DETERMINATION OF ACTIVATION ENERGY OF CEMENT HYDRATION BY ELECTRICAL RESISTIVITY MEASUREMENT

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1 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands DETERMINATION OF ACTIVATION ENERGY OF CEMENT HYDRATION BY ELECTRICAL RESISTIVITY MEASUREMENT Xiaosheng Wei (), Lianzhen Xiao() () School of Civil Engineering and Mechanics, Huazhong University of Science and Technology, Wuhan, China, 37 () School of Materials Science and Engineering, Wuhan Institute of Technology, Wuhan, China, 37 Abstract This study investigates the influence of temperature on the electrical resistivity of Portland cement pastes during first 7 hours. The experiments were conducted on the cement pastes with water cement ratios of.3,.35,.5,.55 and different curing temperatures of 5 C, C and 3 C. The hyperbolic equation for each sample is established to estimate the ultimate resistivity value which can probably be reached. The electrical resistivity results at the temperatures are used to determinate the activation energy for liquid solution in pores (E as ) and the activation energy for hydration reaction (E ar ). The electrical resistivity curves obtained at 5 C and 3 C are corrected to C for deleting the effect of temperature on the ionic mobility in the liquid solutions. E as is increased with the decrease of W/C due to a higher ion concentration in lower W/C paste solution, and E as ranges from to kj/mol. The activation energy of cement hydration is determined to be 39 kj/mol, which is close to the value recommended by the ASTM 7 standard method. Keywords: Electrical resistivity; Cement hydration; Activation energy; Hydration. INTRODUCTION When ordinary Portland cement (OPC) is mixed with water, a serial of chemical reactions take place. The reactions of cement with water proceed at different rates for the various mineral phases and involve both hydrolysis and hydration processes. For a given cement paste the curing temperature is the major external factor affecting the rate of hydration and thus the strength development of concrete. The electrical resistivity (ER) measurements are conducted with ions movement in porosity, which is relative with the formation of porosity. The development of the electrical resistivity with time can demonstrate the physical and chemical change from cement paste part both at the micro- and macro- scale. The electrical resistivity measurement has been often used to characterize hydration process of cement and the transport properties of cement-based materials [-3]. 3

2 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands Morsy et al [,5] studied the effect of temperature on the electrical conductivity of blended cement pastes. McCarter et al [] investigated the influence of thermal cycling on conductivity and the activation energy was studied. However, electrical measurements exhibit temperature dependence due to changing ionic mobility and hydration rate with temperature. When considering temperature, the influence of temperature on ionic mobility and microstructural development should be separated. Temperature has two different effects on the bulk electrical resistivity. The first effect is the temperature dependence of ion mobility and relative with the activation energy for liquid solution in pores (E as ).The second effect is the temperature dependence of reaction rates due to the pore structure refinement of hydration with time. This effect is relative with the activation energy for hydration reaction (E ar ). The objective of this work is to determine the apparent activation energy of pastes according to the electrical resistivity measurement. This study is carried out at temperatures from 5 C to 3 C. Another attempt is to investigate the effect of temperature on early age resistivity.. RAW MATERIALS AND SAMPLE PREPARATION Type I Portland cement (ASTM C5-95) was used in this study. The specific gravity values of cement is 3.5. Seven pastes are prepared as shown in Table. The cement pastes curing at C with various water-cement ratios (W/C) of.3,.35,.5 and.55 are marked as P.3, P.35, P.5 and P.55, respectively. The paste samples with water-cement ratio of. were cured isothermally at three different temperatures: 5 C, C and 3 C and marked as P.(5), P.() and P.(3), respectively. Each sample was mixed for minutes in a planetary-type mixer at 5 rpm and for a further minutes at 9 rpm under a required temperature for the tests. Table : Mix proportions and maximum resistivity Code W/C Cement Curing temperature, C P.3.3 P P.5.5 P P.(). P.(5). 5 P.(3) CORRECTION OF ELECTRICAL RESISTIVITY CURVES BY TEMPERATURE The electrical resistivity (ER) of the cement paste was measured by a non-contact electrical resistivity apparatus []. The curve of electrical resistivity with time presents a hyperbolic

3 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands evolution after setting [3]. Curing temperature not only influences the hydration reaction rates (i.e. maturity) but also ionic mobility, so the correction of electrical resistivity measurement due to temperature is proposed. A higher temperature decreases the viscosity of a pore solution and increases the mobility of the ions. This leads to a decrease in electrical resistivity of the porous material. The conductivity (resistivity) is a function of the mobility of the ions in the concrete liquid phase, it can be expected that the resistivity varies with temperature in the same manner in which the diffusion of ions vary in a liquid phase. This variation is governed by the Arrhenius equation [7] as expressed by Equation (). E ρ = exp R as 73 + T 73 + T REF ρ REF () Where ρ is the resistivity for a reference temperature, ρ REF is the resistivity for a reference temperature, E as is the activation energy for liquid solution in pores, R is the universal gas constant (.3 J/mol K), A is a material constant, T is the temperature in C, T REF is the reference temperature in C and dt is the time interval during which temperature T was measured.. RESULTS AND ANALYSIS. Influence of temperature on the electrical resistivity of cement pastes When the resistivity of cement-based material is measured under a varying temperature, the value will be changed. In this part, the resistivity of cement pastes is measured at different temperatures to obtain the reliable resistivity values corresponding to a reference temperature. The activation energy for liquid solution in pores (E as ) is determined. The mechanism is illustrated in Fig.. The higher temperature results in a higher ionic mobility for same paste structure. Fig.(a)-Fig.(d) show that the electrical resistivity value curing at temperature C is measured at different temperature after curing at day, days and 3 days. Figure: The ionic mobility increases with the temperature 5

4 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands Resistivity (ohm.m) P.3 Portland cement paste, W/C=.3 Isothermal curing at ºC (93ºK) Resistivity measured at Temperature: ºK, 93ºK, 33ºK Temperature(K) d Resistivity (ohm.m) P.35 Portland cement paste, W/C=.35 Isothermal curing at ºC (93ºK) Resistivity measured at Temperature: ºK, 93ºK, 33ºK d Temperature(K) (a) sample P.3 (b) sample P.35 Resistivity (ohm.m) P.5 Portland cement paste, W/C=.5 Isothermal curing at ºC (93ºK) Resistivity measured at Temperature: ºK, 93ºK, 33ºK Temperature(K) d Resistivity (ohm.m) Portland cement paste, W/C=.55 Isothermal curing at ºC (93ºK) Resistivity measured at Temperature: ºK, 93ºK, 33ºK P.55 d Temperature(K) (c) sample P.5 (d) sample P.55 Figure : The electrical resistivity measured at different temperatures after curing day, days and 3 days at C From Fig. it is noted that the resistivity is decreased with the increase of temperature. This is consistent with the increase in ionic mobility with the increase of temperature. To determine the activation energy of the solution in pores (E as ), the measured value of electrical resistivity was used to plot (Ln ρ ) versus (/T) as shown in Fig.3 (a) to Fig.3 (d), and its slope is the value of E as /R. Ln, Resistivity (ohm.m) P.3. d /Temperature (/K) Ln, Resistivity (ohm.m) P /Temperature (/K) d (a) sample P.3 (b) sample P.35

5 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands.5.. Ln, Resistivity (ohm.m).5.5 P.5 d Ln, Resistivity (ohm.m)..... P.55 d /Temperature (/K) /Temperature (/K) (c) sample P.5 (d) sample P.55 Figure 3: The logarithm of resistivity vs. times the reciprocal of temperature The temperature range of 5 ºC to 3 ºC and the reference temperature is T = ºC. The average E as in different samples is found to be E as =57 J/K for W/C=.3, E as = J/K for W/C=.35, E as =5 J/K for W/C=.5, E as =5 J/K for W/C=.55, respectively. The relationship between E as and W/C is shown in Equation (3) and plotted in Fig.. It is noticed that E as is decreased with the increase of W/C. E as = 335W / C (3) 3 Ea (J/mol) y = -335W/C R = W/C Figure : The relationship between E as and W/C The activation energy for liquid solution in pores (E as ) is increased with the decrease of W/C due to a higher ion concentration in a lower W/C paste solution.. Electrical resistivity development curing at different temperature Fig.5 shows the effect of curing temperature on the resistivity curves. Fig5.(a) presents the resistivity curves of samples at different temperatures. Fig.5(b) presents the reference curve for the electrical resistivity at C, and the curves were corrected to take into account 7

6 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands temperature using Equation (). The activation energy for liquid solution in pores (E as ) for W/C=. paste is.9kj/mol based on Equation (). It is close to Ea=.5kJ/mol in the paper by Silva []. A higher temperature causes a higher ionic concentration and a higher ionic mobility and then a lower resistivity. P.(3) Electrical resistivity (Ohm.m) P.(3) P.() P.(5) The ions have a higher mobility Ions in a fluid state after several minutes Time (hours) The ions have a lower mobility (a) P.(3)_corrected P.()_corrected P.(5)_corrected. t t 3. t t. t 5. + t Electrical resistivity (Ohm.m) Corrected resistivity curve P.(3)_corrected P.()_corrected P.(5)_corrected The ions have the same mobility Time (hours) (b) Figure 5: Effect of curing temperature (w/c =.) on the resistivity development curves Before corrected (b) Corrected resistivity under the same reference temperature at C From Fig.5(b), we can see that the difference between upper and lower curves after corrected measurements is larger than the difference of resistivity before corrected measurement as shown as Fig.5(a). It can be found that the corrected curves follow a hyperbolic trend. The hyperbolic equations are fitted during the hardening period from 5 h to 7 h for each curve shown in Fig.5(b). As time t approaches infinity, the ultimate resistivity (R ult ) is obtained. From the equations in Fig.5(b), the values of R ult for samples P.(5) _corrected, P.() _corrected and P.(3) _corrected are., 3. and. ohm.m, respectively. The data above shows that the paste cured at 3 C has the lowest electrical resistivity at later ages, whereas the paste cured at 5 C exhibits the highest potential electrical resistivity at later age. It can be estimated that the electrical resistivity curves develop to be crossed at late ages as the hyperbolic equation predicted. It can be concluded that a higher curing temperature will result in a higher early degree of hydration or strength, but a lower long-term hydration or strength which agree with the results in reference [9].

7 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands.3 The activation energy for hydration reaction Based on the data in Fig5(b), the results of resistivity with temperature at the age of hours, hours and 7 hours were used to plot the curve of the logarithm of resistivity versus times the reciprocal of temperature in Kelvin as shown in Fig.. Its slope is the value of E ar /R and can be used to calculate E ar according to Equation (5). The slope of three lines at the age of hours, hours and 7 hours in Fig. are 5.3,.5, and 3.797, respectively. The average activation energy for hydration reaction (E ar ) is determined to be ( )/3*.3=39 kj/mol for the cement paste. 3.5 Portland cement paste, W/C=. Isothermal curing at 5ºC, ºC, 3ºC Reference Temperature: ºC Ln,Resistivity.5 7h h y = -3.79x y = -.5x h y = -5.3x /Temperature Figure : The activation energy of cement hydration by electrical resistivity The activation energy of cement hydration obtained to be 39 kj/mol is close to the value of kj/mol recommended as the default value by the ASTM 7 standard []. The value is also similar to the values determined by using heat of hydration []. The earlier research found that compressive strength and electrical resistivity of cement paste exhibit similar trends with age. There is positive relation between the electrical resistivity and compressive strength []. The electrical resistivity curve not only dynamically provides information of hydration process but also can be used to obtain the activation energy for hydration reaction (E ar ). The results of the activation energy by electrical resistivity can be used to develop new maturity functions which can predict strength development under non-isothermal conditions. 5. CONCLUSIONS ) The electrical resistivity measurement is used to determinate the activation energy of the liquid solution in the pores and the activation energy for the hydration reaction of cement. The activation energy for liquid solution in pores is increased with the decrease of W/C due to a higher ion concentration in a lower W/C paste solution. )When curing temperature is increased, the electrical resistivity is increased during early age. It is estimated that the resistivity curves at the different temperatures will be crossed at late ages as hyperbolic equations predicted. The lower curing temperature paste has a higher ultimate resistivity for certain temperature range. 3) The activation energy of cement hydration has been determined to be 39 kj/mol and is close to the value recommended by the ASTM 7 standard and the value determined using heat of hydration. 9

8 nd International Symposium on Service Life Design for Infrastructure - October, Delft, The Netherlands ACKNOWLEDGEMENT The National Natural Science Foundation of China (No. 5777) is acknowledged. REFERENCES [] McCarter W.J., Current P.N., The electrical response characteristics of setting cement pastes, Mag. Concr. Res. 3 () (9) 9. [] LI Z., WEI X., LI W. Preliminary Interpretation of Hydration Process of Portland Cement Using Resistivity Measurement[J]. ACI Materials Journal. (3)( 3) [3] WEI Xiaosheng, XIAO Lianzhen, LI Zongjin. Hyperbolic Method to Analyze the Electrical Resistivity Curve of Portland Cements with Superplasticizer. Journal of Wuhan University of Technology (Materials Science Edition). 3() () 5-. [] Morsy MS. Effect of temperature on electrical conductivity of blended cement pastes[j]. Cement and Concrete Research. 9(999) 3. [5] Heikal M., Morsy M. S., Aiad I. Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes. Cement and Concrete Research. 35( ) (5) -7. [] McCarter W.J., Starrs G., Chrisp T.M.Electrical conductivity, diffusion, and permeability of Portland cement-based mortars. Cement and Concrete Research. 3 (9) () [7] Hammond E, Robinson TD. The comparison of electrical properties of various cements and concretes. The Engineer. 99(995) 7-. [] Silva BJ, Jalali S, Ferreira RM. Estimating electrical resistivity based on early age measurements. In: Proceedings of the international RILEM workshop performance based evaluation and indicators for concrete durability, Madrid,. [9] Kim, JK, Moon YH, Eo SH. Compressive strength development of concrete with different curing time and temperature. Cement and Concrete Research. () (99) [] ASTM C7, Standard practice for estimating concrete strength by the maturity method, Book of ASTM standards vol.., American Society for Testing and Materials, Philadelphia (99). [] Dale P. Bentz, Gaurav Sant, Jason Weiss. Early-Age Properties of Cement-Based Materials. I: Influence of Cement Fineness. Journal of Materials in Civil Engineering., 5-5. [] Wei X., Tian K., Xiao L. Prediction of compressive strength of Portland cement paste based on electrical resistivity measurement [J]. Advances in Cement Research. (3) ()