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Natural and accelerated CO2 binding kinetics in cement paste at different relative humidities
Cement and Concrete Research 49 (2013) 21–28
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Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp
Natural and accelerated CO2 binding kinetics in cement paste at different relative humidities Isabel Galan a,⁎, Carmen Andrade b, Marta Castellote b a b
University of Aberdeen, Department of Chemistry, Meston Walk, AB24 3UE, United Kingdom Eduardo Torroja Institute CSIC, Serrano Galvache 4, 28033 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 24 May 2012 Accepted 15 March 2013 Keywords: Thermal analysis (B) Carbonation (C) CO2 combination
a b s t r a c t Natural carbonation and accelerated carbonation are compared in terms of CO2 combination. Thermogravimetrical analysis, neutron diffraction and weight measurements were used to quantify the amounts of CO2 in samples exposed to different CO2 and relative humidity (RH) conditions. An exponential equation is proposed to describe temporal evolution: combination rates and maximum values are obtained both for natural and accelerated processes. The influence of RH in these parameters is analyzed. At 100% CO2 an increase of RH from 53 to 75% diminishes the combination rate considerably, but increases slightly the maximum CO2 binding. At 0.5% CO2 both the rate and the maximum are higher for intermediate RH, with a linear relation between both variables. Equivalences between the different processes have been calculated: the amount of CO2 combined in 1 h at 100% CO2 and 65% RH carbonation is equivalent to approximately 36 days of natural exposure outside and 54 days inside. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Natural carbonation processes are generally very slow due to the low content of CO2 in the atmosphere, around 0.04% volume. As a consequence, very dense concrete elements do not show any carbonation effect in years or even decades. The slowness of the process is the reason why many researchers have performed accelerated tests using higher percentages of CO2, trying to predict the long term behavior. In view of the results obtained, the correlation between accelerated and natural tests is not completely clear in all cases; the mechanisms seem to be different depending on the CO2 concentration. In 1958 Verbeck [1] pointed to the indirect influence that CO2 concentration can exert on the concrete internal humidity as a possible cause of the differences between carbonation at different CO2 concentrations. According to Verbeck, high CO2 concentrations can increase the internal humidity due to the water formation during reaction. At low CO2 concentrations, the internal humidity does not reach a value higher than the external humidity: the reaction rate is lower, and the water evolved inside can come to an equilibrium with that in the atmosphere. In general, it is accepted that at concentrations below 3–4% CO2 the process does not change the composition of the products formed during carbonation in air or natural environments. Much work has been carried out to give the ‘carbonation depth’ correspondence between natural and low-concentration accelerated processes. This carbonation depth refers to the depth where pH has been reduced to below 8–9, that is, the ‘transparent zone’ indicated by the phenolphthalein coloration. In 1987, Ho et al. [2] proved that the carbonation depth in specimens ⁎ Corresponding author. Tel.: +44 1224272916. E-mail address: [email protected] (I. Galan). 0008-8846/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2013.03.009
carbonated at 4% CO2 for one week was similar to the one measured in specimens carbonated in laboratory environment for one year. In 2003, Sanjuan et al. [3] concluded that performing tests for 7–15 days at 4–5% CO2 is an adequate procedure to calculate carbonation ‘rates’, as a very similar depth is measured after one year in natural carbonation. Regarding the differences between the microstructure generated in natural and accelerated processes, Groves et al. [4] observed carbonated C3S pastes with transmission electron microscopy and nuclear magnetic resonance, concluding that in air the C–S–H gel continues to polymerize to a high degree without formation of silica gel, while a similar level of carbonation in pure CO2 leads to a less polymerized C–S–H gel, together with substantial amounts of silica gel. Goñi et al. [5] compared CaCO3 polymorphs formed as well as porosity evolution in samples submitted to natural and 100% CO2 carbonation. In natural carbonation they detected calcite, vaterite and aragonite, while at 100% CO2 calcite was the only polymorph present. As for porosity, its decrease was much more pronounced in accelerated than in natural carbonation. Anstice et al. [6] concluded that the mineralogy and the pore structure of the solid phases varied significantly with the environmental CO2 concentration used for carbonation. The work from Castellote et al. [7] from 2009, more focused on the phases' reaction, confirms that low CO2 concentrations, below 3%, do not modify drastically the microstructure obtained in natural carbonation: a similar C–S–H gel with lower Ca/Si ratio is remained. However, at higher concentrations, 10 and 100%, the microstructure is completely different: the C–S–H gel totally disappears. The final concern of the studies mentioned above is to establish the correspondence between natural carbonation and accelerated carbonation in order to be able to predict the behavior of steel reinforced concrete, that is, to prevent corrosion caused by carbonation. However, not much attention has been paid to an important aspect of the carbonation
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I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
Table 1 Chemical composition and limestone content of the cement CEM I 42.5R (weight %). CaO
SiO2
Al2O3
SO3
Fe2O3
MgO
K2O
TiO2
P2O5
Cl−
Free CaO
CaCO3 filler
63.8
20.2
4.5
3.5
2.6
2.3
0.95
0.27
0.04
0.01
1.23
3.0
process, namely CO2 fixation by combination with the hydrated cement phases, and the variables involved. There are some recent studies [8–16] that have tried to calculate the scope of this phenomenon, but none of them has a deep experimental study of the fitting of temporal evolution data, that is, CO2 maximum values, reaction rates and the equivalences between natural carbonation and accelerated carbonation. There are few studies that consider the temporal evolution of the phases involved in the process. Parrott [17] followed the weight and the CaCO3 in samples exposed to natural carbonation. Goñi [5] followed the CaCO3 content in samples exposed to natural and 100% CO2 carbonation. Castellote [18] and Galan [19] followed the evolution of the main phases in 100% CO2 carbonation and constant RH. The aim of the present work is to study in more detail the correspondence between natural and accelerated carbonation evolution from the point of view of CO2 combination, and its relation to the RH. 2. Experimental Cement paste specimens were prepared with cement CEM I 42.5R (Table 1) and three water/cement (w/c) ratios, 0.45, 0.5 and 0.6. Two geometries were used: prismatic, 1 × 1 × 6 cm, and cylindrical, 1 × 5 cm. The specimens were subjected to three different carbonation processes: natural with 0.04% CO2 concentration, accelerated in 0.5% CO2 atmosphere and accelerated in 100% CO2 atmosphere. The natural carbonation took place inside and outside, both sheltered and not sheltered from the rain. The accelerated carbonation was performed in constant relative humidity (RH) chambers from 11% to 90%. Prior to accelerated carbonation, the specimens were stabilized in constant RH chambers for 50 days.
Fig. 2. Experimental set-up for in situ carbonation between neutron beam and detector.
To follow the combination of CO2 in the samples due to carbonation weight measurements, Thermogravimetrical Analysis (TGA) and Neutron Diffraction (ND) techniques were used. The initial weight, i.e. the weight after stabilization in constant RH and prior to carbonation also in constant RH atmospheres, was taken as reference for calculating the weight evolution. As for TGA data, the temperature range in which the decomposition of CaCO3 takes place was evaluated for each sample. In order to standardize, TGA results are expressed per gram of cement at 1000 °C. The specimens characterized with ND were prepared using deuterated water, D2O, in order to reduce the high background intensity produced by the hydrogen nuclei due to their strong incoherent scattering. After pre-treatment, the specimens were subjected to 100% CO2 fluxes for 8–12 h, also in the same constant humidity conditions as before. Measurements were performed at D1B instrument at the Institut Laue Langevin (ILL). ND spectra were collected every fifteen minutes throughout the experiments. The experimental set-up used for the in
Table 2 Carbonation processes description. Carbonation
% CO2
Environment/RH
Time
Technique
Natural
0.04
Outside sheltered 57% RH (27–97% RH)
4 years
Thermogravimetrical Analysis (TGA)
1 year
Weight measurements
8–12 h
Neutron Diffraction (ND)
Outside not sheltered 57% RH (27–97% RH) Inside 38% RH (24–68% RH) Accelerated
0.5
11% RH 23% RH 33% RH 53% RH 65% RH
Accelerated
100
33% RH 53% RH 65% RH 75% RH
Fig. 1. Experimental set-up for in situ carbonation measured with neutron diffraction.
90% RH
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
CO2 (% per cement g)
25
23
Table 3 Fitting parameters of function represented in Fig. 3: CO2 evolution in samples exposed to natural carbonation.
20
0.6 w/c
0.45 w/c
CO2,m − CO2,i (%) τ (days) CO2,m − CO2,i (%) τ (days)
15
Outside sheltered 28.2 Outside not sheltered 31.4 Inside 20.0
10
176.0 232.5 187.8
28.3 – 21.0
174.7 – 132.4
5
exposed inside. The data of CO2 per gram of cement can be fitted to the exponential function
0 0
200
400
600
800
1000
1200
1400
1600
t (days) Fig. 3. CO2 versus time in specimens exposed to natural carbonation during 4 years: exponential behavior.
situ carbonation experiments is described in [19] and it is shown in Figs. 1 and 2. About 3 cm3 of the solid sample were radiated with the neutrons. The carbonation experiments were measured with 1.28 Å neutrons for those at 33, 53 and 65% RH and with 2.53 Å neutrons for those at 75 and 90% RH. The experiments were performed in two steps: the wavelength used corresponds to the one set at the equipment by the time the two experimental parts were programmed. The evolution of portlandite and calcite during the carbonation processes was followed by means of the intensity–angle–time diffractograms collected in the five experiments. Table 2 is a summary of the processes' characteristics and techniques used. To complement results and discussion, ‘carbonation depth’ was measured by phenolphthalein coloration after CO2 exposure. A brief summary of the results is presented for each process. 3. Results 3.1. Natural carbonation 3.1.1. CO2 evolution Fig. 3 represents the general trend of the CO2 evolution during 4 years in the prismatic specimens exposed to natural carbonation. In particular, the data from Fig. 3 correspond to 0.6 w/c ratio specimens
CO2 ðt Þ ¼ CO2;m þ CO2;i −CO2;m ⋅ expð−t=τ Þ
ð1Þ
where CO2,m represents the maximum value reached, that is, the maximum CO2 combined in the sample including the corresponding to the limestone addition in the cement; and CO2;i represents the initial value, that is, the CO2 combined in the limestone present in the cement. The difference between CO2;m and CO2;i corresponds to the maximum CO2 combined in the sample due to carbonation. Finally, τ, the ‘time constant’, is the time needed to reach 63.2% CO2;m −CO2;i plus the initial value. The inverse of τ can be considered as the ‘combination rate’. The parameters obtained from the fittings are given in Table 3. This equation applies for the 0.6 w/c specimens, for the three environments considered (inside, outside sheltered from the rain, and outside not sheltered), and for those with 0.45 w/c exposed inside and outside sheltered from the rain. The trend of the 0.45 w/c specimen placed outside not sheltered is similar to the rest, but not enough data are available to calculate the fitting parameters. As can be seen when considering the values of maximum combination, the w/c ratio does not seem to play in this case a very important role compared to the influence of the environment: the amount of CO2 combined outside is much higher than inside. As for τ, it varies between 132 and 232 days in the five cases shown. The values of τ inside are higher for the 0.6 w/c specimens; outside sheltered from the rain there are almost no differences between both w/c ratios. After one year exposure, the ‘carbonation depth’ measured with phenolphthalein coloration in 0.45 w/c specimens was slightly bigger for the specimens sheltered (2.7 mm) than for the ones unsheltered (2.4 mm). The specimens placed inside presented the lowest depth
12 0.45 w/c
11 10
23% RH
9
33% RH
8
53% RH
Weight increase (%)
Weight increase (%)
12
11% RH
65% RH
7 6 5 4 3 2
0.5 w/c
11
11% RH
10
23% RH 33% RH
9
53% RH
8
65% RH
7 6 5 4 3 2
1
1
0 0
50
100
150
200
250
300
350
400
t (days)
0 0
50
100
150
200
250
300
350
400
t (days) Fig. 4. Weight increase evolution in 0.45 w/c specimens exposed to 0.5% CO2 atmospheres: at 11 and 23% RH maximum weight is reached; greater weight increase is achieved at 53% RH.
Fig. 5. Weight increase evolution in 0. 5 w/c specimens exposed to 0.5% CO2 atmospheres: specimens at 33 and 65% RH behave similarly.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
Table 4 Fitting parameters of functions represented in Figs. 4 and 5: CO2 evolution in samples exposed to 0.5% CO2 carbonation. 0.45 w/c
0.5 w/c
RH (%)
wim (%)
τ (days)
wim (%)
τ (days)
11 23 33 53 65
1.6 3.7 9.7 14.3 7.9
28.2 57.0 241.2 389.6 221.4
1.7 3.7 8.9 13.7 8.6
23.7 50.0 199.6 270.9 209.9
25
CO2 gain (%)
24
20 15 10 5 0 0
2
4
6
8
10
12
14
16
Weight gain (%) (2.1 mm). In the specimens with 0.6 w/c only the samples exposed outside not sheltered showed two different zones: uncolored and purple red. Samples outside sheltered were completely uncolored after the coloration; the surface of the ones inside was soft pink, indicating pH values between 8 and 9. After two years all samples presented uncolored surfaces, that is, pH values below 8. 3.2. Accelerated carbonation 0.5% CO2 3.2.1. Weight evolution All specimens lost weight during the humidity stabilization period, the losses being higher in the 0.5 than in the 0.45 w/c specimens, and increasing the losses as the RH diminished. Figs. 4 and 5 represent the weight increments of CEM I 42.5R cylindrical specimens exposed to carbonation during one year in 0.5% CO2 atmosphere and 5 different RH: 11, 23, 33, 53 and 65%. Fig. 4 corresponds to 0.45 and Fig. 5 to 0.5 w/c specimens, respectively. In all cases the weight evolution can be fitted to exponential functions like Eq. (1), substituting CO2 by weight. Here, as all values refer to the weight before starting carbonation, the initial value is zero. The fitting parameters obtained are given in Table 4. The specimens carbonated in 11 and 23% RH atmospheres reach their maximum CO2 values before the end of the year. The ones carbonated in 33, 53 and 65% RH, however, do not reach it before the exposure time is finished. Although the differences between both w/c ratios are very small, it should be noticed that the values of τ are in all cases higher in the 0.45 w/c specimens, that is, lower w/c implies lower rate. In both cases the highest value of τ corresponds to the specimens carbonated at 53% RH. These specimens are also the ones that reach higher maximum values. The total weight gain after 1 year carbonation as a function of the RH (Fig. 6) follows a parabolic behavior, with a maximum around 50% RH. Between 11 and 33% RH the values for both w/c ratios are very similar; however, at 53 and 65% the weight gains are greater for the specimens with the highest w/c. As can be seen in Fig. 7, these total weight gain data, expressed per gram of cement, are linearly related to the amounts of CO2 combined 12
Weight gain (%)
10 8 6 4
0.45 w/c 0.5 w/c
2 0 0
10
20
30
40
50
60
70
RH (%) Fig. 6. Weight gain after 1 year in 0.5% CO2 carbonation versus RH: parabolic behavior, maximum value reached near 50% RH.
Fig. 7. CO2 gain versus weight gain after 1 year 0.5% CO2 carbonation: linear relationship.
in the specimens after one year. The linear relationship is independent of the w/c ratio. Only specimens submitted to carbonation at 53 and 65% RH presented two-colored surfaces after the phenolphthalein test. The depth measured was 3.1 ± 0.3 and 1.9 ± 0.4 mm in the 0.45 w/c samples carbonated at 53 and 65%, respectively. At 0.6 w/c, the values were 3.6 ± 0.5 and 2.6 ± 0.4 mm. The surface of the rest of the specimens was totally red purple indicating no pH lowering below 9.
3.3. Accelerated carbonation 100% CO2 3.3.1. CO2 evolution The in situ ND measurements of the carbonation process in CEM I 42.5R and 0.5 w/c specimens allowed to follow the evolution of the calcium carbonate and portlandite in different RH atmospheres. Figs. 8 and 9 present the spectra obtained at 65% (1.28 Å) and 90% RH (2.53 Å), respectively. As explained in [19], at 33% RH almost no changes were observed. At 90% RH, while the portlandite peak remained constant, a slightly increase in the calcite peaks was observed (Fig. 9). At intermediate RH, 53, 65 and 75%, the portlandite decrease and the calcite increase could be clearly observed. Fitting the main diffraction peaks of calcite to Pseudo-Voigts functions, their corresponding areas throughout the carbonation experiments were calculated. At these three intermediate RH both the decrease of portlandite with time and the increase of calcite follow an exponential function similar to those obtained in the previous sections for natural and 0.5% CO2 carbonation. Using the TG data obtained before and after carbonation, the functions fitted for the peak areas can be converted into CO2 combination and portlandite consumption evolution with the form of Eq. (1). Fig. 10 represents the evolution of CO2 and portlandite per cement gram for these three RH. The values of the corresponding parameters are in Table 5. The maximum combination of CO2, that is, CO2,m − CO2,i, varies between 23 and 25%, the only one that reached its maximum at the end of the test being the specimen carbonated in 53% RH. These values are considerably higher than the corresponding amplitude of portlandite, that is, the difference between the initial value and minimum. This implies that, at a certain time, more calcite is being produced than portlandite is disappearing. The values of τ increase with the RH: the specimen carbonated at 75% RH shows the slowest combination rate. In the three cases the values are higher for calcite than for portlandite, which means that calcite needs more time to reach its maximum than portlandite to reach its minimum. In order to relate weight gains with CO2 gains, similar specimens to the ones used for the 0.5% CO2 carbonation were used for 100% CO2 carbonation (CEM I 42.5R, 0.45 and 0.5 w/c), also in the same RH atmospheres, from 11 to 65%. The correlation between the values is shown in Fig. 11. In this case, a linear trend can also be seen, but the values are not as evenly distributed as in the 0.5% CO2 carbonation. Here, the values close to the x-axis correspond to 11, 23 and 33% RH.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
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Fig. 8. ND spectra for 65% RH carbonation: portlandite decrease, calcite increase.
After the phenolphthalein test none of the samples presented uncolored zones. Specimens carbonated at 33 and 90% RH showed a complete red purple surface. In the ones at 53, 65 and 75% RH two zones could be distinguished: one red purple and one soft pink, with the thickness of the soft pink zone decreasing as the RH increases. 4. Discussion This paper aims to deepen on the knowledge of the combination process of CO2 with cement paste. The evolution of combination with time and the influence of the two main variables, RH and environmental concentration of CO2, are discussed in the following paragraphs. 4.1. Influence of the RH in the carbonation processes Weight increases and CO2 gain due to carbonation are conditioned by the environmental RH. As previously found by other researchers, at very low RH there is not enough water in the pores to dissolve the phases involved and to allow their reaction. At intermediate RH, optimal conditions for carbonation are reached: enough water for reaction and also enough space for CO2 diffusion. From a certain value of RH, the pores start to be saturated with water, making diffusion difficult. This fact can be confirmed by the results obtained both for 0.5 (Fig. 6) and 100% CO2 carbonation. Regarding the influence of RH in the temporal evolution, the relation between τ and RH is first considered. Fig. 12 represents the values for the three types of carbonation, with the y-axis in minutes corresponding to 100% CO2 carbonation and the one in days to natural and 0.5% CO2 processes. For the natural values, average RH has been considered for the inner and outer environments, 38 and 57%, respectively.
In 0.5% CO2 carbonation, τ reaches a maximum at around 50% RH, following a similar behavior as the weight increase versus the RH. At low RH there is almost no difference between the two w/c ratios. Between 53 and 75% RH, τ values are higher for the 0.45 w/c than for 0.5 w/c specimens. This means specimens with greater w/c ratio need less time to reach their maximum CO2 combination, which can be attributed to the greater porosity that facilitates the access, dissolution and reaction. In 100% CO2 carbonation between 53 and 75% RH, τ increases with RH. At 53% RH the rate of combination is between 5 and 6 times greater than at 65 and 75%. At very low and very high RH no fitting to exponential functions could be obtained from the data collected, because the changes in the patterns were almost unnoticeable. However, it is expected that the values of τ in both extremes of the RH scale will be lower as in the case of 0.5% CO2 carbonation. Apart from the different CO2 concentrations, the natural and accelerated carbonation processes considered here differ on the variability of environmental RH. In the case of natural carbonation, both inside and outside, there are considerable variations in RH along the year, with the lower values in the inside. As can be seen in Fig. 12, τ reaches relatively similar values in both cases, between 150 and 250 days, and is slightly higher in the specimens with the highest w/c ratio. The influence of environmental humidity in τ is not as pronounced as in the cases of accelerated carbonation at constant RH. Fig. 13 shows the relation between the maximum CO2 that could be combined in the samples and RH for the three processes. To calculate the values of the 0.5% CO2 carbonation, the linear relation found in Fig. 7 between weight gain and CO2 gain has been used. As in the case of τ, for the natural values, average RH has been considered for the inner and outer environments. The maximum CO2 combined in specimens carbonated in 100% CO2 atmospheres is very similar, around 24%, for the three cases considered; it only increases slightly with RH. According to these
Fig. 9. ND spectra for 90% RH carbonation: slight increase of calcite, no changes in portlandite.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
25
30% CO2 53% RH
25%
20
CO2 gain (%)
Weight (% per cement gram)
26
CO2 65% RH
20%
CO2 75% RH
15% 10% PORTLANDITE 65% RH
0
PORTLANDITE 53% RH
0% 0
100
10 5
PORTLANDITE 75% RH
5%
15
0
200
300
400
500
2
4
6
600
8
10
12
14
16
Weight gain (%)
t (min) Fig. 11. CO2 gain versus weight gain after 24 h 100% CO2 carbonation. Fig. 10. CO2 versus time in specimens exposed to 100% CO2 carbonation during 10 h, exponential fitting.
Table 5 Fitting parameters of functions represented in Fig. 10: CO2 and portlandite (CH) evolution in samples exposed to 100% CO2 carbonation. RH (%)
CO2;m −CO2;i ð%Þ
τ (min)
CH2,i − CH2,min (%)
τ (min)
53 65 75
22.7 24.2 25.1
74.4 356.9 448.0
17.2 15.9 14.7
45.4 151.1 354.6
leading to a lower maximum, as it seems to happen in the case of 33 and 53% RH carbonation. Note that the relation in Fig. 7, used to calculate the values in Fig. 13, is independent of the w/c ratio, which explains the similarity in the amount of bound CO2 for both w/c. These graphs (Figs. 12–13) indicate that the more CO2 a specimen can gain the longer time it will need to reach the maximum. Fig. 14 shows the linear relation between τ and the maximum CO2 combined for the RH considered, between 11 and 65%.
4.2. Similarities and differences between the three carbonation processes. Equivalences The evolution of the amounts of CO2 combined in the course of time in paste specimens submitted to 100% CO2 follows the same exponential behavior as in natural carbonation (see Eq. (1)). In the case of 100% CO2 carbonation, the equation applies for atmospheres with RH between 53 and 75% RH. The maximum values reached in accelerated carbonation, between 23 and 25%, are slightly higher than the ones reached in natural carbonation inside, between 20 and 23%, and slightly lower than the ones outside, between 28 and 31%. This means that, on average, by means of 100% CO2 carbonation, very similar amounts of CO2 can be combined as in natural carbonation. Regarding the time constant, in natural carbonation the values reached are between 500 and 4500 times higher than the ones in 100% CO2 carbonation. With the parameters obtained in both types of carbonation, the time equivalences, in terms of CO2 combination, can be calculated (Table 6).
500
500 0.5% CO2 - 0.45 w/c
τ (days)
450
450
0.5% CO2 - 0.5 w/c
400
100% CO2 - 0.5 w/c
400
350
natural - 0.6 w/c
350
natural - 0.45 w/c
300
300
250
250
ONS
200
I
150
200
OS
τ (min)
relations (Figs. 12–13), the specimens carbonated in 100% CO2 at the lowest RH, namely 53%, will need less time to reach the maximum, but if the specimens are left enough time at the corresponding RH, between 53 and 75%, all of them will reach almost the same value of combined CO2. The completion degree of the carbonation process can be calculated comparing the maximum values (Fig. 13) with those reached at the end of the experiments (Fig. 10). In the 53% RH sample, calcite has almost reached its maximum and portlandite its minimum. In the 65% RH specimen, the minimum value of portlandite is nearly attained, while calcite has only accomplished 80% of the reaction. The specimen carbonated at 75% RH has the lowest completion degree, 75% of calcite and 90% of portlandite. In the three RH considered it is evidenced that not only portlandite is being consumed, but that other phases may also carbonate. To evaluate the influence of the RH in the carbonation of other phases, the ratio between the times for attaining maximum calcite and minimum portlandite has been calculated: 1.6, 2.5 and 1.2 for 53, 65 and 75% RH, respectively. According to these values, it might be inferred that the carbonation of other phases is favored in 65% RH atmospheres. In the natural carbonated samples, the maximum values of CO2 reached are considerably different with regard to the environment. Inside, with a lower average RH, the maximum amount of CO2 that can be combined, around 20%, is much lower than outside, between 28 and 31%. The higher combination found outside not sheltered compared to sheltered environment should be explained together with the fact that the carbonation depth was greater in specimens sheltered than in those not sheltered. When exposed to rain, although there are periods during which CO2 diffusion is not allowed in the pores due to rain water block, there are also periods during wetting and drying when optimal RH conditions for combination may be reached. This means that in outside not sheltered specimens CO2 penetrates less distance but combines more CO2; outside sheltered, CO2 penetrates more, as there are no water blocks due to rain, but the environmental RH may not always allow the CO2 combination. In the case of 0.5% CO2 carbonation, the maximum CO2 trend with regard to RH is very similar to that of τ, showing a maximum at around 50% RH. Nevertheless, in this case the difference between both w/c ratios is almost nonexistent, that is, higher w/c leads to higher rate but it does not imply greater combination. The greater pore volume facilitates diffusion; however, the ‘carbonatable’ material may be smaller,
150
I
100
100
50
50
0
0 10
20
30
40
50
60
70
80
RH (%) Fig. 12. Time constant τ versus RH in the three carbonation processes. Left τ-axis applies for 0.5% CO2 and natural carbonation; right τ-axis applies for 100% CO2 carbonation. In natural carbonation the points labeled with OS, ONS and I correspond to the environments outside sheltered, outside not sheltered and inside, respectively. Carbonation times were 4 years, 1 year and 8–12 h for natural, 0.5 and 100% CO2 carbonation processes, respectively.
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Table 6 Time equivalence between natural and 100% CO2 carbonation in terms of CO2 uptake.
0.5% CO2 - 0.45 w/c 0.5% CO2 - 0.5 w/c
35
100% CO2 - 0.5 w/c
Max CO2 (%)
30
natural - 0.6 w/c natural - 0.45 w/c
ONS
Equivalent times in natural carbonation (days)
OS
Inside
Outside sheltered
Outside not sheltered
183 44 34
121 35 27
136 40 30
25
1 h 100% CO2 53% RH 1 h 100% CO2 65% RH 1 h 100% CO2 75% RH
I I
20 15 10 5 0 0
10
20
30
40
50
60
70
80
90
RH (%) Fig. 13. Maximum CO2 gain versus RH in the three carbonation processes. In natural carbonation the points labeled with OS, ONS and I correspond to outside sheltered, outside not sheltered and inside, respectively. Carbonation times were 4 years, 1 year and 8–12 h for natural, 0.5 and 100% CO2 carbonation processes, respectively.
In the 0.5% CO2 carbonation, the time evolution of weight follows as well an exponential behavior in atmospheres with RH between 11 and 65%, and there is a linear relation between weight gain and CO2 gain, with a slope ‘weight gain - CO2 gain’ of 0.53. In [17], Parrott relates weight and CO2 gains in samples carbonated at constant RH and 0.045% CO2, finding a linear relationship with a slope of 0.57. According to Parrott, the kinetic aspects of carbonation can be evaluated from the time evolution of weight gain, fitting the curves to functions proportional to a power of time in which the exponent would depend on RH. As the average value is 0.5, Parrott considers that this function is in agreement with the evolution of the ‘carbonation depth’ proportional to the square root of time. Goñi et al. [5] also propose the square root of time to describe temporal evolution of CaCO3 formation both for natural carbonation and accelerated carbonation. Although the ‘carbonation depth’ and the weight (or CO2 or CaCO3) gains measured at a certain time behave similarly with respect to RH, with parabolic shape reaching the maximum at intermediate RH, their time evolution is different. In the first case, the square root of time law is used to describe the advance of the pH change front in the cementitious material; this solution considers that the process is diffusion controlled, with fast reaction kinetics, and assumes depletion does not occur. In the second case, what is described is the temporal evolution of the CO2 combined in a certain volume of material. For this, an equation with an upper limit should be used: the maximum amount of CO2 combined is controlled, among others, by CaO in cement present in the volume considered. Introducing size as a parameter modifies the boundary conditions and makes the square root of time no longer valid.
450
τ (days)
27
400
0.45 w/c
350
0.5 w/c
300 250 200 150 100 50 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Max CO2 (%) Fig. 14. Time constant τ versus maximum CO2 combined in 0.5% CO2 carbonation: linear relationship for each w/c ratio.
Comparing the two accelerated processes, it can be seen that, although the range of values for weight gain in both types of carbonation is very similar, between 2 and 11%, there are important differences in the behavior as a function of RH. One of these differences is the minimum humidity needed to start the process: at 100% CO2, RH greater than 33% is needed to obtain significant weight increases. This can be explained considering that the slower the carbonation, the more time is available for water present to take part in the reaction that, in turn, produces water and allows the continuity of the process. In very rapid processes at low RH, no matter how much CO2 is in the atmosphere, there is no time for using the scarce available water. Regarding the amounts of CO2 combined in 0.04% CO2 atmospheres (natural carbonation) after four years, they are greater than those in 0.5% CO2 after one year. Only in specimens carbonated at 53 and 65% RH the values reached are similar to the quantities measured in specimens placed inside. In addition, the specimens carbonated in air atmospheres, inside and outside, after four years have reached their maximum values, while in 0.5% CO2 carbonation, only the ones in 11 and 23% RH atmospheres have attained their top value. The results obtained have demonstrated that the evolution of CO2 combination in atmospheres with different CO2 concentrations, 0.04, 0.5 and 100%, follows a similar trend, which is not contradictory to the fact that there might be a different order in the reactions of the CO2 with the phases in the cement paste for each CO2 concentration. Besides this, the results shown prove that carbonating at constant or variable RH does not influence the shape of the fitting function. The different concentrations, time and RH do influence significantly in the values reached of weight and CO2 gain, as well as in the time constant or combination rate.
5. Conclusions From the results presented in this paper, obtained using complementary techniques, namely TGA, weight measurements and ND, the following conclusions can be drawn: 1. The combination of CO2 can be fitted to an exponential function CO2(t) = CO2,m + (CO2,i − CO2,m) ⋅ exp(− t/τ) both in natural carbonation and accelerated carbonation. 2. In natural carbonation, the greater combination and smaller carbonation depth in specimens not sheltered from the rain is attributed to the effect of wetting and drying. Although CO2 diffusion is prevented while the pores are full with rain water, in the accessible zones the more favorable conditions promote the reaction. 3. In slow accelerated carbonation, 0.5% CO2, higher w/c leads to higher combination rate but not necessarily to higher CO2 combined: the greater porosity facilitates diffusion, but the ‘carbonatable’ material may decrease with the w/c. 4. The maximum values reached in 100% CO2 accelerated carbonation are in the same range as the ones in natural carbonation, around 23–25%. In present experiments, one year of inside exposure is equivalent, in terms of CO2 combined, to 2, 7 and 9 h at 100% CO2 and 53, 65 and 75% RH, respectively. 5. Future experimental work should involve more intermediate environmental CO2 concentrations in order to find the equations that relate the combination rate with the CO2 concentration for every RH.
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References [1] G.J. Verbeck, Carbonation of hydrated Portland cement, ASTM Spec. Tech. Publ. 205 (1958) 17–36. [2] D.W.S. Ho, R.K. Lewis, Carbonation of concrete and its prediction, Cem. Concr. Res. 17 (1987) 489–504. [3] M.A. Sanjuan, C. Andrade, M. Cheyrezy, Concrete carbonation tests in natural and accelerated conditions, Adv. Cem. Res. 15 (2003) 171–180. [4] G.W. Groves, A. Brough, I.G. Richardson, C.M. Dobson, Progressive changes in the structure of hardened C3S cement pastes due to carbonation, J. Am. Ceram. Soc. 74 (1991) 2891–2896. [5] S. Goñi, M.T. Gaztañaga, A. Guerrero, Role of cement type on carbonation attack, J. Mater. Res. 17 (2002) 1834–1842. [6] D.J. Anstice, C.L. Page, M.M. Page, The pore solution phase of carbonated cement pastes, Cem. Concr. Res. 35 (2005) 377–383. [7] M. Castellote, L. Fernandez, C. Andrade, C. Alonso, Chemical changes and phase analysis of OPC pastes carbonated at different CO2 concentrations, Mater. Struct. 42 (2009) 515–525. [8] J. Gajda, Absorption of Atmospheric Carbon Dioxide by Portland Cement Concrete, PCA, R&D, Chicago, 2001. (Serial no. 2255a). [9] K. Pommer, C. Pade, Guidelines-Uptake of Carbon Dioxide in the Life Cycle Inventory of Concrete, Danish Technological Institute, Taastrup, Denmark87-7756-757-9, 2005.
[10] B. Lagerblad, Carbon Dioxide Uptake During Concrete Life Cycle—State of the Art, Swedish Cement and Concrete Research Institute, CBI, Stockholm91-976070-0-2, 2005. [11] C.J. Engelsen, J. Mehus, C. Pade, D.H. Saether, Carbon Dioxide Uptake in Demolished and Crushed Concrete, Norwegian Building Research Institute, Oslo82-536-0900-0, 2005. [12] K. Kjellsen, M. Guimaraes, A. Nilsson, The CO2 Balance of Concrete in a Life Cycle Perspective, Danish Technological Institute, DTI, Taastrup, Denmark87-7756-758-7, 2005. [13] G. Jonsson, Information on the Use of Concrete in Denmark, Sweden, Norway and Iceland, Icelandic Building Research Institute, Reykjavk9979-9174-7-4, 2005. [14] C. Pade, M. Guimaraes, The CO2 uptake of concrete in a 100 year perspective, Cem. Concr. Res. 37 (2007) 1348–1356. [15] C.A. Clear, T. De Saulles, BCA Recarbonation Scoping Study, British Cement Association, Camberly, UK, 2007. [16] I. Galan, C. Andrade, P. Mora, M.A. Sanjuan, Sequestration of CO2 by concrete carbonation, Environ. Sci. Technol. 44 (2010) 3181–3186. [17] L.J. Parrott, Carbonation, moisture and empty pores, Adv. Cem. Res. 4 (1991) 111–118. [18] M. Castellote, C. Andrade, X. Turrillas, J. Campo, G.J. Cuello, Accelerated carbonation of cement pastes in situ monitored by neutron diffraction, Cem. Concr. Res. 38 (2008) 1365–1373. [19] I. Galan, C. Andrade, M. Castellote, N. Rebolledo, J. Sanchez, L. Toro, I. Puente, J. Campo, O. Fabelo, Neutron diffraction for studying the influence of the relative humidity on the carbonation process of cement pastes, J. Phys. Conf. Ser. 325 (2011) 012015.
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Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp
Natural and accelerated CO2 binding kinetics in cement paste at different relative humidities Isabel Galan a,⁎, Carmen Andrade b, Marta Castellote b a b
University of Aberdeen, Department of Chemistry, Meston Walk, AB24 3UE, United Kingdom Eduardo Torroja Institute CSIC, Serrano Galvache 4, 28033 Madrid, Spain
a r t i c l e
i n f o
Article history: Received 24 May 2012 Accepted 15 March 2013 Keywords: Thermal analysis (B) Carbonation (C) CO2 combination
a b s t r a c t Natural carbonation and accelerated carbonation are compared in terms of CO2 combination. Thermogravimetrical analysis, neutron diffraction and weight measurements were used to quantify the amounts of CO2 in samples exposed to different CO2 and relative humidity (RH) conditions. An exponential equation is proposed to describe temporal evolution: combination rates and maximum values are obtained both for natural and accelerated processes. The influence of RH in these parameters is analyzed. At 100% CO2 an increase of RH from 53 to 75% diminishes the combination rate considerably, but increases slightly the maximum CO2 binding. At 0.5% CO2 both the rate and the maximum are higher for intermediate RH, with a linear relation between both variables. Equivalences between the different processes have been calculated: the amount of CO2 combined in 1 h at 100% CO2 and 65% RH carbonation is equivalent to approximately 36 days of natural exposure outside and 54 days inside. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Natural carbonation processes are generally very slow due to the low content of CO2 in the atmosphere, around 0.04% volume. As a consequence, very dense concrete elements do not show any carbonation effect in years or even decades. The slowness of the process is the reason why many researchers have performed accelerated tests using higher percentages of CO2, trying to predict the long term behavior. In view of the results obtained, the correlation between accelerated and natural tests is not completely clear in all cases; the mechanisms seem to be different depending on the CO2 concentration. In 1958 Verbeck [1] pointed to the indirect influence that CO2 concentration can exert on the concrete internal humidity as a possible cause of the differences between carbonation at different CO2 concentrations. According to Verbeck, high CO2 concentrations can increase the internal humidity due to the water formation during reaction. At low CO2 concentrations, the internal humidity does not reach a value higher than the external humidity: the reaction rate is lower, and the water evolved inside can come to an equilibrium with that in the atmosphere. In general, it is accepted that at concentrations below 3–4% CO2 the process does not change the composition of the products formed during carbonation in air or natural environments. Much work has been carried out to give the ‘carbonation depth’ correspondence between natural and low-concentration accelerated processes. This carbonation depth refers to the depth where pH has been reduced to below 8–9, that is, the ‘transparent zone’ indicated by the phenolphthalein coloration. In 1987, Ho et al. [2] proved that the carbonation depth in specimens ⁎ Corresponding author. Tel.: +44 1224272916. E-mail address: [email protected] (I. Galan). 0008-8846/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2013.03.009
carbonated at 4% CO2 for one week was similar to the one measured in specimens carbonated in laboratory environment for one year. In 2003, Sanjuan et al. [3] concluded that performing tests for 7–15 days at 4–5% CO2 is an adequate procedure to calculate carbonation ‘rates’, as a very similar depth is measured after one year in natural carbonation. Regarding the differences between the microstructure generated in natural and accelerated processes, Groves et al. [4] observed carbonated C3S pastes with transmission electron microscopy and nuclear magnetic resonance, concluding that in air the C–S–H gel continues to polymerize to a high degree without formation of silica gel, while a similar level of carbonation in pure CO2 leads to a less polymerized C–S–H gel, together with substantial amounts of silica gel. Goñi et al. [5] compared CaCO3 polymorphs formed as well as porosity evolution in samples submitted to natural and 100% CO2 carbonation. In natural carbonation they detected calcite, vaterite and aragonite, while at 100% CO2 calcite was the only polymorph present. As for porosity, its decrease was much more pronounced in accelerated than in natural carbonation. Anstice et al. [6] concluded that the mineralogy and the pore structure of the solid phases varied significantly with the environmental CO2 concentration used for carbonation. The work from Castellote et al. [7] from 2009, more focused on the phases' reaction, confirms that low CO2 concentrations, below 3%, do not modify drastically the microstructure obtained in natural carbonation: a similar C–S–H gel with lower Ca/Si ratio is remained. However, at higher concentrations, 10 and 100%, the microstructure is completely different: the C–S–H gel totally disappears. The final concern of the studies mentioned above is to establish the correspondence between natural carbonation and accelerated carbonation in order to be able to predict the behavior of steel reinforced concrete, that is, to prevent corrosion caused by carbonation. However, not much attention has been paid to an important aspect of the carbonation
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I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
Table 1 Chemical composition and limestone content of the cement CEM I 42.5R (weight %). CaO
SiO2
Al2O3
SO3
Fe2O3
MgO
K2O
TiO2
P2O5
Cl−
Free CaO
CaCO3 filler
63.8
20.2
4.5
3.5
2.6
2.3
0.95
0.27
0.04
0.01
1.23
3.0
process, namely CO2 fixation by combination with the hydrated cement phases, and the variables involved. There are some recent studies [8–16] that have tried to calculate the scope of this phenomenon, but none of them has a deep experimental study of the fitting of temporal evolution data, that is, CO2 maximum values, reaction rates and the equivalences between natural carbonation and accelerated carbonation. There are few studies that consider the temporal evolution of the phases involved in the process. Parrott [17] followed the weight and the CaCO3 in samples exposed to natural carbonation. Goñi [5] followed the CaCO3 content in samples exposed to natural and 100% CO2 carbonation. Castellote [18] and Galan [19] followed the evolution of the main phases in 100% CO2 carbonation and constant RH. The aim of the present work is to study in more detail the correspondence between natural and accelerated carbonation evolution from the point of view of CO2 combination, and its relation to the RH. 2. Experimental Cement paste specimens were prepared with cement CEM I 42.5R (Table 1) and three water/cement (w/c) ratios, 0.45, 0.5 and 0.6. Two geometries were used: prismatic, 1 × 1 × 6 cm, and cylindrical, 1 × 5 cm. The specimens were subjected to three different carbonation processes: natural with 0.04% CO2 concentration, accelerated in 0.5% CO2 atmosphere and accelerated in 100% CO2 atmosphere. The natural carbonation took place inside and outside, both sheltered and not sheltered from the rain. The accelerated carbonation was performed in constant relative humidity (RH) chambers from 11% to 90%. Prior to accelerated carbonation, the specimens were stabilized in constant RH chambers for 50 days.
Fig. 2. Experimental set-up for in situ carbonation between neutron beam and detector.
To follow the combination of CO2 in the samples due to carbonation weight measurements, Thermogravimetrical Analysis (TGA) and Neutron Diffraction (ND) techniques were used. The initial weight, i.e. the weight after stabilization in constant RH and prior to carbonation also in constant RH atmospheres, was taken as reference for calculating the weight evolution. As for TGA data, the temperature range in which the decomposition of CaCO3 takes place was evaluated for each sample. In order to standardize, TGA results are expressed per gram of cement at 1000 °C. The specimens characterized with ND were prepared using deuterated water, D2O, in order to reduce the high background intensity produced by the hydrogen nuclei due to their strong incoherent scattering. After pre-treatment, the specimens were subjected to 100% CO2 fluxes for 8–12 h, also in the same constant humidity conditions as before. Measurements were performed at D1B instrument at the Institut Laue Langevin (ILL). ND spectra were collected every fifteen minutes throughout the experiments. The experimental set-up used for the in
Table 2 Carbonation processes description. Carbonation
% CO2
Environment/RH
Time
Technique
Natural
0.04
Outside sheltered 57% RH (27–97% RH)
4 years
Thermogravimetrical Analysis (TGA)
1 year
Weight measurements
8–12 h
Neutron Diffraction (ND)
Outside not sheltered 57% RH (27–97% RH) Inside 38% RH (24–68% RH) Accelerated
0.5
11% RH 23% RH 33% RH 53% RH 65% RH
Accelerated
100
33% RH 53% RH 65% RH 75% RH
Fig. 1. Experimental set-up for in situ carbonation measured with neutron diffraction.
90% RH
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
CO2 (% per cement g)
25
23
Table 3 Fitting parameters of function represented in Fig. 3: CO2 evolution in samples exposed to natural carbonation.
20
0.6 w/c
0.45 w/c
CO2,m − CO2,i (%) τ (days) CO2,m − CO2,i (%) τ (days)
15
Outside sheltered 28.2 Outside not sheltered 31.4 Inside 20.0
10
176.0 232.5 187.8
28.3 – 21.0
174.7 – 132.4
5
exposed inside. The data of CO2 per gram of cement can be fitted to the exponential function
0 0
200
400
600
800
1000
1200
1400
1600
t (days) Fig. 3. CO2 versus time in specimens exposed to natural carbonation during 4 years: exponential behavior.
situ carbonation experiments is described in [19] and it is shown in Figs. 1 and 2. About 3 cm3 of the solid sample were radiated with the neutrons. The carbonation experiments were measured with 1.28 Å neutrons for those at 33, 53 and 65% RH and with 2.53 Å neutrons for those at 75 and 90% RH. The experiments were performed in two steps: the wavelength used corresponds to the one set at the equipment by the time the two experimental parts were programmed. The evolution of portlandite and calcite during the carbonation processes was followed by means of the intensity–angle–time diffractograms collected in the five experiments. Table 2 is a summary of the processes' characteristics and techniques used. To complement results and discussion, ‘carbonation depth’ was measured by phenolphthalein coloration after CO2 exposure. A brief summary of the results is presented for each process. 3. Results 3.1. Natural carbonation 3.1.1. CO2 evolution Fig. 3 represents the general trend of the CO2 evolution during 4 years in the prismatic specimens exposed to natural carbonation. In particular, the data from Fig. 3 correspond to 0.6 w/c ratio specimens
CO2 ðt Þ ¼ CO2;m þ CO2;i −CO2;m ⋅ expð−t=τ Þ
ð1Þ
where CO2,m represents the maximum value reached, that is, the maximum CO2 combined in the sample including the corresponding to the limestone addition in the cement; and CO2;i represents the initial value, that is, the CO2 combined in the limestone present in the cement. The difference between CO2;m and CO2;i corresponds to the maximum CO2 combined in the sample due to carbonation. Finally, τ, the ‘time constant’, is the time needed to reach 63.2% CO2;m −CO2;i plus the initial value. The inverse of τ can be considered as the ‘combination rate’. The parameters obtained from the fittings are given in Table 3. This equation applies for the 0.6 w/c specimens, for the three environments considered (inside, outside sheltered from the rain, and outside not sheltered), and for those with 0.45 w/c exposed inside and outside sheltered from the rain. The trend of the 0.45 w/c specimen placed outside not sheltered is similar to the rest, but not enough data are available to calculate the fitting parameters. As can be seen when considering the values of maximum combination, the w/c ratio does not seem to play in this case a very important role compared to the influence of the environment: the amount of CO2 combined outside is much higher than inside. As for τ, it varies between 132 and 232 days in the five cases shown. The values of τ inside are higher for the 0.6 w/c specimens; outside sheltered from the rain there are almost no differences between both w/c ratios. After one year exposure, the ‘carbonation depth’ measured with phenolphthalein coloration in 0.45 w/c specimens was slightly bigger for the specimens sheltered (2.7 mm) than for the ones unsheltered (2.4 mm). The specimens placed inside presented the lowest depth
12 0.45 w/c
11 10
23% RH
9
33% RH
8
53% RH
Weight increase (%)
Weight increase (%)
12
11% RH
65% RH
7 6 5 4 3 2
0.5 w/c
11
11% RH
10
23% RH 33% RH
9
53% RH
8
65% RH
7 6 5 4 3 2
1
1
0 0
50
100
150
200
250
300
350
400
t (days)
0 0
50
100
150
200
250
300
350
400
t (days) Fig. 4. Weight increase evolution in 0.45 w/c specimens exposed to 0.5% CO2 atmospheres: at 11 and 23% RH maximum weight is reached; greater weight increase is achieved at 53% RH.
Fig. 5. Weight increase evolution in 0. 5 w/c specimens exposed to 0.5% CO2 atmospheres: specimens at 33 and 65% RH behave similarly.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
Table 4 Fitting parameters of functions represented in Figs. 4 and 5: CO2 evolution in samples exposed to 0.5% CO2 carbonation. 0.45 w/c
0.5 w/c
RH (%)
wim (%)
τ (days)
wim (%)
τ (days)
11 23 33 53 65
1.6 3.7 9.7 14.3 7.9
28.2 57.0 241.2 389.6 221.4
1.7 3.7 8.9 13.7 8.6
23.7 50.0 199.6 270.9 209.9
25
CO2 gain (%)
24
20 15 10 5 0 0
2
4
6
8
10
12
14
16
Weight gain (%) (2.1 mm). In the specimens with 0.6 w/c only the samples exposed outside not sheltered showed two different zones: uncolored and purple red. Samples outside sheltered were completely uncolored after the coloration; the surface of the ones inside was soft pink, indicating pH values between 8 and 9. After two years all samples presented uncolored surfaces, that is, pH values below 8. 3.2. Accelerated carbonation 0.5% CO2 3.2.1. Weight evolution All specimens lost weight during the humidity stabilization period, the losses being higher in the 0.5 than in the 0.45 w/c specimens, and increasing the losses as the RH diminished. Figs. 4 and 5 represent the weight increments of CEM I 42.5R cylindrical specimens exposed to carbonation during one year in 0.5% CO2 atmosphere and 5 different RH: 11, 23, 33, 53 and 65%. Fig. 4 corresponds to 0.45 and Fig. 5 to 0.5 w/c specimens, respectively. In all cases the weight evolution can be fitted to exponential functions like Eq. (1), substituting CO2 by weight. Here, as all values refer to the weight before starting carbonation, the initial value is zero. The fitting parameters obtained are given in Table 4. The specimens carbonated in 11 and 23% RH atmospheres reach their maximum CO2 values before the end of the year. The ones carbonated in 33, 53 and 65% RH, however, do not reach it before the exposure time is finished. Although the differences between both w/c ratios are very small, it should be noticed that the values of τ are in all cases higher in the 0.45 w/c specimens, that is, lower w/c implies lower rate. In both cases the highest value of τ corresponds to the specimens carbonated at 53% RH. These specimens are also the ones that reach higher maximum values. The total weight gain after 1 year carbonation as a function of the RH (Fig. 6) follows a parabolic behavior, with a maximum around 50% RH. Between 11 and 33% RH the values for both w/c ratios are very similar; however, at 53 and 65% the weight gains are greater for the specimens with the highest w/c. As can be seen in Fig. 7, these total weight gain data, expressed per gram of cement, are linearly related to the amounts of CO2 combined 12
Weight gain (%)
10 8 6 4
0.45 w/c 0.5 w/c
2 0 0
10
20
30
40
50
60
70
RH (%) Fig. 6. Weight gain after 1 year in 0.5% CO2 carbonation versus RH: parabolic behavior, maximum value reached near 50% RH.
Fig. 7. CO2 gain versus weight gain after 1 year 0.5% CO2 carbonation: linear relationship.
in the specimens after one year. The linear relationship is independent of the w/c ratio. Only specimens submitted to carbonation at 53 and 65% RH presented two-colored surfaces after the phenolphthalein test. The depth measured was 3.1 ± 0.3 and 1.9 ± 0.4 mm in the 0.45 w/c samples carbonated at 53 and 65%, respectively. At 0.6 w/c, the values were 3.6 ± 0.5 and 2.6 ± 0.4 mm. The surface of the rest of the specimens was totally red purple indicating no pH lowering below 9.
3.3. Accelerated carbonation 100% CO2 3.3.1. CO2 evolution The in situ ND measurements of the carbonation process in CEM I 42.5R and 0.5 w/c specimens allowed to follow the evolution of the calcium carbonate and portlandite in different RH atmospheres. Figs. 8 and 9 present the spectra obtained at 65% (1.28 Å) and 90% RH (2.53 Å), respectively. As explained in [19], at 33% RH almost no changes were observed. At 90% RH, while the portlandite peak remained constant, a slightly increase in the calcite peaks was observed (Fig. 9). At intermediate RH, 53, 65 and 75%, the portlandite decrease and the calcite increase could be clearly observed. Fitting the main diffraction peaks of calcite to Pseudo-Voigts functions, their corresponding areas throughout the carbonation experiments were calculated. At these three intermediate RH both the decrease of portlandite with time and the increase of calcite follow an exponential function similar to those obtained in the previous sections for natural and 0.5% CO2 carbonation. Using the TG data obtained before and after carbonation, the functions fitted for the peak areas can be converted into CO2 combination and portlandite consumption evolution with the form of Eq. (1). Fig. 10 represents the evolution of CO2 and portlandite per cement gram for these three RH. The values of the corresponding parameters are in Table 5. The maximum combination of CO2, that is, CO2,m − CO2,i, varies between 23 and 25%, the only one that reached its maximum at the end of the test being the specimen carbonated in 53% RH. These values are considerably higher than the corresponding amplitude of portlandite, that is, the difference between the initial value and minimum. This implies that, at a certain time, more calcite is being produced than portlandite is disappearing. The values of τ increase with the RH: the specimen carbonated at 75% RH shows the slowest combination rate. In the three cases the values are higher for calcite than for portlandite, which means that calcite needs more time to reach its maximum than portlandite to reach its minimum. In order to relate weight gains with CO2 gains, similar specimens to the ones used for the 0.5% CO2 carbonation were used for 100% CO2 carbonation (CEM I 42.5R, 0.45 and 0.5 w/c), also in the same RH atmospheres, from 11 to 65%. The correlation between the values is shown in Fig. 11. In this case, a linear trend can also be seen, but the values are not as evenly distributed as in the 0.5% CO2 carbonation. Here, the values close to the x-axis correspond to 11, 23 and 33% RH.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
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Fig. 8. ND spectra for 65% RH carbonation: portlandite decrease, calcite increase.
After the phenolphthalein test none of the samples presented uncolored zones. Specimens carbonated at 33 and 90% RH showed a complete red purple surface. In the ones at 53, 65 and 75% RH two zones could be distinguished: one red purple and one soft pink, with the thickness of the soft pink zone decreasing as the RH increases. 4. Discussion This paper aims to deepen on the knowledge of the combination process of CO2 with cement paste. The evolution of combination with time and the influence of the two main variables, RH and environmental concentration of CO2, are discussed in the following paragraphs. 4.1. Influence of the RH in the carbonation processes Weight increases and CO2 gain due to carbonation are conditioned by the environmental RH. As previously found by other researchers, at very low RH there is not enough water in the pores to dissolve the phases involved and to allow their reaction. At intermediate RH, optimal conditions for carbonation are reached: enough water for reaction and also enough space for CO2 diffusion. From a certain value of RH, the pores start to be saturated with water, making diffusion difficult. This fact can be confirmed by the results obtained both for 0.5 (Fig. 6) and 100% CO2 carbonation. Regarding the influence of RH in the temporal evolution, the relation between τ and RH is first considered. Fig. 12 represents the values for the three types of carbonation, with the y-axis in minutes corresponding to 100% CO2 carbonation and the one in days to natural and 0.5% CO2 processes. For the natural values, average RH has been considered for the inner and outer environments, 38 and 57%, respectively.
In 0.5% CO2 carbonation, τ reaches a maximum at around 50% RH, following a similar behavior as the weight increase versus the RH. At low RH there is almost no difference between the two w/c ratios. Between 53 and 75% RH, τ values are higher for the 0.45 w/c than for 0.5 w/c specimens. This means specimens with greater w/c ratio need less time to reach their maximum CO2 combination, which can be attributed to the greater porosity that facilitates the access, dissolution and reaction. In 100% CO2 carbonation between 53 and 75% RH, τ increases with RH. At 53% RH the rate of combination is between 5 and 6 times greater than at 65 and 75%. At very low and very high RH no fitting to exponential functions could be obtained from the data collected, because the changes in the patterns were almost unnoticeable. However, it is expected that the values of τ in both extremes of the RH scale will be lower as in the case of 0.5% CO2 carbonation. Apart from the different CO2 concentrations, the natural and accelerated carbonation processes considered here differ on the variability of environmental RH. In the case of natural carbonation, both inside and outside, there are considerable variations in RH along the year, with the lower values in the inside. As can be seen in Fig. 12, τ reaches relatively similar values in both cases, between 150 and 250 days, and is slightly higher in the specimens with the highest w/c ratio. The influence of environmental humidity in τ is not as pronounced as in the cases of accelerated carbonation at constant RH. Fig. 13 shows the relation between the maximum CO2 that could be combined in the samples and RH for the three processes. To calculate the values of the 0.5% CO2 carbonation, the linear relation found in Fig. 7 between weight gain and CO2 gain has been used. As in the case of τ, for the natural values, average RH has been considered for the inner and outer environments. The maximum CO2 combined in specimens carbonated in 100% CO2 atmospheres is very similar, around 24%, for the three cases considered; it only increases slightly with RH. According to these
Fig. 9. ND spectra for 90% RH carbonation: slight increase of calcite, no changes in portlandite.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
25
30% CO2 53% RH
25%
20
CO2 gain (%)
Weight (% per cement gram)
26
CO2 65% RH
20%
CO2 75% RH
15% 10% PORTLANDITE 65% RH
0
PORTLANDITE 53% RH
0% 0
100
10 5
PORTLANDITE 75% RH
5%
15
0
200
300
400
500
2
4
6
600
8
10
12
14
16
Weight gain (%)
t (min) Fig. 11. CO2 gain versus weight gain after 24 h 100% CO2 carbonation. Fig. 10. CO2 versus time in specimens exposed to 100% CO2 carbonation during 10 h, exponential fitting.
Table 5 Fitting parameters of functions represented in Fig. 10: CO2 and portlandite (CH) evolution in samples exposed to 100% CO2 carbonation. RH (%)
CO2;m −CO2;i ð%Þ
τ (min)
CH2,i − CH2,min (%)
τ (min)
53 65 75
22.7 24.2 25.1
74.4 356.9 448.0
17.2 15.9 14.7
45.4 151.1 354.6
leading to a lower maximum, as it seems to happen in the case of 33 and 53% RH carbonation. Note that the relation in Fig. 7, used to calculate the values in Fig. 13, is independent of the w/c ratio, which explains the similarity in the amount of bound CO2 for both w/c. These graphs (Figs. 12–13) indicate that the more CO2 a specimen can gain the longer time it will need to reach the maximum. Fig. 14 shows the linear relation between τ and the maximum CO2 combined for the RH considered, between 11 and 65%.
4.2. Similarities and differences between the three carbonation processes. Equivalences The evolution of the amounts of CO2 combined in the course of time in paste specimens submitted to 100% CO2 follows the same exponential behavior as in natural carbonation (see Eq. (1)). In the case of 100% CO2 carbonation, the equation applies for atmospheres with RH between 53 and 75% RH. The maximum values reached in accelerated carbonation, between 23 and 25%, are slightly higher than the ones reached in natural carbonation inside, between 20 and 23%, and slightly lower than the ones outside, between 28 and 31%. This means that, on average, by means of 100% CO2 carbonation, very similar amounts of CO2 can be combined as in natural carbonation. Regarding the time constant, in natural carbonation the values reached are between 500 and 4500 times higher than the ones in 100% CO2 carbonation. With the parameters obtained in both types of carbonation, the time equivalences, in terms of CO2 combination, can be calculated (Table 6).
500
500 0.5% CO2 - 0.45 w/c
τ (days)
450
450
0.5% CO2 - 0.5 w/c
400
100% CO2 - 0.5 w/c
400
350
natural - 0.6 w/c
350
natural - 0.45 w/c
300
300
250
250
ONS
200
I
150
200
OS
τ (min)
relations (Figs. 12–13), the specimens carbonated in 100% CO2 at the lowest RH, namely 53%, will need less time to reach the maximum, but if the specimens are left enough time at the corresponding RH, between 53 and 75%, all of them will reach almost the same value of combined CO2. The completion degree of the carbonation process can be calculated comparing the maximum values (Fig. 13) with those reached at the end of the experiments (Fig. 10). In the 53% RH sample, calcite has almost reached its maximum and portlandite its minimum. In the 65% RH specimen, the minimum value of portlandite is nearly attained, while calcite has only accomplished 80% of the reaction. The specimen carbonated at 75% RH has the lowest completion degree, 75% of calcite and 90% of portlandite. In the three RH considered it is evidenced that not only portlandite is being consumed, but that other phases may also carbonate. To evaluate the influence of the RH in the carbonation of other phases, the ratio between the times for attaining maximum calcite and minimum portlandite has been calculated: 1.6, 2.5 and 1.2 for 53, 65 and 75% RH, respectively. According to these values, it might be inferred that the carbonation of other phases is favored in 65% RH atmospheres. In the natural carbonated samples, the maximum values of CO2 reached are considerably different with regard to the environment. Inside, with a lower average RH, the maximum amount of CO2 that can be combined, around 20%, is much lower than outside, between 28 and 31%. The higher combination found outside not sheltered compared to sheltered environment should be explained together with the fact that the carbonation depth was greater in specimens sheltered than in those not sheltered. When exposed to rain, although there are periods during which CO2 diffusion is not allowed in the pores due to rain water block, there are also periods during wetting and drying when optimal RH conditions for combination may be reached. This means that in outside not sheltered specimens CO2 penetrates less distance but combines more CO2; outside sheltered, CO2 penetrates more, as there are no water blocks due to rain, but the environmental RH may not always allow the CO2 combination. In the case of 0.5% CO2 carbonation, the maximum CO2 trend with regard to RH is very similar to that of τ, showing a maximum at around 50% RH. Nevertheless, in this case the difference between both w/c ratios is almost nonexistent, that is, higher w/c leads to higher rate but it does not imply greater combination. The greater pore volume facilitates diffusion; however, the ‘carbonatable’ material may be smaller,
150
I
100
100
50
50
0
0 10
20
30
40
50
60
70
80
RH (%) Fig. 12. Time constant τ versus RH in the three carbonation processes. Left τ-axis applies for 0.5% CO2 and natural carbonation; right τ-axis applies for 100% CO2 carbonation. In natural carbonation the points labeled with OS, ONS and I correspond to the environments outside sheltered, outside not sheltered and inside, respectively. Carbonation times were 4 years, 1 year and 8–12 h for natural, 0.5 and 100% CO2 carbonation processes, respectively.
I. Galan et al. / Cement and Concrete Research 49 (2013) 21–28
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Table 6 Time equivalence between natural and 100% CO2 carbonation in terms of CO2 uptake.
0.5% CO2 - 0.45 w/c 0.5% CO2 - 0.5 w/c
35
100% CO2 - 0.5 w/c
Max CO2 (%)
30
natural - 0.6 w/c natural - 0.45 w/c
ONS
Equivalent times in natural carbonation (days)
OS
Inside
Outside sheltered
Outside not sheltered
183 44 34
121 35 27
136 40 30
25
1 h 100% CO2 53% RH 1 h 100% CO2 65% RH 1 h 100% CO2 75% RH
I I
20 15 10 5 0 0
10
20
30
40
50
60
70
80
90
RH (%) Fig. 13. Maximum CO2 gain versus RH in the three carbonation processes. In natural carbonation the points labeled with OS, ONS and I correspond to outside sheltered, outside not sheltered and inside, respectively. Carbonation times were 4 years, 1 year and 8–12 h for natural, 0.5 and 100% CO2 carbonation processes, respectively.
In the 0.5% CO2 carbonation, the time evolution of weight follows as well an exponential behavior in atmospheres with RH between 11 and 65%, and there is a linear relation between weight gain and CO2 gain, with a slope ‘weight gain - CO2 gain’ of 0.53. In [17], Parrott relates weight and CO2 gains in samples carbonated at constant RH and 0.045% CO2, finding a linear relationship with a slope of 0.57. According to Parrott, the kinetic aspects of carbonation can be evaluated from the time evolution of weight gain, fitting the curves to functions proportional to a power of time in which the exponent would depend on RH. As the average value is 0.5, Parrott considers that this function is in agreement with the evolution of the ‘carbonation depth’ proportional to the square root of time. Goñi et al. [5] also propose the square root of time to describe temporal evolution of CaCO3 formation both for natural carbonation and accelerated carbonation. Although the ‘carbonation depth’ and the weight (or CO2 or CaCO3) gains measured at a certain time behave similarly with respect to RH, with parabolic shape reaching the maximum at intermediate RH, their time evolution is different. In the first case, the square root of time law is used to describe the advance of the pH change front in the cementitious material; this solution considers that the process is diffusion controlled, with fast reaction kinetics, and assumes depletion does not occur. In the second case, what is described is the temporal evolution of the CO2 combined in a certain volume of material. For this, an equation with an upper limit should be used: the maximum amount of CO2 combined is controlled, among others, by CaO in cement present in the volume considered. Introducing size as a parameter modifies the boundary conditions and makes the square root of time no longer valid.
450
τ (days)
27
400
0.45 w/c
350
0.5 w/c
300 250 200 150 100 50 0 0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30
Max CO2 (%) Fig. 14. Time constant τ versus maximum CO2 combined in 0.5% CO2 carbonation: linear relationship for each w/c ratio.
Comparing the two accelerated processes, it can be seen that, although the range of values for weight gain in both types of carbonation is very similar, between 2 and 11%, there are important differences in the behavior as a function of RH. One of these differences is the minimum humidity needed to start the process: at 100% CO2, RH greater than 33% is needed to obtain significant weight increases. This can be explained considering that the slower the carbonation, the more time is available for water present to take part in the reaction that, in turn, produces water and allows the continuity of the process. In very rapid processes at low RH, no matter how much CO2 is in the atmosphere, there is no time for using the scarce available water. Regarding the amounts of CO2 combined in 0.04% CO2 atmospheres (natural carbonation) after four years, they are greater than those in 0.5% CO2 after one year. Only in specimens carbonated at 53 and 65% RH the values reached are similar to the quantities measured in specimens placed inside. In addition, the specimens carbonated in air atmospheres, inside and outside, after four years have reached their maximum values, while in 0.5% CO2 carbonation, only the ones in 11 and 23% RH atmospheres have attained their top value. The results obtained have demonstrated that the evolution of CO2 combination in atmospheres with different CO2 concentrations, 0.04, 0.5 and 100%, follows a similar trend, which is not contradictory to the fact that there might be a different order in the reactions of the CO2 with the phases in the cement paste for each CO2 concentration. Besides this, the results shown prove that carbonating at constant or variable RH does not influence the shape of the fitting function. The different concentrations, time and RH do influence significantly in the values reached of weight and CO2 gain, as well as in the time constant or combination rate.
5. Conclusions From the results presented in this paper, obtained using complementary techniques, namely TGA, weight measurements and ND, the following conclusions can be drawn: 1. The combination of CO2 can be fitted to an exponential function CO2(t) = CO2,m + (CO2,i − CO2,m) ⋅ exp(− t/τ) both in natural carbonation and accelerated carbonation. 2. In natural carbonation, the greater combination and smaller carbonation depth in specimens not sheltered from the rain is attributed to the effect of wetting and drying. Although CO2 diffusion is prevented while the pores are full with rain water, in the accessible zones the more favorable conditions promote the reaction. 3. In slow accelerated carbonation, 0.5% CO2, higher w/c leads to higher combination rate but not necessarily to higher CO2 combined: the greater porosity facilitates diffusion, but the ‘carbonatable’ material may decrease with the w/c. 4. The maximum values reached in 100% CO2 accelerated carbonation are in the same range as the ones in natural carbonation, around 23–25%. In present experiments, one year of inside exposure is equivalent, in terms of CO2 combined, to 2, 7 and 9 h at 100% CO2 and 53, 65 and 75% RH, respectively. 5. Future experimental work should involve more intermediate environmental CO2 concentrations in order to find the equations that relate the combination rate with the CO2 concentration for every RH.
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