydration
reaction. This phenomenon was also observed in previous study
[13] and needs further investigation.
3.2. CO2 uptake in cement-bonded fiberboards
CO2 uptake results of fiberboards determined by mass gain
method (MG) and mass curve method (MC) are summarized in
Table 5. The results by the mass curve method were found consistently higher than that by the mass gain method in all batches. This
might be caused by the vapor loss after opening the chamber with
the mass gain method because not all water evaporated due to carbonation could be collected. It is likely that CO2 uptake determined
by mass curve method provides the upper bound value and CO2
uptake by mass gain method yields the lower bound value. The
averages of CO2 uptake in 2 h carbonation were in a range from
14.3% to 19.5%, based on dry cement mass. They were lower than
23.1%, the maximum possible CO2 uptake by cement solution carbonation (Table 4). The average of CO2 uptake in 8 h carbonation
was 24.4% (CFB 4), which was below the 28.5%, the maximum possible CO2 uptake in 24 h carbonation (Table 4). Longer carbonation
time can promote more carbonation reaction. However the curing
process is usually limited to 24 h to be economic. To work within
this process window, either oven drying 150 min at 50 °C followed
by 8 h carbonation or fan drying 18 h followed by 0.5 h carbonation
can be adopted. The half hour carbonation after 18 h fan drying
could reach a CO2 uptake of 20%, a 70% efficiency in comparison
to maximum possible value in Table 4. Fan drying also makes carbonation curing of fiberboard feasible in industry scale.
Water content in fiberboards after precondition is a critical
parameter in maximizing carbonation reaction. It is similar to the
moisture content that is required for weathering carbonation of
matured concrete. The previous study on weathering carbonation
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
Table 5
CO2 uptake by mass curve method (MC) and mass gain method (MG).
CFB 1
CFB 2
CFB 3
CFB 4
CFB 5
CFB 6
CFB 7
CFB 8
1.7
1.5
1.6
15.2
13.5
14.3
18.1
16.0
17.1
20.1
18.9
19.5
25.1
23.6
24.4
20.6
19.1
19.9
15.6
14.4
15.0
20.4
20.1
20.2
–
20.5
20.5
of concrete indicated that moisture content should be maintained
at about 60% to facilitate carbonation reaction [17]. In early carbonation, it is equivalent to a water removal of 40% with reference
to initial water in saturated green body. It can be achieved by both
compact pressure and preconditioning drying. Fig. 5 shows the
effect of compact pressure and oven drying on water content. It
was clear higher compact pressure had reduced total water content. However the samples were still saturated. To make free space
for gas to penetrate and carbonates to precipitate, preconditioning
by drying is necessary. To justify the water content, oven drying
was adopted for a fast production. For fiberboards compact formed
by 1.4 MPa, 1.0 MPa and 0.7 MPa (CFB 1, 2, 3), the corresponding
drying time was selected at 40, 60 and 150 min for a fixed temperature of 60 °C. The combined action resulted in a water content of
55, 62 and 42% respectively. For constant compact pressure of
0.7 MPa and constant drying time of 150 min, the water content
could be maintained at 42–59%. It should be noted that CFB 4
was preconditioned at lower temperature (50 °C), leading to a
higher water content. In comparison to oven drying, fan drying
(CFB 7 and 8) could also reach a water content of 40–45% but
required a longer process time (Table 2).
The relationship between CO2 uptake and water content after
precondition is presented in Fig. 6. Saturated fiberboards were difficult to be carbonated. The CO2 uptake for the reference batch
without drying (CFB 0) was only 1.6%. It was evident that carbonation of saturated sample was not effective. With precondition drying either by oven or by fan, the uptake was tremendously
increased. Fig. 6 shows, if batch CFB 4 was removed from the plot,
the correlation between CO2 uptake and water content was clear.
The lower the water content, the higher the carbon dioxide uptake.
It was valid when carbonation duration was 2 h or less. CFB 4 was
carbonated 8 h and was not in line with the trend. This trend did
not continue when water content was further decreased to below
40% since carbonation reaction happened in an aqueous pore solution [17]. As long as the water content was in the range of 40–60%
with reference to the saturated green body (Table 1), the CO2
uptake of 14–20% with reference to cement mass could be
achieved if carbonation of fiberboards is limited to 2 h or less.
It was interesting to notice that relatively higher water content
(59%) in batch CFB 4 led to a slower reaction but a higher uptake in
160
150 150 150 150
CFB1@60C
140
120
CFB2@60C
CFB3@60C
Maganitude
CFB4@50C
100
CFB5@60C
CFB6@60C
80
60
60
40
40
20
1.4
62
59
55
42
46 46
1.0 0.7 0.7 0.7 0.7
0
Compact pressure (MPa)
Drying time (min)
Water content (%)
Fig. 5. Effect of compaction and oven drying on water content.
25.00
Carbon dioxide uptake (%)
MC,%
MG,%
Ave,%
CFB 0
CFB7
20.00
CFB5
CFB8
CFB3
CFB2
15.00
CFB6
CFB1
10.00
5.00
y = -0.3005x + 32.391
R² = 0.8869
0.00
0.0
20.0
40.0
60.0
80.0
CFB0
100.0
120.0
Water content after precondition (%)
Fig. 6. Relationship between water content and CO2 uptake.
a prolonged carbonation. The ultimate carbon dioxide uptake by
CFB 4 had reached 24.4% in 8 h. The higher water content helped
avoid fast temperature rise, reduced reaction rate, resulting in even
higher CO2 uptake. The intensive reaction heat for cellulose fiberboard by carbonation allowed the carbonation to proceed with
higher water content. The conclusion was valid for batches made
with same compact pressure (0.7 MPa for CFB 3 to 6). Although
CFB 2 also exhibited a relatively high water content of 62%, the
uptake was only 17.1%. This was attributed to the high compact
pressure (1 MPa) in forming that made less potential space for carbonate precipitation.
When the water content was reduced to 40 – 46%, its corresponding CO2 uptake could reach 20%. It happened in CFB 3, 5, 7,
8. They were fiberboards with same bleached fibers but different
fiber contents, drying methods and carbonation duration. It
appeared that 18 h fan drying could make same uptake as
150 min oven drying. Even 30 min carbonation could produce
comparable uptake as 2 h carbonation. The process window was
quite wide, allowing the trade-off between the uptake and the cost.
It was conclusive that water content after preconditioning drying
was the most critical parameter. It could be justified by oven drying or fan drying for a different duration to maximize the uptake
within a 24 h process window. Fan drying seems to be promising
in scale-up production. Longer carbonation time was always beneficial for higher uptake. Nevertheless fast production was more
economically attractive.
Compared with batch CFB 3 of 12% fiber/cement ratio, CFB 5 of
8% fibers by weight had a comparable uptake value (Fig. 6). Batches
CFB 5 and CFB 6 were identical with a fiber cement ratio of 8%
except the treatment of fibers. CFB 5 used bleached cellulose fibers
and CFB 6 employed unbleached cellulose fibers. The CO2 uptake
was quite different. It was 19.9% in CFB 5 with bleached fibers
and 15.0% in CFB 6 with unbleached fibers. In comparison to the
CO2 uptake of 12.5% in cement paste without fibers [18], the use
of cellulose fibers helped facilitate more carbonate precipitation.
It seemed that the fiber/cement ratio had no direct effect on CO2
uptake, but the bleaching treatment of cellulose fiber played a critical role in promoting carbonation reaction.
57
Z. He et al. / Construction and Building Materials 213 (2019) 51–60
3.3. Strength gain of carbonated cellulose fiberboard
Table 6 summarizes the flexural strengths (fs) and compressive
strengths (fc) of carbonated cellulose fiberboards. For early age
strength immediately after carbonation, the ages at the test of each
batch were quite different. The age of the sample was equal to the
preconditioning time plus carbonation time. The hydration reference (CFB control) strength at the age of 10.5 h was fs = 2.0 MPa
and fc = 3.1 MPa. Obviously carbonation curing had doubled the
flexural strength and tripled the compressive strengths within
24 h.
Table 6 demonstrates the relationship between strength and
compact pressure. Higher flexural and compressive strengths for
CFB1 were directly related to the higher compaction pressures of
1.4 MPa. It was more apparent in compressive strength than in
flexural strength.
For the same compact pressure, compressive strength appeared
to be more correlated to CO2 uptake than flexural strength after
0.5 h, 2 h or 8 h carbonation. There was a clear trend to relate compressive strength to CO2 uptake at the constant compact pressure
of 0.7 MPa. The significant increase in carbonation strength over
hydration strength was suggestive that high degree of carbonation
was the effective way to improve early age compressive strength.
For flexural strength, however, its relationship with CO2 uptake
was not as clear as compressive strength. The immediate flexural
strength after carbonation was in a range between 4.1 MPa and
8.3 MPa. The large discrepancy was caused by the difference in
the age of test, the compact pressure and the degree of carbonation. It seemed that 18 h precondition drying by a fan followed
by half hour carbonation was an option to trade off between flexural strength and cost within a 24 h process window.
Flexural strength was found to be more dependent on the fiber/
cement ratio than the compressive strength. CFB 5 had a lower
fiber/cement ratio of 8% but showed a higher early age flexural
strength in comparison to CFB 3 with 12% fiber. This could be
attributed to the higher cement content in CFB 5, which formed
a stronger cement matrix and better bond in fiberboard. The
unbleached fibers in CFB 6 made fiberboard weaker at early age
because of the low carbonation degree. CFB 7 and 8 displayed
higher flexural strength. The short carbonation (0.5 h) was compensated by a longer preconditioning time (18 h).
Twenty-eight-day strengths of fiberboards after 28 days subsequent hydration are also presented in Table 5. Compressive
strengths had seen an increase in all batches. It was evident that
subsequent hydration was not hindered by early carbonation. For
fiberboard with 8% fiber cement ratio, the subsequent strength
gain was significant, exceeding those with 12% fibers. This was
likely attributed to the high cement content. The development of
flexural strengths at 28 days followed similar trend as observed
in immediate strength. The scale-up production made fiberboards
relatively higher in flexural strength and lower in compressive
strength. Size effect in production appeared to play a role.
The strength of CFB control after hydration of 28 days was
fs = 6.5 MPa and fc = 15.6 MPa. The commercial cement fiberboard
had shown a flexural strength of 6.9 MPa and a compressive
strength of 14.8 MPa [19]. With these two references, most carbonated fiberboards had exhibited strength comparable with or better
than the control. The compressive strength of fiberboards from
scale-up production needs to be improved. In comparison with
commercial products, the carbonated fiberboards had shown more
than 70% of the ultimate strength within 24 h after casting. Carbonation curing has shown the potential to replace autoclave curing for fast strength gain.
3.4. Microstructure of carbonated cellulose fiberboard
SEM micrographs of typical carbonated cellulose fiberboards
from Batch CFB 4 are shown in Fig. 7. Large and densely distributed
carbonate crystals with the grain size of 5 mm were found to grow
both out of the paste matrix (Fig. 7a) and on fiber surface (Fig. 7b).
Cement paste was found to diffuse into the fiber cell walls and cavities and carbonate crystals to protrude from those thin cement
layers. All these features were seen throughout the entire thickness
of the fiberboard.
SEM micrograph of hydrated fiberboard is shown in Fig. 8 as a
reference. Hydration products such as ettringite needles can be
seen. It is apparent that the microstructures of carbonated and
hydrated fiberboards are different in that carbonated products
comprise of a large number of calcium carbonate crystals embedded in conventional hydration products, leading to a densified
matrix.
3.5. Durability of carbonated cellulose fiberboards
Freeze-thaw and wet-dry durability of carbonation cured fiberboards were studied using CFB 8 made from scale-up production.
They are important in out-door exposed applications. Fig. 9 shows
the effect of curing on thickness increase caused by freeze-thaw
cycling. The commercial products by Hatcheck process and autoclave curing exhibited significant expansion along the thickness
if compared to the carbonated counterparts. It was 20% by ASTM
test and 75% by CSA test. The carbonated fiberboards experienced
only 3% and 11% expansion along the thickness in the two tests.
Fig. 10 shows the photos of final thickness after freeze-thaw tests.
Clearly there was delamination in commercial products which was
responsible for the expansion in thickness. Carbonated fiberboards
were more dimensionally stable. It was also attributed to the
monolithic compression molding in carbonation process instead
of laminate stacking in Hatcheck process. The flexural strength
reduction after freeze-thaw cycling is compared in Fig. 11. The
Table 6
Flexural and compressive strength of carbonated fiberboards.
CFB 2
CFB 3
CFB 4
CFB 5
CFB 6
CFB 7
CFB 8
Immediate after carbonation
Age (h)
2.6
Compact (MPa)
1.4
Uptake, %
14.3
fs (MPa)
5.7 ± 0.7
fc (MPa)
13.6 ± 1.2
CFB 1
3.0
1.0
17.1
5.5 ± 0.8
10.2 ± 0.3
4.5
0.7
19.5
4.1 ± 0.1
10.5 ± 0.4
10.5
0.7
24.4
5.8 ± 0.4
12.5 ± 0.4
4.5
0.7
19.9
6.0 ± 0.5
11.0 ± 0.4
4.5
0.7
15.0
4.9 ± 0.3
7.6 ± 0.3
18.5
0.7
20.2
8.3 ± 0.3
11.2 ± 0.4
18.5
0.7
20.5
6.5 ± 0.5
10.5 ± 1.5
After 28 days subsequent hydration
Age (d)
28
fs (MPa)
6.3 ± 0.6
fc (MPa)
15.8 ± 0.1
28
7.9 ± 0.3
15.9 ± 0.2
28
6.1 ± 0.2
15.2 ± 1.5
28
8.5 ± 0.6
18.4 ± 0.7
28
7.6 ± 0.4
23.1 ± 0.3
28
6.9 ± 0.7
20.2 ± 0.1
28
12.2 ± 0.2
15.8 ± 0.1
28
8.5 ± 0.3
12.1 ± 0.5
Note: fs = flexural strength; fc = compressive strength. For hydration reference of CFB control at 28 days: fs = 6.5 MPa, fc = 15.6 MPa. For commercial fiberboards: fs = 6.9 MPa,
fc = 14.8 MPa.
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
(a) In cement paste
Fig. 9. Thickness increase due to freeze-thaw cycling.
ple had maintained 100% and 89% strength in comparison to 81%
and 79% remaining in autoclaving cured samples.
4. The CO2 sequestration capacity and energy analysis
(b) On fiber surface
Fig. 7. Microstructure of carbonated fiberboard.
Fig. 8. Microstructure of hydrated fiberboard.
autoclaving cured commercial products had seen a faster reduction
than the carbonation cured samples. After 200 cycles in water, the
final flexural strength of autoclave products was decreased to 37%,
while the carbonation cured samples still maintained 98% of original strength. For test in saline water by 20 cycles, the residue flexural strength was only 6% in commercial products but was 56% in
carbonated boards. The carbonated fiberboards are more resistant
to freeze-thaw damage. It was possibly due to the precipitation of
calcium carbonates by carbonation.
Effect of wet-dry cycling on flexural strength is displayed in
Fig. 12. The flexural strength was tested after 5 and 10 cycles
and compared to the original strength. The carbonation cured sam-
The United States fiber cement market is estimated to be 250
million m2/year in year 2022 [4]. Assuming a typical thickness of
12 mm, a density of 1200 kg/m3 and a cement content of 50%,
the annual production of cement fiberboards in US will consume
approximately 1.8 million tonnes of cement. If all fiber cement productions in US adopt carbonation curing in place of autoclaving,
the annual carbon dioxide utilization in fiber cement products
could reach 360,000 tonnes using CO2 at an uptake rate of 20%.
The global market is much bigger. The production of cementbond fiberboards can serve as CO2 sink for carbon sequestration
and storage. Instead of being injected into geologic formation for
storage, carbon dioxide gas recovered from industry sources such
as cement plants can be beneficially used in building products
for accelerated strength and improved durability. It will reduce
the dependence on natural gas for high pressure and high temperature steam.
Energy analysis was performed to compare carbon dioxide curing with autoclave curing. It was well documented that autoclaving
of 1 m3 concrete would consume 0.712 GJ of energy through the
combustion of natural gas [20]. Its electricity equivalent is
198 kWh/m3. The use of energy in carbonation curing occurred
during preconditioning by initial hydration. Oven drying or fan
drying was used to precondition the fiberboards to remove approximately 50% free water and create space for carbonates to precipitate. Oven drying could reach the goal in 3 h but was energyintensive. On the other hand, fan drying took 18 h but required
much less energy. Fan drying is preferred. Energy consumption
by fan drying is dependent on the relative humidity (RH) of the
site. A quantitative energy consumption was measured. Table 7
summarizes the energy consumption during preconditioning by
initial hydration of one panel (CFB 8, 305 610 12 mm) with
volume of 0.0022 m3. For a fixed initial hydration time of 18 h,
three RH conditions were tested: 25%, 50% and 80%. A special electric fan with adjustable wind velocity and power direct-reading
function was used to control the evaporation rate and record the
energy consumption. A wind box was built on a digital balance
inside an environmental chamber with selected RH. The panel
was placed in wind box with two ends open. When the target wind
speed was reached based on the criterion of 50% water removal at
the given RH in 18 h, the electric fan power was measured. The corresponding energy consumption can be calculated based on the fan
drying time and power used in a unit of Wh. For the worst scenario
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Z. He et al. / Construction and Building Materials 213 (2019) 51–60
Fig. 10. Dimensional change in thickness.
120
Original strength
After 200 cycles by ASTM C666
After 20 cycles by CSA.A231
Flexural strength, %
100
80
RH,%
Air
drying h
Fan
drying h
Wind speed
m/s
Electric fan
power, W
Total
energy Wh
25 ± 5
50 ± 5
80 ± 5
17
0
0
1
18
18
0.5
0.5
1.7
0.28
0.28
3.04
0.28
5.04
54.7
Initial hydration temperature = 25 °C.
60
less energy. Carbonation curing will show significant economic
and environmental benefits.
40
20
5. Conclusions
0
Commercial products
Carbonation cured
Fig. 11. Effect of freeze-thaw cycles on flexural strength.
120
100
Flexural strength, %
Table 7
Energy consumption in initial hydration of one panel (305 610 12 mm).
0 Cycle
5 Cycles
10 Cycles
80
60
40
20
0
Commercial products
Carbonaon products
Fig. 12. Effect of wet-dry cycles on flexural strength.
(wet site with RH of 80%), the total energy required for precondition was 54.7 Wh/panel, or 24.8 kWh/m3. Compared with
198 kWh/m3 by autoclaving, carbonation curing consumes much
In this study, CO2 sequestration in cement-bonded cellulose
fiberboard has been investigated. The following conclusions are
drawn:
(1) The maximum possible CO2 uptake by ordinary Portland
cement is determined in a cement solution carbonation. It
is obtained in best combination of material and process
parameters and in an ideal condition. With 18 h initial
hydration, the maximum possible carbon dioxide uptake
can reach 23.2% by 2 h carbonation and 28.5% by 24 h
carbonation.
(2) Precondition by initial hydration is helpful to promote carbonation and can be achieved using oven drying or fan drying. Oven drying is fast but energy-intensive, while fan
drying is slow but consumes less energy. For full scale production, fan drying is more feasible.
(3) Cement-bonded cellulose fiberboards have exhibited excellent CO2 absorption behavior. The use of cellulose fibers
enhances the carbonate precipitation in cement matrix,
leading to a possible CO2 uptake of 24.4% based on cement
mass. This uptake value represents 85% reaction efficiency
if compared with the maximum possible in solution carbonation. It is achieved within 24 h process window.
(4) The key parameter for promoting carbonation degree is the
water content in fiberboards. It is found that the ideal range
of water content in fiberboard carbonation is 40–60% with
reference to the original water.