Ind. Eng. Chem. Res. 2006, 45, 4985-4992
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Modification of Composition and Microstructure of Portland Cement Pastes as a Result of Natural and Supercritical Carbonation Procedures Carlos A. Garcı´a-Gonza´ lez,† Ana Hidalgo,‡ Carmen Andrade,‡ M. Cruz Alonso,‡ Julio Fraile,† Ana M. Lo´ pez-Periago,† and Concepcio´ n Domingo*,† Instituto de Ciencia de Materiales de Barcelona, Consejo Superior de InVestigaciones Cientı´ficas (CSIC), Campus de la UAB s/n, E-08193 Bellaterra, Spain, and Instituto de Ciencias de la Construccio´ n Eduardo Torroja, CSIC, Serrano GalVache, 4, E-28033 Madrid, Spain
This study aims to analyze the effects of supercritical carbonation (CO2 at 20 MPa and 318 K) on the physicochemical properties of ordinary Portland cement pastes. The evolution of the main crystalline phases of the cement pastes during carbonation was determined by means of X-ray diffraction and thermogravimetric analysis. The pore structure was analyzed by low-temperature N2 adsorption-desorption and mercury intrusion porosimetry techniques. Finally, the microstructure of the samples was observed by using scanning electron microscopy coupled with energy-dispersive X-ray detection for chemical analysis. For a natural carbonation process, diffusion of CO2 into the pores of the cement paste is considered as the rate-controlling step. Instead, the accelerated reaction kinetics of calcium carbonate precipitation in the supercritical process was chemically controlled by the detachment of calcium ions from solid portlandite or CSH gel. The total pore volume of the studied cement pastes decreased with carbonation, which was associated with the deposition of the formed CaCO3. Samples carbonated under the supercritical conditions developed a higher volume of gel pores than those obtained by natural carbonation. 1. Introduction The formation of solid calcium carbonate (CaCO3) from aqueous solutions or slurries containing calcium and carbon dioxide (CO2) is a complex process of considerable importance in the geochemical and biological areas. Concrete, based on ordinary Portland cement, is one of the most widely used materials on earth with applications ranging from construction to waste immobilization.1,2 A well-known phenomenon associated with cement-based materials placed in natural environmental locations is the carbonation that occurs naturally over numerous years owing to the reaction with atmospheric CO2.3 In the case of steel reinforced concrete, this is one of the major causes of concrete structure deterioration over time, because the reduction of the pH in the cement paste pore solution due to carbonation enables the corrosion of steel.4 On the other hand, concrete carbonation allows the use of other nonexpensive reinforcement products, such as glass, plastics, vegetal fibers, and so forth.5 In general, carbonation increases the cement paste density and reduces the overall permeability, which are all desirable properties for matrixes intended to immobilize waste materials.6 Hazardous wastes, contaminated with heavy metals and other inorganic pollutants or even radioactive compounds, can be stabilized with concrete to reduce their toxicity and to decrease the long-term dissolution of the toxic compounds and their release into the environment. Contradictory beneficial and deleterious effects of carbonation on contaminant elements mobility solidified into cement-based systems have been described.7-11 Concrete encapsulated waste stability appears to be dependent on the type of waste, the type of cement paste or binder, the carbonation method, and the severity of the carbonation treatment, as well as on the environmental conditions. Therefore, different physical and chemical properties of * To whom correspondence should be addressed. Tel.: 0034 935801853. E-mail:
[email protected]. † Instituto de Ciencia de Materiales de Barcelona. ‡ Instituto de Ciencias de la Construccio´n Eduardo Torroja.
carbonated cement pastes, such as the pH of the pore water and phase constitution, permeability, and microstructure, should be evaluated to fully understand the effects of the treatment and to assess the suitability of a carbonation technique to stabilize waste. The most widely used methods for cement paste carbonation involved the natural and the accelerated process.3,11-13 In the first case, samples are stored in contact with the atmosphere. In artificially intensified carbonation, samples are exposed to pure CO2. The present work, which forms part of a broader study, attempts to enlighten some of the issues of accelerated supercritical carbonation. This can have a direct effect in industry because, by speeding the carbonation reaction using compressed CO2, the process can be commercially feasible for the production of a broad range of desirable cement-based materials.14 Supercritical CO2 (SCCO2) has been used on an industrial scale for a number of processes, the most important being its use as a solvent for extraction and for pharmaceutical particles engineering.15 SCCO2 exhibits physicochemical properties tunable with pressure. The additional degree of freedom related to the CO2 compressibility facilitates that technologies based on this solvent can manufacture products in a more efficient way. Additionally, the low viscosity and high diffusivity occurring at supercritical conditions result in enhanced reaction rates for processes carried out involving these media. Although the advantages of treating hardened cement pastes with highpressure CO2 and SCCO2 have been demonstrated, literature concerned with the analysis of carbonation effects at elevated pressures is less common.10,11,16-18 The aim of this work is to provide further evidence and improved understanding of the effects of supercritical carbonation on the physicochemical properties of two types of hardened Portland cement pastes. The results were compared with those obtained for natural carbonation. 2. Experimental Section 2.1. Materials. Two Spanish cements, an ordinary Portland cement (CEM I) from Cementos Goliat S.A., Ma´laga, and a
10.1021/ie0603363 CCC: $33.50 © 2006 American Chemical Society Published on Web 06/13/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006
Figure 1. Process flow diagram of the supercritical equipment.
sulfate resisting Portland cement (CEM I-SR) from Cia. Valenciana de Cementos Portland S.A., Alcala´ de Guadaira, were used for the testing program. Cement pastes were fabricated with a 0.4 water-to-cement ratio and hydrated for 7 days in sealed conditions (98% relative humidity and 293 K). The cement pastes used in this study were mainly granulated powder, although some monolith pieces (∼0.25 cm3) were also tested. CO2 (99.995% purity) was supplied by Carburos Meta´licos S.A. (Spain). 2.2. Processes. Cement paste treatment methods carried out were either natural carbonation with air in an atmospheric vessel or artificially intensified carbonation through the addition of compressed CO2 into a high-pressure reactor. 2.2.1. Natural Carbonation. A first set of powdered samples was stored for a period of 200 days exposed to laboratory atmosphere at ∼291 K (samples labeled nat-200d). A second set of powdered samples was stored for a period of 30 days in a convection air oven at 318 ( 1 K (samples labeled nat-30d) to carbonate samples at a temperature similar to that used for the supercritical process. It has to be pointed out that, after the working experimental periods of 200 and 30 days, the process of natural carbonation of cement paste monoliths was not appreciable in the bulk. 2.2.2. Artificially Intensified Carbonation. High-pressure carbonation experiments were carried out in a stainless steel apparatus (Figure 1). Liquefied CO2 was compressed to the operating pressure by means of a membrane pump (1, Lewa EK-M-210). A CO2 preheating spiral (2) and two tubular reactors (Re1 and Re2, 10 mL each) were placed into an air oven (3). The system pressure was controlled with a back pressure regulator (4, Tescom 26-1761). Cement paste particles (∼1 g) and small pieces of a similar monolithic sample were enclosed in cylinders made of 0.45 µm pore filter paper and added to the tubular reactors together with glass wool. The carbonation procedure was initiated by raising the temperature and pressure to 318 K and 20 MPa, respectively. After system stabilization, the experimental conditions were maintained, at a relatively low flow rate of SCCO2 (∼1-2 standard L min-1), for a period of time of either 2 or 7 h (samples labeled SC-2h or SC-7h, respectively). At the end of the carbonation period, the system was depressurized, and the samples were recovered from the reactors. 2.3. Characterization. Raw and treated cement pastes were analyzed by X-ray diffraction (XRD) from a 2θ value of 5-55° with a Rigaku Rotaflex RU200 B instrument using a step of
0.02° and Cu KR1 radiation. The sample thermal behavior was measured using thermogravimetric (TG) analysis (SEIKO 320U) performed under a N2 atmosphere raising the temperature at a rate of 10 K min-1 in a range of 293-1500 K. Alumina powder was used as the reference material. Low-temperature N2 adsorption-desorption analysis (BET, ASAP 2000 Micromeritics, Inc.) and mercury intrusion porosimetry (MIP, Micromeritics Autopore IV 9500 v.1.05 porosimeter) were used for samples surface and porosity analysis. For BET measurements, samples were first dried under reduced pressure ( 900 K. On both the raw and the natural carbonated samples, the CO2 started to be expelled from the carbonate at approximately 900 K and ended at 975 K (Figure 3a,c). However, in the SCCO2 treated samples, the decarbonation ended at 1050 K (Figure 3b,d). It is suggested that in the SCCO2 treated samples the precipitated calcite was more crystalline than in the atmospherically carbonated ones. Hence, more crystalline samples would be expected to start undergoing decarbonation at a high temperature. The endotherms shown at 675-800 K were the result of dehydroxylation of Ca(OH)2, and they were observed for all the samples with the exception of the fully carbonated CEM I SC-2h sample. TG results also showed that the remaining hydration water after carbonation of the samples was different depending on the processing method. The samples treated under supercritical conditions were highly dehydrated, because the remaining water at 375 K was less than 2 wt %. This contrasts with the value found for the raw materials that was about 7 wt % and for the naturally carbonated samples, where the remaining water was ∼5.5 wt % in samples treated at 291 K and ∼4 wt % in samples treated at 318 K. Environmental factors such as humidity, temperature, and CO2 concentration and factors concerning materials such as the chemical composition of the cement have been known as agents affecting the progress of carbonation as well as the final cement paste constituents and the resulting microstructure. For the samples studied in this work, XRD and TG analysis showed that CEM I was significantly carbonated by natural treatment and totally carbonated after 2 h of supercritical treatment. On the other hand, CEM I-SR was practically not carbonated by atmospheric air and only partially carbonated by compressed CO2, even after 7 h of treatment. It should be taken into account that the CEM I had the higher content of alkali in comparison
to that of CEM I-SR (Table 1). The alkali in the cement paste always favors the concentration of OH- in the pore solution for counteracting the positive ionic charge of K+ and Na+ ions. This favored the formation of CO32- ions. In this sense, many authors have pointed out that alkali in cement paste favors its carbonation, but only at the first step, because the influence of K+ and Na+ ions on the carbonation reaction is limited by the total content of alkali ions in the matrix.26,27 These ions are consumed by incorporation into the CSH gel during decalcification replacing the Ca2+ in the sites where uncompensated charge arises. Hence, carbonation will occur at a higher rate the higher the pH of the pore solution and the longer this high value of pH is maintained. In the case of CEM I-SR, the alkali and the OH- concentration in the pore solution was relatively high for a shorter period of time than for CEM I. On the other hand, it has to be taken into account that, in cement paste with high alkali content, the Ca2+ concentration should be low as a result of the K+ and Na+ ion presence. For the CEM I sample with the high alkali content, an initial massive precipitation of the CaCO3 covering the Ca(OH)2 and inhibiting its dissolution was not likely to occur in the first reaction stage. Therefore, a less extended rim of CaCO3 was formed, and continuous Ca(OH)2 dissolution was favored, resulting in a fully carbonated sample in a short period of time. 3.2. Cement Paste Carbonation by Using Atmospheric CO2. In the natural carbonation treatment, samples were stored in an open vessel exposed to atmosphere. In this condition, only a small amount of CO2 was absorbed in the pore water and was converted into CO32-. The amount of CO2 dissolved and, hence, available CO32- for carbonation was restricted by the low solubility of atmospheric CO2 in water. In the experiments carried out at 291 K, the water produced from the reaction in
Ind. Eng. Chem. Res., Vol. 45, No. 14, 2006 4989 Table 3. Analysis of Porosity Using Hg (MIP) and N2 (BET) Adsorption/Desorption Techniques CEM I raw
nat-200d
CEM I-SR
nat-30d
SC- 2h
raw
0.09 32 9
0.13 40 13
0.06 65 6 0.006 38 14
0.06 60 16 0 15 0
nat-200d
nat-30d
SC-2h
SC-7h
MIP
Vp-MIP(10nm-20µm) [cm3 g-1] Pd-MIP [nm] Sa-MIP [m2 g-1]
0.14 41 14
Vp-BET(2-300nm) [cm3 g-1] Vp-BET(2-10nm)/Vp-BET(2-50nm) [%] Pd-BET [nm] Vmp-BET [cm3 g-1] Sa-BET [m2 g-1] Sa(mp)-BET [m2 g-1]
0.08 60 17 0 18 0
0.09 39 6
BET 0.04 63 12 0 12 0
0.05 59 14 0 14 0
eq 1 was accumulated in the pores restricting further migration of the CO2 into the sample. For experiments carried out at 318 K, the excess of water in the pore was more easily evaporated facilitating CO2 diffusion to the interior, but the rise of the temperature from 291 to 318 K decreased the CO2 solubility in the remaining water. Moreover, Ca(OH)2 solubility was reduced by increasing reaction temperature because of its retrograde solubility behavior. Natural carbonation has been defined as a CO2 diffusional phenomenon, where the carbonation front moves the concrete inward at a rate proportional to the square root of time.28-31 3.3. Cement Paste Carbonation by Using Compressed CO2. By exposing a Portland cement paste to SCCO2 it has been found that the carbonation reaction can be accelerated with respect to a natural carbonation reaction. By using compressed CO2, the first effect was a huge rise (near 100-fold) of CO2 solubility in water with the increase in pressure from 0.1 to 20 MPa. The reaction acceleration was also due to the ease of penetration and diffusion of the SCCO2 into the micropores of the cement paste, providing continuous availability of fresh reactant. Because the density of SCCO2 at the working temperature and pressure was similar to that of water, less critical energy was required to overcome the surface tension than at the atmospheric pressure, thus, enhancing CO2 mass transfer. Moreover, the pressure parameter is susceptible to influence a reaction rate in a positive way depending on the activation volume. Finally, in the dynamic system used in this work, where a continuous flow of CO2 was used, the flowing system could eliminate water formed in the reaction in relatively significant amounts (eq 1 was shifted to the right). Previous studies have shown that increasing the pressure and the temperature in the supercritical conditions did not increase significantly the degree of carbonation.16 Hence, the process was not CO2 diffusion controlled. In general, under the supercritical experimental conditions studied, the global carbonation process was not very fast, despite the concentration of solutes being high; therefore, high nucleation rates were expected.21,22 A period of 2 h of treatment was necessary to fully carbonate CEM I, whereas for CEM I-SR longer reaction times were needed to complete the reaction. In the initial stages of the precipitation process carried out infusing SCCO2, high initial concentrations of CO32- and Ca2+ were reached, which led to a fast and extended initial precipitation of CaCO3, covering the surface of Ca(OH)2 and inhibiting its dissolution to some extent. At this stage, the slowest reaction step was the generation of calcium ions from Ca(OH)2 dissolution. The CaCO3 precipitation progressed, at relatively lower concentration of Ca2+ with respect to CO32-. As the original level of portlandite available for dissolution was depleted, the carbonation reaction was likely supported by calcium hydroxide provided by the decalcification of CSH gel. In this case, CaCO3
0.03 54 13 0 10 0
0.04 60 15 0 10 0
0.05 70 9 0.003 25 8
0.05 79 5 0.004 41 10
has been described to precipitate between the fibrils of CSH in the paste outer product. For SCCO2 treated samples, totally carbonated CEM I, and partially carbonated CEM I-SR, the ettringite phase vanished from the sample before Ca(OH)2 depletion. This fact was explained on the basis of the relatively low pH of the cement paste pore water during supercritical treatment, because equilibrium in eq 9 was shifted to the left (ettringite dissolution starts at pH ∼ 10.5).32 3.4. Analysis of Porosity and Surface Area. The microstructure of the hardened cement paste after it has totally or partially reacted with CO2 has been relatively little explored, although it is of considerable practical interest, because it affects the properties of carbonated cement paste, such as strength and permeability.33,34 For example, the engineering properties of concrete are directly influenced or controlled by the number, type, and size of pores present. The total volume of pores, not their size or connectivity, affects the strength and elasticity of concrete, whereas concrete permeability is also influenced by pore size and connectivity. As the pore structure of cementitious materials appears to be strongly influenced by exposure to carbonation, this aspect was deeply studied in this work using adsorption isotherms and pore volume accessible to both N2 (BET method) and Hg (MIP method). It should be taken into account that, in general, these different analysis techniques do not measure the same surface area or pore volume for a given type of sample.35-37 In this work, we have assumed that each one of the used techniques evaluates accurately a different fraction of the surface and pore volume on the basis of pore size. Classification of pores in hardened cement paste has been done according to diameter following Mindess and Young38 and Taylor:1 large capillaries or macroporosity (> 50 nm), medium and small capillaries (1050 nm), hydrated phases gel porosity (2.5-10 nm), and microporosity (
