Environ. Sci. Technol. 2008, 42, 5329–5334
Potential for Carbon Adsorption on Concrete: Surface XPS Analyses LIV M. HASELBACH* AND SHUGUO MA University of South Carolina, 300 Main Street, Columbia, South Carolina 29208
Received March 11, 2008. Revised manuscript received May 2, 2008. Accepted May 5, 2008.
The concrete industry is a contributor to the global carbon cycle particularly with respect to the contribution of carbon dioxide in the manufacturing of cement (calcination). The reverse reaction of carbonation is known to occur in concrete, but is usually limited to exterior surfaces exposed to carbon dioxide and humidity in the air. As alternate concrete uses expand which have more surface area, such as crushed concrete for recycling, it is important to understand surface adsorption of carbon dioxide and the positive impacts it might have on the carbon cycle. X-ray photoelectron spectroscopy (XPS) is used in this study to evaluate carbon species on hydrated cement mortar surfaces. Initial estimates for carbon absorption in concrete using other techniques predict the potential for carbonate species to be a fraction of the calcination stoichiometric equivalent. The XPS results indicate that there is a rapid and substantial uptake of carbon dioxide on the surfaces of these mortars, sometimes exceeding the calcination stoichiometric equivalents, indicative of carbon dioxide surface complexation species. On pure calcite, the excess is on the order of 30%. This accelerated carbon dioxide surface adsorption phenomenon may be important for determining novel and effective carbon sequestration processes using recycled concrete.
Introduction Concrete is a contributor to the carbon cycle, both during the construction process and during the manufacture of cement, one of its main ingredients. In fact, the manufacture of cement is thought to contribute to more than 5% of the current anthropogenic carbon dioxide releases worldwide (1). This is not expected to decrease as countries such as China and India develop their infrastructure on a massive scale. Although the percentage varies, approximately half of the cement manufacturing carbon dioxide releases are from energy use and half are from the actual chemical reaction (calcination) in many countries (2). There are many options to consider in lowering these impacts including both energy and material substitutions, but the focus herein, and in much of the literature, is on carbon dioxide absorption or adsorption in concrete both through the reverse chemical reaction of calcination (carbonation) and other opportunities for carbon sequestering in concrete during its life cycle (3, 4). The carbonation reaction is diffusion limited and dependent on many variables such as relative humidity (water content), pH, concrete mix, concrete age, and pore structure (surface area) (5, 6). For pervious concrete, with its open pore structure and wetting * Corresponding author phone: (803) 917-3800; fax: (803) 7770670; e-mail:
[email protected]. 10.1021/es800717q CCC: $40.75
Published on Web 06/11/2008
2008 American Chemical Society
cycles during precipitation events, there is a potential for a substantial amount of the carbonation to take place over its service life, making the chemical reaction contribution fairly carbon neutral. For many concrete uses, it is estimated that carbonation is not very extensive over their useful initial service lives, but research investigations into accelerating carbonation before and during a secondary “recycled” life as a form of carbon sequestering are being explored. There are several researchers focusing on this reverse calcination (carbonation) process in concrete, but the theory presented herein is that, just as in aquatic and environmental chemistry, there may be other complexation and differently bound carbon entities within the hydrated cement paste. Some might afford the opportunity for additional carbon dioxide adsorption or absorption, while there are others which may be exploited to accelerate the initial uptake of carbon dioxide. This paper focuses on surface specific adsorption potentials in concrete. The interest in this phenomenon is based on the recent increase in the forms of concrete which have much larger surface areas, such as recycled concrete fines. Some initial experiments had been performed in the laboratories of the researchers using X-ray photoelectron spectroscopy (XPS) to analyze the amounts of carbon on the surface of fairly young, hydrated cement pastes. The results indicated that there were quantities of carbon far in excess of what would be expected if only carbonation had occurred. The objective of this research is to determine what carbon types exist on these surfaces and if some of them may be carbon dioxide surface adsorption species possibly in the form of carbonate complexes. Typical Initial Compositions of Portland Cement and Concrete. The main ingredients in Portland cement are limestone and clays. They are heated in a kiln and the major reaction of interest herein is the calcination of the limestone (calcium carbonate) to CaO with the release of carbon dioxide as in the following reaction: CaCO3 f CaO + CO2
(1)
The limestone usually contributes all of the CaO and the clays contribute the SiO2, Al2O3, and Fe2O3 (gypsum and some other ingredients, if used, may add some Ca without adding CO2). There are also small amounts of other impurities such as magnesium, sodium, potassium, and sulfur (7). Water, aggregate (large and small), cement and various admixtures are then mixed together to form concrete. Concrete represents a very dynamic, complex chemical system and the cement paste (cement plus water phase) will undergo many changes to its composition with age and with exposure to other compounds. Concrete also may contain many other cementitious materials which may reduce its attributed carbon content, and other additives that interact with the cement compounds differently and alter its composition. Researchers have been trying to study and model the changing composition of the cement paste for various mixes and under various environmental conditions for well over a half-century. Due to its very dynamic nature, and the constant changes to mix compositions that are being promoted to attain various performance goals, this endeavor will probably continue as long as concrete is such an important material. Table 1 gives an example composition of Portland cement and hydrated cement paste (mortar) when the water is first added, although the ratios vary according to mix preferences and uses (7). Note that the concrete industry uses a special shorthand notation to represent the various oxides in concrete and these letters are listed in Table 1. VOL. 42, NO. 14, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Example Composition of Portland Cement and Initial Hydrated Cement Paste (Assume Add Two Moles of Water for Each Mole of Calcium) oxide letter
chemical formula
MW
S A F C j S rest H sum
SiO2 Al2O3 Fe2O3 CaO SO3 MgO etc. H2O
60 102 160 56 80 40a 18
a
cement massb percent
cement mol in 100 g
initial hydrated cement mol
initial hydrated cement mass (g)
initial hydrated cement mass %
21 6 4 65 2 2
0.350 0.059 0.025 1.161 0.025 0.050 0 1.670
0.350 0.059 0.025 1.161 0.025 0.050 2.321 3.991
21 6 4 65 2 2 41.8 141.8
14.8 4.2 2.8 45.8 1.4 1.4 29.5 100
100
Assume approximate molecular weight of MgO.
b
Ref (7).
Various reactions take place over time within the cement paste phase and some water may be lost. A simplification of the main reactions can be summarized in the following equations: CaO + H2O f Ca(OH)2
(2)
x CaO + y H2O + z SiO2 f C-S-H where C-S-H is a complex gel based on the 3 oxides (3) The cement paste is constantly changing, but many researchers have found distinct regions of C-S-H with what appear to be hydrate (water) layers between and distinct regions of calcium hydroxide in maturing cement paste (7). Due to the presence of significant amounts of calcium hydroxide, cement mortars are generally highly basic with pHs usually greater than 12 (7, 8). Carbonation. When calcium hydroxide is exposed to dissolved carbon dioxide under high pH conditions, the carbon dioxide will react with the calcium hydroxide rather quickly to form the more stable calcium carbonate solid. Variations in pH, concentrations, temperature, and other factors will all impact this reaction rate. For the purposes of this discussion, the main carbonation equation can be summarized as follows: Ca(OH)2 + CO2 f CaCO3 + H2O
(4)
There are several other phases in concrete which can carbonate. Huntzinger acknowledges a much slower carbonation reaction from the weathering of the calcium-silicate (C-S-H) minerals (9). Other minor elements such as magnesium can also form carbonates. It has been known for a long time that complex monocarbonate and hemicarbonate aluminate hydrates can exist in concrete (10, 11). Carbonation in concrete has been studied for many decades. The reaction is mainly limited by the diffusion rate of carbon dioxide into the interior of the concrete mass in order for carbonation to take place. For most applications, carbonation is limited to a small depth along the exterior exposed face of the structure where the conditions for carbonation are favorable (12). In fact, having carbonation limited to these areas is usually a benefit for many applications of concrete, since the carbonation reaction lowers the pH. This changes the usually highly alkaline nature of the cement paste, which is thought to protect reinforcement bars from oxidation (8). There are also advantages to carbonation. For instance, it has been found that carbonation can increase the strength of concrete (13). Recently, due to concerns with the massive anthropogenic releases of greenhouse gases (GHGs) in the developed and developing world, there is interest in exploring ways to accelerate carbonation in concrete forms where it is not a concern, such as in recycled concrete and other uses not using iron reinforcement. There 5330
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are also studies about using accelerated carbonation to immobilize hazardous wastes (14). Unfortunately, one of the obstacles to enhancing carbonation may be carbonation itself since carbon dioxide is a much larger molecule than water (molecular weights of 44 and 18, respectively). If there is a small water phase layer in concrete and carbon dioxide dissolves, then the larger carbonate ion may effectively block the pathway for further carbonation. This has been shown to happen in some experiments when high carbon dioxide concentrations are exposed to cement pastes (13). Due to the potential impact that carbonation in concrete may have on the global carbon cycle, there is currently an explosion in research into the extent and potential of carbonation under various circumstances. Many researchers are estimating the impact of concrete carbonation on regional or global bases. In all this, the first question that needs to be answered is how much of the carbon dioxide that is released during calcination can in fact be carbonated, or in other words, what is the maximum carbonation possible? Most researchers agree that, under the right conditions, all the calcium within the paste in the calcium hydroxide form can be carbonated, but there are varying predictions on how much carbonation can take place in the C-S-H gel (5, 15). In addition, there is the potential for other carbonate species to form as previously mentioned with respect to the monocarbonate and hemicarbonate aluminates (10, 11). Lagerblad predicts that in fact all the calcium can be carbonated given enough time and the proper circumstances (5). However, Engelsen et al. estimate that the proportion of 75% is more likely and this is approximately the percent of the calcium hydroxide phase in hydrated cement paste (15). There are many experimental methods used to study the extent of carbonation in cement paste from simpler phenolphthalein studies which measure pH changes to the use of thermal reaction analyses and X-ray diffraction techniques. Many researchers are using thermal gravimetric analysis (TGA) to measure the amount of carbon in a sample. This technique measures mass loss as a sample is heated and various compounds either evaporate or decompose and lose mass. Most make the assumption that carbon dioxide is only released at fairly elevated temperatures when calcium carbonate is known to decompose (16–18). In these cases carbon is assumed to mainly exist as a calcium carbonate species. The interest is mainly in the amount of carbon dioxide within the cement paste in bulk. The X-ray diffraction techniques are used to analyze the crystalline structure within the hydrated cement (19). As interest is expanding as to the feasibility of using various forms of concrete for carbon sequestering, more research is needed. Initial estimates using other experimental techniques focus on absorbed carbon dioxide and carbonation. This research also focuses on the potential for alternative surface
FIGURE 1. C1s XPS of three samples: (a) a standard calcium carbonate sample; (b) a hydrated cement paste sample aged for 0.5 month; (c) a hydrated cement paste sample aged for 2.5 months. carbon species, in addition to those balancing the cement manufacturing chemical reaction, since some of these other forms of concrete, such as recycled concrete fines and pervious concrete, may have a much larger exposed surface. Therefore research into the surface concentration of carbonate species and adsorbed species is needed. The surface technique proposed herein is X-ray photoelectron spectroscopy (XPS). XPS has often been excluded from being used to study the carbon species in hydrated cement paste due to the difficulties of interpreting carbon contamination from the air and equipment. In this study, the various carbon species will be looked at separately for comparative purposes.
Experimental Methods The experimental process for XPS is that X-rays (normally from an Al KR or Mg KR source) are directed onto a surface resulting in the emission of electrons, called photoelectrons, due to the interaction of photons with electrons. The kinetic energies, which can be converted into binding energies, of photoelectrons are measured through electron energy analyzing and detecting systems. The binding energy of electrons in a core level of an element is normally different for different oxidation states. XPS is used to identify the chemical compositions and oxidation states of elements on a surface. The escape depth of photoelectrons is limited because of the interaction with other electrons and atoms and only photoelectrons coming off several surface layers deep can be detected without losing their energy. Therefore, XPS is a surface sensitive technique. XPS would not normally be as useful for determining cement mortar composition, since its composition can vary widely with depth. The reason for using XPS as an investigative tool is that in order to determine carbonation with respect to depth in typical samples, compositions at these depths need to be determined and XPS can be used to analyze the composition of many surfaces cut from various depths of a cement sample. In addition, since some of the options being investigated for carbon sequestration in concrete may use forms that have much higher surface areas, such as pervious
concrete or fines from crushing recycled concrete aggregate (RCA), the surface phenomena may become more significant if properly exploited for sequestration. It is well-known that carbon is found on many surfaces and since XPS is a method to measure surface compounds, it is very difficult to determine the true carbon composition of the surface due to the potential for significant carbon contamination. The carbon can come from many sources including the air, handling contamination, or even the oil in the equipment. Most of this contamination is typically referred to as “adventitious” carbon and may many times be very difficult to differentiate from other carbon species. Sometimes the contamination is further differentiated into adventitious and more polar carbon contamination (C-O or CdO bonds). It has been found that adventitious carbon is common on oxides including SiO2, Al2O3, and other oxide surfaces. Many researchers have studied how to use XPS for determining carbon taking these concerns into account (20–23). To analyze data from XPS measurements on cement, and determine the carbon species sources, the XPS measurements in this research were also performed on known carbonate species. The three standard species used were laboratorygrade calcium carbonate, laboratory-grade calcium acetate monohydrate, and mined dolomite ore. In addition, the differentiation between some of the contamination forms will be attempted to be included in the XPS study results. XPS measurements were carried out on the Kratos AXIS Ultra DLD XPS system equipped with a hemispherical energy analyzer and a monochromatic Al KR source. The monochromatic Al KR source was operated at 15 keV and 150 W. The pass energy was fixed at 40 eV for the detail scans and the spectrum focused on the photoelectron energy in the range typical for electrons originating in the 1s orbital of carbon (C1s). Powdered cement samples were supported on doublesided transparent tape for XPS analysis. One concern with analyzing samples attached to transparent tape is that there may be a potential contribution of C1s signal from the substrate. To determine if this would be of concern, the standards that came in powdered form (calcium carbonate and calcium acetate monohydrate) were also analyzed in pellet form. The pellets were prepared by pressing some powder between two stainless steel plates in a hydraulic press. Peak fitting was conducted with the XPSPEAK4.1 software (24).
Results The XPS analyses were performed on a variety of cement paste samples prepared in the laboratory with Portland cement and water. Small amounts were placed in cuvets of approximately 10 mm × 10 mm with 4-5 mm depth. These were allowed to cure under various conditions for one-half month, which represents young hydrated cement samples, unlikely to have extensive carbonation, yet cured enough for limited use in some applications. The samples were either left in the ambient conditions of the laboratory (A) or placed in an oven (O) at 50 °C. Some of each were left open (O) so that they had a typical ambient source of carbon dioxide, while others were placed in covered (C) cuvets limiting the carbon dioxide availability. The relative humidity was not controlled. The samples were then scraped either on the top (t) or bottom (b) and these scrapings were analyzed. The tops had more exposure to carbon dioxide than the corresponding bottoms. This differentiation was adopted to determine if the XPS method could distinguish between samples which had been exposed to slightly different levels of carbonation, even with the other sources of carbon contamination in the analysis method. About half of the samples were analyzed VOL. 42, NO. 14, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. XPS Results atomic mole percent ofa C C adventitious C-O
sample AC1b AO1t AO1b OC1t OC1b OO5t AC3t AC3b chunk pre sputtering chunk post sputtering AO2tc AO2bc OO3tc OO4tc OO4bc OO5bc OC5tc OC5bc AC5bc standard calcium carbonate in pellet form standard calcium carbonate in powder form-1 standard calcium carbonate in powder form-2 standard calcium acetate in pellet form standard calcium acetate in powder form standard dolomite sample 1 standard dolomite sample 2
C C)O
mole ratio to Ca b
C C C carbonate Ca 2p adventitious C-O
C C)O
C C carbonate total
41.2 32.2 34.0 39.2 38.3 37.3 38.5 42.4 32.0 24.2 35.5 31.9 34.7 30.9 25.7 30.4 32.5 31.7 33.3
5.9 3.2 10.3 1.1 9.9 0.9 10.1 0.8 9.9 2 11.9 1.7 13.8 1.2 12.3 2.2 7.8 1.1 7.2 in C-O 7.8 1.3 8.6 0.8 11.9 12.8 1.2 8.78 2.2 11.8 3.1 10.2 2.1 11.9 0.6 8.0 0.9
16.9 27.0 26.4 19.8 17.8 20.0 22.2 16.3 29.2 32.3 28.0 29.0 26.9 25.4 25.7 25.6 28.1 27.2 30.8
30.3 27.9 28.3 27.6 30.0 25.5 22.6 24.5 28.9 35.1 25.3 29.0 24.0 27.4 27.7 28.0 26.1 28.4 26.4
1.36 1.15 1.20 1.42 1.28 1.46 1.71 1.73 1.11 0.69 1.40 1.10 1.44 1.13 0.93 1.09 1.24 1.12 1.26
0.19 0.11 0.37 0.04 0.35 0.03 0.37 0.03 0.33 0.07 0.47 0.07 0.61 0.05 0.50 0.09 0.27 0.04 0.21 in C-O 0.31 0.05 0.3 0.03 0.49 0.47 0.04 0.32 0.08 0.42 0.11 0.39 0.08 0.42 0.02 0.3 0.04
0.56 0.97 0.94 0.72 0.59 0.79 0.98 0.67 1.01 0.92 1.11 1.00 1.12 0.93 0.93 0.91 1.08 0.96 1.17
2.22 2.53 2.52 2.54 2.27 2.78 3.35 2.99 2.43 1.82 2.87 2.43 3.05 2.57 2.25 2.53 2.79 2.51 2.77
32.6
8.5
0
33.1
24.2
1.35
0.35
0
1.37
3.07
28.1
6.7
0
36.6
27.6
1.02
0.24
0
1.33
2.59
24.6
13.2
0
35.2
27.0
0.91
0.49
0
1.30
2.70
54.7
4.0
29.1
12.3
4.46
0.32
2.37
7.15
54.0 26.8 29.9
4.9 7.0 2.7
28.5 33.8 32.6
12.6 18.8 18.8
4.28 1.43 1.59
0.38 0.37 0.14
2.26 1.8 1.74
6.92 3.67 3.73
1.3 4.9
0.07 0.26
a Atomic mole percents are only based on elements chosen for the analysis and are not inclusive of all the elements in the sample. Only the ratios of the atomic mole percents are descriptive of the actual surface composition makeup. b The adventitious form for the acetate samples would also include the carbon in the organic component of the standard compound. c Sample analyzed two months after scraping.
TABLE 3. Average Ratios for the Scraped Cement Paste Samples in the XPS Studies all samples Ctotal/Ca 2.64 Ccarbonate/Ca 0.91
top
bottom 0.5 months 2.5 months
2.81 2.50 0.96 0.86
2.65 0.78
2.64 1.02
within 24 h while the rest were put in covered cuvets for two months prior to analysis. One special sample from the initial cuvets was also analyzed in a “chunk”. This chunk held a significant amount of water in it and it took two days for the sample to degas in the XPS chamber prior to completion of the chunk analyses. The chunk was analyzed twice, both before and after sputtering with Ar+ at the beam energy of 2 keV. The XPS analyses were also performed on samples of the three standards: laboratory-grade calcium carbonate, laboratorygrade calcium acetate, and ore-grade dolomite. Figure 1 shows C1s XPS for a standard calcium carbonate sample (a) and for hydrated cement paste samples aged for 0.5 and 2.5 months, (b) and (c), respectively. Since all the samples are nonconductive, the peak positions were corrected by positioning the lowest binding energy peaks at 284.5 eV. The counts were segregated into four carbon groups. The first, labeled adventitious carbon refers to organic carbons, including carbon from contamination around 284.5 eV, the C-O carbon was that around the 286 eV, the CdO carbon was that around 287.5 eV, and the rest was included in the carbonate 5332
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category with approximately a 289.5 eV peak. A summary of all the results for all samples is displayed in Table 2. Table 2 helps in determining many important preliminary results. The first thing to consider is the consistency of the experiments and the sources of contamination. The standard samples best describe the testing consistency. In both dolomite samples, the mole ratio of total carbon to calcium is approximately 3.7. The individual carbon component ratios vary somewhat, but this can be expected since the totals are based on integrating within the intensity peaks and this is subject to interpretation of where to start one species and end the other. The calcium carbonate and calcium acetate monohydrate standards both have very consistent sets of ratios for the carbonate to calcium analyses which also points toward a consistency of analysis. It is interesting to note that in both cases the pellet samples have slightly greater adventitious and total ratios. This is indicative of two things: there may be a slight additional contamination in the pressing process, and the concern about the glue to the substrate falsely increasing these ratios for the powder forms is insignificant. This suggests that future testing should proceed with powder samples not being pressed. Based on these results, the method appears consistent, although not necessarily accurate in determining both calcium carbonate bonds and total carbon amounts. The standard reference sample results of calcium carbonate in the powder form will be used for comparison to the hydrated cement samples. For preliminary estimates, the stable calcium carbonate bonds on the surface of the hydrated
cement samples will be determined using a correction factor of (1.33)-1 or 0.75 as in the following equation: (C-carb ⁄ Ca)predicted ) 0.75 × (C-carb ⁄ Ca)experimental (5) These calcium carbonate bonds are considered to be a combination of the three types of calcium carbonate: vaterite, aragonite, and calcite (25). Gopinath et al. have shown that these have different bonding energies with vaterite the lowest and calcite the highest, but all are still in this higher range (22). However, the 1.33 mol ratio of carbon to calcium as seen in the standard calcium carbonate samples is not necessarily a calibration error. Stipp and Hochella showed that calcite exposed to water, even just humidity in the air, may also have a surface concentration of a hydrated carbonate-calcite species (23). They refer to this species as S.CO3H. These were found even after the surfaces had been exposed to a vacuum for a long time. This peak is even higher than the various calcium carbonate energy levels producing a shoulder on the higher side. In the Stipp and Hochella study, the additional surface carbonate on the calcite ranged from 25 to 150% of the carbonate directly associated with the calcium carbonate bond, depending on whether the surfaces were exposed to dissolution and/or precipitation processes (23). The existence of these surface species was also studied by other techniques such as atomic force microscopy (AFM) (26). Later studies by Stipp also indicate that there are chemisorbed bicarbonate species on calcite surfaces (27). Carbonate dissolution and precipitation are very probable processes within the chemistry of concrete with exposure to water and carbon dioxide. Therefore, it is very likely that the S.CO3H species also exists on the surface of a hydrated cement paste sample. Another important finding from other surface studies on calcite is that the surface may have the capacity to also adsorb contaminants from water sources and become an important tool in water remediation. Some example studies looked into surface adsorption of rare earth elements such as europium and fluoride (28, 29). The evidence then suggests that this same behavior can probably be expected for other elements of interest. As previously mentioned, in uncarbonated, hydrated cement paste, the calcium is not always associated with hydroxide as in calcium hydroxide. A significant portion is associated with the C-S-H gel. The amount of CaO associated with C-S-H based in Table 1 is 30%. This is why many researchers predict that the maximum amount of carbonation possible with respect to the calcium hydroxide reaction is from 70 to 75% and that only after a significant amount of time, would additional carbonation associated with mineralization of the C-S-H gel raise this percent. However, in Table 2 it can be seen that the percent of carbonates on the surface far exceeds this 70% for most of the samples. This indicates that there may indeed be a significant amount of the S.CO3H species on the surface. There are a few other trends that can be interpreted from these results. The results in Table 2 were averaged with respect to several parameters and these are depicted in Table 3. The average carbonate carbon to calcium ratio is as mentioned higher than would be expected for the main carbonation reaction. This ratio is also higher for the top surfaces which would have had additional exposure to carbon dioxide in the air, and is higher for the older samples, which also had additional time for exposure and reaction in the covered cuvets. With respect to the total carbon, the average ratio of carbon to calcium on the surface is consistent for all the samples and is identical to the ratio for the standard calcium carbonate powder sample. This is a very interesting result. Although not fully carbonated, most of the cement samples show the
same overall affinity for retaining carbon on their surface as calcium carbonate, including the stable carbonated form. This result is further indication that there may be ways to accelerate overall carbon sequestering to hydrated cement paste by taking advantage of surface adherence of carbon in a more loosely bound form, without having to wait for the slower carbonation process of the C-S-H gel or other phases to take place. The total carbon to calcium ratio is slightly higher for the top samples, which may be explained by additional opportunities for contamination Implications for CO2 Adsorption on Concrete. There is evidence that concrete cement pastes have the capacity to adsorb more carbon dioxide than is stoichiometrically calculated from the calcination reactions, although this capacity is highly surface dependent. This implies that with an increase in surface area, recycled concrete cement mortars may have the potential to sequester additional carbon dioxide and other carbon species fairly rapidly as compared to the carbonation reaction. Modified design of sequestering processes with respect to particle size, water content, and exposure which include the physical and chemical reactions related to these species might be used to improve the rate and magnitude of carbon dioxide sequestering in recycled concrete. However, many more tests would need to be performed before any statistically significant conclusions could be made with respect to these parameters.
Acknowledgments This work is based upon work supported by the National Science Foundation under Grant 0725268.
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