Effects of microstructure and pore water on electrical conductivity of cement slurry during early hydration

Composites Part B 177 (2019) 107435

Contents lists available at ScienceDirect

Composites Part B journal homepage: www.elsevier.com/locate/compositesb

Effects of microstructure and pore water on electrical conductivity of cement slurry during early hydration Kaiqiang Liu a, b, Xiaowei Cheng a, b, *, Jingxue Li a, Xianshu Gao a, c, Yan Cao a, Xiaoyang Guo b, Jia Zhuang a, **, Chunmei Zhang a a b c

School of Materials Science and Engineering, Southwest Petroleum University, 61500, China State Key Laboratory of Oil & Gas Reservoir Geology and Exploitation, Southwest Petroleum University, 610500, China State Key Laboratory of Green Building Materials, China Building Materials Academy, Beijing, 1002204, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrical conductivity Microstructure Pore characteristics Hydration Oil-well cement

In this study, inductance-type electrical conductivity measurement (IECM) was performed to study the electrical conductivity of different formulations of cement slurry. Low-field nuclear magnetic resonance, in situ X-ray micro-tomography, and environmental scanning electron microscopy were used to investigate the effects of the microstructure, pore characteristics, and pore water of cement slurry on its electrical conductivity. Experimental results showed that the pore characteristics and ion concentration governed the initial electrical conductivity of fresh cement slurries. The sedimentation stability of fresh cement slurries increased the number of inter­ connected pores, and the initial electrical conductivity increased from 16.8 mS/cm to 19.4 mS/cm. When the CaCl2 accelerator content was 2 wt%, the initial electrical conductivity of the fresh cement slurry increased to 26.9 mS/cm, while the electrical conductivity rapidly decreased with the formation of hydration products. With increasing hydration time, large amounts of hydration products were formed in the pores, which changed the microstructure and pores of the cement slurry from connected large volume pores to many small volume pores and subsequently changed the distribution of pore water. The experimental results revealed that the connected porosity of the cement slurry was proportional to the electrical conductivity. An exponential relationship was found between the electrical conductivity and the evaporable water content, whereas a linear relationship was found between the electrical conductivity and the free water content.

1. Introduction The hydration of Portland cement slurry is a complex physical and chemical process, and its electrical conductivity is a key property for understanding its hydration and hardening process [1,2]. Mixing of cement with water causes release of ions such as Ca2þ, OH , and SO2þ 4 into the water and a consequent increase in the electrical conductivity of the resulting cement slurry [3,4]. Subsequently, hydration products such as hydrated calcium silicate (C–S–H) and ettringite (Aft) are formed, which results in a decrease in the electrical conductivity of the cement slurry [5,6]. The electrical conductivity of cement slurry has long been a topic of interest to researchers studying cement and con­ crete. Various electrochemical methods such as electrical resistivity measurement, non-contact impedance measurement, and inductance-type electrical conductivity measurement (IECM) have been

used to study changes in the electrical conductivity of cement slurry during early hydration [6–9]. Results of such works have provided some valuable information for understanding the setting time, pore charac­ teristics, mechanical properties, volume shrinkage, and early hydration process of cement slurry [4]. Many works have been conducted on the electrical conductivity and electrical properties of cement-based materials. On the one hand, re­ searchers studied the effects of different additives on the properties of cement-based materials by testing the materials’ electrical properties. For example, the electrical conductivity of cement-based materials was measured in order to study the effects of the water–cement ratio, flash ash, slag, and temperature on the hydration process [6,8,9] and to predict the degree of hydration, pore structure, and compressive strength of cement paste [10–12]. Moreover, Wang et al. [7,13] inves­ tigated the effects of addition of superplasticiser and latex on

* Corresponding author. School of Materials Science and Engineering, Southwest Petroleum University, 61500, China. ** Corresponding author. E-mail addresses: [email protected] (X. Cheng), [email protected] (J. Zhuang). https://doi.org/10.1016/j.compositesb.2019.107435 Received 21 January 2019; Received in revised form 19 August 2019; Accepted 10 September 2019 Available online 11 September 2019 1359-8368/© 2019 Elsevier Ltd. All rights reserved.

K. Liu et al.

Composites Part B 177 (2019) 107435

cement-based materials by testing the materials’ electrical conductivity. On the other hand, several researchers studied the mechanism of change in the electrical conductivity of cement slurry. For example, some works [14–16] proposed that changes in the degree of hydration, pore char­ acteristics, and hydration products can affect the electrical conductivity of cement slurry. The generally accepted view is that cement slurry is a two-component material, and the pore characteristics (pore solution, porosity, pore size distribution, and pore connectivity) of cement slurry are key factors influencing its electrical conductivity in the early hy­ dration stage [4,9,13,17], particularly in its liquid–solid transition stage. According to a previous study [4], the electrical conductivity of cement slurry reduces rapidly when it is in the liquid–solid transition stage. With the aim of determining the relationship between the pore characteristics and the electrical conductivity of cement slurry in the liquid–solid transition stage, previous works applied models such as the CEMHYD3D hydration model and a multi-phase phenomenological model [9,18–20] to simulate the microstructure and pore characteristics. Further, it was proposed that a change in pore connectivity would have greater influ­ ence than a decrease in porosity would on the conductivity of cement slurry in the early hydration stage [18]. However, because cement slurry in the liquid–solid transition stage does not have a fixed shape, evalu­ ation of the pore characteristics of cement slurry by conventional methods (mercury porosimeter method, gas adsorption, etc.) is difficult. Thus far, few studies have experimentally examined the quantitative relationship between the pore characteristics and the electrical con­ ductivity of cement slurry in the liquid–solid transition stage. In cementing engineering of oil wells and natural gas wells, Portland oil-well cement slurries are injected into the annulus space between a metal casing and rock formations in order to isolate the formations and protect the metal casing. However, when the cement slurry is in the liquid–solid transition stage, ‘gas migration’ accidents frequently occur and the cementing quality degrades [21,22]. Therefore, it is important to understand the early hardening process and pore characteristic evo­ lution of oil-well cement slurries to ensure successful cementing oper­ ations. Cement slurry is a solid–liquid two-phase material, which conducts electrons via the directional movement of ions in pore water. A change in the pore characteristics of cement slurry with increasing hy­ dration time will induce changes in the physical state and distribution of pore water [23]. The present study investigated the electrical conductivity of cement slurries with different properties by IECM. In situ X-ray micro­ tomography (μCT) was used in combination with image reconstruction methods for in situ testing of the three-dimensional (3D) microstructure and pore characteristics of cement slurry at different hydration times. Then, low-field nuclear magnetic resonance (LF-NMR) and mass con­ servation methods were applied to quantitatively analyse the contents of various types of pore water in cement slurries at different hydration times. From these experimental results, the pore spatial distribution, porosity, pore shape, connected porosity, and distribution of pore water in the cement slurry in the early hydration stage were estimated. The relationship of the pore characteristics and pore water with the electrical conductivity of the cement slurry was also determined. The obtained experimental results are expected to provide a theoretical basis for the study of the hydration process of cement slurry and the design of an oilwell cement slurry system.

2. Experimental materials and methods 2.1. Materials The cement slurries investigated in this study were prepared using Gclass oil-well cement provided by Sichuan Jiahua Corporation, China, according to the API 10-A standard [24]. The chemical composition of this cement is presented in Table 1. An acrylic acid/acrylamide/2-acrylamido-2-methylpropanesulfonic acid (AMP­ S/AM/AA) terpolymer fluid-loss additive was used in the preparation of the cement slurry. Fig. 1 shows the chemical structure of this fluid-loss additive; the figure reveals that its chemical structure includes hydro­ philic groups (i.e. amide (–CONH2), carboxyl (–COOH), and sulfonic acid (–SO3H) groups), which can increase the viscosity of the pore water. Further, the –COOH and –CONH2 groups react with calcium (Ca2þ) and hydroxyl (OH ) ions under high-alkalinity and high-Ca2þ conditions in the cement slurry to form a ‘net-like’ structure (as shown in Fig. 10 (a)) [25]. The increase in the viscosity of the pore water and the formation of the ‘net-like’ structure prevent sedimentation of cement particles and reduce fluid loss of the cement slurry, as has also been reported in other works [26,27]. An accelerator (analytically pure CaCl2) was used to control the hydration reaction rate of the cement slurry [28,29]. Both these chemicals (i.e. the additive and accelerator) were purchased from Weihui Chemical Co., China. 2.2. Testing methods 2.2.1. Preparation and properties of cement slurry The cement slurry formulations used in the experiments and the properties of these formulations are listed in Table 2. All the additive contents were calculated by the weight of the cement. The cement slurry was mixed according to the procedure outlined in chapter 1 of the API RP 10B-2 standard [30]. A water-to-cement ratio of 0.44 was maintained throughout the experiments, and the experimental temperature was maintained at 30 � C. The density and sedimentation stability of the cement slurry were determined according to the procedures outlined in chapters 6 and 12, respectively, of the API RP 10B-2 standard [30]. Sedimentation stability is a key parameter for determining whether or not the cement slurry undergoes particle sedimentation. The sedimen­ tation experiment required a sedimentation tube (shown in Fig. 2), which had an inner diameter of 25 � 0.5 mm and a length of 200 mm in this study. The sedimentation experiment involved the following steps. ① Pour cement slurry into the sedimentation tube, gently tap the tube

Fig. 1. Chemical structure of fluid-loss additive. Table 1 Chemical composition of oil-well cement (wt.%) used to prepare cement slurry in this study. Oxide

CaO

SiO2

Al2O3

Fe2O3

MgO

Na2O

K2O

SO3

Loss

Cement

63.74

22.48

3.41

4.01

1.47

0.9

1.55

1.40

1.04

2

K. Liu et al.

Composites Part B 177 (2019) 107435

2.2.2. Electrical conductivity The electrical conductivity of the cement slurry formulations was tested via IECM using an apparatus manufactured by Yantai Chemins Instrument Co., Ltd. A direct-current power source was used to apply a working voltage of 24 V, and a measurement range of 0–100 mS/cm was used. Fig. 3 shows photographs of the IECM apparatus and experimental process. In the experiment, 150 mL of cement slurry was poured into a 200 mL glass beaker, and the IECM apparatus was then submerged in the cement slurry. The electrical conductivity of the cement slurry was measured using the IECM data acquisition system at 30 � C. In addition, the electrical conductivity of the pore solution in the cement slurry was tested. This test involved the following steps. ① Mix the cement slurry according to the API RP 10B-2 standard [30], and pour 400 mL of the cement slurry into a high-pressure, high-temperature filter press. ② Cure the cement slurry at 30 � C, and when the hydration time was expected time the pore solution was obtained through the cement slurry at high pressure (>6.9 MPa). ③ The pore solution is collected directly into a glass beaker for subsequent electrical conduc­ tivity measurements. This procedure was also adopted by Ridha et al. [31]. Meanwhile, according to the Cherif’s results [32], it can be seen that main ion in cement slurry includes the Ca2þ, Naþ, and Kþ. Thus, the ion concentration (Ca2þ, Naþ and Kþ) of the pore solution was tested using inductively coupled plasma optical emission spectroscopy (ICP-OES) (Optima 7300 V, PerkinElmer USA).

Table 2 Properties of cement slurry formulations. Material

Density, g/cm3

Cement 3.15 G333S (wt.%) 1.45 CaCl2 (wt.%) 2.15 Water (wt.%) 1.00 Cement slurry density (g/cm3) Sedimentation stability (α) (%)

Formulation F-1

F-2

F-3

F-4

F-5

/ 0 0 44 1.898 5.29

/ 1.0 0 44 1.894 0.53

/ 2.0 0 44 1.890 0

/ 2.0 1.0 44 1.892 0

/ 2.0 2.0 44 1.893 0

2.2.3. Hydration reaction and microstructure analysis 2.2.3.1. Hydration heat analysis. The hydration reaction of Portland cement is an exothermic process, which can be measured via analysis of the heat and heat flow of the cement slurry. A previous study [33] re­ ported that the early hydration of cement slurry increased with an in­ crease in the mixing speed and that it was also affected by the shearing conditions. Therefore, in order to ensure experimental accuracy in this study, the cement slurry for the hydration heat tests was prepared ac­ cording to the API 10B-2 standard [30]. The cement slurry was mixed at 4000 � 250 r/min for about 15 s, after which mixing was continued at 12000 � 250 r/min for 35 s. Then, the resulting slurry (about 6 g) was injected into a sampling bottle for isothermal calorimetry analysis, which was then sealed and placed in a calorimeter (TAM Air, TA In­ struments) at 30 � C to acquire data on the hydration heat of the cement slurry.

Fig. 2. Sedimentation tube.

containing the slurry to dislodge any air bubbles, and then fill the tube completely. ② Place the filled tube in a chamber in a vertical position at a set temperature of 30 � C. ③ Cure the slurry for 24 h before removing the tube from the chamber. Then, remove the slurry from the tube. ④ Measure and record the length of the slurry specimen. Mark the spec­ imen at a position approximately 20 mm from the bottom and a position approximately 20 mm from the top (mark these parts as specimens B1 and T1, respectively), and cut specimen B1 and T1. ⑤ Determine the relative densities of specimens B1 and T1 by applying Archimedes’ principle (1): M ρ ¼ air Mwater

2.2.3.2. X-ray diffraction analysis. X-ray diffraction (XRD) spectra of dried cement slurry (drying method is described in Section 2.2.4) were acquired using a powder diffractometer (DX-2700, Haoyuan Instru­ ment). A Cu-kα X-ray tube with an input voltage of 40 kV and a current of 30 mA was employed. The cement slurry samples were scanned in the 2θ range of 5� –70� with a scanning rate of 0.04� /s. Then, the XRD results were analysed using the JADE 6.0 software. 2.2.3.3. Microstructure analysis. Liquid nitrogen freezing and vacuum freeze drying methods were used to prepare samples for environmental scanning electron microscopy (ESEM; Quanta 450, FEI, USA) in order to observe the microstructure of the cement slurry at different hydration times. A detailed description of sample preparation for ESEM has been provided elsewhere [25]. On the one hand, ESEM requires a sample to be dry before testing, and the drying process may alter the microstructure and pore structure of the sample [34]. On the other hand, cement slurry in the early hydration stage does not have a fixed shape and determi­ nation of the pore structure by some conventional approaches is diffi­ cult. Therefore, in order to accurately evaluate the microstructure and pore characteristics of the cement slurry, in situ μCT (Phoenix Nanotom, GE, USA) was used to study the effects of the microstructure on the electrical conductivity of the cement slurries. In this experiment, the room temperature was set to 30 � C and the prepared cement slurry was

(1)

Here, ρ is the relative density of the specimen, Mwater is the mass of the specimen in water (g), and Mair is the mass of the specimen in air (g). Then, the sedimentation stability of the cement slurry can be determined according to (2):

α¼

ρB1 ρT 1 � 100% ρslurry

(2)

where α is the sedimentation stability of the cement slurry (%), ρB1 is the relative density of B1, ρT1 is the relative density of T1, and ρslurry is the cement slurry density.

3

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 3. Photographs of apparatus used for inductance-type electrical conductivity measurement.

filled into a syringe with an inner diameter of 5 mm and length of 75 mm. Next, the syringe with the cement slurry was fixed to the μCT bench. The total scanning time was set to 12 min (the sample was scanned from 0� to 360� with 2 scans per degree, and the time of each scan was 1 s), and the X-ray intensity radiographs were measured every 2 h using the μCT scanner. Then, the radiographs were reconstructed using the Phoenix Datos X reconstruction software (GE) to obtain 3D images of the cement slurry. Additional details of the sample prepara­ tion procedure and reconstruction process of the 3D microstructure can be found elsewhere [35,36].

gel pores in hydration products, and T2 > 7.5 m s for large-volume pores in cement slurry (Fig. 4). In the cement slurry in the present study, all these pores were filled with water. Hence, the LF-NMR data could be analysed according to the mass conservation law in order to quantify the free water, capillary water, and gel water contents of the cement slurry.

2.2.4. Pore water distribution analysis In this experiment, the mass conservation law and LF-NMR [37,38] were used to study the relationship between the pore water and the electrical conductivity of the cement slurry. Muller et al. [23] reported that in the early hydration stage, pore water is present in the form of free water in macropores, water in capillary pores, water in gel pores, and chemical water, i.e. water chemically bound to other molecules. Free water, capillary water, and gel water together constitute the so-called evaporable water [10].

2.2.4.2. Chemical water content. Chemical water in cement slurry in­ cludes the constitutional water and the crystal water of the hydration products; the chemical water content of cement slurry can be analysed by thermal decomposition methods [43]. After testing of the evaporable water content of the cement slurry, the same cement slurry specimen was used to analyse the chemical water content. Thermogravimetric analysis (TGA) was used to measure the mass change in the cement slurry specimen in the temperature range of 100–1000 � C. Lothenbach et al. [44] proposed that the hydration products (C–S–H, Ca(OH)2, AFt, etc.) decomposed before 600 � C. Therefore, the mass change that occurred from 105 � C to 600 � C was considered as the chemical water content. The specimen mass at 105 � C was denoted as m105� C;t and that at 600 � C was denoted as m600� C;t . Then, the chemical water content of the cement slurry was calculated according to (5):

1 S ¼ϕ T2 V

where ϕ is the relaxivity of the pore surfaces, S is the pore area, and V is the pore volume.

2.2.4.1. Evaporable water content. The evaporable water content of the cement slurry at different hydration times was measured according to the mass conservation law. The experimental procedure was as follows. First, 5–10 mL of cement slurry was poured into a sampling bottle with an internal diameter of 10 mm. The mass of the empty bottle was measured and denoted as m1 . The total mass of the bottle containing the cement slurry was measured and denoted as m2;t . The bottle containing the cement slurry was sealed and placed in a water bath at 30 � C. At hydration times of 2, 4, 6, 8, 10, 12, and 14 h, the bottle was removed and plunged into liquid nitrogen ( 196 � C) for more than 30 min. Then, the frozen cement slurry was immediately placed in a freeze vacuum dryer maintained at a temperature of 90 � C. The freeze-dried cement slurry was again dried at 105 � C until the sample mass stopped changing; this procedure is similar to that reported previously [39,40]. The mass of the dried sample was denoted as m3;t . The evaporable water content was calculated according to (3): wð1; tÞ ¼

m2;t m1 m3;t � 100% m2;t m1

(4)

wð2; tÞ ¼

m105� C;t m2;t

m600� C;t � 100% m1;t

(5)

where wð2; tÞ is the chemical water content of the cement slurry at different hydration times (%). 3. Results 3.1. Electrical conductivity 3.1.1. Effects of sedimentation stability on electrical conductivity The electrical conductivities of cement slurries with different for­ mulations are shown in Fig. 5 and Fig. 6. Previous works [45–47] have reported that when cement is mixed with water, ions such as Naþ, Kþ, Ca2þ, SO24 , and OH dissolve into the pore water, which increases the ion concentration in the pore water. Figs. 5 and 6 show that the initial electrical conductivity was high and that it followed an increasing trend with increasing hydration time for fresh cement slurry. Then, the elec­ trical conductivity decreased with increasing hydration time. Fig. 5 shows the effect of the fluid-loss additive content on the electrical conductivity of the cement slurry. From the properties of the cement slurry formulations presented in Table 2, it can be seen that the sedi­ mentation stability of the cement slurry increased with increasing fluid-loss additive content; this indicates that the cement particles were

(3)

Here, m1 is the mass of the empty sampling bottle (g), m2;t is the total mass of the bottle containing the cement slurry (g), m3;t is the total mass of the dried cement slurry (g), and wð1; tÞ is the evaporable water con­ tent of the cement slurry at different hydration times (%). LF-NMR was used to estimate the pore water distribution via mea­ surement of the transverse relaxation time (T2) of pore water in pores with different volumes. (4) shows the relationship between T2 and the pore characteristics of the cement slurries. Bede et al. [41,42] found 1.5 < T2 < 7.5 m s for capillary pores in cement slurry, T2 < 1.5 m s for 4

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 4. Relationship between T2 and pore characteristics.

Fig. 5. Effects of fluid-loss additive on electrical conductivity of cement slurry.

Fig. 6. Effects of CaCl2 content on electrical conductivity of cement slurry.

5

K. Liu et al.

Composites Part B 177 (2019) 107435

dispersed homogenously. Using the previously developed CEMHYD3D model [15,18], we found that this homogenous dispersion causes an increase in the cross-sectional area (AðtÞ) of pore water in a unit cross-sectional area of cement slurry. The electrical conductance GðtÞ of the cement slurry also increases with increasing AðtÞ, as given by (6): GðtÞ ¼

AðtÞ ϕðtÞ � L

reveal that the electrical conductivity of the pore solution was much higher than that of F-4. Because in the cement slurry the conductivity of the solid component is low, the AðtÞ (as shown in (6)) of the pore solu­ tion was increased. These results are similar to those obtained by Xiao et al. [10]. Additionally, when the hydration time of the cement slurry was 2, 6, and 10 h, the results of ion concentration show that with an increase in hydration time the ion concentration of Ca2þ reduces, the ion concentration of Naþ and Kþ increases in the pore solution, even the total ion concentration of Ca2þ, Naþ and Kþ has a slight increasing. However, the factors influencing the electrical conductivity of the pore solution include the concentration and electronic charge of the ion, and the change of ion species can change the electrical conductivity of the pore solution. Combining the electrical conductivity of F-4 cement slurry formu­ lation (see Fig. 6), when the hydration time increased from 2 h to 10 h, the electrical conductivity of the pore solution reduced from 39 mS/cm to 32.4 mS/cm, decreased by 16.9%. However, the electrical conduc­ tivity of F-4 formulation reduced from 20.24 mS/cm to 2.65 mS/cm, decreased by 86.9%. Therefore, the results in Figs. 6 and 7 conclude that the pore solution of fresh cement slurry has a significant influence on the electrical conductivity, but when the hydration reaction is induction period the pore solution has little influence on the electrical conduc­ tivity of F-4 cement slurry. Therefore, understanding the hydration process and internal microstructure of the cement slurry is crucial to know the electrical conductivity of cement slurry during early hydration.

(6)

Here, ϕðtÞ is the resistivity of cement slurry (Ω⋅m), AðtÞ is the crosssectional area of the conductive medium (m2), and L is the length of the conductive medium (m). In addition, the rate of change in the electrical conductivity of the cement slurry decreased with increasing fluid-loss additive content. Ma et al. [27] reported that the fluid-loss additive can be adsorbed onto the surface of cement particles (see the microstructure in Fig. 10), which may retard the hydration process and decrease the rate of change in the electrical conductivity. 3.1.2. Effects of hydration reaction on electrical conductivity A cement slurry system with good sedimentation stability was used to investigate the effect of the hydration rate on the electrical conduc­ tivity of cement slurry. CaCl2 is a commonly used cement accelerator that can increase the early hydration reaction rate of cement slurry [29, 48]. Fig. 6 shows the electrical conductivity curves of cement slurry formulations with different CaCl2 contents, from which it is clear that the addition of CaCl2 increased the initial electrical conductivity of the cement slurry. CaCl2 easily dissolves in water, and when added to the cement slurry, it causes a rapid increase in the ion concentration (and hence the electrical conductivity) of the pore water. Previous studies [48,49] have reported that CaCl2 reacts with tricalcium aluminate (C3A) to form Friedel’s salt or other hydration products, which results in rapid consumption of CaCl2 and an increase in the hydration reaction rate; this consequently leads to changes in the slurry microstructure and pore characteristics and a decrease in the electrical conductivity. Fig. 6 shows that an increase in the CaCl2 content of the cement slurry results in a faster decrease in the electrical conductivity during early hydration.

3.1.4. Hydration reaction Fig. 8 shows the hydration heat and heat flow of the F-4 formulation. The heat flow curve in this figure has two peaks, the first of which is observed between 20 min and 60 min. In previous works [4,14], this peak was attributed to the dissolution of cement particles and the for­ mation of hydration products, such as AFt and Friedel’s salt. In the present study, some hydration products cover the surface of the cement particles, which reduces the heat flow of the cement slurry. The cement slurry rapidly hydrates with increasing hydration time and consequently releases heat, which is the source of the second peak. At this stage, cement clinker, e.g. C3S and C2S, rapidly hydrates and forms hydration products. Fig. 9 shows the XRD spectra of the F-4 formulation at hydration times of 0, 4, 8, and 12 h. From the results in Fig. 9 (a), tricalcium silicate (Ca3SiO5), dicalcium silicate (Ca2SiO4), and brownmillerite (Ca2(Al,

3.1.3. Electrical conductivity of pore solution The electrical conductivity of pore solution in the F-4 cement slurry formulation was estimated as a function of the hydration time. Fig. 7 shows the experimental results of the electrical conductivity and ion concentration of the pore solution in F-4. The experimental results

Fig. 7. Electrical conductivity and ion concentration of pore solution in F-4 cement slurry formulation. 6

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 8. Hydration heat and heat flow of F-4 cement slurry.

Feþ3)2O5) are found to be the main phases of unhydrated cement. In the XRD spectra, the peaks at 29.472� and 32.634� correspond to Ca3SiO5, the peaks at 31.023� and 32.247� correspond to Ca2SiO4, and the peak at 33.728� corresponds to Ca2(Al, Feþ3)2O5. When the hydration time in­ creases to 4 h, a peak is observed at 2θ of 18.06� , and this peak corre­ sponds to portlandite (Ca(OH)2). Fig. 9 (b) shows the XRD spectra of the cement slurry at hydration times of 4, 8, and 12 h in the 2θ range of 26� –36� . Analysis of the XRD spectra reveals that the intensities of the peaks at 29.472� , 32.634� , and 34.406� decrease with an increase in the hydration time, which can confirm that Ca3SiO5 reacts with water rapidly in the early hydration stage of the cement slurry. Furthermore, hydration products such as Ca(OH)2 and C–S–H are formed in the cement slurry.

3.2.2. μCT 3D microstructure The 3D microstructure and pore characteristics of the F-4 cement slurry formulation at hydration times of 4, 6, 8, 10, and 12 h were analysed. Fig. 11 (a) shows the reconstructed 3D microstructure in a volume of 150 � 150 � 200 voxels in the cement slurry at the hydration time of 6 h. Each voxel was a cube with a side length of 2.4 μm. The VGStudio Max 3.2 software was used to determine the pore character­ istics according to the global grey threshold algorithm, additional details of the global grey threshold algorithm have been reported elsewhere [36,51]. Fig. 11 (b) shows the pore characteristics and pore spatial distribution; it can be seen that the pores in the cement slurry do not have any fixed shape. On the basis of the pore volume in the cement slurry, pores with a volume smaller than 50 voxels and those with a volume greater than 150 voxels were analysed, as shown in Fig. 11 (c) and Fig. 11 (d), respectively. The results in Fig. 11 (c) and Fig. 11 (d) reveal that the spatial distribution of the smaller-volume pores was wider than that of the larger-volume pores. Visual analysis showed that the smaller-volume pores were connected together to form the larger-volume pores. Fig. 12 shows the characteristics and spatial distribution of pores with a volume greater than 150 voxels in F-4 at different hydration times. Both the pore volume and the pore connectivity decreased with increasing hydration time. The connected porosity of the cement slurry was then calculated using the ‘burning’ algorithm [52]. Fig. 13 shows the calculation results of the connected porosity of the cement slurry in the z-axis direction (the coordinate direction indicated by the arrow in Fig. 11 (a)) at a hydration time of 6 h. From the results in Fig. 13, we could determine the mean connected porosity of the F-4 cement slurry formulation. Table 3 presents the results of the <50-voxel porosity (i.e. porosity resulting from <50-voxel pores), >150-voxel porosity (i.e. porosity resulting from >150-voxel pores), total porosity, and mean connected porosity of F-4 at different hydration times. From the results in Table 3, it was found that when the hydration time increased from 4 h to 12 h, the porosity of F-4 decreased from 10.9% to 9.67% (i.e. a decrease of 11.28%). It has been previously reported that testing of sub-micrometre and nano-sized pores by μCT is difficult because of the limitation on the μCT resolution [36]. When the hydration time increased from 4 h to 12 h, the >150-voxel porosity of F-4 decreased from 4.42% to 3.14% (i. e. a decrease of 28.96%); however, the <50-voxel porosity increased from 5.43% to 5.46%. From the results of the mean connected porosity (Table 3), it was found that the connected porosity decreased from 5.93% to 3.64% (i.e. a decrease of 38.62%) as the hydration time increased from 4 h to 12 h. These experimental results confirm that the formation of hydration products in the pores and on the surface of the

3.2. Microstructure 3.2.1. ESEM microstructure To study the relationship between the electrical conductivity and the microstructure of the cement slurry at different hydration times, the ESEM microstructure of cement slurry was analysed in the stage of electrical conductivity reduction (see Fig. 10). Fig. 10 (a) shows that the fluid-loss additive (AMPS/AM/AA polymer) in the cement slurry formed a ‘net-like’ structure that prevented the sedimentation of cement parti­ cles and there were many pores in the cement slurry. With increasing hydration time, C–S–H and other hydration products were continuously formed on the surface of the cement particles and on that of the fluid-loss additive polymer (see Fig. 10 (b) and Fig. 10 (c)). At a hydration time of 8 h, the surfaces of both the fluid-loss additive polymer and the cement particles were almost completely covered with hydration products, and some of the pores were filled with C–S–H gel (Fig. 10 (d)). During this process, large amounts of hydration products were formed in the pores, which reduced the porosity and pore volume of the cement slurry. As the amount of hydration products increased with increasing hydration time, some small-sized pores were observed at hydration times of 10 h and 12 h (see Fig. 10(e) and 10 (f), respectively). Previous studies [41,50] reported these small-sized pores as being gel pores. From Figs. 6 and 10, it was found that the pore size of the cement slurry decreased with decreasing electrical conductivity, which is consistent with previously reported results [9,10,18,20]. However, ESEM observations can provide only the 2D microstructure and pore structure of cement slurry. From these results (Fig. 10), analysis of the connectivity, shape, and spatial distribution of pores in cement slurry is difficult.

7

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 9. XRD spectra and phases of cement slurry at different hydration times. (a) XRD spectra in 2θ range of 5� –70� (b) XRD spectra in 2θ range of 26� –36� .

cement particles induced a transition of pore volume from large to small and caused a decrease in the degree of connectivity of pores in the cement slurry.

order to investigate the relationship between the pore water distribution and the electrical conductivity of the cement slurry, firstly, TGA was used to evaluate the mass change of the cement slurry specimen between 105 � C and 600 � C, as shown in Fig. 14. The chemical water content of the cement slurry was calculated using (3), and the results are shown in Fig. 15. It can be seen that the chemical water content increased with increasing hydration time and that the chemical water content increased rapidly at the point where the electrical conductivity reduced; this in­ dicates that the hydration reaction of the cement slurry accelerated in the stage of electrical conductivity reduction.

3.3. Pore water distribution 3.3.1. Chemical water The physical state of pore water changed with the formation of hy­ dration products and with changes in the microstructure and pore characteristics (as shown in Figs. 10 and 12). Some of the pore water and cement particles reacted to produce chemical water, whereas some amount of pore water occupied the pores with different volumes. In 8

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 10. ESEM microstructure of F-4 cement slurry formulation at different hydration times.

Fig. 11. Three-dimensional microstructure and pore characteristics of cement slurry at hydration time of 6 h: (a) 3D microstructure, (b) pore characteristics and pore spatial distribution, (c) spatial distribution of pores with volume smaller than 50 voxels, and (d) spatial distribution of pores with volume greater than 150 voxels.

Fig. 12. Characteristics and spatial distribution of pores with volume greater than 150 voxels.

3.3.2. Evaporable water The results shown in Fig. 15 demonstrate that in the early hydration stage of the cement slurry contained only a small amount of chemical water whereas the pores with different volumes contained a large amount of molecular water. Depending on the pore volume, pore water can be categorised as free water in large-volume pores, capillary water in capillary pores, and gel water in gel pores; all these types of pore water together constitute evaporable water [23]. Fig. 16 shows the evaporable water content of F-4 at different hydration times. In the fresh

cement slurries, the evaporable water content was 29.93%; when the hydration time of the cement slurry was 14 h, the evaporable water content was 24.31%. Moreover, LF-NMR was used to quantitatively evaluate the free water content, capillary water content, and gel water content of the cement slurry at different hydration times. Fig. 17 shows the transverse relaxation time (T2) curves of F-4 at different hydration times; it can be seen that T2 decreased with increasing hydration time. On the basis of the results of Bede et al. [41,42] (described in Section 2.2.4), the 9

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 13. Connected porosity of F-4 cement slurry formulation in z-axis direction at hydration time of 6 h.

slurry at different hydration times according to (7):

Table 3 Estimation results of pore characteristics of cement slurry at different hydration times. Pore characteristic <50-voxel porosity (%) >150-voxel porosity (%) Total porosity (%) Mean connected porosity (%) Connectivity degree (%)

wðx; tÞ ¼

Hydration time (h) 4

6

8

10

12

5.43 4.42 10.90 5.93 54.40

5.37 4.30 10.79 5.74 53.20

5.40 4.01 10.41 4.71 45.24

5.44 3.47 10.08 4.26 42.26

5.46 3.14 9.67 3.64 37.64

Ax;t � wð1; tÞ Atotal;t

(7)

where wðx; tÞ is the content of a given type of pore water (%), Ax;t is the T2 peak area of that type of pore water, Atotal; t is the T2 peak area of total amount of pore water, and w(1,t) is the evaporable water content (%). Table 4 lists the calculated free water content, capillary water con­ tent, and gel water content of the cement slurry at different hydration times. It can be seen that fresh cement slurry mainly contained free water and capillary water. At the point where the electrical conductivity reduced, a large amount of free water transformed into capillary water and gel water.

contents of the different types of pore water described above were calculated from the peak areas of the T2 curves (hereafter referred to as ‘T2 peak areas’). The results of the evaporable water content (Fig. 16) and those of T2 (Fig. 17) were then used to calculate the free water content, capillary water content, and gel water content of the cement

Fig. 14. TGA curves of F-4 cement slurry formulation at different hydration times. 10

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 15. Chemical water content as a function of hydration time.

Fig. 16. Evaporable water content as a function of hydration time.

4. Discussion

cement slurry. Some hydration products cover the surface of the cement particles and hinder further hydration, whereas the ion concentration in the pore water is in dynamic equilibrium with the dissolution and hy­ dration reactions. Then, in the acceleration period of the hydration re­ action, pore water is consumed to form large amounts of hydration products, and this process causes a change in the microstructure and pore characteristics of the cement slurry [4,18,54,55]. The electrical conductivity of the cement slurry subsequently begins to decrease rapidly. Therefore, changes in the microstructure and pore character­ istics of the cement slurry may be the main contributing factors to the decrease in the electrical conductivity.

4.1. Relationship between hydration reaction and the electrical conductivity The relationship between the hydration reaction and the electrical conductivity of F-4 cement slurry formulation was analysed (as shown in Fig. 18). The results in Fig. 18 reveal that the dissolution of CaCl2 into the cement slurry releases a considerable amount of heat, and the Ca2þ and Cl ions present in the pore water cause a rapid increase in the electrical conductivity. Next, some Ca2þ and Cl ions are consumed in the formation of hydration products [48,49,53], which causes a reduc­ tion in the ion concentration in the pore water and a corresponding reduction in the electrical conductivity of the cement slurry. From the obtained heat flow results, it can be seen that the hydration time of 1–4 h corresponds to the induction period of the hydration reaction of the

4.2. Relationship between microstructure and electrical conductivity The cement slurry is a two-component material, and its pores are filled with water. Fig. 19 shows the microstructural change of the 11

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 17. T2 curves of F-4 cement slurry formulation at different hydration times. Table 4 Types of pore water and their contents in cement slurry. Type of pore water Evaporable water Free water Capillary water Gel water

Water content (wt.%) 0h

2h

4h

6h

8h

10 h

12 h

14 h

29.90 17.91 11.99 0.00

29.71 18.12 11.59 0.00

29.53 17.40 12.13 0.00

28.75 13.84 14.91 0.00

27.33 6.67 20.66 0.00

26.20 0.77 23.43 2.00

25.22 0.03 16.52 8.67

24.31 0.00 9.68 14.63

Fig. 18. Electrical conductivity and heat flow of F-4 cement slurry formulation as functions of hydration time.

cement slurry. In the acceleration stage of hydration of the cement slurry, large amounts of hydration products were formed in the cement slurry. However, the density of the hydration products is less than that of the cement clinker according to Scrivener’s results [56]. The formation of the products occupied these large-volume pores; this caused a reduction in the porosity and mean connected porosity of the cement slurry because the large-volume pores were broken up into numerous small-volume pores and some previously connected pores were sepa­ rated (see Fig. 19). However, in the early hydration stage, the pores in the cement slurry were filled with pore water, and the pore water is a main conductive medium (see Figs. 6 and 7). The evolution of pore

characteristics led to a change in the physical state of the pore water [21]. Combining the results of Table 3 and Fig. 6, the mean connected porosity of the cement slurry was found to be almost proportional to the electrical conductivity. Within the measurement region, a decrease in the connected porosity of the cement slurry caused a reduction in the area of the conductive pore water, as shown in Fig. 19. According to (6), a decrease in the connected porosity of the cement slurry would cause a decrease in AðtÞ and the electrical conductivity, and the microstructural and pore structure changes caused a change in the electrical conduc­ tance of the cement slurry. In cement slurry, the pore water distribution changed with the 12

K. Liu et al.

Composites Part B 177 (2019) 107435

Fig. 19. Microstructural change of the cement slurry with increasing hydration time.

Fig. 20. Relationship between pore water distribution and electrical conductivity. (a) relationship between chemical water content and electrical conductivity (b) relationship between evaporable water content and electrical conductivity (c) relationship between free water content and electrical conductivity.

formation of hydration products and with changes in the microstructure and pore characteristics. From the results of Fig. 15, it can be seen that in the cement slurry the chemical water content increases with increasing hydration time. In combination with Figs. 6 and 15, an exponential relationship was observed between the electrical conductivity and the chemical water content of the cement slurry, as shown in Fig. 20 (a). However, the water-to-cement ratio of the cement slurry used in this experiment was 0.44, whereas the total water content was 30.56%. When the hydration time was 14 h, the chemical water content was only 5.2%. This suggests that the evaporable water may be a key factor influencing the electrical conductivity of cement slurry. Thus, the rela­ tionship between the evaporable water and the electrical conductivity was analysed, and it also showed an exponential trend (see Fig. 20 (b)).

The electrical conductivity of the cement slurry increased with an in­ crease in evaporable water content, which is similar to the relationship between porosity and electrical conductivity reported by Sanish et al. [54]. Combining with the results of Fig. 6 and Table 4, the analysis of the relationship between the free water content and the electrical conduc­ tivity of the cement slurry revealed a linear relationship (see Fig. 20 (c)), which indicated that the free water content was a key factor influencing the electrical conductivity of the cement slurry. In the early hydration stage, the free water occupied large-volume connected pores is the main conductive medium in the cement slurry [23,41]. In the present study, the formation of hydration products within the pores and on the surface of cement particles caused a large-volume pore to break up into many small-volume pores, which 13

K. Liu et al.

Composites Part B 177 (2019) 107435

resulted in a decrease in the degree of connectivity of the pores and a decrease in the amount of free water contained in large-volume pores. According to these experimental results, it can be seen that the con­ nected porosity and the free water content of the cement slurry were proportional to its electrical conductivity. According to above the experimental results and analysis, we can obtain that in the early hydration stage the modification of the pore characteristics of the cement slurry resulted in a change of pore water, which is crucial for reducing the electrical conductivity of the cement slurry during early hydration. And through the experimental results, when the cement slurry is in the early hydration stage, a linear rela­ tionship between the electrical conductivity and connected porosity, and a linear relationship between the electrical conductivity and free water content can be obtained. These results can provide some funda­ mental data and theories for the study of the electrical conductivity, microstructure, and pore characteristics of the cement slurry in the early hydration stage. However, some experimental and theoretical works to determine the pore connectivity of the cement slurry using the electrical conductivity should be performed in the future. Because understanding the pore connectivity of the cement slurry in the early hydration stage is crucial to know the reason of ‘gas migration’ accidents in cementing operation of natural gas wells.

References [1] Salem TM. Electrical conductivity and rheological properties of ordinary Portland cement – silica fume and calcium hydroxide – silica fume pastes. Cement Concr Res 2002;32:1473–81. https://doi.org/10.1016/S0008-8846(02)00809-8. [2] Heikal M, Morsy MS, Aiad I. Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes. Cement Concr Res 2005;35:680–7. https://doi.org/10.1016/j.cemconres.2004.06.012. [3] Wei X, Li Z. Early hydration process of Portland cement paste by electrical measurement. J Mater Civ Eng 2006;18:99–105. https://doi.org/10.1061/(ASCE) 0899-1561. 2006)18:1(99). [4] Tang SW, Cai XH, He Z, Zhou W, Shao HY, Li ZJ, et al. The review of early hydration of cement-based materials by electrical methods. Constr Build Mater 2017;146:15–29. https://doi.org/10.1016/j.conbuildmat.2017.04.073. [5] Tamas FD. Electrical conductivity of cement pastes. Cement Concr Res 1982;12: 115–20. [6] Tang SW, Li ZJ, Shao HY, Chen E. Characterization of early-age hydration process of cement pastes based on impedance measurement. Constr Build Mater 2014;68: 491–500. https://doi.org/10.1016/j.conbuildmat.2014.07.009. [7] zhen Xiao L, jin Li Z, sheng Wei X. Selection of superplasticizer in concrete mix design by measuring the early electrical resistivities of pastes. Cement Concr Compos 2007;29:350–6. https://doi.org/10.1016/j.cemconcomp.2006.12.015. [8] Topcu IB, Uygunoglu T, Hocaoglu I. Electrical resistivity of fly ash blended cement paste at hardening stage. Mater Sci 2016;22:458–62. https://doi.org/10.5755/j01. ms.22.3.10771. [9] He R, Ma H, Hafiz RB, Fu C, Jin X, He J. Determining porosity and pore network connectivity of cement-based materials by a modified non-contact electrical resistivity measurement: experiment and theory. Mater Des 2018;156:82–92. https://doi.org/10.1016/j.matdes.2018.06.045. [10] Xiao L, Li Z. Early-age hydration of fresh concrete monitored by non-contact electrical resistivity measurement. Cement Concr Res 2008;38:312–9. https://doi. org/10.1016/j.cemconres.2007.09.027. [11] Ridha S, Irawan S, Ariwahjoedi B. Strength prediction of Class G oilwell cement during early ages by electrical conductivity. J Pet Explor Prod Technol 2013;3: 303–11. https://doi.org/10.1007/s13202-013-0075-9. [12] Backe KR, Lile OB, Lyomov SK, Science NU. Characterizing curing cement slurries by electrical conductivity. SPE Drill Complet 2001;16:201–7. https://doi.org/ 10.2118/74694-PA. [13] Wang M, Chung DDL. Understanding the increase of the electric permittivity of cement caused by latex addition. Compos B Eng 2018;134:177–85. https://doi. org/10.1016/j.compositesb.2017.09.068. [14] Shen P, Lu L, He Y, Wang F, Hu S. Hydration monitoring and strength prediction of cement-based materials based on the dielectric properties. Constr Build Mater 2016;126:179–89. https://doi.org/10.1016/j.conbuildmat.2016.09.030. [15] McCarter WJ, Starrs G, Chrisp TM. Electrical conductivity, diffusion, and permeability of Portland cement-based mortars. Cement Concr Res 2000;30: 1395–400. https://doi.org/10.1016/S0008-8846(00)00281-7. [16] Levita G, Marchetti A, Gallone G, Princigallo A, Guerrini GL. Electrical properties of fluidified Portland cement mixes in the early stage of hydration. Cement Concr Res 2000;30:923–30. https://doi.org/10.1016/S0008-8846(00)00282-9. [17] Mendoza Reales OA, Dias Toledo Filho R. A review on the chemical, mechanical and microstructural characterization of carbon nanotubes-cement based composites. Constr Build Mater 2017;154:697–710. https://doi.org/10.1016/j. conbuildmat.2017.07.232. [18] Liu Z, Zhang Y, Jiang Q. Continuous tracking of the relationship between resistivity and pore structure of cement pastes. Constr Build Mater 2014;53:26–31. https:// doi.org/10.1016/j.conbuildmat.2013.11.067. [19] Yu P, Duan YH, Chen E, Tang SW, Wang XR. Microstructure-based fractal models for heat and mass transport properties of cement paste. Int J Heat Mass Transf 2018;126:432–47. https://doi.org/10.1016/j.ijheatmasstransfer.2018.05.150. [20] Weiss J, Snyder K, Bullard J, Bentz D. Using a saturation function to interpret the electrical properties of partially saturated concrete. J Mater Civ Eng 2012;25: 1097–106. https://doi.org/10.1061/(asce)mt.1943-5533.0000549. [21] Liu K, Cheng X, Zhang X, Li Z, Zhuang J, Guo X. Relationship between the microstructure/pore structure of oil-well cement and hydrostatic pressure. Transp Porous Media 2018;124:463–78. https://doi.org/10.1007/s11242-018-1078-2. [22] Li Z, Vandenbossche J, Iannacchione A, Brigham J, Kutchko B. Theory-based review of limitations with static gel strength in cement/matrix characterization. SPE Drill Complet 2016;31:145–58. https://doi.org/10.2118/178923-PA. [23] Muller ACA, Scrivener KL, Gajewicz AM, McDonald PJ. Densification of C-S-H measured by 1H NMR relaxometry. J Phys Chem C 2013;117:403–12. https://doi. org/10.1021/jp3102964. [24] American Petroleum Institute (API). API 10A, Specification for cements and materials for well cementing. 2002. [25] Cheng X, Liu K, Li Z, Wu Q, Duan W, Guo X. Effects of ammonium hydrolyzed polyacrylonitrile on oil-well cement slurry. J Mater Civ Eng 2017;29:1–6. https:// doi.org/10.1061/(ASCE)MT.1943-5533.0001933. [26] Hurnaus T, Plank J. Synthesis, characterization and performance of a novel phosphate-modified fluid loss additive useful in oil well cementing. J Nat Gas Sci Eng 2016;36:165–74. https://doi.org/10.1016/j.jngse.2016.10.011. [27] Ma J, Zheng H, Tan M, Liu L, Chen W, Guan Q, et al. Synthesis, characterization, and flocculation performance of anionic polyacrylamide P (AM-AA-AMPS). J Appl Polym Sci 2013;129. https://doi.org/10.1002/app.38900. 1984–91. [28] Zhang J, Weissinger EA, Peethamparan S, Scherer GW. Early hydration and setting of oil well cement. Cement Concr Res 2010;40:1023–33. https://doi.org/10.1016/ j.cemconres.2010.03.014.

5. Conclusions We investigated the effects of microstructure, pore characteristics, and pore water distribution on the electrical conductivity of cement slurries in the liquid–solid transition stage. The conclusions of this study are summarised as follows. (1) In fresh cement slurries, the pore characteristics and ion con­ centration were key factors influencing the electrical conductiv­ ity. In slurries with good sedimentation stability, the crosssectional areas of conductive pore water were larger, which led to an increase in the electrical conductivity of the slurry. In­ creases in the free water content of and ion concentration in the pore water also resulted in an effective increase in the electrical conductivity of the fresh cement slurry. (2) During the hydration of the cement slurry, the hydration reaction rate, pore characteristics, and free water content were key factors influencing its electrical conductivity. In the acceleration stage of the hydration reaction, the electrical conductivity of the cement slurry decreased with increasing hydration time, where some of the pore water and cement particles reacted to form chemical water in the hydration products. These hydration products occupied the pores, which caused the large-volume pores to break up into many small-volume pores and separation of some previ­ ously connected pores. The connected porosity was proportional to the electrical conductivity of the cement slurry. (3) Modification of the pore characteristics resulted in a change in the physical state of the pore water; free water in the largevolume pores transformed into capillary water and gel water. Further, the free water content and electrical conductivity were linearly correlated. Acknowledgments This work supported by the National Key R&D Program of China (No. 2016YFB0303600) and China Scholarship Council (No. 201808510194). We also like to acknowledge the Advanced Cementing Materials Research Centre of SWPU.

14

K. Liu et al.

Composites Part B 177 (2019) 107435 [44] Lothenbach B, Winnefeld F, Alder C, Wieland E, Lunk P. Effect of temperature on the pore solution , microstructure and hydration products of Portland cement pastes. Cement Concr Res 2007;37:483–91. https://doi.org/10.1016/j. cemconres.2006.11.016. [45] He Z, Li ZJ. Non-contact resistivity measurement for characterisation of the hydration process of cement-paste with excess alkali. Adv Cem Res 2004;16:29–34. https://doi.org/10.1680/adcr.16.1.29.36261. [46] Chen W, Li Y, Shen P, Shui Z. Microstructural development of hydrating portland cement paste at early ages investigated with non-destructive methods and numerical simulation. J Nondestruct Eval 2013;32:228–37. https://doi.org/ 10.1007/s10921-013-0175-y. [47] Liao Y, Wei X. Penetration resistance and electrical resistivity of cement paste with superplasticizer. Mater Struct 2014;47:563–70. https://doi.org/10.1617/s11527013-0079-4. [48] Talero R, Trusilewicz L, Delgado A, Pedrajas C, Lannegrand R, Rahhal V, et al. Comparative and semi-quantitative XRD analysis of Friedel’s salt originating from pozzolan and Portland cement. Constr Build Mater 2011;25:2370–80. https://doi. org/10.1016/j.conbuildmat.2010.11.037. [49] Abate C, Scheetz BE. Aqueous phase equilibria in the system CaO-Al2O3-CaCl2H2O: the significance and stability of friedel’s salt. J Am Ceram Soc 1995;78: 939–44. https://doi.org/10.1111/j.1151-2916.1995.tb08418.x. [50] Zhao L, Guo X, Liu Y, Zhao Y, Chen Z, Zhang Y, et al. Hydration kinetics, pore structure, 3D network calcium silicate hydrate, and mechanical behavior of graphene oxide reinforced cement composites. Constr Build Mater 2018;190: 150–63. https://doi.org/10.1016/j.conbuildmat.2018.09.105. [51] Fusseis F, Xiao X, Schrank C, De Carlo F. A brief guide to synchrotron radiationbased microtomography in (structural) geology and rock mechanics. J Struct Geol 2014;65:1–16. https://doi.org/10.1016/j.jsg.2014.02.005. [52] Liu C, Liu G, Liu Z, Yang L, Zhang M, Zhang Y. Numerical simulation of the effect of cement particle shapes on capillary pore structures in hardened cement pastes. Constr Build Mater 2018;173:615–28. https://doi.org/10.1016/j. conbuildmat.2018.04.039. [53] Peterson K, Julio-Betancourt G, Sutter L, Hooton RD, Johnston D. Observations of chloride ingress and calcium oxychloride formation in laboratory concrete and mortar at 5 � C. Cement Concr Res 2013;45:79–90. https://doi.org/10.1016/j. cemconres.2013.01.001. [54] Sanish KB, Neithalath N, Santhanam M. Monitoring the evolution of material structure in cement pastes and concretes using electrical property measurements. Constr Build Mater 2013;49:288–97. https://doi.org/10.1016/j. conbuildmat.2013.08.038. [55] Zhang M, Jivkov AP. Micromechanical modelling of deformation and fracture of hydrating cement paste using X-ray computed tomography characterisation. Compos B Eng 2016;88:64–72. https://doi.org/10.1016/j. compositesb.2015.11.007. [56] Scrivener KL, Juilland P, Monteiro PJM. Advances in understanding hydration of Portland cement. Cement Concr Res 2015;78:38–56. https://doi.org/10.1016/j. cemconres.2015.05.025.

[29] Wang C, Chen X, Wang R. Do chlorides qualify as accelerators for the cement of deepwater oil wells at low temperature? Constr Build Mater 2017;133:482–94. https://doi.org/10.1016/j.conbuildmat.2016.12.089. [30] American Petroleum Institute (API). API 10B-2, Recommended practice for testing well cements. second ed. 2013. [31] Ridha S, Irawan S, Ariwahjoedi B. Prediction equation for permeability of class G oilwell cement under reservoir conditions. Arabian J Sci Eng 2014;39:5219–28. https://doi.org/10.1007/s13369-014-1028-4. [32] Cherif R, Hamami AA, Aït-Mokhtar A, Meusnier JF. Study of the pore solution and the microstructure of mineral additions blended cement pastes. Energy Procedia 2017;139:584–9. https://doi.org/10.1016/j.egypro.2017.11.257. [33] Juilland P, Kumar A, Gallucci E, Flatt RJ, Scrivener KL. Effect of mixing on the early hydration of alite and OPC systems. Cement Concr Res 2012;42:1175–88. https://doi.org/10.1016/j.cemconres.2011.06.011. [34] Zhang Z, Zhu Y, Zhu H, Zhang Y, Provis JL, Wang H. Effect of drying procedures on pore structure and phase evolution of alkali-activated cements. Cement Concr Compos 2019;96:194–203. https://doi.org/10.1016/j.cemconcomp.2018.12.003. � [35] Zhang H, Savija B, Lukovi�c M, Schlangen E. Experimentally informed micromechanical modelling of cement paste: an approach coupling X-ray computed tomography and statistical nanoindentation. Compos B Eng 2019;157: 109–22. https://doi.org/10.1016/j.compositesb.2018.08.102. [36] Lei DY, Guo LP, Chen B, Curosu I, Mechtcherine V. The connection between microscopic and macroscopic properties of ultra-high strength and ultra-high ductility cementitious composites (UHS-UHDCC). Compos B Eng 2019;164: 144–57. https://doi.org/10.1016/j.compositesb.2018.11.062. [37] She AM, Yao W, Yuan WC. Hydration dynamics of portland cement studied by magnetic resonance. Appl Mech Mater 2012;193–194:509–12. https://doi.org/10. 4028/www.scientific.net/AMM.193-194.509. [38] Liu K, Cheng X, Zhang C, Gao X, Zhuang J, Guo X. Evolution of pore structure of oil well cement slurry in suspension–solid transition stage. Constr Build Mater 2019; 214:382–98. https://doi.org/10.1016/j.conbuildmat.2019.04.075. [39] Zhou C, Ren F, Zeng Q, Xiao L, Wang W. Pore-size resolved water vapor adsorption kinetics of white cement mortars as viewed from proton NMR relaxation. Cement Concr Res 2018;105:31–43. https://doi.org/10.1016/j.cemconres.2017.12.002. [40] Knapen E, Cizer O, Van Balen K, Van Gemert D. Effect of free water removal from early-age hydrated cement pastes on thermal analysis. Constr Build Mater 2009;23: 3431–8. https://doi.org/10.1016/j.conbuildmat.2009.06.004. [41] Bede A, Scurtu A, Ardelean I. NMR relaxation of molecules confined inside the cement paste pores under partially saturated conditions. Cement Concr Res 2016; 89:56–62. https://doi.org/10.1016/j.cemconres.2016.07.012. [42] McDonald PJ, Rodin V, Valori A. Characterisation of intra- and inter-C-S-H gel pore water in white cement based on an analysis of NMR signal amplitudes as a function of water content. Cement Concr Res 2010;40:1656–63. https://doi.org/10.1016/j. cemconres.2010.08.003. [43] Neves Junior A, Ferreira SR, Toledo Filho RD, Fairbairn E de MR, Dweck J. Effect of early age curing carbonation on the mechanical properties and durability of high initial strength Portland cement and lime-pozolan composites reinforced with long sisal fibres. Compos B Eng 2019;163:351–62. https://doi.org/10.1016/j. compositesb.2018.11.006.

15