# Effect of CNT-COOH Addition on the Compressive Strength, Chloride Resistance, and Microstructure of Cement Mortar

Authors: Xiaofan Wang; Yao Shi; Xueyang Pei; Lipeng Cui; Hua Zhu
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

This work evaluated the effect of multiwalled carboxyl carbon nanotube (CNT-COOH), upon cement mortar. In order to obtain a homogeneous suspension, the CNT-COOH was dispersed in distilled water, and it then underwent an ultrasonication treatment. Samples of cement mortar were prepared with varying the additions of this suspension at 0.01%, 0.05%, and 0.10% by weight of the cement. These samples were tested in compression and for chloride migration in terms of diffusion coefficient. An increase in the CNT-COOH content led to an increase in the compressive strength. While there was an increase in the chloride diffusion coefficient up to 0.05% weight fraction, higher dosage led to a drop in the chloride diffusion coefficient with a further increase in the compressive strength. Using mercury intrusion porosimetry (MIP), it is shown that at a weight fraction of 0.10%, CNT-COOH effects a pore-size refinement by eliminating macropores (≥50 nm) and a 34% reduction in porosity. The (scanning electron microscopy) SEM and (X-ray diffraction) XRD analyses confirm that CNT-COOH leads to mature hydration products, nucleation effect, and a refined microstructure.


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## Body

## 1. Introduction

Cement-based composites are inherently porous and quasi-brittle and so are inevitably susceptible to aggressive substances including chlorides, sulfates, and carbonation [1]. Much attention is now focused on durability in coastal regions and extreme climates [2]. A host of nanomaterial is known to increase the strength and durability of concrete, to achieve a longer service life [3, 4].Carbon nanotube (CNT) is acknowledged as one of the most promising materials due to its nanoscale, and noticeable mechanical [5–9], electrical [10, 11], thermal [12] and uniquely favorable chemical properties [9]. CNT is an allotrope of cylindrical carbon-nanostructure, composing of C–C bonds, which are the strongest bonds in nature [13]. With its length-to-diameter ratio as high as 132000000 : 1 and specific surface area generally between 50 and 1315 m2/g, it offers remarkably reinforcing ability compared to other nanosized materials. Studies indicate that adding CNT as reinforcement in cement-based composites promotes hydration-nucleation, pore-filling, pore-connectivity, and crack-bridging, all of which leads to an increase in the strength, fracture toughness, and densification [14–18].Prior studies reveal that an optimal content of CNT is in the range 0.01∼0.15% by weight of cement. This is shown to increase the compressive strength by 50% [18–22]. The strength enhancement was attributed to the changes in the microstructure and improvement to the C–S–H phase as shown by Nochaiya [23] and Konsta-Gdoutos [22–26], respectively. Moreover, the filling and bridging effects of CNT can effectively improve pore distribution and densify microstructure of cement-based composites, so that to reduce permeability and benefit long-term durability [27–30]. Wang et al. [31] dosed up to 0.15 wt% CNT and found optimal performance under chloride and sulfate attack with 0.10% CNT by weight fraction. Lu et al. [32] reported that 0.05 wt% CNT incorporation achieved the best chloride resistance in ultrahigh-strength concrete. Similar conclusions were obtained also by Han [33] and Lin [34] et al. However, few works have reported no improvement in chloride resistance arising from CNT [35, 36]. It is suspected that excessive CNT incorporation will limit the benefits from CNT.As known, the benefits from CNT to mechanical performance and durability of cementitious mixtures depend upon the amount and dispersion of CNT [22, 37]. As stated earlier, there is an optimal dosage for CNT. Excessive and/or too few CNT will interfere the fiber dispersion and inhibit the hydration reaction. On the contrary, being a nanomaterial, CNT is facile to agglomerate due to its strong Van der Waals forces between tubes, high aspect ratio, large surface area, and bundle-like entanglement properties [38–42], which in turn result in poor dispersion and reagglomeration in aqueous solution, leading to further decrease in the reinforcing effect of CNT [43, 44]. Therefore, better techniques must be explored to ensure efficient CNT dispersion. Some researchers chose to treat the CNT with sonication together with surfactants [45], and it achieves satisfactory dispersion with no damage to the structure of CNT [46]. However, the introduction of surfactants can increase the porosity of the resulting concrete, thereby degrading mechanical properties and resistance of water and corrosive substances [42, 47, 48]. Another concern is that CNT is inherently hydrophobic and hard to form any chemical bonds with the cementitious matrix.As a result, again there is some compromise with water tightness and crack-bridging [17, 49]. Mudimela et al. [50] introduced a chemical vapor deposition (CVD) method to resolve the poor bond between CNT and silica-rich materials. Grafting functional groups (-COOH or –OH) onto the CNT surface by CVD modification, not only achieves a better dispersion of CNT in the cement matrix [51, 52], but also strengthens the covalent bond between the CNT and the cement paste, through adhesion with C–S–H and C–H [53]. In this work, the multiwalled carbon nanotubes are first pretreated with carboxyl functional groups and then subjected to ultrasonic treatment for 45 minutes. Upon adding to a cement mortar, the resulting mixtures were examined for compressive strength and chloride penetration per the rapid chloride migration (RCM) method after various durations of curing. In addition, the pore size distribution and micromorphology within the mortar were evaluated through mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), and the effect upon hydration was explored using X-ray Diffraction (XRD).


## 2. Materials and Methods of Testing

### 2.1. Raw Materials

Portland cement conforming to PO.42.5 [54] was used in this study, and the physical properties and chemical composition are listed in Tables 1 and 2, respectively. Standard sand, in accordance with ISO 679 [55], was used as the fine aggregate. An industrial grade multi-walled carboxyl carbon nanotube (CNT-COOH) was sourced locally, which was provided from Shenzhen Nanoport Co., Ltd. The specific properties and XRD trace for this CNT-COOH materials are shown in Table 3 and Figure 1(a). In ordinary CNT, the (002) peak is at 26.3° [56], which corresponds to the space between the graphitic layers in carbon-related materials. However, for CNT-COOH, the (002) peak as witnessed at 25.7°, which could be attributed to the introduction of carboxylic function groups, which was confirmed by FTIR in Figure 1(b). It can be observed that the extending peak of 1380 cm−1 represents (-C=C stretching), constituting the basic network structure of CNT. Furthermore, the other significant absorption peaks appeared at about 1580 cm−1and 1730 cm−1, indicating the C=O- stretching vibration and bending vibration in the COOH group [57]. As well, Figures 2(a) and 2(b) show the CNT-COOH suspension, before and after ultrasonication, respectively. Obviously, as displayed in Figure 2(b), the final morphology of the CNT-COOH was a uniform and well-dispersed network of fibers.Table 1
Main physical properties of cement.
Water requirement of normal consistency (%)Setting time (min)Compressive strength (MPa)Flexural strength (MPa)Specific surface area (m2/kg)InitialFinal7 d28 d7 d28 d26.216422831.852.36.88.9349Table 2
Chemical compositions of cement.
ComponentSiO2Al2O3Fe2O3CaOMgOSO3Na2OLOIContents21.146.144.6765.941.150.350.480.13Table 3
Properties of carboxyl multiwalled carbon nanotube (CNT-COOH).
Raw materialDiameter (nm)Length (μm)Purity (wt, %)Special surface area (m2/g)Tap density (g/cm3)Ash content (%)-COOH content (wt, %)CNT-COOH5–1510–30>95200–2400.221<3∼9.18Figure 1
(a) The XRD patterns. (b) The FTIR spectra of CNT-COOH.
(a)(b)Figure 2
The SEM images of CNT-COOH (a) powder and (b) suspension after ultrasonic treatment.
(a)(b)


### 2.2. Mix Proportions and Sample Preparation

In this study, four mixtures of mortar were prepared with varying amounts of CNT-COOH. The details of mixture composition and proportions are shown in Table4, where the mix identifier follows the dosage of CNT-COOH namely, 0%, 0.01%, 0.05% and 0.10% by weight of cement. The water-to-binder ratio was kept constant at 0.45 for all mixtures; however, the water content in the CNT suspension was included in the overall target water.Table 4
Mix proportion of CNT-COOH-dosed mortar samples.
Samples water/binderContent (kg/m3) of constituents of the mix.WaterCementSandCNTC00.4522550015000C10.4522550015000.050C50.4522550015000.250C100.4522550015000.500Prior to its addition to the mortar, the CNT-COOH suspension underwent ultrasonication for a period of 2 hours, which is known to enable a well-dispersed regime in the cement slurry. The preparation of specimens containing different CNT-COOH contents was as follows, and the procedure is illustrated in Figure3:(1)
Pouring CNT-COOH into distilled water and mixing by using ultrasonic cleaner for 2 h at 60°C to achieve a well-dispersed suspension(2)
Adding cement and sands in blender, and mixing dry for 30 seconds(3)
Pouring CNT-COOH suspension (at a concentration of 6 g/L) and residual water (removing the water content from CNT-COOH suspension) into the blender, with mixing for another 3 min(4)
The mixture was poured into molds; cubes for compressive strength and cylinders for RCM test. The molds were compacted on a vibration table for 30 s(5)
Smoothening the specimens and curing for 24 hours in shaded place(6)
Demolding all specimens and placing in a curing chamber (RH ≥ 90%,T: 20 ± 2°C) until further testingFigure 3
The procedure of casting CNT-COOH-modified cement mortar.


### 2.3. Methods of Testing

#### 2.3.1. Testing for Compressive Strength

The compressive strength testing of the mortar samples were determined after 7, 28, and 60 days curing time, having dimensions of 70.7 mm × 70.7 mm × 70.7 mm. Each set of experiment was repeated 3 times and in accordance with JGJ/T70-2009 [58]. The results were averaged from the three measured values for each set.


#### 2.3.2. Testing for Chloride Diffusion

Rapid chloride migration coefficient method (RCM) tests of mortar samples were explored for 7 and 28 days in accordance with Chinese standard. Samples measuring three discs Φ100 × 50 mm were sawn from the original cylindrical specimens after 6 and 21 days and then kept in a curing chamber until the predetermined time, respectively. This test, RCM-NTB [59], is used for testing the depth of chloride ingress in mortar samples. The test yields the coefficient of chloride diffusion, which is calculated as follows [60]:(1)DRCM,0=2.872×10−6ThXd−αXdt,α=3.338×10−3Th,where DRCM.0 is the diffusion coefficient of chloride ion; T is the average temperature in Kelvin of the positive ion solution NaOH; h is the thickness of the specimen, in m; Xd is the average penetration depth, in m; t is the testing time, in seconds; and α is the auxiliary variable.


#### 2.3.3. Testing for Phase Composition and Microstructure

After the compression test, the broken samples were crushed into small fragments and soaked in concentrated ethyl alcohol to terminate the hydration reaction till testing. The pore size distribution was evaluated by mercury intrusion porosimetry (MIP), using the AUTOPORE IV 9500 instrument. Prior to the MIP test, the samples were kept in a vacuum oven at 60°C for 24 h. The crystalline phases of the CNT-COOH-modified cement mortar were characterized by X-ray diffraction using a copper source, which recorded data in the 2θ range from 3° to 90° at a speed of 10°/min. The microstructure of cement mortar doped with CNT-COOH was observed by using a field emission SEM instrument (Hitachi SU8220).


## 2.1. Raw Materials

Portland cement conforming to PO.42.5 [54] was used in this study, and the physical properties and chemical composition are listed in Tables 1 and 2, respectively. Standard sand, in accordance with ISO 679 [55], was used as the fine aggregate. An industrial grade multi-walled carboxyl carbon nanotube (CNT-COOH) was sourced locally, which was provided from Shenzhen Nanoport Co., Ltd. The specific properties and XRD trace for this CNT-COOH materials are shown in Table 3 and Figure 1(a). In ordinary CNT, the (002) peak is at 26.3° [56], which corresponds to the space between the graphitic layers in carbon-related materials. However, for CNT-COOH, the (002) peak as witnessed at 25.7°, which could be attributed to the introduction of carboxylic function groups, which was confirmed by FTIR in Figure 1(b). It can be observed that the extending peak of 1380 cm−1 represents (-C=C stretching), constituting the basic network structure of CNT. Furthermore, the other significant absorption peaks appeared at about 1580 cm−1and 1730 cm−1, indicating the C=O- stretching vibration and bending vibration in the COOH group [57]. As well, Figures 2(a) and 2(b) show the CNT-COOH suspension, before and after ultrasonication, respectively. Obviously, as displayed in Figure 2(b), the final morphology of the CNT-COOH was a uniform and well-dispersed network of fibers.Table 1
Main physical properties of cement.
Water requirement of normal consistency (%)Setting time (min)Compressive strength (MPa)Flexural strength (MPa)Specific surface area (m2/kg)InitialFinal7 d28 d7 d28 d26.216422831.852.36.88.9349Table 2
Chemical compositions of cement.
ComponentSiO2Al2O3Fe2O3CaOMgOSO3Na2OLOIContents21.146.144.6765.941.150.350.480.13Table 3
Properties of carboxyl multiwalled carbon nanotube (CNT-COOH).
Raw materialDiameter (nm)Length (μm)Purity (wt, %)Special surface area (m2/g)Tap density (g/cm3)Ash content (%)-COOH content (wt, %)CNT-COOH5–1510–30>95200–2400.221<3∼9.18Figure 1
(a) The XRD patterns. (b) The FTIR spectra of CNT-COOH.
(a)(b)Figure 2
The SEM images of CNT-COOH (a) powder and (b) suspension after ultrasonic treatment.
(a)(b)


## 2.2. Mix Proportions and Sample Preparation

In this study, four mixtures of mortar were prepared with varying amounts of CNT-COOH. The details of mixture composition and proportions are shown in Table4, where the mix identifier follows the dosage of CNT-COOH namely, 0%, 0.01%, 0.05% and 0.10% by weight of cement. The water-to-binder ratio was kept constant at 0.45 for all mixtures; however, the water content in the CNT suspension was included in the overall target water.Table 4
Mix proportion of CNT-COOH-dosed mortar samples.
Samples water/binderContent (kg/m3) of constituents of the mix.WaterCementSandCNTC00.4522550015000C10.4522550015000.050C50.4522550015000.250C100.4522550015000.500Prior to its addition to the mortar, the CNT-COOH suspension underwent ultrasonication for a period of 2 hours, which is known to enable a well-dispersed regime in the cement slurry. The preparation of specimens containing different CNT-COOH contents was as follows, and the procedure is illustrated in Figure3:(1)
Pouring CNT-COOH into distilled water and mixing by using ultrasonic cleaner for 2 h at 60°C to achieve a well-dispersed suspension(2)
Adding cement and sands in blender, and mixing dry for 30 seconds(3)
Pouring CNT-COOH suspension (at a concentration of 6 g/L) and residual water (removing the water content from CNT-COOH suspension) into the blender, with mixing for another 3 min(4)
The mixture was poured into molds; cubes for compressive strength and cylinders for RCM test. The molds were compacted on a vibration table for 30 s(5)
Smoothening the specimens and curing for 24 hours in shaded place(6)
Demolding all specimens and placing in a curing chamber (RH ≥ 90%,T: 20 ± 2°C) until further testingFigure 3
The procedure of casting CNT-COOH-modified cement mortar.


## 2.3. Methods of Testing

### 2.3.1. Testing for Compressive Strength

The compressive strength testing of the mortar samples were determined after 7, 28, and 60 days curing time, having dimensions of 70.7 mm × 70.7 mm × 70.7 mm. Each set of experiment was repeated 3 times and in accordance with JGJ/T70-2009 [58]. The results were averaged from the three measured values for each set.


### 2.3.2. Testing for Chloride Diffusion

Rapid chloride migration coefficient method (RCM) tests of mortar samples were explored for 7 and 28 days in accordance with Chinese standard. Samples measuring three discs Φ100 × 50 mm were sawn from the original cylindrical specimens after 6 and 21 days and then kept in a curing chamber until the predetermined time, respectively. This test, RCM-NTB [59], is used for testing the depth of chloride ingress in mortar samples. The test yields the coefficient of chloride diffusion, which is calculated as follows [60]:(1)DRCM,0=2.872×10−6ThXd−αXdt,α=3.338×10−3Th,where DRCM.0 is the diffusion coefficient of chloride ion; T is the average temperature in Kelvin of the positive ion solution NaOH; h is the thickness of the specimen, in m; Xd is the average penetration depth, in m; t is the testing time, in seconds; and α is the auxiliary variable.


### 2.3.3. Testing for Phase Composition and Microstructure

After the compression test, the broken samples were crushed into small fragments and soaked in concentrated ethyl alcohol to terminate the hydration reaction till testing. The pore size distribution was evaluated by mercury intrusion porosimetry (MIP), using the AUTOPORE IV 9500 instrument. Prior to the MIP test, the samples were kept in a vacuum oven at 60°C for 24 h. The crystalline phases of the CNT-COOH-modified cement mortar were characterized by X-ray diffraction using a copper source, which recorded data in the 2θ range from 3° to 90° at a speed of 10°/min. The microstructure of cement mortar doped with CNT-COOH was observed by using a field emission SEM instrument (Hitachi SU8220).


## 2.3.1. Testing for Compressive Strength

The compressive strength testing of the mortar samples were determined after 7, 28, and 60 days curing time, having dimensions of 70.7 mm × 70.7 mm × 70.7 mm. Each set of experiment was repeated 3 times and in accordance with JGJ/T70-2009 [58]. The results were averaged from the three measured values for each set.


## 2.3.2. Testing for Chloride Diffusion

Rapid chloride migration coefficient method (RCM) tests of mortar samples were explored for 7 and 28 days in accordance with Chinese standard. Samples measuring three discs Φ100 × 50 mm were sawn from the original cylindrical specimens after 6 and 21 days and then kept in a curing chamber until the predetermined time, respectively. This test, RCM-NTB [59], is used for testing the depth of chloride ingress in mortar samples. The test yields the coefficient of chloride diffusion, which is calculated as follows [60]:(1)DRCM,0=2.872×10−6ThXd−αXdt,α=3.338×10−3Th,where DRCM.0 is the diffusion coefficient of chloride ion; T is the average temperature in Kelvin of the positive ion solution NaOH; h is the thickness of the specimen, in m; Xd is the average penetration depth, in m; t is the testing time, in seconds; and α is the auxiliary variable.


## 2.3.3. Testing for Phase Composition and Microstructure

After the compression test, the broken samples were crushed into small fragments and soaked in concentrated ethyl alcohol to terminate the hydration reaction till testing. The pore size distribution was evaluated by mercury intrusion porosimetry (MIP), using the AUTOPORE IV 9500 instrument. Prior to the MIP test, the samples were kept in a vacuum oven at 60°C for 24 h. The crystalline phases of the CNT-COOH-modified cement mortar were characterized by X-ray diffraction using a copper source, which recorded data in the 2θ range from 3° to 90° at a speed of 10°/min. The microstructure of cement mortar doped with CNT-COOH was observed by using a field emission SEM instrument (Hitachi SU8220).


## 3. Results and Discussion

### 3.1. Compressive Strength

The compressive strength of the all mortars at 7 and 28 day are shown in Figure4. Compared with the reference mixture that contained no CNT, it can be observed that the incorporation of CNT-COOH was beneficial to the compressive strength of cement mortar. The compressive strength of the mortar increased with an increase in the CNT-COOH content, consistently. The greatest rise was 22% increase after 28 curing days with 0.10% CNT-COOH by weight fraction of cement. Similar conclusions were obtained by Xu et al. [25] via integrated method with surfactant and ultrasonic treatments. The increase in compressive strength may be attributed to the nanotubes reinforcing cement mortar and promoting nucleation for hydration, due to their nanosize and the presence of carboxyl groups. Sindu et al. [61], who used a similar CNT but also included a surfactant, found that the compressive strength while rising for small doses begins to drop as the CNT content exceeds 0.08%. This drop may be due to the higher porosity associated with a surfactant. The present findings clearly highlight the benefit of avoiding a surfactant, as was done in this study.Figure 4
Compressive strength of CNT-mortar samples with the various dosage of CNT-COOH after 7 and 28 days of curing time (C0: 0%, C1: 0.01%, C5: 0.05%, and C10 : 0.10% of CNT-COOH by weight of cement).


### 3.2. Chloride Penetration Resistance Analysis

The chloride diffusion depth at 28-day and coefficient at 7-, 28-day are shown in Figures5 and 6, respectively. With an increase in the CNT-COOH content, the chloride diffusion depth and the diffusion coefficient, both rose gradually up to 0.05% weight fraction. The two parameters drop together for a CNT-COOH content equal to 0.10%. Note that the 28-day compressive strength was the highest at 0.10% CNT-COOH content. At 28th day, the chloride diffusion depth and the diffusion coefficient in the reference mixture was 11.84 mm and 8.32 × 10−12 m2/s, respectively. Their values increased by 15% and 33%, respectively, to 13.57 mm and 11.05 × 10−12 m2/s for 0.05% CNT-COOH dosage. However, these values decreased by 13% and 24%, respectively, to 10.32 mm and 6.32 × 10−12 m2/s upon adding 0.10% CNT-COOH to the plain mixture.Figure 5
Chloride diffusion depth of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time.Figure 6
Chloride diffusion coefficient of CNT-mortar samples with the various dosage of CNT-COOH after 7 and 28 days of curing time.While the compressive strength rose consistently with an increase in the CNT-COOH dosage, the chloride diffusion increases up to 0.05% weight fraction. Only for the highest dosage as studied here, namely, 0.10%, did the chloride diffusion drop sharply, such that the diffusion coefficient was less than what was noted for the reference mixture. A closer look at the internal microstructure is therefore warranted. In what follows, the air-void network is examined in order to understand the possible underlying reasons for this apparent inconsistency.


### 3.3. Pore Size Distribution Analysis

The pore size distribution was characterized by mercury intrusion porosimetry (MIP), using samples of 3∼6 mm size fragment obtained after the compression test. According to Mindess et al. [62] and Zhao et al. [63], pores of size between 50 and 10000 nm may be classified as macropores; those between 10 and 50 nm may be classified as mesopores and those less than 10 nm are denoted as gel pores. It is acknowledged that the macropores have the most significant influence upon the permeability of cement-based materials [64]. In other words, cement-based materials with more coarse pores and higher porosity mean lower durability properties. The differential intrusion across the various mixtures is shown in Figure 7. Note that there was a consistent drop in macropores in comparison with the reference mixture, C0. While the micropores (<50 nm) increase with an addition of CNT-COOH, the macropores (>50 nm) are seen to increase for up to 0.05%. The macropores decrease substantially for 0.10% CNT-COOH content. Evidently, the differential intrusion is consistently more than C0 for all mixtures up to 0.05% weight fraction of CNT-COOH. The part of macropores vastly decreases at 0.10% CNT-COOH, which may be attributed to the reduction in the volume of macropores. While the compressive strength is dependent upon overall porosity, the water tightness depends not just upon the porosity but more significantly upon the fraction of macropores and clearly, the latter is almost eliminated at 0.10% addition of CNT-COOH.Figure 7
The log differential intrusion with pore diameter of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time.The output from the MIP test is listed in Table5. It is clear that the porosity of C10 was the lowest, being about 34% lower than that for C0. On the contrary, neither mixture C1 nor mixture C5 depicts any noticeable difference with respect to the reference mixture C0. Again, the values of total pore area, median pore diameter, and the average pore diameter, all were the lowest in case of mixture C10. Since 0.10% weight fraction was the highest dosage of CNT-COOH investigated in this study, it is not clear at present what if any is the optimal dosage for this form of modified CNT. Fiber reinforcement is known to introduce meso- and macropores, due to the introduction of the fiber-matrix interface [65]. Therefore, there appears to be a threshold dosage level beyond which the system becomes sufficiently impermeable to chemical diffusion.Table 5
MIP analysis of CNT-mortar at 28 day.
SamplesPorosity (%)Total pore area (m2/g)Median volume pore diameter (nm)Average pore diameter (nm)C012.495.05126.046.2C111.845.18824.440.6C512.355.58329.851.8C108.234.58321.533.7


### 3.4. The Effect of CNT-COOH upon the Hydration of Mortars

Figure8 represents the XRD patterns of all mortar mixtures after 28 days of standard curing. Some essential phases derived from the hydration reaction can be identified as characteristic peaks. These characteristic peaks are the principal hydration products, including calcium hydroxide (CH), calcium carbonate (CC), and ettringite (AFt). Note that there is no new phase appearing in mixtures that contain CNT-COOH, when compared with the reference plain mixture. It is also seen that the peak intensity of CH and CC increases consistently with an increase in the CNT-COOH content. The peak intensity is a qualitative measure of the degree of crystallinity. According to the two main cement hydration chemical equations as following, the production of CH can imply the cement hydration rate to a certain extent, so that the variation of CH peak can explain that the various dosage of CNT promotes the hydration reaction of cement-based composites or not. So that, one observes that the phases in the mixture C10 show a significantly higher degree of crystallinity for CH, over those in the reference mixture, C0. As such, the introduction of CNT-COOH appears to promote hydration, most likely by acting as nucleation sites for the reactions in hydrating cement paste:(2)3CaO·SiO2+nH2O=xCaO·SiO2·yH2O+3−xCaOH2,2CaO·SiO2+nH2O=xCaO·SiO2·yH2O+2−xCaOH2.Figure 8
The XRD patterns of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time.


### 3.5. Microstructure of CNT-COOH Reinforced Mortar

In order to understand the effect of CNT-COOH upon the hydration and microstructure of the cement mortar, mixtures C5 and C10 were examined under scanning electron microscopy (SEM). The microscopic images for mixtures C5 and C10 are shown in Figures9(a) and 9(d), respectively. The CNT-COOH was well dispersed, and no CNT-COOH clumping was observed in either mixture. However, while there is clearly visible matrix of calcium silicate hydrate (C–S–H) in mixture C10, this is not visible in mixture C5. Evidently, at 0.10% CNT-COOH, the degree of hydration was significantly superior. Both Figures 9(b) and 9(c) display fiber bridging, even for 0.05% CNT-COOH dosage. However, Figures 9(e) and 9(f) clearly indicate a higher degree of filling and crack bridging. As well, having a higher dosage increases the number of fibers that appear at a fracture surface. Together with its inherent hydrophilic nature, the presence of higher dosage of CNT-COOH greatly benefits both mechanical performance and water tightness in cementitious systems. This study did not measure the effect of the carboxyl group, for the amount grafted was preset by the manufacturing source. Further studies are underway to examine this parameter. Overall, it is clear from these SEM images that for 0.10% CNT-COOH by weight of cement, the mortar achieves a densely compacted microstructure, partly through the filler effect and partly through promoting the hydration process.Figure 9
The SEM images of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time: (a–c) C5; (d–f) C10.


## 3.1. Compressive Strength

The compressive strength of the all mortars at 7 and 28 day are shown in Figure4. Compared with the reference mixture that contained no CNT, it can be observed that the incorporation of CNT-COOH was beneficial to the compressive strength of cement mortar. The compressive strength of the mortar increased with an increase in the CNT-COOH content, consistently. The greatest rise was 22% increase after 28 curing days with 0.10% CNT-COOH by weight fraction of cement. Similar conclusions were obtained by Xu et al. [25] via integrated method with surfactant and ultrasonic treatments. The increase in compressive strength may be attributed to the nanotubes reinforcing cement mortar and promoting nucleation for hydration, due to their nanosize and the presence of carboxyl groups. Sindu et al. [61], who used a similar CNT but also included a surfactant, found that the compressive strength while rising for small doses begins to drop as the CNT content exceeds 0.08%. This drop may be due to the higher porosity associated with a surfactant. The present findings clearly highlight the benefit of avoiding a surfactant, as was done in this study.Figure 4
Compressive strength of CNT-mortar samples with the various dosage of CNT-COOH after 7 and 28 days of curing time (C0: 0%, C1: 0.01%, C5: 0.05%, and C10 : 0.10% of CNT-COOH by weight of cement).


## 3.2. Chloride Penetration Resistance Analysis

The chloride diffusion depth at 28-day and coefficient at 7-, 28-day are shown in Figures5 and 6, respectively. With an increase in the CNT-COOH content, the chloride diffusion depth and the diffusion coefficient, both rose gradually up to 0.05% weight fraction. The two parameters drop together for a CNT-COOH content equal to 0.10%. Note that the 28-day compressive strength was the highest at 0.10% CNT-COOH content. At 28th day, the chloride diffusion depth and the diffusion coefficient in the reference mixture was 11.84 mm and 8.32 × 10−12 m2/s, respectively. Their values increased by 15% and 33%, respectively, to 13.57 mm and 11.05 × 10−12 m2/s for 0.05% CNT-COOH dosage. However, these values decreased by 13% and 24%, respectively, to 10.32 mm and 6.32 × 10−12 m2/s upon adding 0.10% CNT-COOH to the plain mixture.Figure 5
Chloride diffusion depth of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time.Figure 6
Chloride diffusion coefficient of CNT-mortar samples with the various dosage of CNT-COOH after 7 and 28 days of curing time.While the compressive strength rose consistently with an increase in the CNT-COOH dosage, the chloride diffusion increases up to 0.05% weight fraction. Only for the highest dosage as studied here, namely, 0.10%, did the chloride diffusion drop sharply, such that the diffusion coefficient was less than what was noted for the reference mixture. A closer look at the internal microstructure is therefore warranted. In what follows, the air-void network is examined in order to understand the possible underlying reasons for this apparent inconsistency.


## 3.3. Pore Size Distribution Analysis

The pore size distribution was characterized by mercury intrusion porosimetry (MIP), using samples of 3∼6 mm size fragment obtained after the compression test. According to Mindess et al. [62] and Zhao et al. [63], pores of size between 50 and 10000 nm may be classified as macropores; those between 10 and 50 nm may be classified as mesopores and those less than 10 nm are denoted as gel pores. It is acknowledged that the macropores have the most significant influence upon the permeability of cement-based materials [64]. In other words, cement-based materials with more coarse pores and higher porosity mean lower durability properties. The differential intrusion across the various mixtures is shown in Figure 7. Note that there was a consistent drop in macropores in comparison with the reference mixture, C0. While the micropores (<50 nm) increase with an addition of CNT-COOH, the macropores (>50 nm) are seen to increase for up to 0.05%. The macropores decrease substantially for 0.10% CNT-COOH content. Evidently, the differential intrusion is consistently more than C0 for all mixtures up to 0.05% weight fraction of CNT-COOH. The part of macropores vastly decreases at 0.10% CNT-COOH, which may be attributed to the reduction in the volume of macropores. While the compressive strength is dependent upon overall porosity, the water tightness depends not just upon the porosity but more significantly upon the fraction of macropores and clearly, the latter is almost eliminated at 0.10% addition of CNT-COOH.Figure 7
The log differential intrusion with pore diameter of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time.The output from the MIP test is listed in Table5. It is clear that the porosity of C10 was the lowest, being about 34% lower than that for C0. On the contrary, neither mixture C1 nor mixture C5 depicts any noticeable difference with respect to the reference mixture C0. Again, the values of total pore area, median pore diameter, and the average pore diameter, all were the lowest in case of mixture C10. Since 0.10% weight fraction was the highest dosage of CNT-COOH investigated in this study, it is not clear at present what if any is the optimal dosage for this form of modified CNT. Fiber reinforcement is known to introduce meso- and macropores, due to the introduction of the fiber-matrix interface [65]. Therefore, there appears to be a threshold dosage level beyond which the system becomes sufficiently impermeable to chemical diffusion.Table 5
MIP analysis of CNT-mortar at 28 day.
SamplesPorosity (%)Total pore area (m2/g)Median volume pore diameter (nm)Average pore diameter (nm)C012.495.05126.046.2C111.845.18824.440.6C512.355.58329.851.8C108.234.58321.533.7


## 3.4. The Effect of CNT-COOH upon the Hydration of Mortars

Figure8 represents the XRD patterns of all mortar mixtures after 28 days of standard curing. Some essential phases derived from the hydration reaction can be identified as characteristic peaks. These characteristic peaks are the principal hydration products, including calcium hydroxide (CH), calcium carbonate (CC), and ettringite (AFt). Note that there is no new phase appearing in mixtures that contain CNT-COOH, when compared with the reference plain mixture. It is also seen that the peak intensity of CH and CC increases consistently with an increase in the CNT-COOH content. The peak intensity is a qualitative measure of the degree of crystallinity. According to the two main cement hydration chemical equations as following, the production of CH can imply the cement hydration rate to a certain extent, so that the variation of CH peak can explain that the various dosage of CNT promotes the hydration reaction of cement-based composites or not. So that, one observes that the phases in the mixture C10 show a significantly higher degree of crystallinity for CH, over those in the reference mixture, C0. As such, the introduction of CNT-COOH appears to promote hydration, most likely by acting as nucleation sites for the reactions in hydrating cement paste:(2)3CaO·SiO2+nH2O=xCaO·SiO2·yH2O+3−xCaOH2,2CaO·SiO2+nH2O=xCaO·SiO2·yH2O+2−xCaOH2.Figure 8
The XRD patterns of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time.


## 3.5. Microstructure of CNT-COOH Reinforced Mortar

In order to understand the effect of CNT-COOH upon the hydration and microstructure of the cement mortar, mixtures C5 and C10 were examined under scanning electron microscopy (SEM). The microscopic images for mixtures C5 and C10 are shown in Figures9(a) and 9(d), respectively. The CNT-COOH was well dispersed, and no CNT-COOH clumping was observed in either mixture. However, while there is clearly visible matrix of calcium silicate hydrate (C–S–H) in mixture C10, this is not visible in mixture C5. Evidently, at 0.10% CNT-COOH, the degree of hydration was significantly superior. Both Figures 9(b) and 9(c) display fiber bridging, even for 0.05% CNT-COOH dosage. However, Figures 9(e) and 9(f) clearly indicate a higher degree of filling and crack bridging. As well, having a higher dosage increases the number of fibers that appear at a fracture surface. Together with its inherent hydrophilic nature, the presence of higher dosage of CNT-COOH greatly benefits both mechanical performance and water tightness in cementitious systems. This study did not measure the effect of the carboxyl group, for the amount grafted was preset by the manufacturing source. Further studies are underway to examine this parameter. Overall, it is clear from these SEM images that for 0.10% CNT-COOH by weight of cement, the mortar achieves a densely compacted microstructure, partly through the filler effect and partly through promoting the hydration process.Figure 9
The SEM images of CNT-mortar samples with the various dosage of CNT-COOH after 28 days of curing time: (a–c) C5; (d–f) C10.


## 4. Concluding Remarks

This study examines the effect of carbon nanotubes, grafted with carboxyl groups upon the compressive strength, chloride diffusion, pore-size distribution and microstructure of cement-based mortar. The carboxyl group is expected to impart hydrophilic nature to the carbon nanotube and the resulting CNT-COOH suspension was introduced at weight fraction of 0%, 0.01%, 0.05%, and 0.10% by cement. The principal findings are summarized as follows:(1)
Compared with the reference plain mixture, CNT-COOH reinforced mortar consistently displayed higher compressive strength for both 7th day and 28th day. As expected therefore, the maximum increase was 22% at 28 day, with 0.10% CNT-COOH dosage.(2)
With an increase in the CNT-COOH dosage, the depth of chloride diffusion and coefficient of chloride diffusion, both rose consistently up to 0.05% weight fraction. However, at 0.10% dosage, both chloride diffusion and its depth record a drop with respect to the reference mixture.(3)
Mercury intrusion porosimetry illustrates that the total porosity decreases consistently with an increase in the CNT-COOH dosage. However, there was a rise in the volume of macropores up to 0.05% weight fraction. Again, at 0.10% CNT-COOH dosage, there was a distinct drop in the volume of macropores, even when compared with the unreinforced reference mixture.(4)
Taken together, XRD and SEM outputs illustrate that at 0.10% CNT-COOH dosage, there is a pronounced increase in the degree of crystallinity among the hydration products. The SEM images show that at 0.10% CNT-COOH, the microstructure displays a combination of densification and crack-bridging. Together, these transformations to the microstructure likely explain the increase in strength and resistance to chloride diffusion.



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*Source: 3345279-2022-04-13.xml*