New insights on the use of supercritical carbon dioxide for the accelerated carbonation of cement pastes

Available online at www.sciencedirect.com

J. of Supercritical Fluids 43 (2008) 500–509

New insights on the use of supercritical carbon dioxide for the accelerated carbonation of cement pastes Carlos A. Garc´ıa-Gonz´alez a , Nadia el Grouh a , Ana Hidalgo b , Julio Fraile a , Ana M. L´opez-Periago a , Carmen Andrade b , Concepci´on Domingo a,∗ b

a Instituto de Ciencia de Materiales de Barcelona (CSIC), Campus de la UAB s/n, E-08193 Bellaterra, Spain Instituto de Ciencias de la Construcci´on Eduardo Torroja (CSIC), Serrano Galvache 4, E-28033 Madrid, Spain

Received 22 December 2006; received in revised form 6 July 2007; accepted 27 July 2007

Abstract This study aims to analyze the effects of supercritical carbon dioxide (SCCO2 ) on the carbonation of Portland cement pastes, with and without mineral addition. The process is based on the advantages of the low viscosity and surface tension of SCCO2 that allow the complete wetting of complex substrates with intricate geometries, including internal surfaces of agglomerates such of these found in cement pastes. The supercritical treatment alters the bulk chemical and structural properties of cement pastes by accelerating natural carbonation reactions, while at the same time it reduces both free and bound water. The observed overall effects of supercritical carbonation on Portland cement pastes were the neutralization of pore water alkalinity, the formation of calcium carbonate and the reduction in the Ca/Si ratio of the calcium silicate hydrate (CSH) gel. Moreover, the massive precipitation of calcium carbonate inside microcracks after supercritical treatment caused the refinement of the microstructure, thus, reducing water permeability to a large extent. Supercritical carbonation method can be completed in few hours, rendering it technically interesting. This work also contributes to the idea of increasing the durability of concrete by means of a preventive hydrophobic supercritical treatment against water ingress. A generic SCCO2 method for concrete silanization is proposed here. © 2007 Elsevier B.V. All rights reserved. Keywords: Supercritical CO2 ; Cement; Carbonation; Silanization; Permeability; Waste management

1. Introduction 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]. Concrete is used in radioactive containment structures as a barrier, liner or encasement container [3,4]. In the case of low-level radioactive waste, attention is paid to the use of cementitious materials for its immobilization and stabilization. Many designs for highlevel radioactive waste confinement, particularly in deep and humid geological underground disposal, envisage the presence of cementitious materials in conjunction with clay (bentonite) barriers. In realistic conditions, confinement durability is limited by concrete interaction with clays, host rock and groundwater. The evolution of the microstructure and the leaching processes of cementitious materials in contact with groundwater depend

∗

Corresponding author. Tel.: +34 935801853; fax: +34 935805729. E-mail address: [email protected] (C. Domingo).

0896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2007.07.018

on the type of cement used to produce concrete. Recent improvements in concrete mechanical properties and compactness have been achieved by means of the optimization of the granular packing and the adjustment of pozzolans (mineral additions). Pozzolans are defined as substances which are not themselves cementitious, but form hydraulic compounds through a pozzolanic reaction with calcium hydroxide when mixed with cement in presence of water [5–8]. The formation of solid calcium carbonate (CaCO3 ) from aqueous solutions or slurries containing calcium and carbon dioxide is a complex process of considerable importance in the geochemical and biological areas. Natural carbonation of concrete consists of the chemical reaction between the CO2 present in the air and the calcium contained in the water of the pores of the cement paste (pore solution) in a dynamic equilibrium with the cement hydration products [9,10]. The carbonation effect in the cement paste chemical composition, porosity and permeability has been widely reported in the literature [11–13]. Carbonation of cementitious materials usually increases strength and reduces permeability, which are desirable properties for

C.A. Garc´ıa-Gonz´alez et al. / J. of Supercritical Fluids 43 (2008) 500–509

matrices used to immobilize waste. One of the main consequences of the carbonation reaction is the modification of the pH in the pore solution from a standard value between 12.5 and 13.5 for Portland cement pastes, to a value below 9 in the carbonated zones [13]. The most widely used methods for cement carbonation involve the natural and the accelerated process [9,14–16]. In the first case, samples are stored in contact with atmosphere, while in the artificially intensified carbonation process, samples are exposed to either atmospheric or compressed pure CO2 . The possibilities of enhancement of concrete mechanical properties and durability by treatment with supercritical carbon dioxide (SCCO2 ) have been already described for applications in cement-waste stabilization [14,17–23]. The first objective of this on-going work is to provide further evidence and a better understanding of the effects of supercritical carbonation on the physicochemical properties of Portland cement pastes with and without mineral addition. Moreover, our interest is focused on the preparation of cement pastes with a pH similar to that of the bentonite pore water (7–9) that will not cause instability in the clay when used for high-level radioactive waste confinement. This work also aims to support the concept already proposed that indicates an increase of the durability of concrete by means of a preventive hydrophobic treatment against water ingress [24–27]. Bifunctional silanes, particularly trialkoxysilanes, are widely used to form hydrophobic coatings, since they are non-toxic and environmentally compliant chemicals [28–30]. The effectiveness of the hydrophobic treatment should be evaluated using the criteria of deep penetration to obtain a strongly improved long-term protection [29,30]. The penetration of the hydrophobic ingredient depends on the porosity and the permeability of the concrete substrate. Besides, the penetration capacity of the waterproofing agent and its concentration gradient depend on system specific parameters, such as the molecular size of the active hydrophobic compound, the type of solvent used for dilution and the treatment technology. Deposition of a great variety of silane molecules on different surfaces has been extensively studied using either gaseous- or liquid-phase reac-

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tions [29,31,32]. An important difficulty in using the traditional solution-phase silanization processes is that the solution containing the silane turns out to be very viscous during reaction and solvent evaporation step. Therefore, kinetics became restricted by the mass transport of the silane to the inner porous surface of the agglomerates and, eventually, transport could be hindered by channels blockage [33]. As an alternative, a generic SCCO2 method for cement pastes silanization is proposed here. The advantage of the supercritical method is that the low viscosity and surface tension of SCCO2 silane solutions, in comparison with conventional liquid solutions, allow the complete wetting of internal surfaces of cement/concrete agglomerates with intricate geometries [33–36]. In this work, preliminary results of the application of a hydrophobic treatment performed on mortars using the supercritical technology are presented. 2. Experimental 2.1. Materials Raw materials were sulfate resisting Portland cement (CEM I-SR), a class F fly ash (FA with a mean particle size of 18 ␮m), both from Cia. Valenciana de Cementos Portland S.A. (Alcal´a de Guadaira, Spain), and silica fume (SF with a mean particle size of 26 ␮m) from Ferroatl´antica S.L. (A Coru˜na, Spain). Formulations of studied pastes were 100% CEM I-SR (CEM), 56% CEM I-SR + 35% FA + 9% SF (CEM-35FA9SF), 83% CEM ISR + 17% SF (CEM-17SF) and 83% CEM I-SR + 8% FA + 9% SF (CEM-8FA9SF). Cement pastes were fabricated with a 0.4 water to cement ratio and hydrated during 7 days in sealed conditions (98% relative humidity at 293 ± 2 K). Cylinders (30 mm diameter × 20 mm height) of mortars of the CEM-35FA9SF sample were prepared with a 0.5 sand to cement ratio. The sand was a standard material (UNI ENV 196-1) with particle size from 80 to 2000 ␮m. The cement pastes and mortar used in this study were mainly granulated powder, although some monolith pieces (∼0.2 cm3 ) were also processed using the supercritical method. Octyltriethoxysilane (C14 H32 O3 Si, Fluka) was used for

Fig. 1. Process flow diagram of the supercritical equipment: (a) 10 mL tubular reactors (Re1 and Re2) and (b) 70 mL autoclave (Re3).

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the hydrophobic treatment of mortar. CO2 (99.995% purity) was supplied by Carburos Met´alicos S.A. (Spain). 2.2. Processes 2.2.1. Natural carbonation A set of powdered samples of the different cement pastes (samples labeled raw) was stored for a period of 200 days exposed to laboratory atmosphere at ∼291 K (samples labeled nat). 2.2.2. Supercritical carbonation Experiments were carried out in a stainless steel apparatus (Fig. 1). Liquefied CO2 was compressed to the operating pressure by means of a membrane pump (P1, Lewa EK-M-210). The system pressure was controlled with a back pressure regulator (V5, Tescom 26-1761). Cement paste particles (∼1 g) were enclosed in cylinders made of 0.45 ␮m pore filter paper and added to 10 mL tubular reactors (Re1 and Re2 in Fig. 1a) placed into a circulating air oven. In a different equipment setup, a mortar cylinder, small monolithic pieces and granulated mortar particles were added to a 70 mL high-pressure autoclave (Re3 in Fig. 1b) with magnetic stirring and heated using resistances. In all the experiments, the carbonation procedure was initiated by raising the temperature and pressure to 318 K and 20 MPa, respectively (samples labeled SC). A one-pass SCCO2 (∼1–2 L min−1 at normal conditions) flow technique was used for cement pastes carbonation. Samples were carbonated for either 2 or 7 h (samples SC2h and SC7h, respectively). Only mortar carbonation (granulated and monolithic small pieces) was carried out in the batch mode and for a period of 3 h (sample SC3h). 2.2.3. Mortar silanization The 3 h SCCO2 -treated mortar sample was silanized using the 70 mL autoclave (Re3 in Fig. 1b). Two millilitres of liquid silane was added to the bottom of the reactor and the mortar sample was located on the top. The silanization process was carried out in the batch mode at 343 K and 8.5 MPa during 3 h (samples labeled SC-s). 2.3. Characterization Chemical composition of raw cement pastes were obtained by Atomic Absorption Spectrometry (AAS, Perkin-Elmer 1100B). Raw and treated cement pastes were analyzed by X-ray diffraction (XRD) from a 2θ value of 5–45◦ with a Rigaku Rotaflex RU200B instrument using a step of 0.02◦ and Cu K␣1 radiation. Samples thermal behavior was studied by thermogravimetric (TGA) and differential thermogravimetric analysis (DTA) using a SEIKO 320U. In DTA measurements, alumina powder was used as the reference material. Analysis were performed under N2 atmosphere raising the temperature at a rate of 10 K min−1 between the range of 293–1500 K. Low-temperature N2 adsorption–desorption analysis (BET method, ASAP 2000 Micromeritics INC) and Hg intrusion porosimetry (MIP method, 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 (<1 mPa) at 378 K for 5 h. Specific surface area (Sa-N2 ) was determined by the BET method. The volume of small capillary pores and gel pores (Vp-N2 ) was estimated by the BJH method and the micropore volume (Vmp-N2 < 2 nm) by the t-curve method [37]. The average pore diameter (Dp-N2 ) was calculated as 4Vp-N2 /Sa-N2 . For MIP measurements, samples were first dried over night under reduced pressure and at laboratory temperature. Data on surface area (Sa-Hg ), average diameter (Dp-Hg ) and volume of large capillary pores together with macroporosity (Vp-Hg ) was obtained from MIP analysis. Particle size cumulative volume distribution was determined for mortar granulated samples (sieved first through a 200 ␮m mesh) by a light scattering method (Coulter LS 130 instrument) after dispersing the powder in hydrophobic petroleum (Fluka, purum). To confirm the presence of the silane in the mortar, samples were mixed with KBr and analyzed over the wavenumber range of 600–4000 cm−1 using a Fourier transform infrared spectrometer (Shimadzu FTIR8300). A water permeability test was applied to the mortar cylinders using a home-made equipment described elsewhere [38]. The coefficient of water permeability, Kw (m s−1 ), was calculated by applying Darcy’s law [39]. Simultaneously, the pH was measured in the percolated water. 3. Results and discussion 3.1. Cement pastes composition before and after carbonation reaction Portland cement contains four main components [1]: anhydrous tricalcium silicate (Ca3 SiO5 or C3 S), anhydrous ␤-dicalcium silicate (Ca2 SiO4 or C2 S), tricalcium aluminate (Ca3 Al2 O6 ) and tetracalcium aluminoferrite (Ca2 (Al,Fe)2 O5 ). The major products of Portland cement hydration include calcium hydroxide or portlandite (Ca(OH)2 or CH, 20–25 wt.%), calcium silicate hydrates (2CaO·SiO2 ·nH2 O or CSH gel, 40–60 wt.%) and calcium sulfoaluminate hydrates (AF(m,t), 10–20 wt.%) that can be either monosulfate (3CaO·Al2 O3 · CaSO4 ·12H2 O) or ettringite (3CaO·Al2 O3 ·3CaSO4 ·32H2 O) [40]. The main glassy component of SF and FA mineral additions was SiO2 , which in the presence of water reacted with calcium hydroxide to produce CSH [7,8]. For FA mineral addition, some calcium aluminate gel (CAH) or mixtures of calcium aluminate silicate gel (CSAH) were likely formed simultaneously to CSH gel by reaction of FA aluminum phases with the cement paste [6]. It is generally recognized that setting and hardening of cement pastes are due to CSH (or CAH) gel formation. The chemical composition of the studied cement pastes is represented in Fig. 2. In the principal reaction involved on the carbonation of hydrated cement pastes, CO2 reacts with the dissolved Ca2+ and sparingly soluble CaCO3 is precipitated in the cement pore network. In a normal system, it is described that, initially, in the pore water there is abundance of Ca2+ due to Ca(OH)2 dissolution. During carbonation, the original concentration level of portlandite is reduced and Ca2+ is then provided to the medium by the decalcification of the CSH gel phase. Simultaneously with

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Fig. 2. Chemical composition of the studied cement pastes.

these processes, Ca2+ can also be supplied by the dissolution of AF(m,t) and/or residual C3 S or C2 S phases [9,11]. The evolution of the main crystalline phases during carbonation was determined by X-ray diffraction analysis (patterns not shown). Since the crystallinity of CSH and CAH gels was rather poor, the XRD investigation could focus only on the transformation of crystalline portlandite and ettringite (AFt) phases to calcium carbonate. In CO2 -treated samples, either natural or intensified, calcite was the main carbonate polymorph formed [22]. The main XRD peaks of portlandite were still evident in all of the samples analyzed after the different carbonation treatments, indicating that complete carbonation of this phase did not take place. For SCCO2 -treated samples, XRD reflections of ettringite phase vanished from the sample before Ca(OH)2 depletion, but not from the naturally carbonated samples. This fact was explained on the basis of a relatively low pH value of the cement pore water during supercritical treatment (ettringite dissolution starts at pH ∼ 10.5) [11]. C2 S and C3 S reflections also diminished in the SCCO2 -treated samples. The weight loss during TGA between specific temperatures (obtained from DTA) allowed for the quantification of the percentage of portlandite and calcite in raw and treated cement paste samples. The presence of CaCO3 formed during treatment was shown by a large endotherm appearing at T > 900 K that correspond to its decarbonation. CaCO3 was also present in the raw samples, due to the usual reaction of hydrated cement pastes with atmospheric CO2 during their preparation. An endotherm appearing at ca. 675–775 K was the result of dehydroxylation of Ca(OH)2 and was observed for all the samples, confirming the XRD result which indicated that complete carbonation of this phase did not take place. In order to perform a quantitative analysis of the transformation of Ca(OH)2 to CaCO3 , the TGA values for precipitated CaCO3 and reacted Ca(OH)2 were calculated from the CO2 and H2 O weight loss, respectively, and are expressed as CaO equivalents in Fig. 3a. The amount of precipitated calcite became higher when processing the cement paste samples with SCCO2 in comparison to natural carbonated samples, being quite similar for samples treated during either 2 or 7 h under supercritical conditions. However, the reacted amount of portlandite was considerably larger for samples stored under atmospheric conditions (Fig. 3a). These results suggested

Fig. 3. Calculated thermal weight loss: (a) initial Ca(OH)2 and CaCO3 in the raw sample, reacted Ca(OH)2 calculated by subtracting the measured amount of Ca(OH)2 after processing from the initial amount in the raw material, and precipitated CaCO3 calculated by subtracting the initial amount of CaCO3 in the raw material from the measured one after carbonation (data is expressed as CaO equivalent for both phases), and (b) free water (at 378 K) and combined water (378–698 K) in raw and treated samples.

that during the 200-day treatment at atmospheric conditions, the precipitated calcite came mainly from the carbonation of the portlandite phase, since the amounts of Ca(OH)2 reacted and CaCO3 formed were very similar, when expressed both as a CaO equivalent. The transformation of Ca(OH)2 crystals into additional CSH gel due to pozzolanic reaction can be deduced from data presented in Fig. 3a, where it can be observed that a relatively large amount of Ca(OH)2 vanished during the 200 days natural carbonation of samples with mineral addition without the formation of additional CaCO3 . For samples with mineral addition, some Ca(OH)2 crystals were consumed due to pozzolanic reaction with the FA and SF mineral addition during the 200-day treatment period, since the cement pastes were allowed to cure before carbonation only for 7 days. On the other hand, the measured amount of precipitated calcite after supercritical treatment did not correspond to what it would be expected if only carbonation of the reacted portlandite took place (Fig. 3a). Instead, the amount of calcite formed was about four- to fivefold higher than that corresponding to full carbonation of reacted portlandite. Therefore, other solid phases containing Ca2+ , and not only portlandite, were carbonated and contributed to calcite formation in these cases. The loss of free water (at 378 K) and the loss of bound water (378–698 K) were also quantified (Fig. 3b). The structurally bound water derived mainly from CSH and CAH gels, but could include some contribution from aluminate and ferroaluminate hydrates. The amount of remnant free and bound water in the samples decreased significantly when carbonating using supercritical conditions. For the naturally treated cement pastes, the

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amount of free water diminished only slightly, while the amount of combined water either did not decrease or even increased in some of the samples. The major increase in the amount of combined water after natural carbonation was measured for the sample CEM-35FA9SF (the one with the largest percentage of mineral addition), indicating again the formation of CSAH gel through pozzolanic reaction during the 200-day storing period.

increased (Fig. 3a). As the original surface of portlandite available for dissolution was depleted, the carbonation reaction was likely supported by Ca2+ provided by the decalcification of CSH or CAH gel and ettringite as shown by XRD analysis [22,47]. For the naturally treated samples, with low initial supersaturation levels due to low CO2 concentration, a less extended rim of CaCO3 was formed around portlandite crystals. Therefore, continuous Ca(OH)2 dissolution was favored and gel phases were less carbonated.

3.2. Natural and supercritical carbonation reaction 3.3. Cement pastes microstructure The pattern with regards to depletion of Ca(OH)2 and formation of CaCO3 observed during XRD studies was confirmed by TGA analysis. Both analysis techniques indicated that samples treated under supercritical conditions during few hours were carbonated to a larger extent than those naturally carbonated during 200 days. In the natural carbonation treatment, samples were stored in an open vessel exposed to atmosphere. In these conditions, the amount of CO2 dissolved in the pore water, and hence available CO3 2− for carbonation, was restricted by the low solubility of atmospheric CO2 in water (0.06 mol% at 291 K and 0.1 MPa) [41]. Natural carbonation has been defined as a CO2 diffusional phenomenon, where the carbonation front moves inwards the cement paste at a rate proportional to the square root of time [42,43]. By using compressed CO2 , the first effect was a very high increment (near 100-fold) of CO2 solubility in water with the increase in pressure from 0.1 to 20 MPa (2.4 mol% at 318 K and 20 MPa) [41]. Moreover, in the dynamic system used in this work, where a continuous flow of CO2 was used, the flowing system eliminated water produced in the carbonation reaction in relatively significant amounts (50–75 wt.% of the water present in the raw cement). This fact was confirmed by TGA analysis of the remnant free water (Fig. 3b) after supercritical carbonation. In general, carbonation is highly accelerated when water is continuously diffused from the centre part of the specimen towards the exterior, since otherwise excess of water can restrict the CO2 entry to the interior of the sample [13,43]. Therefore, the reaction acceleration was also due to the ease of penetration and diffusion of large amounts of SCCO2 into the micropores of the cement paste, providing continuous availability of fresh reagent [18,22]. Previous studies have shown that an increase of the pressure in the supercritical conditions did not raise significantly the degree of carbonation [14,44]. Hence, the supercritical process was not CO2 diffusion controlled. At the initial stages of the precipitation process carried out infusing SCCO2 , CO2 invaded the entire sample and high initial concentrations of CO3 2− and Ca2+ were reached, which led to a fast and pervasive initial precipitation of CaCO3 . Proceeding with the supercritical carbonation, a large amount of calcium carbonate precipitated covering mainly the surface of Ca(OH)2 and inhibiting its dissolution to some extent. This effect has been also observed studying the SCCO2 carbonation of Ca(OH)2 suspensions [45,46]. The net result was that the carbonation reaction begins rapidly, but then drastically slowed down with time. Indeed, by increasing the supercritical processing period from 2 to 7 h, the amount of precipitated calcite was only slightly

As the pore structure of cementitious materials appears to be strongly influenced by exposure to carbonation, this aspect was investigated in this work using adsorption isotherms and pore volume accessible to 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 [48–50]. Classification of pores in cement paste has been done according to diameter after Mindess and Young [51]: large capillaries or macroporosity (>50 nm), medium and small capillaries (10–50 nm), hydrated phases gel porosity (2.5–10 nm) and microporosity (<2.5 nm). BET method has been considered suitable to accurately estimate the volume of medium and small capillaries, gel pores and micropores, while MIP method has been used to describe the macroporosity. Results obtained using BET technique showed that the fine pore size distribution of hardened cement pastes, either with or without mineral addition, carbonated during 200 days at atmospheric conditions, had a similar shape to that of the raw materials. The total pore volume (Vp-N2 ) and surface area (Sa-N2 ) decreased by about 25 and 15%, respectively after natural carbonation (Fig. 4). This effect was associated with the precipitation of CaCO3 on the pore walls, thus reducing pore volume. In general, calcium carbonate precipitation helps to fill in pore space and densify the cement paste since the molar volume of calcite is 17% larger than that of portlandite. Contrarily, the Vp-N2 for samples treated at supercritical conditions only diminished slightly for the sample without mineral addition and even increased for samples with mineral addition (Fig. 4a). This fact may be explained by the deposition of calcite in the walls of the macropores during supercritical carbonation, giving rise to smaller pores within the pore range measured using BET technique. Moreover, the supercritical carbonated samples had a significantly larger part of the porosity attributed to cylinders with small pore radii indicated by decreased Dp-N2 (Fig. 4b). Increasing the duration of the supercritical treatment from 2 to 7 h, the pore size distribution was shifted to smaller pores and lower values of Dp-N2 . Supercritical carbonated samples even developed microporosity (Vmp-N2 ) that was not observed in raw or naturally treated samples (Fig. 4a). In addition, the mesoand micropore structure developed under supercritical carbonation conditions could be associated with the carbonation of the less dense CSH gel portion, either the outer shell or the interphase with the mineral addition, and the formation of gels with low Ca/Si ratio. The highly cross-linked silica gel has a rela-

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tively open nature at the nanometric level compared to CSH slits [19,22,52]. TGA results had already indicated that for the supercritically treated samples a large amount of CO2 reacted with the Ca2+ in the gel phase instead of with the Ca2+ in the Ca(OH)2 to give CaCO3 (Fig. 3a). This fine porosity is not easily observed in carbonations carried out using a flow of air since the smallest pores (with the highest capillary pressure) are the most likely to collapse. This is the result of pore collapse during the drying step due to large capillary forces imposed at the liquid–vapor interface. In the supercritical system, pores were filled with SCCO2 , which has a high density and very low surface tension, thus, reducing the capillary pressure. Moreover, after processing, SCCO2 was eliminated rapidly from the system without the formation of a liquid–vapor interface, avoiding the collapse of the structure. The surface area measured using BET technique (Sa-N2 ) also increased for samples treated under supercritical conditions (Fig. 4c) reflecting the increase in the number of fine pores with low volume but large surface area. The supercritical carbonation of the gel phases is supposed to consist of the deposition of fine crystals of CaCO3 within the fibrils of the low-density gel. In the area of macroporosity measured using MIP technique (>50 nm), the main effect of supercritical carbonation was a decrease of total macropore volume (Fig. 4a), while the mean macropore diameter was almost not influenced (Fig. 4b). The decrease in macropore volume was reflected in a slight decrease of total surface area (Fig. 4c).

Fig. 4. N2 (BET method) and Hg (MIP method) adsorption/desorption data obtained from cement paste and mortar samples: (a) pore volume, (b) pore diameter, and (c) surface area.

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3.4. Mortars permeability and pore water pH The study of water permeability was performed for mortars cylinders constituted of 50 wt.% CEM-35FA9SF/sand, untreated and carbonated under supercritical conditions during 3 h. First, this material was studied in regards to its composition and porosity. TGA and DTA curves of the raw and supercritically carbonated mortar are shown in Fig. 5. The endothermic peak at 670–750 K is due primarily to decomposition of portlandite. Surprisingly, the amount of portlandite increased from a value of 2.5 wt.% in the raw material to a value of 3.7 wt.% after supercritical treatment. This lead to a negative values of reacted Ca(OH)2 calculated following the method explained in Fig. 3a. This fact appears to be due to the prolonged reaction of residual anhydrous C3 S (or ␤-C2 S) with the water produced in the carbonation reaction during the supercritical treatment. It should be taken into account that mortar carbonation was performed in a batch mode and not in the dynamic mode used for cement pastes carbonation. Therefore, the water produced during the batch of mortar carbonation remained in the reactor, probably in the cement paste pores since its solubility in SCCO2 is relatively low [41], and lead to the formation of portlandite and CSH. The loss of combined water (378–700 K) attributed mainly to CSH gel indicated that the amount of this phase diminished only slightly from 2.6 to 2.4 wt.% (Fig. 3b). Calcium carbonate showed the distinctive decomposition peak in TGA analysis at the temperature range of 900–1020 K (Fig. 5) and its percentage increased from an initial value of 2.5–8 wt.% after supercritical carbonation (Fig. 3a). Results obtained using BET analysis showed that the cement paste CEM-35FA9SF and its granulated mortar derivate had a

Fig. 5. TGA and DTA curves obtained for the raw, supercritically carbonated (SC3h) and silanized (SC-s) granulated mortar samples.

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The pH of the supercritically carbonated mortar was reduced to about 8.5 (measured in the percolated water during the permeability test at the achieved near steady state) from an initial value of about 13.5 for the raw material. It is known that when the mechanism of carbonation is dissolution of Ca(OH)2 , as occurs in the case of natural carbonation of Portland cement, the solubility of portlandite maintains the pH of the pore solution at about 12.5 [13]. In the supercritical-treated samples a large amount of non-reacted portlandite remained within the material (Fig. 3a), but it was presumably enclosed by solid products of carbonation in view of the low pH measured for the pore solution [20]. 3.5. Mortar silanization

Fig. 6. Low-temperature N2 adsorption/desorption (BET method) pore size distribution on a volume basis for raw, supercritically carbonated (SC3h) and silanized (SC-s) granulated mortar samples.

comparable behavior before and after supercritical treatment. For both samples, the value of Vp-N2 was almost not influenced, the Dp-N2 decreased and the Sa-N2 increased (Fig. 4). The Vp-N2 for treated and raw mortar samples had the similar value of 0.04 cm3 g−1 , but the supercritical carbonated sample had a significantly larger part of the porosity attributed to cylinders with very small pore radii (Fig. 6), which is expected to influence the permeability. Permeability, defined as the movement of a fluid through a porous medium under an applied pressure head, is considered as the most important property of concrete governing its longterm durability [53]. Immobilization efficiency of waste into concrete depends on its permeation characteristics. The modification on the microstructure of the hardened cement pastes after it has partially reacted with CO2 affects the permeability. Both porosity and interconnectivity of pores in the cement paste influence the permeability [54]. Water permeability coefficients (Kw ) were determined for the CEM-35FA9SF mortar cylinder when near steady-state flow was attained (water flow rate at inlet equals water flow rate at outlet). Fifteen days of treatment and a water pressure of 0.02 MPa applied to samples was enough to attain near steady state for the reference raw mortar. Alternatively, 80 days of treatment and a water pressure of 0.05 MPa was needed for the carbonated mortar in order to reach a measurable flow through this sample and to reach an steady flow of water. Obtained Kw values were of 4 × 10−7 m s−1 for the raw mortar and of 2 × 10−10 m s−1 for the supercritical carbonated sample. Hence, the water permeability test showed that the supercritically carbonated mortar had significantly better water permeability resistant behavior than the non-carbonated one. Although the total pore volume measured using BET technique was similar in both studied samples, the permeability diminished because a more refined and disconnected structure occurs in the supercritically carbonated mortar as also found by other authors [53,54].

The open porosity of mortars causes permeability, which in turn, is responsible of their deterioration. Although it was observed that the supercritical treatment decreased the mortar permeability significantly, further treatment will be necessary to avoid weathering as much as possible and, thus, to improve mortar performance as a container. The SCCO2 method for cement pastes silanization is expected to reduce the absorption and transport of liquid water through the concrete [26,27]. Silanes with silanol groups (–Si(OH)x ) and polymers with siloxane bonds (Si–O–Si) are highly soluble in SCCO2 , presumably due to the interaction between CO2 , a weak Lewis acid, and the strong electron–donor capacity of the siloxane group [55]. Fig. 7 shows the FTIR spectra for the reference and treated granulated mortars. The band at ∼3450–3500 cm−1 was attributed to the stretching mode of hydrogen bonded OH species (O–H· · ·O–H) adsorbed on the surface. In hydrated Portland cements, the main IR band for C–S–H gels appears at

Fig. 7. FTIR spectra for the raw, supercritically carbonated (SC3h) and silanized (SC-s) granulated mortar samples. In the SC-s spectrum, the arrow indicates the most representative silane absorption bands corresponding to –CH2 – and –CH3 in the silane chain.

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Fig. 8. Optical photographs of monolith mortar pieces added to a layer of water: (a) raw mortar, (b) supercritically carbonated mortar, and (c) supercritically silanized mortar samples.

∼1000 cm−1 [47]. The characteristic bands of calcite could be found at 2550, 1420, 875, and 710 cm−1 . The presence of silane in the prepared mortar sample was also confirmed by FTIR, although the interpretation of all appropriate silane bands was a difficult task since there was a strong overlapping with cement constituents bands. The most representative silane absorption bands found corresponded to the C–H stretching modes of the –CH2 – and –CH3 groups (2950–2850 cm−1 ) of the long-chain alkyl group. For the silanized mortar sample, an endothermic weight loss was observed in the TGA analysis between 740 and 900 K, which provided a wide ATD peak centered at 825 K (Fig. 5, SC-s sample). This is the habitual temperature range for the decomposition (through cleavage of C–C or Si–C bonds) of surface bonded or chemisorbed silanes [28]. Siloxane head groups (Si–O substrate) remain on the surface until about 1200 K. The amount of unreacted silane or physically trapped polysiloxane, corresponded to the loss of weight at temperatures around 540 K (boiling point for the used silane) and it was regarded as insignificant in the studied sample. Surface reaction of the trifunctional alkoxysilane with hydroxylated surfaces involves several habitual steps [29]. The silanization reaction is initiated by hydrolyzing one or more of the alkoxy groups (Si–OR) giving place to Si–OH groups. Note that water is an indispensable reactant to initiate the reaction. The supercritical procedure is considered as an anhydrous method in which only moisture previously present in the pores was added to the reaction medium. The hydrolyzed groups might then form hydrogen bond with hydroxyl groups of the inorganic surfaces (schematized as HO-#) in the mortar pores (Si–OH· · ·HO-#), favoring siloxane bonds between adjacent molecules placed on the surface (#-OH· · ·HO–Si–O–Si–OH· · ·HO-#). Finally, during curing, covalent links are formed with the substrate (Si–O-#) with concomitant loss of water. Silane covalent bonding is expected to form preferentially around pozzolanic particles (SiO2 , Al2 O3 ) [30], although even chemical reactions have been described to take place between the silane and CaCO3 [56]. Another important fact which encounters great interest for concrete containers located in humid environment is the water rising phenomenon. As regards of the capillary rise, the occurrence is expected to be reduced for hydrophobic mortars. BET analysis indicated that after mortar silanization the total pore volume was only of 0.015 cm3 g−1 in comparison with the initial value for the untreated or carbonated mortar of 0.04 cm3 g−1

Fig. 9. Cumulative volume particle size distribution for raw, supercritically carbonated (SC3h) and silanized (SC-s) granulated mortar samples measured in petroleum.

(Fig. 4a). The large reduction in the pore volume occurred in pores with very small diameter (Fig. 6), which is expected to reduce the capillary rise. The water absorption due to capillary rise of untreated and silanized mortar samples was estimated by placing monolith small pieces of these samples in a layer of water and weighing them after 1 h. The water absorption of the untreated specimen was almost 3 wt.%, while for the silanized sample the water absorption was only about 0.5 wt.%. Fig. 8 shows that the reference and supercritically carbonated mortar sample sank inside the water, while the silanized sample floated on the water surface. In addition, the silanized pieces rearranged their position in the water layer in a micellar-like arrangement in order to minimize their contact surface with water. Finally, light scattering analysis showed that the silanized sample was more easily dispersed in a hydrophobic liquid (petroleum in this case) than the raw and supercritically carbonated mortars (Fig. 9), as expected for particles hydrophobized on the surface. 4. Conclusions The supercritical treatment altered the bulk chemical and structural properties of cement by accelerating natural carbonation reaction, while at the same time reduced both free and

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bound water. Results suggested that during supercritical carbonation the rate of reaction of portlandite slowed down and was overtaken by the rate of reaction of CSH gel. The overall effects of cement paste carbonation were the formation of calcite and the reduction in the Ca/Si ratio of the CSH gel. Supercritical carbonation of mortars reduced water permeability to a large extent. The refinement of pores due to supercritical carbonation decreases the chance of transportation of aggressive fluids through pores and eventually increases the durability of concrete. Moreover, carbonation neutralizes the alkaline nature of the hydrated cement paste avoiding the propagation of an alkaline plume towards other components of the barrier. The pore water pH of carbonated matrices was 2–3 units below the pH of 12.5 normally associated with the dissolution of portlandite. Further treatment with a supercritical silane coating technique showed a remarkable effectiveness in making the mortars hydrophobic. The penetration capacity of the low-viscosity SCCO2 silane solutions inside of the mortar pores was improved compared to conventional liquid solvents containing silane. Mortar water absorption due to capillary rise was also reduced after silanization. Acknowledgements The financial support of EU Project STRP SurfaceT NMP2-CT-2005-013524 is greatly acknowledged. Carlos A. Garc´ıa-Gonz´alez gives acknowledgement to CSIC for its funding support through a I3P fellowship. References [1] H.F.W. Taylor, Cement Chemistry, Academic Press, New York, 1990. [2] S. Diamond, Aspects of concrete porosity revisited, Cement Concrete Res. 29 (8) (1999) 1181–1188. [3] C.C.D. Coumes, S. Courtois, D. Nectoux, S. Leclercq, X. Bourbon, Formulating a low-alkalinity, high-resistance and low-heat concrete for radioactive waste repositories, Cement Concrete Res. 36 (12) (2006) 2152–2163. [4] A. Macias, A. Kindness, F.P. Glasser, Impact of carbon dioxide on the immobilization potential of cemented wastes: chromium, Cement Concrete Res. 27 (2) (1997) 215–225. [5] S.K. Malhotra, N.G. Dave, Investigations into the effect of addition of fly ash and burnt clay pozzolana on certain engineering properties of cement composites, Cement Concrete Compos. 21 (1999) 285–291. [6] A. Guerrero, S. Go˜ni, A. Mac´ıas, M.P. Lux´an, Effect of the starting fly ash on the microstructure and mechanical properties of fly ash-belite cement mortars, Cement Concrete Res. 30 (2000) 553–559. [7] X. Fu, Z. Wang, W. Tao, C. Yang, W. Hou, Y. Dong, X. Wu, Studies on blended cement with a large amount of fly ash, Cement Concrete Res. 32 (7) (2002) 1153–1159. [8] I. Janotka, T. N¨urnbergerov´a, Effect of temperature on structural quality of the cement paste and high-strength concrete with silica fume, Nucl. Eng. Design 235 (2005) 2019–2032. [9] B. Bary, A. Sellier, Coupled moisture–carbon dioxide–calcium transfer model for carbonation of concrete, Cement Concrete Res. 34 (10) (2004) 1859–1872. [10] A.C. Garrabrants, F. S´anchez, D.S. Kosson, Changes in constituent equilibrium leaching and pore water characteristics of a Portland cement mortar as a result of carbonation, Waste Manage. 24 (2004) 19–34. [11] B. Johannesson, P. Utgenannt, Microstructural changes caused by carbonation of cement mortar, Cement Concrete Res. 31 (6) (2001) 925–931.

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