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Influence of slag incorporation on electrochemical behavior of carbonated cement
Construction and Building Materials 147 (2017) 661–668
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Influence of slag incorporation on electrochemical behavior of carbonated cement Qiwen Qiu, Zhentao Gu, Jiaqi Xiang, Canjie Huang, Shuxian Hong, Feng Xing, Biqin Dong ⇑ Guangdong Province Key Laboratory of Durability for Marine Civil Engineering, School of Civil Engineering, Shenzhen University, Shenzhen 518060, Guangdong, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Slag effect on electrochemical
behavior of carbonated cement is studied. Slag cement exhibits larger semicircles of Nyquist curve than plain cement paste. An increase of slag content can hinder the ion transport process in cement system. Slag cement has less resistance of ion transport than fly ash cement.
a r t i c l e
i n f o
Article history: Received 30 December 2016 Received in revised form 24 April 2017 Accepted 1 May 2017
Keywords: Slag blended cement Carbonation Electrochemical behavior Electrochemical impedance spectroscopy Prediction
a b s t r a c t A research work is carried out to understand the influence of slag incorporation on the electrochemical behavior of carbonated cement systems. Measurement of carbonation depth and electrochemical impedance spectroscopy is conducted. A suitable equivalent circuit model is applied to fit the measured electrochemical impedance data (Nyquist curve) and quantitatively obtain the electrochemical property for carbonated slag blended cement. Test results demonstrate that increasing the slag replacement ratio in cement can increase the carbonation depth, enlarge the semi-circle of the Nyquist curve, and improve the resistance of ion transfer at the ‘‘solid-liquid phase” interface. We also compare the slag incorporation effect on the electrochemical system of carbonated cement with another common mineral admixture (i.e. fly ash). It is found that slag blended cement exhibits smaller semi-circle of Nyquist curve and larger carbonation depth than fly ash blended cement. Based on the electrochemical modification, we also predict the carbonation depth of blended cements with different slag contents. Ó 2017 Published by Elsevier Ltd.
1. Introduction As an essential component of concrete, ordinary Portland cement is one of the most widely used building materials around the world. In China, the cement production has grown rapidly
⇑ Corresponding author. E-mail address: [email protected] (B. Dong). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.008 0950-0618/Ó 2017 Published by Elsevier Ltd.
alongside the national economy since the 1980s [1]. However, a large amount of carbon dioxide (CO2) is emitted from the fossil fuel combustion and the calcination process in cement industry [2]. Generally, one ton cement production can release one and a half tons CO2 to the atmosphere, and it is estimated that CO2 emission will alarmingly increase to 38.8 109 tons by 2025 [3]. Such high CO2 emission would lead to escalate the global greenhouse effect. Moreover, cement production consumes a large amount of energy
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and accounts for a large portion of the financial expenditure in the construction market [4–8]. To remedy these problems, a promising solution is using the mineral admixtures which partially replace the cement constituent. With the development of construction technology, slag has been widely used as a mineral admixture for concrete materials. It is obtained as a by-product or waste material from the ferrous and nonferrous metal industries [9–16]. Slag has numerous advantages which favor it as a constituent of cement, for example, the low initial capital cost of the raw material and the associated energy savings [17,18]. Despite the above beneficial aspects, aggressive chemical substances from the outside environment can penetrate into the blended cement through the capillary pores [5,19–23]. When CO2 penetrates the cementitious materials in this way, carbonation occurs. Carbonation is a neutralization reaction that can lead to reduce the alkalinity of pore solution in cement and concrete materials owing to the consumption of hydrate cement compounds [24–27], which is an unwanted chemical phenomenon for steel-reinforced concrete structures [28,29]. Based on the alkalinity change, conventional testing methods (e.g. phenolphthalein indicator [30,31], pH meter [32]) have been applied for carbonation characterization. However, these methods require the damage of specimen, and the measurement accuracy depends on the test duration and the experience of inspectors. Repairing the damage after measurement by using phenolphthalein indicator also requires much time and heavy workload. In addition to the chemical modification, previous studies have indicated that carbonation can tremendously affect the electrochemical behavior (e.g. resistance of ion transfer and diffusion) of various cement systems [33–36]. This indicates that the carbonation assessment can be achieved by obtaining electrochemical characteristics of carbonated cement systems with the use of electrochemical measurement techniques like electrochemical impedance spectroscopy (EIS). EIS method is non-destructive, steady-state, sensitive, highly reproducible, and relatively easy to be used in construction field. The equipment and devices used in EIS measurement are easily available from electrochemical community, which are not so expensive especially for the portable EIS instruments. The data analysis and post-treatment can be performed with computer program and the results can be visualized automatically. From the engineering point of view, investigating the factors influencing the electrochemical behavior of carbonated cementitious materials is an essential step to develop a reliable electrochemical-based approach for analysis of carbonation of different cement-based materials. From the review of past research work, effects of water-to-cement ratio [36], river sand aggregate [37], and fly ash incorporation [38] on the electrochemical performance of carbonated cementitious material have been reported. Nevertheless, the role of slag on the electrochemical behavior of cement under carbonation has not yet been well clarified thus far. As compared to fly ash, slag material is self-cementing and does not require calcium hydroxide to form cementitious products, resulting in a distinct pore structure in cement system [39–42]. As a result, a special carbonation behavior as well as electrochemical behavior can be formed for slag blended cement. Understanding the impact of slag on the electrochemical property of carbonated cement would be of great importance for durability design of blended cement lining with an excellent carbonation resistance. The objective and originality of study is to investigate the electrochemical behavior for carbonated cement systems with slag incorporation. The impedance spectra of slag blended cement, fly ash blended cement and plain cement is compared. Factor of slag content on the carbonation depth and the electrochemical system is evaluated. An equivalent circuit model is applied to quantitatively analyze the EIS data. Furthermore, the electrochem-
ical property of slag blended cement is correlated with the carbonation depth. Finally, the prediction of carbonation depth is performed. 2. Materials and methods 2.1. Specimen details Type I ordinary Portland cement was used in accordance with GB175-2007 [43], which was obtained from the Onoda Cement Limited Company in Shenzhen, China. The percentages of calcium silicate (C3S), dicalcium silicate (C2S), tri-calcium aluminate (C3A), and tetra-calcium aluminoferrite (C4AF) in cement were 65.35%, 5.06%, 10.23%, and 10.40%, respectively. Class I slag and fly ash was obtained from the Mawan power plant in Shenzhen, China, according to the standard GB/T 180462008 [44]. The chemical components of cement, slag and fly ash are shown in Table 1. Cement without incorporation of slag was fabricated as a control specimen. In order to figure out the different effects between slag and fly ash, the fly ash blended cement was also fabricated. Four slag contents were considered in the slag blended cement. The mix proportions of all the cements are given in Table 2. The dimensions (width height length) of all specimens were 40 mm 40 mm 160 mm. The cement raw material, mineral admixture and distilled water were mixed in a treater for approximately 90 s. The mixture was then cast in a steel mold and compacted by using a vibrating table. The steel mold was removed after 24 h of the matrix hardening under ambient conditions. Thereafter, the hardened specimens were stored in a water-curing tank for 28 days of curing. Finally, the specimens were transferred into a drying chamber at a temperature of 40 °C. The total number of specimens for each mixture type was 24. For each measurement time, three identical specimens for each mixture type were taken for carbonation depth measurement. Before carbonation depth measurement, one of the above three specimens was selected as the representative for EIS measurement. 2.2. Accelerated carbonation condition and carbonation depth measurement Before exposure to carbon dioxide in the laboratory, the side faces of the specimens were sealed with wax while the two remaining end surfaces were left without wax, as shown in Fig. 1(a). This was to prevent CO2 from penetrating into the cement specimens from the side faces, only allowing the carbonation to proceed inwards from the end surfaces in one-dimensional manner. Under the 28 days underwater curing age, the drying condition and the above surface treatment, it is assumed that the hydration activity of plain cement and blended cement was almost completed and had little impact on the carbonation process and the EIS measurement results. Specimens were placed in an accelerated carbonation chamber where the temperature was 30 °C (±1 °C), the humidity was 65–70% and the concentration of CO2 was 20%. After carbonation condition, the carbonation depth was measured according to the suggestions from these literatures [45–47] and Chinese GBJ820-85 Standard for test methods of long-term performance and durability of ordinary concrete [48]. The carbonation depth measurement was conducted at the ages of 0, 3, 7, 14, 36, 60, 90 and 120 carbonation days. 1% phenolphthalein solution was sprayed on the split cross section to determine the carbonation depth. In order to alleviate reading errors, each specimen was tested at seven equidistant points on each split face, where the carbonation depth was defined as the perpendicular distance from the carbonation front to the exposed surface. The average value was calculated and presented as the carbonation depth, with the precision of 0.1 mm. 2.3. EIS measurement and modelling The EIS measurements were conducted at different carbonation ages (0, 3, 7, 14, 36, 60, 90, 120 carbonation days). During EIS measurement, the specimens were placed between two parallel electrodes (made of stainless steel) mounted in a test mold (Chinese Patent No. ZL 201120473976.2), as shown in Fig. 1(b). In order to Table 1 Chemical composition and physical properties of the cement and fly ash. Composition (Mass% as Oxide)
Cement
Slag
Fly ash
Calcium oxide (CaO) Silica (SiO2) Alumina (Al2O3) Iron Oxide (Fe2O3) Magnesium oxide (MgO) Sulfur trioxide (SO3) Potassium oxide (K2O) Loss on ignition (LOI)
64.67 18.59 4.62 4.17 2.35 3.32 0.92 1.03
37.73 34.62 11.81 2.73 9.43 1.42 0.65 1.2
4.74 62.32 23.95 1.33 2.04 1.25 0.76 3.12
Physical Properties Specific surface area (m2/kg) 80 lm sieving fineness (%)
345 4.15
443 –
391 8.30
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Q. Qiu et al. / Construction and Building Materials 147 (2017) 661–668 Table 2 Mix design of plain cement, slag blended cement and fly ash blended cement. Sample No.
Slag content
Fly ash content
Water-to-binder ratio
SC0 SC10 SC20 SC30 SC40 FC20
0% 10% 20% 30% 40% 0%
0% 0% 0% 0% 0% 20%
0.40 0.40 0.40 0.40 0.40 0.40
make a good contact between the electrode and the specimen, a wet and thin sponge was used to insert into the gap between them. A sinusoidal potential was applied through the specimen between electrodes with the frequency range from 0.01 to 1 MHz. The electrochemical impedance spectra for the carbonated cement were measured by a constant potential/constant current instrument (version: Potentiostat/Galvanostat 283). In addition, ZsimpWin software (Princeton Applied Research, America) was adopted in this study to analyze the EIS data of cement system, given that the software can provide straightforward and versatile equivalent circuit model fitting. An equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) was used to fit the measured electrochemical impedance spectra in order to gain the electrochemical properties (e.g. resistance of ion transport) of carbonated cement, as shown in Fig. 2(a). In this model, Rs refers to the resistance of pore electrolyte in slag blended cement; Q1 refers to the double layer capacitance existing at the ‘‘solid-liquid phase” interface in slag blended cement; Rct1 refers to the resistance of charge transfer at the ‘‘solid-liquid phase” interface in slag blended cement; W1 refers to the Warburg resistance of charge diffusion through slag blended cement; Q2 refers to the double layer capacitance existing between electrodes and the slag blended cement; Rct2 refers to the resistance of charge transfer between electrodes and the slag blended cement; W2 refers to the Warburg resistance of charge diffusion between the electrodes and slag blended cement; ZF1 = Rct1 + W1 refers to the impedance of the Faraday process in slag blended cement; while ZF2 = Rct2 + W2 refers to the impedance of the Faraday process between slag blended cement and electrodes. The physical meanings of all the circuit elements are described in the caption of Fig. 2. This model has been validated for the carbonation study of cement-based materials [36,37], which was adopted in this research to acquire the quantitative information of electrochemical system for slag blended cement under carbonation. The impedance of model Rs(Q1(Rct1W1))(Q2(Rct2W2)) consists of three parts in series: electrical resistance of pore electrolyte (Rs), impedance of solid/liquid double phase ZF1/(1 + jɷZF1Q1), and impedance between cement and electrodes ZF2/(1 + jɷZF2Q2). The impedance can be expressed by the following two equations. High frequency domain:
Fig. 2. Equivalent circuit model for the study of carbonated slag blended cement: (a) schematic diagram of circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)), and (b) Nyquist curve in a complex-plane plot of Cartesian coordinates of circuit model.
purpose to demonstrate that the current model has the best performance to describe the electrochemical behavior of slag blended cement.
3. Results and discussion 3.1. Carbonation depth
2 2 1 Rct1 þ Rct2 Zr Rs ðRct1 þ Rct2 Þ þ Z 2i ¼ 2 2
ð1Þ
Low frequency domain:
Z r ¼ Z i 2r21 Q 1 2r22 Q 2 þ Rs þ Rct1 þ Rct2
ð2Þ
Based on these two equations, a Nyquist diagram in a complex-plane plot of Cartesian coordinates for cement materials can be drawn as Fig. 2(b). The impedance spectra are composed of a semicircle part at high frequency region and a linear part at lower frequency region. In this study, the fitted result by equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) is compared with other conventional circuit models like Rs(Q(RctW)) [49–51] and Rs(Q1Rct1)(Q2Rct2) [52–55], with the
It has been well acknowledged that the carbonation depth is linearly proportional to the square root of the carbonation time t [56–58], as expressed by Eq. (3).
Dk
pffiffi t
ð3Þ
where D denotes the carbonation depth and k denotes the carbonation coefficient. Fig. 3 shows the experimental carbonation depths of slag blended cements, and the results go in agreement with Eq. (3). The regression analysis in Fig. 3 shows that R2 value ranges
Side face sealed with wax
Computer EIS apparatus Test mould End face exposed to CO2
(a)
Specimen
(b)
Fig. 1. Photographs of (a) cement sample sealed with wax and (b) EIS measurement.
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from 0.853 to 0.957 for plain cement and all slag blended cements, which indicates the linear relationship between carbonation depth and square root of time. The k values for SC0, SC10, SC20, SC30 and SC40 are 0.694, 0.695, 09.837, 0.877, and 1.012, respectively. It is found that the k values for SC0 and SC10 are closed. This means that effect of slag incorporation on reducing the carbonation resistance is not significant when the slag content is low. With higher slag content, the k value is shown to increase remarkably. Fig. 4 also claims that carbonation depth is increased by increasing the addition of slag in cement. The phenomenon is attributed to the reduction of Ca(OH)2 in cement hydration due to the slag replacement. This can accelerate the penetration of carbon dioxide through the cement mixture. Carbonation depth of fly ash blended cement (FC20) is also presented to compare the results with SC20. It is indicated the use of slag can lead to a faster carbonation rate than the fly ash incorporation with the same replacement ratio. As reported from the literature [59], the hydration development of slag blended cement is slower than that for fly ash blended cement at an equivalent replacement level. This can further lead to lower resistance of slag blended cement against carbonation.
Fig. 4. Carbonation depth (mm) measured for plain cement, slag blended cements and fly ash blended cement at 60 days carbonation age.
3.2. Nyquist curves at different carbonation ages The progressive Nyquist curves of the slag blended cement (SC30) at 0, 7, 36 and 60 days carbonation are shown in Fig. 5. The radius of the semi-circle of Nyquist curve increases by steps as the carbonation proceeds. Based on our investigation, similar results are also found for other mixture type (i.e. SC0, SC10, SC20 and SC40). This phenomenon indicates that the process of ions transport (e.g. ion transfer and ion diffusion) for slag blended cement is resisted by carbonation. From the electrochemical perspective, a large amount of the OH- ions in the slag blended cement materials are consumed by carbonation. K.A. Snyder et al. [60] have reported that the OH ion has the highest conductive capacity of all the ions in the cementitious system. Therefore, carbonation can lower the conductivity at the solid/liquid interface in slag blended cement materials, as can be characterized by the expansion of the semi-circle of Nyquist curve. The progressive Nyquist curves enables to trace the carbonation degree of slag blended cement. 3.3. Effect of slag contents on the Nyquist curve of carbonated cement The amount of slag content also has a significant influence on the electrochemical impedance behavior of the blended cements under carbonation. Fig. 6 shows the Nyquist curves of the plain cement and slag blended cements with 10% and 40% slag replacement ratios at the carbonation time of 3 days. It can be observed that the
Fig. 5. Nyquist curves of slag blended cement SC30 measured at different carbonation ages.
blended cements containing slag (SC10 and SC40) exhibit larger semi-circles than does the cement without slag (SC0). This phenomenon is also found at other carbonation ages. Additionally, an increase in slag content can further enlarge the semi-circle of the Nyquist curve. This phenomenon may be attributed to the microfiller effect of slag particle. The slag particles can fill in the space within the calcium silicate hydrate (CSH) gel so that they block the capillary pores and hence increase the density of the concrete [61]. Consequently, the processes of ion transfer and diffusion at the ‘‘solid-liquid interfaces” inside the cement sample are resisted significantly. Also, the pore solution is somewhat less alkaline when slag is applied. This means less OH ion concentration and lower conductivity of the electrochemical system in blended cement material. Moreover, as previously discussed, the carbonation process for blended cement with high slag content is faster than that of blended cement with low slag content. This can lead to a stronger microstructural densification of blended cement with high slag content, contributing to its larger radius of semi-circle of the Nyquist curve. 3.4. Comparison of electrochemical impedance spectra for slag blended and fly ash blended cements
Fig. 3. Carbonation depth measured for slag blended cements.
Both slag and fly ash are commonly used as supplementary cementing materials in construction, but they may have different
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influence on the carbonation behavior and the electrochemical property of cement system. This section focuses on identifying the difference of electrochemical impedance spectra under the slag and fly ash effect. Fig. 7 shows the Nyquist curves of slag blended cement (SC20) and fly ash blended cement (FC) at 60 days carbonation time. It is interesting to note that fly ash blended cement has remarkably larger semi-circle than the slag blended cement. Similar results at other carbonation time are also found in this study. Due to the slower hydration development as mention previously, the slag blended cement has lower resistance of ion transport process especially at the non-carbonated region. Besides, the pozzolanic reaction of fly ash is believed to consume a large amount of OH- ions, which significantly reduces the ion conductivity and leads to stronger resistance to ion transport in the blended cement system.
Rs(Q1(Rct1W1))(Q2(Rct2W2)) is adopted to quantitatively analyze the electrochemical system of carbonated slag blended cement. With the purpose to demonstrate the suitability of this model for slag blended cement, we perform a comparative study among different exiting circuit models. Fig. 8(a) and (b) show the measured and fitted electrochemical impedance spectra of slag blended cements under 28 days carbonation. It is found that the Randles model Rs(Q(RctW)) presents a poor fitting to the impedance data. This means that the model Rs(Q(RctW)) is not reliable to interpret the electrochemical system of carbonated blended cement due to without considering the charge interaction at the solid/liquid interface [36]. A better fitting can be observed by a more complicated model Rs(Q1Rct1)(Q2Rct2), but the deviation from the measured EIS curve is still clearly seen. This is because the model Rs(Q1Rct1)(Q2Rct2) does not consider the Warburg impedance especially for the cementitious material in wet conditions (cured in water and kept in carbonation chamber with 95% relative humidity) [37]. Compared with the above circuit models, the model Rs(Q1(Rct1W1))(Q2(Rct2W2)) performs the best fitting to the EIS data. The excellent fitting performance by this model is also found for other carbonation days. The fitted results give a strong evidence that the circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) is effective and reliable for the study of electrochemical system in slag blended cement subjected to carbonation. Given that the ion transfer process at the ‘‘solid-liquid phases” interface in the electrochemical system of cement can be greatly affected by carbonation, the parameter Rct1 is expected to be significantly changed as carbonation processes. In this study, the Rct1 values are extracted by fitting the model Rs(Q1(Rct1W1))(Q2(Rct2W2)) to the measured EIS data at different carbonation ages. From Fig. 9, it can be found that Rct1 value becomes larger when the slag content is increased, which indicates the stronger resistance of ion transfer at the ‘‘solid-liquid phases” interface in blended cement. Also, the results in Fig. 9 demonstrate that the increasing values of Rct1 for all slag blended cements have an approximately linear dependence on the carbonation time t, as expressed by the following equation:
3.5. EIS modelling result
½Rct1 ðtÞ Rct1 ð0Þ t
Fig. 6. Nyquist curves of cements without slag (SC0) and slag blended cements (SC10 and SC40) under carbonation condition.
In order to obtain the quantitative electrochemical properties (e.g. resistance of ion transfer and diffusion), an appropriate electrochemical circuit model can be selected to fit the measured EIS curve. In this research, the recently developed circuit model
ð4Þ
In Fig. 9, the R2 value ranges from 0.968 to 0.988 for all kinds of specimens, which indicates the reliability of the relationship Rct1 ðtÞ Rct1 ð0Þ t 3.6. Prediction of carbonation Based on Eqs. (3) and (4), an equation concerning the functional relationship between carbonation depth D and Rct1 is derived and expressed by Eq. (5)
DK
Fig. 7. Nyquist curves of slag blended cement and fly ash blended cement after 60 days of carbonation.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rct1 ðtÞ Rct1 ð0Þ
ð5Þ
The feasibility of predicting the carbonation depth of slag blended cement is demonstrated in Fig. 10(a) and (b). The upper and lower limits of 95% confidence interval are presented in Fig. 10. The carbonation depth at 90 days for SC10 is estimated to be 6.62 mm, while the measured value is 7.36 mm. The predicted error is calculated as 10.05%. As for the carbonation time of 120 days, the depth for SC10 is estimated to be 7.68 mm, with a predicted error of 8.35%. The predicted error may be attributed to the reading bias in the measurement of the carbonation depth. The predicted results for other slag blended cements are given in Table 3. In this research, the prediction accuracy is satisfactory (with errors less than 11%) for all the carbonated slag blended cements. This indicates that carbonation depth of slag blended cement can be effectively monitored and predicted based on using the electrochemical property measured by EIS.
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Fig. 8. The Nyquist plots of the measured electrochemical impedance for slag blended cement at 28 days carbonation time and the fitted results through the Randles circuit model Rs(Q(RctW)), the circuit model Rs(Q1Rct1)(Q2Rct2) and the new equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)): (a) cement with 0% slag content and (b) cement with 20% slag content.
Fig. 9. The quantitative values of circuit element Rct1 at different carbonation time.
By using the EIS methodology, the carbonation of cement-based materials considering the factors of incorporating mineral admixtures like slag and fly ash has been investigated in our research
work. Effect of fine aggregate addition on the EIS results and the model analysis is also examined in the previous study [37]. However, the effect of coarse aggregate on our model analysis has not yet been clarified. It has been reported by previous studies that defects and capillary pores are increased by incorporating the aggregate and thus the ion transport is facilitated [62–64]. In future work, the EIS study on the carbonated concrete is needed to be carried out. With knowing the EIS results for concrete material and the relationship between these results and the carbonation depth, the application of EIS method for evaluating the concrete structure in the construction field would be more practical and feasible. Based on our investigation, a functional relationship between the carbonation depth and the model parameter Rct1 is observed for slag blended cements with different slag replacement ratios: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D K Rct1 ðtÞ Rct1 ð0Þ. In the EIS methodology, the Rct1 values at different carbonation ages are obtained from fitting the corresponding Nyquist curves by our proposed circuit model. Such Rct1 values are useful to be taken to estimate the long term carbonation depth. Regarding the functional relationship, K values are varied with different slag replacement ratios. In order to apply this technique more practically for field estimation of carbonation depth of slag blended concrete materials, we have presented K values for plain cement specimen (SC0) and slag blended cements (SC10,
Fig. 10. Comparison of the measurement results and predicted results of carbonation depth based on ðD K
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rct1 ðtÞ Rct1 ð0Þ: (a) SC10; (b) SC30.
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Q. Qiu et al. / Construction and Building Materials 147 (2017) 661–668 Table 3 Comparison of average measured carbonation depth (mm) and the prediction results for cements with different slag replacement ratios. Time
Item
SC0
SC10
SC20
SC30
SC40
90 day
Measured carbonation depth Prediction result Prediction error (%) Measured carbonation depth Prediction result Prediction error (%)
7.14 6.82 4.48 7.43 8.06 8.48
7.36 6.62 10.05 8.38 7.68 8.35
8.39 7.95 5.24 9.05 9.21 1.77
9.54 8.53 10.58 9.71 9.82 1.13
8.99 9.08 1.00 9.89 10.44 5.56
120 day
Table 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K values in the functional relationship D K Rct1 ðtÞ Rct1 ð0Þ for carbonation depth prediction of different cement-based materials. Material type
SC0
SC10
SC20
SC30
SC40
K values
0.00623
0.00650
0.00725
0.00761
0.00784
SC20, SC30 and SC40), as shown in Table 4. K values are shown to increase when more slag content is used. However, application of this technique requires that the material category and the mix proportion should be known beforehand. In the future study, the correlation between K value and measured Rct1 value for different cement-based material systems should be investigated and clarified, so that the EIS method can be feasibly adopted for carbonation assessment in concrete structures.
Acknowledgements The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (No. 51508106/51478270/51538007) and the Collaborative Innovation Center for Advanced Civil Engineering Materials, Nanjing, P. R. China. References
4. Conclusions Influence of slag incorporation on the electrochemical behavior for cement systems under carbonation is investigated in this study. Based on the research findings of this work, the following conclusions can be drawn: 1. The carbonation depth of slag blended cement has a linear relationship with the square root of carbonation time. Carbonation depth can be increased by increasing the addition of slag in cement. Resistance of slag blended cement to carbonation is weaker than that of fly ash blended cement. 2. The radius of the semi-circle of Nyquist curve is increased as carbonation proceeds. The progressive Nyquist curves enables to trace the carbonation degree of slag blended cement. 3. Blended cement containing slag exhibits larger semi-circle of Nyquist curve than the cement without slag. Additionally, an increase in slag content can further enlarge the semi-circle of the Nyquist curve. 4. Comparing the effects between slag and fly ash, fly ash blended cement is shown to have remarkably larger semi-circle than the slag blended cement. The slag blended cement has less resistance to the ions transfer process than fly ash blended cement. 5. Equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) performs the best fitting to the EIS data, comparing with other the circuit models Rs(Q(RctW)) and Rs(Q1Rct1)(Q2Rct2). 6. Increasing the slag content can increase the value of Rct1 (an electrochemical parameter representing the ion transfer resistance at the ‘‘solid-liquid phases” interfaces of carbonated cement). It is also shown that values of Rct1 for all slag blended cements have an approximately linear dependence on the carbonation time t: ½Rct1 ðtÞ Rct1 ð0Þ t 7. A functional relationship between the carbonation depth and is established: D K the model parameter Rct1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rct1 ðtÞ Rct1 ð0Þ which can accurately predict the carbonation depth of slag blended cement.
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Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of slag incorporation on electrochemical behavior of carbonated cement Qiwen Qiu, Zhentao Gu, Jiaqi Xiang, Canjie Huang, Shuxian Hong, Feng Xing, Biqin Dong ⇑ Guangdong Province Key Laboratory of Durability for Marine Civil Engineering, School of Civil Engineering, Shenzhen University, Shenzhen 518060, Guangdong, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Slag effect on electrochemical
behavior of carbonated cement is studied. Slag cement exhibits larger semicircles of Nyquist curve than plain cement paste. An increase of slag content can hinder the ion transport process in cement system. Slag cement has less resistance of ion transport than fly ash cement.
a r t i c l e
i n f o
Article history: Received 30 December 2016 Received in revised form 24 April 2017 Accepted 1 May 2017
Keywords: Slag blended cement Carbonation Electrochemical behavior Electrochemical impedance spectroscopy Prediction
a b s t r a c t A research work is carried out to understand the influence of slag incorporation on the electrochemical behavior of carbonated cement systems. Measurement of carbonation depth and electrochemical impedance spectroscopy is conducted. A suitable equivalent circuit model is applied to fit the measured electrochemical impedance data (Nyquist curve) and quantitatively obtain the electrochemical property for carbonated slag blended cement. Test results demonstrate that increasing the slag replacement ratio in cement can increase the carbonation depth, enlarge the semi-circle of the Nyquist curve, and improve the resistance of ion transfer at the ‘‘solid-liquid phase” interface. We also compare the slag incorporation effect on the electrochemical system of carbonated cement with another common mineral admixture (i.e. fly ash). It is found that slag blended cement exhibits smaller semi-circle of Nyquist curve and larger carbonation depth than fly ash blended cement. Based on the electrochemical modification, we also predict the carbonation depth of blended cements with different slag contents. Ó 2017 Published by Elsevier Ltd.
1. Introduction As an essential component of concrete, ordinary Portland cement is one of the most widely used building materials around the world. In China, the cement production has grown rapidly
⇑ Corresponding author. E-mail address: [email protected] (B. Dong). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.008 0950-0618/Ó 2017 Published by Elsevier Ltd.
alongside the national economy since the 1980s [1]. However, a large amount of carbon dioxide (CO2) is emitted from the fossil fuel combustion and the calcination process in cement industry [2]. Generally, one ton cement production can release one and a half tons CO2 to the atmosphere, and it is estimated that CO2 emission will alarmingly increase to 38.8 109 tons by 2025 [3]. Such high CO2 emission would lead to escalate the global greenhouse effect. Moreover, cement production consumes a large amount of energy
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and accounts for a large portion of the financial expenditure in the construction market [4–8]. To remedy these problems, a promising solution is using the mineral admixtures which partially replace the cement constituent. With the development of construction technology, slag has been widely used as a mineral admixture for concrete materials. It is obtained as a by-product or waste material from the ferrous and nonferrous metal industries [9–16]. Slag has numerous advantages which favor it as a constituent of cement, for example, the low initial capital cost of the raw material and the associated energy savings [17,18]. Despite the above beneficial aspects, aggressive chemical substances from the outside environment can penetrate into the blended cement through the capillary pores [5,19–23]. When CO2 penetrates the cementitious materials in this way, carbonation occurs. Carbonation is a neutralization reaction that can lead to reduce the alkalinity of pore solution in cement and concrete materials owing to the consumption of hydrate cement compounds [24–27], which is an unwanted chemical phenomenon for steel-reinforced concrete structures [28,29]. Based on the alkalinity change, conventional testing methods (e.g. phenolphthalein indicator [30,31], pH meter [32]) have been applied for carbonation characterization. However, these methods require the damage of specimen, and the measurement accuracy depends on the test duration and the experience of inspectors. Repairing the damage after measurement by using phenolphthalein indicator also requires much time and heavy workload. In addition to the chemical modification, previous studies have indicated that carbonation can tremendously affect the electrochemical behavior (e.g. resistance of ion transfer and diffusion) of various cement systems [33–36]. This indicates that the carbonation assessment can be achieved by obtaining electrochemical characteristics of carbonated cement systems with the use of electrochemical measurement techniques like electrochemical impedance spectroscopy (EIS). EIS method is non-destructive, steady-state, sensitive, highly reproducible, and relatively easy to be used in construction field. The equipment and devices used in EIS measurement are easily available from electrochemical community, which are not so expensive especially for the portable EIS instruments. The data analysis and post-treatment can be performed with computer program and the results can be visualized automatically. From the engineering point of view, investigating the factors influencing the electrochemical behavior of carbonated cementitious materials is an essential step to develop a reliable electrochemical-based approach for analysis of carbonation of different cement-based materials. From the review of past research work, effects of water-to-cement ratio [36], river sand aggregate [37], and fly ash incorporation [38] on the electrochemical performance of carbonated cementitious material have been reported. Nevertheless, the role of slag on the electrochemical behavior of cement under carbonation has not yet been well clarified thus far. As compared to fly ash, slag material is self-cementing and does not require calcium hydroxide to form cementitious products, resulting in a distinct pore structure in cement system [39–42]. As a result, a special carbonation behavior as well as electrochemical behavior can be formed for slag blended cement. Understanding the impact of slag on the electrochemical property of carbonated cement would be of great importance for durability design of blended cement lining with an excellent carbonation resistance. The objective and originality of study is to investigate the electrochemical behavior for carbonated cement systems with slag incorporation. The impedance spectra of slag blended cement, fly ash blended cement and plain cement is compared. Factor of slag content on the carbonation depth and the electrochemical system is evaluated. An equivalent circuit model is applied to quantitatively analyze the EIS data. Furthermore, the electrochem-
ical property of slag blended cement is correlated with the carbonation depth. Finally, the prediction of carbonation depth is performed. 2. Materials and methods 2.1. Specimen details Type I ordinary Portland cement was used in accordance with GB175-2007 [43], which was obtained from the Onoda Cement Limited Company in Shenzhen, China. The percentages of calcium silicate (C3S), dicalcium silicate (C2S), tri-calcium aluminate (C3A), and tetra-calcium aluminoferrite (C4AF) in cement were 65.35%, 5.06%, 10.23%, and 10.40%, respectively. Class I slag and fly ash was obtained from the Mawan power plant in Shenzhen, China, according to the standard GB/T 180462008 [44]. The chemical components of cement, slag and fly ash are shown in Table 1. Cement without incorporation of slag was fabricated as a control specimen. In order to figure out the different effects between slag and fly ash, the fly ash blended cement was also fabricated. Four slag contents were considered in the slag blended cement. The mix proportions of all the cements are given in Table 2. The dimensions (width height length) of all specimens were 40 mm 40 mm 160 mm. The cement raw material, mineral admixture and distilled water were mixed in a treater for approximately 90 s. The mixture was then cast in a steel mold and compacted by using a vibrating table. The steel mold was removed after 24 h of the matrix hardening under ambient conditions. Thereafter, the hardened specimens were stored in a water-curing tank for 28 days of curing. Finally, the specimens were transferred into a drying chamber at a temperature of 40 °C. The total number of specimens for each mixture type was 24. For each measurement time, three identical specimens for each mixture type were taken for carbonation depth measurement. Before carbonation depth measurement, one of the above three specimens was selected as the representative for EIS measurement. 2.2. Accelerated carbonation condition and carbonation depth measurement Before exposure to carbon dioxide in the laboratory, the side faces of the specimens were sealed with wax while the two remaining end surfaces were left without wax, as shown in Fig. 1(a). This was to prevent CO2 from penetrating into the cement specimens from the side faces, only allowing the carbonation to proceed inwards from the end surfaces in one-dimensional manner. Under the 28 days underwater curing age, the drying condition and the above surface treatment, it is assumed that the hydration activity of plain cement and blended cement was almost completed and had little impact on the carbonation process and the EIS measurement results. Specimens were placed in an accelerated carbonation chamber where the temperature was 30 °C (±1 °C), the humidity was 65–70% and the concentration of CO2 was 20%. After carbonation condition, the carbonation depth was measured according to the suggestions from these literatures [45–47] and Chinese GBJ820-85 Standard for test methods of long-term performance and durability of ordinary concrete [48]. The carbonation depth measurement was conducted at the ages of 0, 3, 7, 14, 36, 60, 90 and 120 carbonation days. 1% phenolphthalein solution was sprayed on the split cross section to determine the carbonation depth. In order to alleviate reading errors, each specimen was tested at seven equidistant points on each split face, where the carbonation depth was defined as the perpendicular distance from the carbonation front to the exposed surface. The average value was calculated and presented as the carbonation depth, with the precision of 0.1 mm. 2.3. EIS measurement and modelling The EIS measurements were conducted at different carbonation ages (0, 3, 7, 14, 36, 60, 90, 120 carbonation days). During EIS measurement, the specimens were placed between two parallel electrodes (made of stainless steel) mounted in a test mold (Chinese Patent No. ZL 201120473976.2), as shown in Fig. 1(b). In order to Table 1 Chemical composition and physical properties of the cement and fly ash. Composition (Mass% as Oxide)
Cement
Slag
Fly ash
Calcium oxide (CaO) Silica (SiO2) Alumina (Al2O3) Iron Oxide (Fe2O3) Magnesium oxide (MgO) Sulfur trioxide (SO3) Potassium oxide (K2O) Loss on ignition (LOI)
64.67 18.59 4.62 4.17 2.35 3.32 0.92 1.03
37.73 34.62 11.81 2.73 9.43 1.42 0.65 1.2
4.74 62.32 23.95 1.33 2.04 1.25 0.76 3.12
Physical Properties Specific surface area (m2/kg) 80 lm sieving fineness (%)
345 4.15
443 –
391 8.30
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Q. Qiu et al. / Construction and Building Materials 147 (2017) 661–668 Table 2 Mix design of plain cement, slag blended cement and fly ash blended cement. Sample No.
Slag content
Fly ash content
Water-to-binder ratio
SC0 SC10 SC20 SC30 SC40 FC20
0% 10% 20% 30% 40% 0%
0% 0% 0% 0% 0% 20%
0.40 0.40 0.40 0.40 0.40 0.40
make a good contact between the electrode and the specimen, a wet and thin sponge was used to insert into the gap between them. A sinusoidal potential was applied through the specimen between electrodes with the frequency range from 0.01 to 1 MHz. The electrochemical impedance spectra for the carbonated cement were measured by a constant potential/constant current instrument (version: Potentiostat/Galvanostat 283). In addition, ZsimpWin software (Princeton Applied Research, America) was adopted in this study to analyze the EIS data of cement system, given that the software can provide straightforward and versatile equivalent circuit model fitting. An equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) was used to fit the measured electrochemical impedance spectra in order to gain the electrochemical properties (e.g. resistance of ion transport) of carbonated cement, as shown in Fig. 2(a). In this model, Rs refers to the resistance of pore electrolyte in slag blended cement; Q1 refers to the double layer capacitance existing at the ‘‘solid-liquid phase” interface in slag blended cement; Rct1 refers to the resistance of charge transfer at the ‘‘solid-liquid phase” interface in slag blended cement; W1 refers to the Warburg resistance of charge diffusion through slag blended cement; Q2 refers to the double layer capacitance existing between electrodes and the slag blended cement; Rct2 refers to the resistance of charge transfer between electrodes and the slag blended cement; W2 refers to the Warburg resistance of charge diffusion between the electrodes and slag blended cement; ZF1 = Rct1 + W1 refers to the impedance of the Faraday process in slag blended cement; while ZF2 = Rct2 + W2 refers to the impedance of the Faraday process between slag blended cement and electrodes. The physical meanings of all the circuit elements are described in the caption of Fig. 2. This model has been validated for the carbonation study of cement-based materials [36,37], which was adopted in this research to acquire the quantitative information of electrochemical system for slag blended cement under carbonation. The impedance of model Rs(Q1(Rct1W1))(Q2(Rct2W2)) consists of three parts in series: electrical resistance of pore electrolyte (Rs), impedance of solid/liquid double phase ZF1/(1 + jɷZF1Q1), and impedance between cement and electrodes ZF2/(1 + jɷZF2Q2). The impedance can be expressed by the following two equations. High frequency domain:
Fig. 2. Equivalent circuit model for the study of carbonated slag blended cement: (a) schematic diagram of circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)), and (b) Nyquist curve in a complex-plane plot of Cartesian coordinates of circuit model.
purpose to demonstrate that the current model has the best performance to describe the electrochemical behavior of slag blended cement.
3. Results and discussion 3.1. Carbonation depth
2 2 1 Rct1 þ Rct2 Zr Rs ðRct1 þ Rct2 Þ þ Z 2i ¼ 2 2
ð1Þ
Low frequency domain:
Z r ¼ Z i 2r21 Q 1 2r22 Q 2 þ Rs þ Rct1 þ Rct2
ð2Þ
Based on these two equations, a Nyquist diagram in a complex-plane plot of Cartesian coordinates for cement materials can be drawn as Fig. 2(b). The impedance spectra are composed of a semicircle part at high frequency region and a linear part at lower frequency region. In this study, the fitted result by equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) is compared with other conventional circuit models like Rs(Q(RctW)) [49–51] and Rs(Q1Rct1)(Q2Rct2) [52–55], with the
It has been well acknowledged that the carbonation depth is linearly proportional to the square root of the carbonation time t [56–58], as expressed by Eq. (3).
Dk
pffiffi t
ð3Þ
where D denotes the carbonation depth and k denotes the carbonation coefficient. Fig. 3 shows the experimental carbonation depths of slag blended cements, and the results go in agreement with Eq. (3). The regression analysis in Fig. 3 shows that R2 value ranges
Side face sealed with wax
Computer EIS apparatus Test mould End face exposed to CO2
(a)
Specimen
(b)
Fig. 1. Photographs of (a) cement sample sealed with wax and (b) EIS measurement.
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from 0.853 to 0.957 for plain cement and all slag blended cements, which indicates the linear relationship between carbonation depth and square root of time. The k values for SC0, SC10, SC20, SC30 and SC40 are 0.694, 0.695, 09.837, 0.877, and 1.012, respectively. It is found that the k values for SC0 and SC10 are closed. This means that effect of slag incorporation on reducing the carbonation resistance is not significant when the slag content is low. With higher slag content, the k value is shown to increase remarkably. Fig. 4 also claims that carbonation depth is increased by increasing the addition of slag in cement. The phenomenon is attributed to the reduction of Ca(OH)2 in cement hydration due to the slag replacement. This can accelerate the penetration of carbon dioxide through the cement mixture. Carbonation depth of fly ash blended cement (FC20) is also presented to compare the results with SC20. It is indicated the use of slag can lead to a faster carbonation rate than the fly ash incorporation with the same replacement ratio. As reported from the literature [59], the hydration development of slag blended cement is slower than that for fly ash blended cement at an equivalent replacement level. This can further lead to lower resistance of slag blended cement against carbonation.
Fig. 4. Carbonation depth (mm) measured for plain cement, slag blended cements and fly ash blended cement at 60 days carbonation age.
3.2. Nyquist curves at different carbonation ages The progressive Nyquist curves of the slag blended cement (SC30) at 0, 7, 36 and 60 days carbonation are shown in Fig. 5. The radius of the semi-circle of Nyquist curve increases by steps as the carbonation proceeds. Based on our investigation, similar results are also found for other mixture type (i.e. SC0, SC10, SC20 and SC40). This phenomenon indicates that the process of ions transport (e.g. ion transfer and ion diffusion) for slag blended cement is resisted by carbonation. From the electrochemical perspective, a large amount of the OH- ions in the slag blended cement materials are consumed by carbonation. K.A. Snyder et al. [60] have reported that the OH ion has the highest conductive capacity of all the ions in the cementitious system. Therefore, carbonation can lower the conductivity at the solid/liquid interface in slag blended cement materials, as can be characterized by the expansion of the semi-circle of Nyquist curve. The progressive Nyquist curves enables to trace the carbonation degree of slag blended cement. 3.3. Effect of slag contents on the Nyquist curve of carbonated cement The amount of slag content also has a significant influence on the electrochemical impedance behavior of the blended cements under carbonation. Fig. 6 shows the Nyquist curves of the plain cement and slag blended cements with 10% and 40% slag replacement ratios at the carbonation time of 3 days. It can be observed that the
Fig. 5. Nyquist curves of slag blended cement SC30 measured at different carbonation ages.
blended cements containing slag (SC10 and SC40) exhibit larger semi-circles than does the cement without slag (SC0). This phenomenon is also found at other carbonation ages. Additionally, an increase in slag content can further enlarge the semi-circle of the Nyquist curve. This phenomenon may be attributed to the microfiller effect of slag particle. The slag particles can fill in the space within the calcium silicate hydrate (CSH) gel so that they block the capillary pores and hence increase the density of the concrete [61]. Consequently, the processes of ion transfer and diffusion at the ‘‘solid-liquid interfaces” inside the cement sample are resisted significantly. Also, the pore solution is somewhat less alkaline when slag is applied. This means less OH ion concentration and lower conductivity of the electrochemical system in blended cement material. Moreover, as previously discussed, the carbonation process for blended cement with high slag content is faster than that of blended cement with low slag content. This can lead to a stronger microstructural densification of blended cement with high slag content, contributing to its larger radius of semi-circle of the Nyquist curve. 3.4. Comparison of electrochemical impedance spectra for slag blended and fly ash blended cements
Fig. 3. Carbonation depth measured for slag blended cements.
Both slag and fly ash are commonly used as supplementary cementing materials in construction, but they may have different
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influence on the carbonation behavior and the electrochemical property of cement system. This section focuses on identifying the difference of electrochemical impedance spectra under the slag and fly ash effect. Fig. 7 shows the Nyquist curves of slag blended cement (SC20) and fly ash blended cement (FC) at 60 days carbonation time. It is interesting to note that fly ash blended cement has remarkably larger semi-circle than the slag blended cement. Similar results at other carbonation time are also found in this study. Due to the slower hydration development as mention previously, the slag blended cement has lower resistance of ion transport process especially at the non-carbonated region. Besides, the pozzolanic reaction of fly ash is believed to consume a large amount of OH- ions, which significantly reduces the ion conductivity and leads to stronger resistance to ion transport in the blended cement system.
Rs(Q1(Rct1W1))(Q2(Rct2W2)) is adopted to quantitatively analyze the electrochemical system of carbonated slag blended cement. With the purpose to demonstrate the suitability of this model for slag blended cement, we perform a comparative study among different exiting circuit models. Fig. 8(a) and (b) show the measured and fitted electrochemical impedance spectra of slag blended cements under 28 days carbonation. It is found that the Randles model Rs(Q(RctW)) presents a poor fitting to the impedance data. This means that the model Rs(Q(RctW)) is not reliable to interpret the electrochemical system of carbonated blended cement due to without considering the charge interaction at the solid/liquid interface [36]. A better fitting can be observed by a more complicated model Rs(Q1Rct1)(Q2Rct2), but the deviation from the measured EIS curve is still clearly seen. This is because the model Rs(Q1Rct1)(Q2Rct2) does not consider the Warburg impedance especially for the cementitious material in wet conditions (cured in water and kept in carbonation chamber with 95% relative humidity) [37]. Compared with the above circuit models, the model Rs(Q1(Rct1W1))(Q2(Rct2W2)) performs the best fitting to the EIS data. The excellent fitting performance by this model is also found for other carbonation days. The fitted results give a strong evidence that the circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) is effective and reliable for the study of electrochemical system in slag blended cement subjected to carbonation. Given that the ion transfer process at the ‘‘solid-liquid phases” interface in the electrochemical system of cement can be greatly affected by carbonation, the parameter Rct1 is expected to be significantly changed as carbonation processes. In this study, the Rct1 values are extracted by fitting the model Rs(Q1(Rct1W1))(Q2(Rct2W2)) to the measured EIS data at different carbonation ages. From Fig. 9, it can be found that Rct1 value becomes larger when the slag content is increased, which indicates the stronger resistance of ion transfer at the ‘‘solid-liquid phases” interface in blended cement. Also, the results in Fig. 9 demonstrate that the increasing values of Rct1 for all slag blended cements have an approximately linear dependence on the carbonation time t, as expressed by the following equation:
3.5. EIS modelling result
½Rct1 ðtÞ Rct1 ð0Þ t
Fig. 6. Nyquist curves of cements without slag (SC0) and slag blended cements (SC10 and SC40) under carbonation condition.
In order to obtain the quantitative electrochemical properties (e.g. resistance of ion transfer and diffusion), an appropriate electrochemical circuit model can be selected to fit the measured EIS curve. In this research, the recently developed circuit model
ð4Þ
In Fig. 9, the R2 value ranges from 0.968 to 0.988 for all kinds of specimens, which indicates the reliability of the relationship Rct1 ðtÞ Rct1 ð0Þ t 3.6. Prediction of carbonation Based on Eqs. (3) and (4), an equation concerning the functional relationship between carbonation depth D and Rct1 is derived and expressed by Eq. (5)
DK
Fig. 7. Nyquist curves of slag blended cement and fly ash blended cement after 60 days of carbonation.
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rct1 ðtÞ Rct1 ð0Þ
ð5Þ
The feasibility of predicting the carbonation depth of slag blended cement is demonstrated in Fig. 10(a) and (b). The upper and lower limits of 95% confidence interval are presented in Fig. 10. The carbonation depth at 90 days for SC10 is estimated to be 6.62 mm, while the measured value is 7.36 mm. The predicted error is calculated as 10.05%. As for the carbonation time of 120 days, the depth for SC10 is estimated to be 7.68 mm, with a predicted error of 8.35%. The predicted error may be attributed to the reading bias in the measurement of the carbonation depth. The predicted results for other slag blended cements are given in Table 3. In this research, the prediction accuracy is satisfactory (with errors less than 11%) for all the carbonated slag blended cements. This indicates that carbonation depth of slag blended cement can be effectively monitored and predicted based on using the electrochemical property measured by EIS.
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Fig. 8. The Nyquist plots of the measured electrochemical impedance for slag blended cement at 28 days carbonation time and the fitted results through the Randles circuit model Rs(Q(RctW)), the circuit model Rs(Q1Rct1)(Q2Rct2) and the new equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)): (a) cement with 0% slag content and (b) cement with 20% slag content.
Fig. 9. The quantitative values of circuit element Rct1 at different carbonation time.
By using the EIS methodology, the carbonation of cement-based materials considering the factors of incorporating mineral admixtures like slag and fly ash has been investigated in our research
work. Effect of fine aggregate addition on the EIS results and the model analysis is also examined in the previous study [37]. However, the effect of coarse aggregate on our model analysis has not yet been clarified. It has been reported by previous studies that defects and capillary pores are increased by incorporating the aggregate and thus the ion transport is facilitated [62–64]. In future work, the EIS study on the carbonated concrete is needed to be carried out. With knowing the EIS results for concrete material and the relationship between these results and the carbonation depth, the application of EIS method for evaluating the concrete structure in the construction field would be more practical and feasible. Based on our investigation, a functional relationship between the carbonation depth and the model parameter Rct1 is observed for slag blended cements with different slag replacement ratios: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi D K Rct1 ðtÞ Rct1 ð0Þ. In the EIS methodology, the Rct1 values at different carbonation ages are obtained from fitting the corresponding Nyquist curves by our proposed circuit model. Such Rct1 values are useful to be taken to estimate the long term carbonation depth. Regarding the functional relationship, K values are varied with different slag replacement ratios. In order to apply this technique more practically for field estimation of carbonation depth of slag blended concrete materials, we have presented K values for plain cement specimen (SC0) and slag blended cements (SC10,
Fig. 10. Comparison of the measurement results and predicted results of carbonation depth based on ðD K
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rct1 ðtÞ Rct1 ð0Þ: (a) SC10; (b) SC30.
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Q. Qiu et al. / Construction and Building Materials 147 (2017) 661–668 Table 3 Comparison of average measured carbonation depth (mm) and the prediction results for cements with different slag replacement ratios. Time
Item
SC0
SC10
SC20
SC30
SC40
90 day
Measured carbonation depth Prediction result Prediction error (%) Measured carbonation depth Prediction result Prediction error (%)
7.14 6.82 4.48 7.43 8.06 8.48
7.36 6.62 10.05 8.38 7.68 8.35
8.39 7.95 5.24 9.05 9.21 1.77
9.54 8.53 10.58 9.71 9.82 1.13
8.99 9.08 1.00 9.89 10.44 5.56
120 day
Table 4 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi K values in the functional relationship D K Rct1 ðtÞ Rct1 ð0Þ for carbonation depth prediction of different cement-based materials. Material type
SC0
SC10
SC20
SC30
SC40
K values
0.00623
0.00650
0.00725
0.00761
0.00784
SC20, SC30 and SC40), as shown in Table 4. K values are shown to increase when more slag content is used. However, application of this technique requires that the material category and the mix proportion should be known beforehand. In the future study, the correlation between K value and measured Rct1 value for different cement-based material systems should be investigated and clarified, so that the EIS method can be feasibly adopted for carbonation assessment in concrete structures.
Acknowledgements The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China (No. 51508106/51478270/51538007) and the Collaborative Innovation Center for Advanced Civil Engineering Materials, Nanjing, P. R. China. References
4. Conclusions Influence of slag incorporation on the electrochemical behavior for cement systems under carbonation is investigated in this study. Based on the research findings of this work, the following conclusions can be drawn: 1. The carbonation depth of slag blended cement has a linear relationship with the square root of carbonation time. Carbonation depth can be increased by increasing the addition of slag in cement. Resistance of slag blended cement to carbonation is weaker than that of fly ash blended cement. 2. The radius of the semi-circle of Nyquist curve is increased as carbonation proceeds. The progressive Nyquist curves enables to trace the carbonation degree of slag blended cement. 3. Blended cement containing slag exhibits larger semi-circle of Nyquist curve than the cement without slag. Additionally, an increase in slag content can further enlarge the semi-circle of the Nyquist curve. 4. Comparing the effects between slag and fly ash, fly ash blended cement is shown to have remarkably larger semi-circle than the slag blended cement. The slag blended cement has less resistance to the ions transfer process than fly ash blended cement. 5. Equivalent circuit model Rs(Q1(Rct1W1))(Q2(Rct2W2)) performs the best fitting to the EIS data, comparing with other the circuit models Rs(Q(RctW)) and Rs(Q1Rct1)(Q2Rct2). 6. Increasing the slag content can increase the value of Rct1 (an electrochemical parameter representing the ion transfer resistance at the ‘‘solid-liquid phases” interfaces of carbonated cement). It is also shown that values of Rct1 for all slag blended cements have an approximately linear dependence on the carbonation time t: ½Rct1 ðtÞ Rct1 ð0Þ t 7. A functional relationship between the carbonation depth and is established: D K the model parameter Rct1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Rct1 ðtÞ Rct1 ð0Þ which can accurately predict the carbonation depth of slag blended cement.
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