Carbonation of Portland Cement Mortars Including Metakaolin and Limestone Zhenguo Shi1*, Barbara Lothenbach2, Mette Rica Geiker3, Josef Kaufmann2, Sergio Ferreiro4, Jørgen Skibsted1 1. Department of Chemistry and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, 8000 C Aarhus, Denmark 2. Laboratory for Concrete & Construction Chemistry, Swiss Federal Laboratories for Materials Science and Technology (Empa), Dübendorf 8600, Switzerland 3. Department of Structural Engineering, Norwegian University of Science and Technology (NTNU), Trondheim 7491, Norway 4. Aalborg Portland A/S, Cementir Holding S.p.A., 9100 Aalborg, Denmark
Abstract Carbonation of Portland cement mortars including metakaolin and limestone has been investigated. Mortar bars of a white Portland cement (P) and of three blends with a clinker replacement level of 35 wt.%, i.e., pure limestone (L), pure metakaolin (M), and metakaolin/limestone (ML, 3:1 w/w), were cured saturated at 20 oC for 91 days and then exposed to 1% (v/v) CO2 for another 91 days at a relative humidity of 57 %. Slices with a 15 – 20 mm thickness were split from the mortar bars and the carbonation depths were measured using a 1 % phenolphthalein pH indicator. Thermogravimetric analysis (TGA) was used for quantification of portlandite and calcium carbonate. These data were compared with predictions from thermodynamic modeling. In addition, the microstructure was characterized by mercury intrusion porosimetry (MIP). The lowest and highest performance with respect to carbonation was observed for the L and P mortars, respectively. MIP showed comparable total porosity and threshold pore sizes for the P, M, and ML mortars after carbonation. TGA revealed the formation of comparable amounts of CaCO3 in the outer surface layer of these mortars, indicating similar binding capacity. Thermodynamic modeling predicted that portlandite is initially carbonated, resulting in a slight decrease in pH, and that a partial carbonation of C-S-H induces a pH decrease as reflected in the carbonation depth, indicated by the phenolphthalein indicator. This explains the apparent high carbonation resistance for the P mortar as indicated by phenolphthalein, since the P mortar contains a relative larger amount of portlandite. In contrast, mortars M and ML contain only small amounts of portlandite and the decrease of pH indicated by phenolphthalein occurs at an earlier carbonation time for these two mortars. The low carbonation resistance of the L mortar is explained by the high porosity, high initial pore connectivity and low amount of portlandite compared to the P mortar. Originality A combination of several analytical approaches, describing carbonation depths, microstructural changes and CO2 binding, have been applied to investigate the carbonation performance of mortars including white Portland cement, metakaolin and limestone. The carbonation-induced pH decrease, reflected by the carbonation depths, is attributed to partial carbonation of the C-S-H phase. The P, ML, and M blends are observed to have comparable total porosity and threshold pore sizes determined by MIP after carbonation, and similar apparent CO2 binding measured by TGA. The observed differences in carbonation performance by the phenolphthalein method cannot be explained unless a partial carbonation of the C-S-H phase is taken into account. Thermodynamic modelling predicts that partial carbonation of the C-S-H phase results in a decrease of the pH as indicated by the phenolphthalein method. Keywords: Carbonation; Portland cement; metakaolin; limestone; thermodynamic modeling
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Corresponding author:
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1. Introduction With the aim of reducing CO2 emission associated with Portland cement production, significant research efforts focus at the moment on new Portland cement blends including calcined clays as supplementary cementitious materials (SCM’s). This interest reflects that clays are widely abundant in the Earth’s crust and that significantly lower temperatures are required for their thermal activation compared to those used in a cement kiln for Portland clinker production. Limestone represents another interesting material, which is commonly added in small amounts to Portland cements, where it increases the early strength, reduces the water demand and improves the rheology of the resulting concrete (Herfort D., 2004). Furthermore, limestone provides nucleation sites for the formation and growth of the C-S-H hydration products and it is also partially consumed during hydration, resulting in the formation of monocarbonate (Lothenbach B. et al., 2008a). The combination of limestone with other SCM’s has been used to develop ternary cement blends (De Weerdt K. et al., 2011). For example, a synergetic effect between metakaolin and limestone has been observed in ternary Portland cement blends, as seen by an increase in compressive strength (Steenberg M. et al. 2011; Antoni, M. et al., 2012). The durability of such new cement blends is important and an improved under-standing of their long-term performance under severe environmental conditions, such as sulfate attack, chloride ingress and carbonation, is required before an industrial realization. The present study is a part of a range of durability investigations of Portland cement – SCM’s blends, all employing a replacement level of 35 wt.% of clinker. The present paper focusses on the carbonation of Portland cement – metakaolin blends with or without limestone. For comparison, a reference of pure Portland cement and Portland cement clinker with a 35 wt.% limestone replacement are also investigated. For pure Portland cements, the physical-chemical changes during carbonation are quite well understood. For these systems, the destabilization of the main hydration products, portlandite (CH) and the calcium-silicate-hydrate (C-S-H) phase, during carbonation has been described in terms of kinetics, microstructural changes and moisture properties (Morandeau A. et al., 2014). For Portland cement – SCM blends, the major carbonation process is attributed to carbonation of the C-S-H, since only small amounts of CH may be present in such blends. Although several studies have reported that incorporation of SCM’s in blended cements improve the microstructure of the cement paste, and consequently enhances its durability, this does not apply for carbonation processes. Most of the SCM blended cements have been reported to exhibit poor carbonation resistance (see for example, Papadakis V. G., 2000 and Antoni M., 2013). This underlines the research needs to better understand the differences in carbonation performance for Portland cement and Portland cement – SCM blends. In this work, the phenolphthalein spray method is used to monitor carbonation depths whereas mercury intrusion porosimetry is applied to monitor the changes in microstructure due to carbonation. A combination of thermogravimetric analysis and thermodynamic modeling is employed in this investigation of the actual and potential CO2 binding for different blends and to examine changes of pH in relation to phase changes. Finally, the carbonation behaviors of the studied blends are explained in terms of pore-structure changes and CO2 binding. 2. Experimental 2.1. Materials The binders used in this study were made from a white Portland cement (wPc, CEM I), limestone (LS) and metakaolin (MK). The wPc from Aalborg Portland A/S, Denmark, included 3.1 wt.% LS, 4.1 wt.% gypsum and 1.9 wt.% free lime. The MK was produced in the laboratory by thermal treatment of kaolinite (Kaolinite SupremeTM from Imerys Performance Minerals, UK) at 550 oC for 20 h. The limestone was a Maarstrichtian chalk from Rørdal, Northern Denmark. The chemical compositions of the binder materials, determined by X-ray fluorescence (XRF), are given in Table 1. The wPc contains 62.7 wt.% alite and 17.6 wt.% belite, as determined by 29Si MAS NMR. The sand used to produce the mortars is a CEN reference sand (Normensand GmbH, Germany), which has a silica content of at least 98 wt.%. A superplasticizer (SP, Glenium 27, BASF, Germany) was used to achieve a similar flow for all mortars.
Table 1: Chemical compositions of the binder materials (wt.%) SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
TiO2
P2O5
LOI
wPc
21.81
3.56
0.24
66.13
1.10
0.43
0.041
3.371
0.309
0.042
2.57
LS
3.92
0.33
0.14
53.73
0.35
0.05
0.08
0.05
0.02
0.10
41.8
MK
52.84
39.49
1.42
0.22
0.483
0.998
0.05
0.061
0.88
0.105
3.55
2.2. Binder compositions and mortar preparations The compositions of the blends (Table 2) targeted a replacement of 35 wt.% white Portland clinker by the SCM’s. This resulted in a real binder composition with 31.9 wt.% replacement of white Portland cement (wPc), considering the small amounts of LS and gypsum in the wPc. The blends were used to produce mortars with a constant water to binder ratio (w/b = 0.5) and binder to sand ratio (b/s = 1:3), both ratios by weight. The dosage of SP was adjusted to the content of MK (SP/MK=0.07) by weight to achieve a flow which was within ±5% of the flow of the reference P mortar. Mortars were cast in molds of 40 × 40 × 160 mm3. After demoulding, the mortars were cured in demineralized water at 20 oC for 91 days prior to the exposure to the CO2 environment. From the measured degrees of hydration by 27Al and 29Si MAS NMR analyses on similar blended cement paste samples, the mortars can be considered as nearly fully hydrated (Dai Z. et al., in preparation). Mortar prisms for carbonation were exposed to a controlled atmosphere with 1.0% (v/v) CO2 in an incubator at 20 oC. The relative humidity (RH) was set to target a value of 57%, which matches the RH range ~ 40 – 70% for maximum carbonation reactions (Thiery M. et al., 2007). Table 2: Compositions of the Portland cement – SCM blends (wt.%). Mortar labels wPc MK LS P 100 0 0 L 68.1 0 31.9 ML
68.1
25.5
6.4
M
68.1
31.9
0
2.3. Phenolphthalein spray test The phenolphthalein spray test is a simple colorimetric method to monitor the carbonation depth. The phenolphthalein creates a contrast between a “carbonated” zone (pH < 8.3) and a “non-carbonated” zone (pH > 8.3). The fresh split surface of the slice was sprayed by phenolphthalein and then photographed after the color was stabilized as shown in figure 1. The carbonation depths were measured on the photographs using the commercial software “GetData Graph Digitizer”.
Figure 1. Illustration of the color change (0 - 66 s) after spraying the M mortar with phenolphthalein.
2.4. Mercury intrusion porosimetry The pore structure of the mortars was characterized for the carbonated and non-carbonated mortars using mercury intrusion porosimetry (MIP). Two full subsequent intrusion cycles were performed on a Pascal 140/440 equipment. Cut slices with thickness of about 2 mm close to the surface and slices from the non-carbonated inner core were sampled and dried by solvent exchange with isopropanol for 2 days, followed by drying in an oven at 50 oC for 24 hours. The total porosity (intruded pore volume)
and the pore connectivity were obtained from the intrusion curves. The threshold pore size was acquired from the intersection of the two tangents in the intrusion curve versus pore size as described in an earlier study (Canut, M. M. C., 2011). 2.5. Thermogravimetric analysis The carbonated and non-carbonated samples were investigated by thermogravimetric analysis (TGA) in order to quantify the actual CO2 binding. First, the casting side and its parallel side were removed using a mechanical saw with a thick blade (for fast cutting). The remaining part between these two sides, where the CO2 diffusion can be considered as an one-dimension diffusion process, was then cut layer-by-layer employing a diamond saw with a thin blade (for precise cutting with thickness of about 0.2 mm). Each layer was ground into a fine powder for the TGA measurements. Approximately 50 mg of the powder was heated at 20 oC/min from 30 to 980 oC. Quantification of Ca(OH)2 and CaCO3 was performed by the tangential method. The quantified results were normalized as mass percentage of ignited mortars relative to the weight at 800 oC. 2.6. Thermodynamic modeling Thermodynamic modeling was carried out using the Gibbs free energy minimization program, GEMS3 (Kulik D. et al., 2013, Wagner T. et al., 2012), which calculates the equilibrium phase assemblages in complex chemical systems from its total bulk elemental composition. The default databases are expanded with the CEMDATA07 database (Lothenbach B. et al., 2008b) and data for distinct C-S-H phases (Kulik D., 2011), which contain solubility products of solids relevant for cementitious systems. 3. Results and discussion 3.1. Carbonation depths The carbonation depths as measured by the phenolphthalein spray method are plotted as a function of the square root of time in figure 2. No initial carbonation of the mortars is observed in accordance with the mortars being kept saturated prior to exposure. When these mortars are exposed to 1% CO2 at 57% RH, a significant degree of carbonation is observed already after 7 days of exposure for all mortars except the P mortar and the carbonation depths increase linearly as function of square root of time. Only the reference mortar (P) shows a very slow progress of carbonation and thereby the highest resistance to carbonation whereas the L mortar, containing no MK, is most vulnerable to carbonation. Moreover, the data in figure 2 reveals that neither of the MK containing blends exhibit a good resistance to carbonation.
Figure 2. Carbonation depths determined by the phenolphthalein spray test under the exposure condition of 1% (v/v) CO2 at 57% RH and 20 oC.
3.2. Pore structures Figures 3 and 4 show typical MIP intrusion curves for the non-carbonated mortars after the first and second intrusions. The samples were taken from the non-carbonated inner core of the mortars after 119 days of hydration (i.e., 91 days of curing in demineralized water and 28 days of CO2 exposure). The results in figures 3 and 4 show that the incorporation of MK in the mortars results in a refined microstructure (lower threshold pore size and larger fraction of ink-bottle pores (Kaufmann J. 2010)), as compared to that observed for the reference mortar (P), their total intruded porosities being almost identical (figure 3). When the carbonation depths are measured, it is observed that the mortars show different rates of color evolution after spraying phenolphthalein on the freshly split surface. For the P and L mortars, the change of color occurs immediately (within 1 second after spraying the phenolphthalein) whereas the change of color develops quite slowly for the ML and M mortars, as demonstrated in figure 1. Finally, the pink color of all samples reaches a comparable darkness. Differences in the pore connectivity of the mortars may explain this phenomenon. The P and L mortars have high pore connectivities, which allow for fast access of the phenolphthalein solution and causes an immediate contact between phenolphthalein and the alkaline environment. In contrast, the ML and M mortars have refined pore structures with low connectivity, resulting in an increased time of diffusion for phenolphthalein to reach the alkaline environment and thus a slower color change.
Figure 3. Intrusion curves from the first MIP Figure 4. Intrusion curves from the second MIP intrusion intrusion cycle. Non-carbonated, inner core samples. cycle. Non-carbonated, inner core samples.
Figure 5. Impact of carbonation on the microstructure based on the first MIP intrusion cycle.
Figure 6. Impact of carbonation on the microstructure based on the second MIP intrusion cycle.
The impact of carbonation on the microstructure is examined in figures 5 and 6, which illustrate the intrusion curves from the first and second MIP intrusion cycles, respectively. Figure 5 shows that the pore threshold, i.e., the breakthrough pore radius, for all types of mortars is larger after carbonation and at the same time that no significant changes of total porosity are observed for any of the mortars. The intrusion curves after the second intrusion cycle (figure 6) reveal a similar coarsening effect of the threshold pore radius for all mortars with little impact on the total porosity. It has been reported (e.g., by Morandeau A. et al., 2014) that the pore-size distribution in ordinary Portland cement paste is significantly reduced during accelerated carbonation (10% CO2, RH = 63%) as a result of clogging by the formed CaCO3 of the whole range of pores accessible to MIP (1µm to 4nm). This result is not consistent with the coarsening observed in the present work for all samples (figure 5). We expect that the coarsening effect may result in an acceleration of the carbonation, as observed for the ML and M mortars (figure 2), where the carbonation rate is not slowed down by their initial very fine threshold pore sizes. However, this is not the situation for the P mortar, where no significant acceleration of the carbonation is observed after 91 days of exposure. 3.3. CO2 binding It is clear that the carbonation performance of the different mortars cannot solely be explained on basis of the pore structure analysis. The ability of the hydrated cement to bind CO2 must also be taken into account. The phase assemblages of the hydrated samples have been predicted by thermodynamic modeling, employing the measured degrees of hydration for the principal phases (alite, belite, tricalcium aluminate, metakaolin) for the paste samples, as determined by 27Al and 29Si MAS NMR analyses of the samples after 91 days of hydration (Dai Z. et al., in preparation). Figure 7 illustrates the volume fractions of the predicted phases in the paste samples of the four blends. The major hydrate phases for paste P are C-S-H, ettringite, portlandite, and monocarbonate. Similar phases are predicted for the L paste, however, in smaller amounts as the cement is diluted by the addition of 31.9 wt.% of LS which only reacts to a small extent. For the ML and M samples, portlandite is predicted to be absent, as a result of its pozzolanic reaction with MK. The results also show that there is a relatively little increase in the total volume of C-S-H for the MK samples (note that the volume of C-S-H includes the interlayer space, but not its associated gel porosity, which is expected to be higher for the MK containing samples as compared to the pastes of the P and L blends (Kulik D., 2011)). The phase assemblages calculated by thermodynamic modeling are found to be in good agreement with experimental data (Dai Z. et al., in preparation), although, some small amounts of portlandite are still detected by XRD after 91 days.
Figure 7 Phase assemblages predicted by thermodynamic modeling for paste samples of the four blends cured saturated for 91 days at 20 oC and without any CO2 exposure.
In order to predict the CO2 binding capacity based on the phases present after 91 days of hydration, the amount of the main calcium-bearing phases relevant to cement carbonation has been calculated using molar percentages rather than volumes and related to the mass of ignited mortar (by division with a factor of 4 according to the binder/sand ratio of 3). The total amount of CaO present in the hydrates can be used to predict the maximum CO2 binding capacity for each system and these data are summarized in table 3. It should be noted that the samples used for the CaCO3 measurements were taken from the outer layer of the mortars, which have been in direct contact with the exposing environment, whereas the samples used for the portlandite measurements were taken from the noncarbonated inner core of the mortars. The measured CO2 binding capacity was obtained by quantification of CaCO3 formed during carbonation. The amount of CaCO3 originally incorporated by adding limestone in the blends is deducted from the total measured quantity of CaCO3. The results are normalized as mol percentage of 100 g ignited mortars (TGA temperature at 800 oC) in order to eliminate the impact of moisture content of the samples. The thermodynamic calculations shown in table 3 indicate that the P mortar has a significantly higher CaO content, allowing for a potentially much higher binding of CO2 as compared to the other three blends. However, the experimental data in table 3 show that the amount of CaCO3 bound in the different composite cement mortars is similar after 91 days of carbonation, indicating that carbonation proceeds at a similar rate in the different mortars, although they have different total potential CO2 binding capacities and different phases (CH or C-S-H), which bind the CO2. In this context, it should be recalled that the pore structure data in figure 5 shows that the P, M and ML mortars exhibit similar microstructural changes during carbonation. The changes in both the pore structure and measured CO2 binding data seem to contradict the observed difference in the apparent carbonation depths measured by phenolphthalein, as shown in figure 2 (further comments on this discrepancy will be given the next section 3.4). For the analyzed samples, the presence of portlandite particles with carbonated surfaces and noncarbonated centers has also been observed. This phenomenon has also been reported in other studies and related to the formation of dense calcite layers coating the portlandite crystals, thereby preventing its further carbonation (Morandeau A. et al., 2014). Table 3. Comparison of the amounts of measured CaCO3 (carbonated samples close to the mortar surface) and portlandite (non-carbonated inner core) after 91 days of carbonation and calculated total potential CO2 binding capacity [mol CaO/100g ignited mortar] of the hydrated mortars (Shi Z. et al., in preparation).
Method Phases Calculated by GEMS Ca(OH)2 C-S-H Monocarbonate Total CaO (potential binding capacity) Measured by TGA Surface: CaCO3 (carbonation) Core: Ca(OH)2 Surface: Ca(OH)2
P 0.11 0.12 0.01 0.24 0.12 0.11 0.02
L 0.07 0.10 0.006 0.18 0.10 0.07 0
ML 0.00 0.11 0.06 0.17 0.11 0.01 0
M 0.00 0.11 0.03 0.14 0.12 0.004 0
3.4. pH changes Since the carbonation depths were measured with the phenolphthalein indicator, the differences between the P and M/ML mortars with respect to their apparent carbonation performance may reflect changes in pH related to phase changes. The thermodynamic modeling predicts that portlandite is less stable than C-S-H towards carbonation, implying that carbonation of the C-S-H phase will first occur when portlandite is depleted. However, this is not always observed experimentally by bulk analyses, since portlandite particles carbonated only at the surface may be present (Morandeau A. et al., 2014). When portlandite has been carbonated, the thermodynamic modeling predicts a partial decalcification of the C-S-H phase. Both the carbonation of portlandite and the decalcification of the C-S-H are
accompanied by a decrease in pH, where the carbonation depth indicated by phenolphthalein indicator may principally reflect the pH change corresponding to partial carbonation of the C-S-H phase. The partial carbonation of the C-S-H phase occurs earlier for the ML and M mortars than for the P mortar, due to consumption of portlandite by the pozzolanic reaction with MK. This explains the shallow carbonation depths observed for the P mortar as shown in figure 2. In addition, the Ca/Si ratio of the C-S-H phase is higher for the P mortar than the ML and M mortars (Dai, Z. et al., 2014). Thus, the C-S-H phase in the P mortar has a higher CO2 binding capacity as compared to the low-Ca/Si C-S-H phases in the ML and M mortars, following the results from a recent carbonation study of synthesized C-S-H phases with different Ca/Si ratios (Sevelsted T. F. and Skibsted J. 2015). 4. Conclusions The carbonation performance of well-hydrated mortars including Portland cement, metakaolin and limestone has been investigated by the exposure to a 1% (v/v) CO2 concentration at RH = 57 % for up to 91 days. The highest and lowest carbonation performance was observed for a pure Portland cement mortar and a binary Portland cement – limestone blend, respectively. The following principal conclusions can be drawn: (1) Thermodynamic modeling predicts firstly the carbonation of CH and then carbonation of the C-SH phase which results in a decalcification of the C-S-H. The partial carbonation of the C-S-H may explain the pH decrease reflected by the carbonation depth observed by the phenolphthalein indicator. (2) For Portland cement and Portland cement, metakaolin and limestone mortars, MIP has shown comparable total porosities and threshold pore sizes after carbonation. TGA has revealed that they have similar measured CaCO3 content in the surface layer after 91 days of carbonation, indicating similar CO2 binding. The partial carbonation of the C-S-H phase starts earlier for the metakolincontaining blends due to a much smaller amount of portlandite in these mortars, which led to a deeper carbonation depth measured by the phenolphthalein indicator method. (3) For the Portland cement – limestone mortar, the amount of portlandite is higher than in the metakaolin-containing mortars. However, this did not improve its carbonation resistance, which is ascribed to a very high initial total porosity and pore connectivity for the Portland cement – limestone mortar. (4) A coarsening effect after carbonation has been observed for all mortars with an increase of the threshold pore sizes measured by MIP. Acknowledgements The Danish Council for Strategic Research is acknowledged for financial support to the LowE-CEM project. References - Antoni M. et al., 2012. Cement substitution by a combination of metakaolin and limestone. Cement and Concrete Research, 42, 1579 - 1589. - Antoni, M., 2013. Investigation of cement substitution by blends of calcined clays and limestone. PhD dissertation, École Polytechnique Fédérale de Lausanne. - Canut M.M.C., 2011. Pore structure in blended cement pastes. PhD dissertation, Technical University of Denmark, Department of Civil Engineering. - De Weerdt K. et al., 2011. Synergy between fly ash and limestone powder in ternary cements. Cement and Concrete Composites, 33, 30 - 38. - Dai Z., Tran T.T., Skibsted J., 2014. Aluminum incorporation in the C-S-H phase of white Portland cement – metakaolin blends by 27Al and 29Si MAS NMR spectroscopy. Journal of the American Ceramic Society, 97, 2662 - 2671. - Dai Z., Kunther W., Ferreiro S., Herfort D., Skibsted J., 2015. Investigation of blended systems of supplementary cementitious materials with white Portland cement and limestone, (manuscript in preparation). - Herfort D., 2004. Challenges of cement production. Anna Maria 2004 Workshop on Cement and Concrete, Anna Maria Island, Florida, USA.
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