Effect of carbonation in supercritical CO2 on the properties of hardened cement paste of different alkalinity

Construction and Building Materials 123 (2016) 704–711

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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat

Effect of carbonation in supercritical CO2 on the properties of hardened cement paste of different alkalinity L. Urbonas ⇑, V. Leno, D. Heinz Technische Universität München, Chair of Mineral Engineering, Germany

h i g h l i g h t s
Large increase in compressive strength of hardened cement paste and mortar.
Significant changes in porosity and structure of hardened cement paste.
Unfavourable reduction in carbonation rate and strength due to alkalis.

a r t i c l e

i n f o

Article history: Received 28 April 2016 Received in revised form 12 July 2016 Accepted 15 July 2016 Available online 21 July 2016 Keywords: Portland cement Carbonation Supercritical carbon dioxide Alkalis

a b s t r a c t It is well-known that the strength of cementitious building materials is increased by carbonation. This process was accelerated using supercritical carbon dioxide scCO2 at 150 bar. Hardened Portland cement pastes with different alkali contents and w/c ratios of 0.5 and 0.6 were treated for four hours in scCO2 after preliminary storage at different relative humidities. Strengths were measured and the microstructure investigated by X-ray diffraction, scanning electron microscopy and mercury intrusion porosimetry. Carbonation in scCO2 produces a substantial increase in strength which is favoured by low moisture contents and low-alkali cements. The retarding effect of alkalis on the carbonation reaction was confirmed by thermodynamic calculations. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The natural carbonation of concrete in air is a slow process which lowers the pH of the pore solution of surface concrete until ultimately depassivation of the steel reinforcement occurs and corrosion is initiated. However, carbonation can be advantageous because, under certain conditions, it can increase the strength of cementitious construction materials [1,2]. In the 1990’s American patents were published for the treatment of cementitious and lime-based materials with supercritical CO2 (scCO2) in order to enhance strength [3]. Subsequent scientific publications on the effect of scCO2 dealt mainly with changes in phases and microstructure of hardened cement paste [4–6]. Hidalgo et al. [5] treated hardened cement paste powder and granulate and observed complete decomposition of portlandite, C-S-H-phases and ettringite to form calcium carbonate mainly as the modification calcite. The carbonation of C-S-H phases leads to the formation of a silica gel network [5–7] accompanied by a significant decrease in porosity [4,6,7] and a substantial increase in compressive ⇑ Corresponding author. E-mail address: [email protected] (L. Urbonas). http://dx.doi.org/10.1016/j.conbuildmat.2016.07.040 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.

strength [8]. Short et al. [6] found that the moisture content of hardened cement paste affects the processes occurring during treatment with scCO2; the most extensive change in phase composition occurred after storage at about 35 %RH before treatment. Favourable effects of scCO2 treatment on the strength of cementbonded glass fibres [8], fibreboard [9] and other cementitious materials produced by pressing [10] have been reported. Regarding the feasibility of underground CO2 storage, some recent work has considered the degradation of hardened cement paste in supercritical CO2 or in water saturated with CO2 at high pressures [11,12]. It was found that the initial densification of the microstructure produced under these conditions was followed by degradation and an increase in porosity after longer periods of treatment (about 6 weeks). Little or no attention has been paid in earlier publications to the effect of cement composition on carbonation in scCO2 and strength development. It is expected that especially the alkali content of the cement, which changes pore solution chemistry, will affect the carbonation process. The literature contains different and sometimes conflicting opinions on the effect of alkalis on the rate of carbonation. Whereas according to Goni et al. [13], the rate of carbonation of hardened cement paste is hardly affected by alkalis under

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accelerated conditions, Kobayashi et al. [14] report a distinct reduction in carbonation resistance of concrete produced by increasing the alkali content of the cement. Using Portland cement (CEM I) and Portland cement with high sulphate resistance (CEM SR) in experiments at atmospheric pressure and with scCO2, Garcia-Gonzalez et al. [15] observed an increase in the carbonation rate of hardened cement paste with alkali content. The possible effect of the mineralogical composition and microstructure (i.e. porosity and pore size distribution) of the hardened cement paste was not considered. However, Reschke and Gräf [16] found that higher alkali contents inhibit the progress of carbonation and explained this by the faster initial reaction of cements rich in alkalis. The authors suggest that carbonation is retarded by the higher concentration of hydroxide ions in the pore solution which must be neutralized by the carbonation reaction. Regarding the development of innovative cementitious materials based on treatment with supercritical CO2, the alkali content of the cement and the moisture content of the binder paste are of central importance. The present work focusses on the effect of w/c ratio and moisture content on the phase composition, pore structure and strength of hardened cement paste made with two commercial Portland cements with different alkali contents.

2. Experimental methods and materials The chemical compositions of the cements were determined using ICP OES. The crystalline mineral phases were analysed with X-ray diffraction (theta-theta goniometer 3003 TT, Agfa NDT Pantak Seifert) using the Rietveld refinement (software Autoquan); 20% ZnO was added as an internal standard for the quantitative determination of the amorphous contribution. Thin sections of hardened cement paste specimens were prepared for investigations with scanning electron microscopy (SEM Jeol JSM5900LV). Selected positions were quantitatively analyzed with an EDX system (Röntec). The porosity and pore size distribution (4 nm to 300 lm) of the specimens were determined with mercury intrusion porosimetry (AutoPore III, Micromeritics). The carbonation depths were measured by spraying freshly fractured samples with a phenolphthalein indicator solution with accuracy of ±0.4 mm. This indicator appears pink in contact with alkaline hardened cement paste with pH values in excess of about 9 and colourless at lower levels of pH. This method is therefore not suitable for the characterisation of complete carbonation [10], but only for approximately determination of the carbonation front. The strength of the specimens was determined according to EN 196-1. The compressive strength was determined to within ±1.8 MPa, the flexural strength to within ±0.4 MPa. Individual strengths with values above 10% of the mean were discarded. Two Portland cements CEM I 32.5 R with different alkali contents were used whose chemical and mineralogical compositions listed in Tables 1 and 2. Pastes were mixed with both cements at w/c ratios of 0.5 and 0.6. To prevent sedimentation of the flowable pastes in the moulds (160  40  40 mm3), the fresh pastes were mixed and immediately rotated in wide-mouth bottles until a mini-slump of 7 ± 1 cm was measured before pouring. The bars were demoulded after a period of 24 h at 20 °C and >95 %RH. To avoid cracking, the

Table 2 Mineralogical composition of the Portland cements in wt.%. Cement

C3S

C2S

C3A

C4AF

C1 C2

53.6 45.7

11.3 23.7

6.36 8.7

12.6 8.7

relative humidity of surrounding air then was reduced in slow steps before storage at 20 °C in climatic chambers at relative humidities of 35%, 50% or 65%. The CO2 treatment was then carried out at an age of 16 d. The mass and length of each specimen were measured immediately before CO2 treatment and the residual moisture determined by drying specimen fragments at 40 °C or 105 °C until the change in weight was negligible (Table 3). In addition, mortar bars 40  40  160 mm3 with w/c = 0.5 and a sand/cement s/c ratio of 3 were produced according to EN 196-1 and stored under the same conditions as the hardened cement paste bars. To investigate the effect of alkalis on carbonation in more detail, the alkali content of the Portland cement C2 with (Na2Oeq = 0.56 wt.%) was increased by adding K2SO4 (hardened cement paste) or NaOH (mortar) to the mixing water thus increasing the Na2Oeq to in effect that of C1 (1.02 wt.%). The hardened cement paste bars were treated in a pressure reactor (Fig. 1) with scCO2 (H2O-Amount < 50 ppm) at a pressure of 150 bar and a controlled temperature of 50 °C; the critical point for carbon dioxide is at 72.8 bar and 31.2 °C (Fig. 2). The relatively high pressure of 150 bar was chosen to accelerate the penetration of carbon dioxide into the specimens. The pressure was increased continuously over a period of one hour up to 150 bar. Due to the exothermic carbonation reaction, the temperature reached on average about 75 °C during the pressurization of hardened cement paste specimens and up to 60 °C in the case of the mortar specimens (Fig. 3). The temperature was thermostatically controlled at 50 °C during the treatment after which the pressure was reduced over a period of one hour to the ambient value. Following removal from the pressure reactor, the specimens were covered with damp cloths and cooled to 20 °C before measuring carbonation depths and changes in length, weight and strength. Measurements were also carried out with parallel non-carbonated reference samples of the same age. Isopropanol was used to stop the hydration reaction of specimen fragments, remaining from the strength measurements, for determinations of porosity and pore size distribution (MIP), microstructure (REM) and mineralogical composition (XRD). If the bars were not thoroughly carbonated after treatment, specimens were taken from the carbonated region. The specimens for XRD analysis were dried at 40 °C and ground to <32 lm. 3. Results and discussion 3.1. Carbonation depth The carbonation depths measured after treatment depended on the w/c ratio, the relative humidity of the storage environment before treatment and the alkali content of the cement (Fig. 4). The hardened cement paste specimens with a w/c ratio of 0.6 were fully carbonated after 4 h scCO2 treatment. The carbonation

Table 1 Oxide composition of the Portland cements in wt.%. Cement

SiO2

Al2O3

Fe2O3

CaO

MgO

SO3

K2O

Na2O

Na2Oeq

C1 C2

18.70 22.09

5.01 4.95

3.53 2.80

61.34 62.55

1.64 0.99

2.81 2.70

1.20 0.46

0.24 0.26

1.02 0.56

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Table 3 Residual moisture (with respect to initial weight) of the hardened cement paste specimens at an age of 16 d before carbonation, depending on storage conditions and drying temperature. Storage conditions °C/%RH

Residual moisture [wt.%], drying at 40 °C C1 w/c = 0.5

C1 w/c = 0.6

C2 w/c = 0.5

C2 w/c = 0.6

20/35 20/65

5.9 10.1

6.1 10.2

6.2 9.8

4.9 9.2

20/35 20/65

Residual moisture [M.-%], drying at 105 °C 8.4 8.4 9.2 7.9 12.7 12.5 12.7 12.0

Fig. 3. Typical evolution of pressure and temperature in CO2 reactor.

paste than the low-alkali cement C2. To establish whether the carbonation rate depends mainly on the C3S or alkali content of the cement, the alkali content of the low-alkali cement C2 was – as described above – increased to that of C1 by adding K2SO4 (hardened cement paste) or NaOH (mortar). Alkali addition resulted in lower carbonation depths now corresponding to C1 (Fig. 4), i.e. the carbonation rate decreases with increasing alkali content of the cement. 3.2. Compressive and flexural strength

Fig. 1. Pressure reactor for treatment with scCO2.

p [bar] Supercritical fluid

1000 Solid

Liquid

100 Critical point

10 Triple point

Gas

1 -1

10 -100

-50

0

50

100

T [°C] Fig. 2. Phase diagram of carbon dioxide. Red point – treatment conditions [17].

depth of the specimens with a w/c ratio of 0.5 increased with decreasing storage relative humidity. This is probably because higher moisture contents of the pore system after storage at higher relative humidities led to more rapid filling of the pores with water produced by the carbonation reaction and therefore a significant reduction in the rate of CO2 penetration. The carbonation depths of specimens containing cement C1 with the higher alkali content were generally lower than that of C2. However, cement C1 contains more alite and therefore slightly more portlandite in the hardened

The scCO2 treatment significantly increased the compressive strength of the hardened cement paste specimens (Fig. 5). The compressive strengths of the specimens with a w/c ratio of 0.5 and stored at 35 %RH almost doubled the strengths of the noncarbonated reference specimens. In general, higher strengths were observed for larger carbonation depths. The strengths of the specimens with the low-alkali cement C2 were higher than those with cement C1. The relative humidity during the preliminary storage of specimens with w/c = 0.6 had little effect on strength because CO2 was able to completely penetrate and completely carbonate these specimens. Compared with the other relative humidities, storage at 50 %RH produced a somewhat higher strength gain of 250% with respect to the reference specimens. The larger increase in strength and higher absolute compressive strength were exhibited by the specimens with low-alkali cement C2. Treatment with scCO2 resulted in no appreciable increase in flexural strength of the specimens. The individual values varied greatly which was probably connected with micro cracks formed on the surface of the specimens during the preliminary storage. scCO2 treatment also increased the strength of the mortar specimens by a factor of approximately two (Fig. 6). The compressive strength of the carbonated mortar with cement C2 was at 67.2 MPa substantially higher than the standard strength of this cement of 51.7 MPa at an age of 28 d (EN 196-1). The increase in compressive strength and absolute strength of the mortar (C2M_NaOH) with alkali enriched cement were slightly below those of the mortar with cement C2. This agrees with the lower degree of carbonation occurring at higher alkali contents. Carbonation led to a small decrease in flexural strength compared with the reference specimens. In contrast, Farahi et al. [10,18] report an increase the flexural strength of mortar specimens after scCO2 treatment. However, the mortar composition (Portland cement and lime, partly calcitic aggregate), the specimen preparation (pressing with vacuum dewatering) and preliminary treatment deviate strongly from the present investigations.

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Fig. 4. Carbonation depths of hardened cement paste (C1 and C2, w/c = 0.5) and mortar (C2M) specimens made with cements C1, C2 or alkali enriched cements C2_K2SO4 and C2M_NaOH after preliminary storage at different relative humidities and treatment in scCO2 at 50 °C and 150 bar.

Fig. 5. Compressive strengths of hardened cement paste specimens with and without scCO2 treatment after storage at different relative humidities.

on the incorporation of the reaction water in the silica gel and carbonated C-S-H phases. The carbonation of portlandite results in a volume increase between about 3% and 14% depending on the CaCO3 polymorph. Opposed to this, it is known that the carbonation of the C-S-H-phases leads to shrinkage processes in the hardened cement paste. According to [19] and [20] this shrinkage is mainly due to the formation of highly polymerized crosslinked silica gels and the associated volume reduction. Whether the hardened cement paste swells or shrinks during the carbonation with scCO2 also depends on the porosity of the specimens. In the case of the specimens with w/c = 0.6, the pore space is sufficient to incorporate an additional volume of the solids, without generating internal stress and therefore length changes. Consequently, the carbonation of the C-S-H phases induced the contraction of the samples. 3.4. Porosity and pore size distribution

Fig. 6. Compressive and bending strengths of mortar specimens (cement C2) with and without (reference) scCO2 treatment.

The reference hardened cement paste specimens possessed relatively high total porosities 38 vol.% (w/c = 0.5) and about 45 vol.% (w/c = 0.6) which were only slightly affected by the cement chosen or the addition of alkalis. However, the reference specimens with the low-alkali cement C2 und w/c = 0.6 possessed a higher proportion of coarser pores (d > 100 nm) than the specimens made with cement C1. The specimen with cement C2 and w/c = 0.5 was slightly lower in total porosity, but slightly higher in capillary porosity, than the specimens made with cement C1. The addition of K2SO4 to cement C2 hardly changed the porosity and pore size distribution of the hardened paste. Treatment with supercritical CO2 led to a significant reduction of the porosity due to the formation of CaCO3 and an increase in the proportion of gel pores (<10 nm) due to the formation of silica gel from the C-S-Hphases (Figs. 7 and 8). The reduction in total porosity of specimens with cement C2 was slightly higher than for specimens with cement C1 or C2 with added alkalis.

3.3. Change in volume

3.5. Mineralogical investigations

Regardless of cement composition, an increase in length of about 1 mm/m was observed immediately after the treatment of the hardened cement paste specimens with w/c ratio of 0.5. The mortar specimens (also w/c = 0.5) increased in length to 0.28 mm/m. In contrast, the hardened cement paste specimens with w/c = 0.6, and therefore a higher porosity, decreased in length by about 1 mm/m. The increase in length in the specimens with a w/c ratio of 0.5 may be due to (a) the increase in the volume of the solid phases on the formation of forming CaCO3 from portlandite and (b) swelling

The different w/c ratios and relative humidities during preliminary storage did not appreciably affect the mineralogical composition of hardened cement paste specimens. As can be seen in Fig. 9, the reference specimens contained portlandite, ettringite and, as the main constituent, X-ray amorphous compounds probably attributable to the C-S-H phases. The specimens prepared with cement C1, which contains more alite, also possess slightly higher residual amounts of C3S. Furthermore, calcite and the other residual clinker minerals belite (b-C2S), brownmillerite (C4AF) as well as small amounts of C3A are present.

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Fig. 7. Effect of scCO2 treatment (4 h at 50 °C and 150 bar) on the porosity of the hardened cement paste specimens with w/c = 0.6.

Fig. 8. Effect of Na2Oeq on the porosity of hardened cement paste specimens (w/c = 0.5) with and without scCO2 treatment (4 h at 50 °C and 150 bar).

Fig. 9. Change in the mineralogical composition of hardened cement paste specimens (w/c = 0.5, preliminary storage at 35 %RH) due to treatment with scCO2 (carbonated region of specimens).

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709

scCO2 treatment clearly resulted in a significant change in the mineral phases composition of the hardened pastes. Portlandite was transformed into calcium carbonate phases and the content of C-S-H phases (X-ray amorphous fraction) decreased significantly on the formation of silica gel.

that the C/S ratio of these particles was very low ranging between 0.2 and 0.8.

CaðOHÞ2 þ CO2aq ¼ CaCO3 þ H2 O

The present results show that the carbonation of hardened cement paste with scCO2 is a rapid process which completely reforms the microstructure. Low viscosity and good wetting properties favour the transport of scCO2 into hardened cement paste. The carbonation rate and therefore strength development depend on cement and paste composition. Cement C1 with more alkalis and C3S exhibited a lower rate of carbonation and strength gain. Possible causes for this behaviour are as follows.

ð1Þ

CaO-SiO2 -H2 O þ CO2aq þ ðn þ z  1ÞH2 O ¼ CaCO3 þ SiO2  nH2 O þ zH2 O

ð2Þ

Moreover, the residual clinker minerals also reacted during scCO2 treatment, in particular the calcium silicates. Here calcium dissolved in the carbonic acid thus entering the pore space of the hardened paste. Thus C3S, for example, is replaced by highly crosslinked silica gel with a low calcium content.

3CaO  SiO2 þ 3CO2aq þ nH2 O ¼ 3CaCO3 þ SiO2  nH2 O

ð3Þ

Calcium carbonate, composed of the three polymorphs calcite, aragonite and vaterite, was the main phase after carbonation. The alkali content of the cements had no appreciable effect on the quantitative distribution of the polymorphs. The small amounts of portlandite in the specimens with cements C1 and C2 are due to the hydration of non-carbonated alite and belite directly after scCO2 treatment. As well as the absence of ettringite, crystalline phases with sulphate (e.g. gypsum, syngenite, arcanite) or aluminium hydrates (e.g. gibbsite) were not identified by XRD or SEM in the scCO2treated specimens. According to thermodynamic calculations [21] gypsum and aluminium hydroxide are expected for the given conditions.

3CaO  Al2 O3  3CaSO4  32H2 O þ 3CO2aq ¼ 3CaCO3 þ 2AlðOHÞ3 þ 3ðCaSO4  2H2 OÞ þ 23H2 O

ð4Þ

However, with increasing alkalinity of the pore solution the concentration of sulphate ions is expected to increase and, correspondingly, the amount of gypsum will decrease. Part of the sulphate in the pore solution, originating from ettringite or gypsum produced by carbonation, exists as a counter ion to the alkali metal ions. During scCO2 treatment a considerable amount of pore water formed which flowed out of the specimens thus removing alkali metal sulphates from the specimens. 2 CaSO4 þ CO2 3 ¼ CaCO3 # þSO4

ð5Þ

No crystalline aluminium phases were identified by XRD after treatment with scCO2. Earlier studies using 27Al NMR [7] have shown that aluminium is incorporated in the silica gel produced by carbonation. 3.6. Changes in microstructure Before treatment, the hydrated cement pastes contain residual clinker minerals C3S, C2S and C4AF as well as C-S-H phases and portlandite. No significant differences between the cements were observed by SEM (Fig. 10). The scCO2 carbonation led to significant changes in the microstructure of the hardened pastes. Mainly irregularly shaped compact particles formed in a denser microstructure as confirmed by the decrease in porosity observed by mercury intrusion porosimetry (Fig. 8). The backscattered electron images reveal mainly bright C4AF particles and darker areas rich in Si (SiO2xH2O) embedded in a fine matrix mainly composed of carbonates. Only a few alite and belite particles remained (not shown in the figures). They had reacted with the acidic pore solution to form silica gel particles (up to 30 ml) as relics at the original positions of the alite or belite particles. EDX analysis showed

3.7. Chemical reactions occurring during scCO2 treatment


Faster hydration of the alkali-rich cement C1 at early ages produces a denser microstructure. Slightly lower capillary porosities (32.0 vol.% for C2 and 30.7 vol.% for C1, hcp, w/c = 0.5) were measured for the reference sample with cement C1 after preliminary storage. The amount of amorphous phases (C-S-H) was similar for both cements. However, somewhat more portlandite was present in the specimens produced with cement C1 containing more C3S.
Soluble alkalis affect the rate of carbonation directly. This is supported by the reduction in carbonation rate produced by adding alkalis to cement C2. Thermodynamic calculations were performed to investigate the effect of alkalis on carbonation. Using the hydrogeological simulation program PHREEQC and LLNL database [22], the solubility of CO2 gas in water at 150 bar and 50 °C (2.9 mol/kg) is about 73 times higher than under normal conditions (0.038 mol/kg). However, small amounts of water dissolve in scCO2 [23]. One can assume that scCO2 dissolves in pure water to form a relatively strong carbonic acid with a pH of about 2.9 and species (CO2 2 (aq), H2CO3, HCO 3 , CO3 ) corresponding to the well-known CO2 gas/water equilibrium.

xCO2ðgÞ þ yH2 O () aCO2ðaqÞ þ bðH3 Oþ þ HCO3 Þ þ cð2H3 Oþ þ CO2 3 Þ ð6Þ At the beginning of the scCO2 treatment, penetrating CO2 molecules dissolve in the highly alkaline pore solution whose concentration depends on the alkali content of the Portland cement. The concentration of alkalis in the pore solution of hardened cement paste with w/c = 0.5 and an alkali equivalent Na2Oeq of 1.02 wt.% (cement C1) is about 800 mmol/kg (molal) according to [24] and unpublished own investigations in the case of cement C2 with Na2Oeq = 0.56 wt.%, 450 mmol/kg. Based on weight loss during the preliminary storage at 20 °C and 35 %RH, the alkali concentrations increased by a factor of approximately four. This means that the penetrating CO2 reacts with highly concentrated alkaline pore solutions with OH concentrations of about 3.2 mol/kg (C1) or 1.8 mol/kg (C2). The effect of the alkalinity of the pore solution on the concentration of dissolved carbonates was simulated thermodynamically. Fig. 11 shows the equilibrium concentrations of carbonates as a function of potassium concentration. The total concentration of dissolved carbonate increases with the concentration of alkalis in the solution. Owing to the low pH, carbonate is mainly present as HCO-3. The pH increases with increasing alkali content. At the same time, the concentration of the carbonic acid decreases indicating a decrease in the rate of carbonation. As scCO2 penetrates into the hardened paste, the concentrations of carbonate and calcium (supplied by the dissolution of portlandite and C-S-H phases) in the pore solution increase at the penetration front until the solubility of CaCO3 is exceeded

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C1_ref

C2_ref

C2S

C2S, C3S

CSH

Ca(OH)2

C2S, C3S

CSH C2S, C3S

C1_carb

C2_carb

C4AF SiO2·xH2O

CaCO3

C4AF SiO2·xH2O

SiO2·xH2O C4AF

CaCO3

Fig. 10. Scanning electron micrographs (BSE) of polished sections of hardened cement paste specimens (w/c = 0.5) before and after 4 h in scCO2.

Fig. 11. Effect of alkali concentration on equilibrium concentrations of carbonates and pH at 50 °C and a CO2 pressure of 150 bar. The concentration of CO2 is 3 negligible. As in the usual convention, the H2CO3 concentration includes the dissolved gas CO2(aq).

and it precipitates. Fig. 12 shows the solubility limit of calcite with respect to the addition of CO2 to potassium hydroxide solutions, in the presence of portlandite. Solutions with more potassium are able to contain a larger amount of carbonate before the calcite precipitates. Consequently, the pore solution in the hardened cement paste from the alkali-rich cement (C1, 3.2 mol/kg OH-) can dissolve above 2.4 more CO2 than the cement with the low alkali content (C2, 1.8 mol/kg OH-), i.e. calcite formation is retarded by the alkalis

Fig. 12. Calculated solubility limit for calcite formation in potassium hydroxide solutions in the presence of portlandite.

because the CO2 binding capacity is, in effect, increased. This supports Reschke and Gräf [16] who suggested that higher concentrations of hydroxides in the pore solution retard carbonation. 4. Conclusions The strength increase obtained by treating hardened cement paste with scCO2 is given by the degree of carbonation of the materials. This depends on cement and mix composition, especially the alkali content, as well as the moisture content of the hardened

L. Urbonas et al. / Construction and Building Materials 123 (2016) 704–711

paste. Whereas hardened cement paste specimens with w/c = 0.6 were completely carbonated after 4 h in scCO2, the degree of carbonation of specimens with w/c = 0.5 was lower owing to their lower porosity and smaller proportion of capillary pores. Water formed by the carbonation reaction fills pores thus reducing the transport of scCO2 into the depth of the hardened paste and therefore the rate of carbonation. This is effect is more pronounced at higher moisture contents because the pores already contain water. The rate of carbonation in scCO2 is slower with high-alkali cement. This is explained by the higher solubility of carbonic acid due to the increase pH of pore solutions containing alkalis, i.e. calcite formation is retarded by the alkalis because the CO2 binding capacity is increased. The degradation of hydrates and residual clinker minerals to form CaCO3, mainly calcite, results in a denser microstructure and an increase in compressive strength up to 250%. Larger gains in strength and absolute strengths are obtained with low-alkali cement. Acknowledgments The authors express their thanks to the German Research Foundation (DFG) for supporting this project financially. Thanks are due to Dr. R.E. Beddoe for discussions, helpful suggestions and revision of this paper. References [1] C.F. Chang, J.W. Chen, Strength and elastic modulus of carbonated concrete, ACI Mater. J. 102 (2005) 315–321. [2] C. Shi, F. He, Y. Wu, Effect of pre-conditioning on CO2 curing of lightweight concrete blocks mixtures, Constr. Build. Mater. 26 (2012) 257–267. [3] Cement treated with high-pressure CO2. US Patent Nr. 5,518,540; 1996. [4] C.A. García-González, A. Hidalgo, C. Andrade, C. Alonso, J.F. Sainz, A.M. LópezPeriago, C. Domingo, Modification of composition and microstructure of Portland cement pastes as a result of natural and supercritical carbonation procedures, Ind. Eng. Chem. Res. 45 (2006) 4985–4992. [5] A. Hidalgo, C. Domingo, C.A. García-González, S. Petit, C. Andrade, C. Alonso, Microstructural changes induced in Portland cement-based materials due to natural and supercritical carbonation, J. Mater. Sci. 43 (2008) 3101–3111.

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