Mechanical, Electrical and Fractural Characteristics of Carbon Nanomaterial-Added Cement Composites

Abstract

This study investigates the effects of different carbon nanomaterials (CNMs), namely, carbon nanotubes (CNTs), carbon nanofibers (CNFs), graphene, and graphite nanoplatelets (GNP) on the mechanical, electrical, and fractural characteristics of cement composites. The electrical conductivity results indicated that CNT- and CNF-added composites exhibited percolation threshold ranges of 0.1% to 0.3% and 0.3% to 1.0%, respectively. Regarding the mechanical properties tests, the composite with a 1.0% CNF showed the best results. Furthermore, fractural characteristics results indicated that even additions of the smallest dosage, i.e., 0.1% of CNM, exhibited positive results. Overall, the study highlighted the potential of CNM-added cement composites.

1. Introduction

Cement composites are one of the oldest materials used in construction; their use dates back to 7000 BC [1]. Along with human civilization, cement composites have evolved significantly. Cement-composite materials have good resistance to corrosion, water, and temperature, along with economic advantages, making them one of the most suitable materials for construction. Currently, cement composites, such as concrete, are classified as quasi-brittle materials, meaning that they have relatively high compressive strength but low flexural and tensile strengths [2,3]. Construction has advanced rapidly in modern times. In addition to being fast-paced, construction technology has evolved to possess high quality. Because cement-based composites are the most commonly used materials in construction, ongoing development and research are being conducted to improve their properties. The use of nanomaterials in cement composites is a new technology that is currently being researched and practiced in modern construction.

Carbon nanomaterials (CNMs) are predominantly composed of carbon atoms [4]. They exhibit excellent electrical and thermal conductivity and high mechanical strength [4,5,6]. Carbon nanomaterials can be classified into carbon nanotubes (CNTs), carbon nanofibers (CNFs), and graphene according to their shape, dimensions, and nanostructural features [4,5,6,7]. Graphene oxides (GOs), graphite nanoplatelets (GNPs), carbon dots, nanodiamonds, and fullerenes are examples of carbon nanomaterials.

CNTs were first discovered in 1991 by the pioneer researcher Iijima [8]. CNTs are cylindrical carbon allotropes produced from graphene sheets [2,9]. CNTs can be further classified into multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs). SWCNTs have an exceptionally high tensile strength of 50–500 GPa and elastic modulus of 1.0 TPa [10]. MWCNTs exhibit exceptional characteristics analogous to those of SWCNTs. The tensile strength of MWCNTs ranges from 10 to 60 GPa. Their elastic modulus ranges between 0.3 and 1 TPa, while their electrical resistivity is between 5 and 50 μΩ·cm. Research has demonstrated that incorporating CNTs into cement composites positively influences various properties, such as compressive strength [11,12,13,14,15], flexural strength [11,14,16,17], Young’s modulus [15,18,19], tensile strength [13,20], durability [21,22,23], fracture energy [24,25], fracture toughness [24,25,26], electrical properties [15,23,27,28,29,30], and thermal properties [29,31].

CNFs are quasi-one-dimensional nanomaterials with exceptionally high tensile strength, elastic modulus, and electrical and thermal conductivity [8,16,32,33]. CNFs can exhibit elastic modulus values as high as 0.4–0.6 TPa, tensile strength of 2.7–7.0 GPa, and electrical resistivity of 55 μΩ·cm [10]. Numerous studies have demonstrated substantial enhancements in various properties, such as compressive strength [16,33,34,35,36,37], flexural strength [16,17,36,37,38], Young’s modulus [18,37], fracture toughness [39,40], fracture energy [40], durability [35], electrical properties [29,41,42], and thermal properties [29] of a cement composite.

Graphene is a two-dimensional (2D) layer of covalently bonded carbon atoms [43] and an essential component of graphite, charcoal, carbon nanotubes, fullerenes, and other allotropes [44]. It has excellent properties, such as high thermal conductivity, high electrical conductivity, and high surface area [45,46]. The use of graphene has been proven to significantly improve the compressive strength [44,47,48], flexural strength [47,48,49], Young’s modulus [50], tensile strength [51], durability [52], fracture energy [24], fracture toughness [24], and electrical properties [46,48,53] of cement composites.

GNPs are two-dimensional platelets made up of stacked graphene layers [54,55,56]. They are characterized by low weight, planar structure, excellent mechanical properties, and excellent thermal and electrical conductivities [54,55,56]. GNPs possess elastic modulus, strength, electrical resistivity, diameter, thickness, surface area, and aspect ratio of 1 TPa, approximately 10–20 GPa, 50 μΩ·cm, 1–20 µm, approximately 30 nm, approximately 2630 m²/g, and 50–300, respectively [56]. Multiple studies have demonstrated the positive impact of GNP addition on various properties of cement composites. Studies conducted by numerous researchers have reported significant improvements in the compressive strength [14,15,57,58,59], flexural strength [14,58,59], durability [57], fracture energy [60], electrical properties [15,29,61,62], and thermal properties [29] of cement composites.

The influence of different CNM types on the mechanical properties of cement composites has been extensively investigated. However, studies on the effects of CNM types such as GNP and graphene on the fracture characteristics of cement composites remain scarce. Additionally, research on the fracture characteristics of CNM-added cement composites, where CNMs are dispersed while maintaining low fluidity, is still limited. This study aims to bridge this gap by investigating the fracture characteristics of low-fluidity cement composites incorporating CNTs, CNFs, GNPs, and graphene.

In this research, a comparison was made between four different nanomaterials (CNT, CNF, GNP, and graphene) with content ratios of 0.1, 0.3, 0.6, and 1.0 wt. %, along with a cement-only sample, that were fabricated to examine their mechanical properties (compressive strength, and flexural strength), fracture characteristics (fracture energy, and fracture toughness), electrical properties and porosity. Factors such as the dispersion method, dosage rate, and curing age were consistent in all the prepared cement composites.

2. Materials and Methods

Cement-based composite materials were prepared at different mixing ratios to study the mechanical, electrical, and fracture characteristics of cement composites containing CNMs. The cement paste binder used in this study was made using ordinary Portland cement (OPC) and normal tap water. The CNT, CNF, and graphene used in this study were purchased from Beijing Daoking Co., Ltd. (Beijing, China), and the GNP was purchased from Timenano Co., Ltd. (Chengdu, China). The physical characteristics of the four CNMs (CNT, CNF, graphene, and GNP) employed in this investigation are consistent with those reported in a study conducted by Wang et al., in 2020 [7].

The CNM composite mixture ratios and fabrication techniques adopted in this study are stated as follows: a group of composite samples was prepared using only cement and water, which were termed “cement-only.” Cement-only samples were used to form a reference group. In total, four cement-only composite samples were fabricated. In addition to the cement-only samples, four types of composites, namely, CNT/cement, CNF/cement, graphene/cement, and GNP/cement, with four different mixing ratios were prepared. CNM/cement composites used CNM content ratios of 0.1%, 0.3%, 0.6%, and 1% by cement weight. Four replicate samples were prepared for each CNM ratio. Sixteen samples were fabricated for each type of CNM/cement composite. The mix proportions of the composite materials are listed in

Table 1. Mix proportions.

Composite Type	CNM Ratio	SP * (g)	Water (g)	Cement (g)	W/C Ratio	CNM (g)
CNT/cement	0.1	0	541.4	2165.4	0.25	2.2
	0.3	14				6.5
	0.6	25				13.0
	1	82.7				21.7
CNF/cement	0.1	0	541.4	2165.4	0.25	2.2
	0.3	0				6.5
	0.6	1.8				13.0
	1	7				21.7
Graphene/cement	0.1	0	541.4	2165.4	0.25	2.2
	0.3	0				6.5
	0.6	5.7				13.0
	1	20				21.7
GNP/cement	0.1	0	541.4	2165.4	0.25	2.2
	0.3	0				6.5
	0.6	0				13.0
	1	2.3				21.7
Cement only	0	0	541.4	2165.4	0.25	0

* Note that the W/C ratio refers only to the water-to-cement ratio and does not consider the superplasticizer (SP).

.

The fabrication procedure is shown in Figure 1 and is explained as follows. First, each component of the cement composite, that is cement, water, and CNM, was weighed. After weighing all components, cement and CNM were poured into a steel container and thoroughly mixed for approximately three minutes. After three minutes, water was added, and then the mixture was mixed for another two minutes. After thorough mixing, the freshly prepared CNM/cement composites underwent a flow test in compliance with ASTM C230 [63]. After performing the flow test, superplasticizer (SP) was added as needed to maintain the fluidity within the range of 120–160 mm (corresponding to 20 to 60%). For samples that maintained fluidity within this range, SP was not added. After obtaining the desired fluidity, the composite mixtures were poured into 40 mm × 40 mm × 160 mm steel molds. After the mold was filled, a mechanical vibrator was used for proper compaction. The sample was allowed to settle for 24–48 h at room temperature. After settling, the hardened composite samples were removed from the molds. Then, the composite samples were cured in tap water for 28 days.

A novel method was used in an effort to distribute CNM in cement composites. The method was developed according to research conducted by Nam and Lee, 2015 [27]. In this dispersion method, CNMs were effectively distributed by maintaining a low fluidity of 120–160 mm (corresponding to 20% to 60%) in the freshly prepared cement composites [27].The testing methods are briefly described as follows. Three-point bending tests were performed to measure the flexural strength, fracture toughness, and fracture energy. Four specimens for each nanomaterial content ratio were subjected to the three-point bending test. All composite specimens were tested at 28 d of curing. A notch of 6 mm was made on the middle span using an electric saw in all cured samples. While measuring the flexural strength, a crack opening displacement (COD) gauge capable of measuring displacements with an accuracy of 0.01 mm was fixed at the notch to measure flexural displacement, as shown in Figure 2. A displacement-controlled universal testing machine (UTM) with a 5-t load cell was used at a loading rate of 0.002 mm/s.

A three-point bending test was performed according to ASTM C 293 [64,65] in an effort to evaluate flexural strength.

$${\sigma }_t={\displaystyle \frac{3FL}{2b{\left(h-a\right)}^2}}$$

(1)

where ${\sigma }_t$, $F$, $L$, $b$, $h$, and $a$ represent flexural strength (MPa), maximum load (N), length of span (mm), width of the specimen (mm), height of the specimen (mm), and notch depth (mm), respectively [64,65].

Fracture toughness and fracture energy were calculated using the following equations [66,67]:

$$G_f={\displaystyle \frac{0.75W_0+W_1}{A_{lig}}}$$

(2)

$$W_0={\int }_0^{\delta 0}P\left(\delta \right)d\delta$$

(3)

$$A_{lig}=\left(h-a\right)b$$

(4)

$$W_1=0.75\left({\displaystyle \frac{S{M}_1}{L}}+2M_2\right)gCMO{D}_C$$

(5)

$$K_{ic}={\displaystyle \frac{FS}{b{h}^{\frac{3}{2}}}}f\left({\displaystyle \frac{a}{h}}\right)$$

(6)

$$f\left({\displaystyle \frac{a}{h}}\right)=2.9f{\left({\displaystyle \frac{a}{h}}\right)}^{0.5}-4.6f{\left({\displaystyle \frac{a}{h}}\right)}^{1.5}+21.8f{\left({\displaystyle \frac{a}{h}}\right)}^{2.5}-37.6f{\left({\displaystyle \frac{a}{h}}\right)}^{3.5}+38.7f{\left({\displaystyle \frac{a}{h}}\right)}^{4.5}$$

(7)

$$\ge 2.5{\left({\displaystyle \frac{K_{ic}}{{\sigma }_t}}\right)}^2$$

(8)

where $G_f$ is the fracture energy of the composite (N/m), $K_{ic}$ is the fracture toughness of the composite (MPa/m^1/2), $W_0$ is the area under the load-displacement curve, $P\left(\delta \right)$ is the load–crack mouth opening displacement (CMOD) curve, ${\delta }_0$ is the displacement at the times of failure, $M_1$ is the mass of the composite between supports (Kg), and $M_2$ is mass of the jig, which is not attached to a testing machine but placed on the specimen until rupture (Kg), $g$ is the gravitational acceleration (m/s²), $S$ is the loading span (mm), $L$ is the total length of specimen (mm), $CMO{D}_C$ is the crack mouth opening displacement at the time of rupture (mm), and $F$ is the peak load (N). Note that the $K_{ic}$ value can be deemed valid only when the condition stated in Equation (8) is satisfied.

After performing the three-point bending test, a sample that was split into two was used for compressive strength tests by following ASTM C 349 [68], as shown in Figure 3. A UTM with a capacity of 300 t was used to conduct compressive strength tests. The measurements were taken at a constant head-loading rate of 0.02 mm/s using displacement control. Four specimens were evaluated for each nanomaterial content ratio; the average value was used as the representative compressive strength of each type [12]. The compressive strengths of the specimens were determined as follows:

$$\sigma ={\displaystyle \frac{P}{A}}$$

(9)

where $\sigma$ is the stress (N/mm²), $P$ is the maximum load at the point of failure (N), and $A$ is the cross-sectional area (mm²)

A porosity test was performed to analyze the changes occurring in the pore characteristics of cement composites when CNM was added and analyze the relationship between porosity and mechanical and fractural characteristics. Porosity can be defined as the volume of permeable space in the specimens. Generally, the porosity of cement composites decreases when CNM is added; this phenomenon occurs because CNM acts as a filler agent in the cement composite. Porosity tests were performed according to ASTM C 642 [69].

$$P={\displaystyle \frac{\left(C-A\right)}{\left(C-B\right)}}100$$

(10)

Here, $P$ is the porosity, $A$ denotes the mass of the oven-dried specimen in the air,$B$ represents the mass of the specimen in water after it is immersed and boiled in water, and $C$ denotes the mass of the surface-dried specimen in the air after immersion and boiling [69,70].

The sample used for the three-point bending test, which was split into two parts, was used for the electrical conductivity tests. Silver paste was applied to both sides of the composite to reduce the contact resistance between the copper foil and composite [71]. After applying the silver paste, copper foil tape, which acted as an electrode, was applied over the silver paste. The resistances of the CNM/cement composites were calculated based on Ohm’s law. The following equation was used to determine the DC conductivity [7,70,71]:

$$\sigma ={\displaystyle \frac{1}{\rho }}={\displaystyle \frac{L}{RA}}$$

(11)

where $\sigma$ is the DC conductivity (S/cm), $R$ is the calculated resistance value, $A$ is the cross-sectional area of the electrode in contact with the composites, and $L$ is the interval of the electrodes [7,70,71].

3. Results

3.1. Mechanical Properties

The flexural strength and compressive strength of the CNM/cement composites at various ratios and dosages at 28 days of curing are shown in Figure 4 and Figure 5, respectively.

CNT/cement samples showed an increase in flexural strength with an increase in CNT content ratio of up to 0.6%. On further increasing the CNT content ratio up to 1.0%, the flexural strength value sharply decreased. The flexural strength of cement composite increased to 5.41%, 16.69%, and 45.68% for 0.1%, 0.3%, and 0.6% CNT content, respectively, compared with the cement-only sample. Compressive strength results showed that the inclusion of 0.3% CNT of cement weight exhibited the highest increase in CNT/cement composite samples. The compressive strength of the composites was 119.45 MPa at 0.3 wt.% CNT, which denoted a higher compressive strength than that of all other CNM/cement sample ratios, corresponding to a 112.80% increase compared with that of the cement-only sample.

CNT addition resulted in significant improvements in both mechanical properties compared to those of the cement-only samples. This overall improvement in mechanical properties can be attributed to the reinforcing behavior of CNT, i.e., crack bridging, gap filling, modification of the microstructure, and nucleation effects of CNT on the cement matrix [72,73]. Criteria such as the water/cement ratio, dispersion of CNT, and low porosity [70,73,74,75,76] are also crucial in improving the mechanical properties of composites. However, in the current study, CNT/cement composites with 1.0% CNT content exhibited decreased flexural strength, and compressive strength, which could be due to improper dispersion of the high CNT content. It has been reported that due to van der Waals forces, CNTs tend to agglomerate, and the likelihood of agglomeration increases with higher CNT dosages in cement composites [77]. Microstructural observations conducted by Mohsen et al. [78] revealed significant CNT agglomeration at a 0.5% dosage compared to 0.15% and 0.25%. Similarly, Yazdani and Emon (2016) [11], as well as Manzur and Yazdani (2015) [17], reported a reduction in the mechanical properties of cement composites at higher CNT contents, attributing the deterioration to CNT agglomeration at elevated concentrations.

For all mechanical properties, a CNT content ratio between 0.3% and 0.6% showed more positive results compared with those obtained at other ratios. Other studies have also reported the same pattern. Manzur, Yazdani, and Emon, 2016 [11], and Manzur and Yazdani, 2015 [17], concluded that the optimum concentration of CNT for an increase in compressive strength is between 0.1% and 0.3%. Similarly, a study conducted by Evangelista et al., 2019 [13], illustrated that compared to 0.2% and 0.6% CNT content of cement weight, 0.4% CNT content increased the compressive strength of cement mortar. From these results, a conclusion can be drawn that the optimum concentration of CNT for increasing the mechanical strength of cement composites should be in the range of 0.1–0.6%. GNP/cement samples exhibited the maximum increase in flexural strength of 7.56 MPa at a 1.0% GNP content ratio. Adding GNP content at a ratio of 1.0% improved the flexural strength of cement composites by 27.17% compared to that of the cement-only sample. The flexural strength of cement composites increased up to 4.01% for 0.1% GNP content compared to that of the cement-only sample, which proved that even a minimum amount of GNP could improve the flexural strength of cement composites. GNP/cement samples showed a different trend from that of CNT- and CNF-added cement samples, and maximum compressive strength was observed when 0.1% GNP ratio of cement weight was added to the cement composite. Above that, the compressive strength decreased with an increase in the ratio. The compressive strength of cement composites increased by 69.90% for 0.1% GNP content compared to that of the cement-only sample. The GNP/cement composite showed significant improvements in both mechanical properties, which were attributed to the refinement of the pore structure and its filler effect in the cement matrix [57].

The flexural strength of CNF/cement exhibited the maximum value at a 1.0% CNF content ratio, which was 9.06 MPa. This corresponded to a 52.38% increase compared to the flexural strength of the cement-only sample. CNF ratios of 0.1%, 0.3%, and 0.6% exhibited an increase of 22.25%, 29.05%, and 20.33%, respectively, in the flexural strength of the cement composite compared to that of the cement-only sample. For the CNF/cement samples, the compressive strength gradually increased with increasing CNF content ratios, as shown in Figure 5. The compressive strength at a 1.0% CNF ratio was 118.94 MPa, an 111.9% increase compared with that of the cement-only sample. CNF ratios of 0.1%, 0.3%, and 0.6% showed an increase of 77.28%, 88.04%, and 68.35%, respectively, in compressive strength compared to that of the cement-only sample. Multiple studies have concluded that the improvement in mechanical properties can be attributed to the CNF nanostructure. The outer surface of the CNF consists of a conical graphite plane that is angled in relation to the fiber’s longitudinal axis, which improves the mechanical properties by anchoring and creating an interfacial bond between the cement and CNF [18,79,80]. The proper dispersion of CNF also improves the mechanical properties by restricting the propagation of nanocracks in the cement composites, due to pore filling [39,81].

The flexural strength of the graphene-incorporated sample was similar to that of the GNP- and CNF-incorporated samples. The maximum flexural strength value for graphene/cement samples was 8.951 MPa at the 1% content ratio, which is an increase of 50.42% compared to that of the cement-only sample. The flexural strength of the composite increased by 40.29%, 24.16%, and 24.72% for 0.1%, 0.3%, and 0.6% graphene content, which indicated that graphene at all dosages could improve the flexural strength of cement composites. The compressive strength test results indicated that the graphene/cement samples exhibited similar characteristics to those of the GNP/cement samples. At a 0.1% graphene content ratio, the compressive strength of the sample was 115.75 MPa, which was the maximum compressive strength compared to other content ratios. When the graphene content was maintained at 0.1%, 0.3%, 0.6%, and 1% of cement weight, the compressive strengths of the graphene/cement samples increased by 106.22%, 75.82%, 86.52%, and 100.74%, respectively, compared to that of the cement-only sample. The overall improvement in the graphene/cement composite can be attributed to graphene providing a nanofiller effect that limits microcrack formation owing to crack bridging and graphene’s ability to promote the production of calcium-silicate-hydrate (C-S-H) gel, which enhances the mechanical properties of cement composites [45].

3.2. Fractural Characteristics

Fracture Energy and Fracture Toughness

As shown in Figure 6a,b, the CNM/cement composites significantly improved the fracture energy and toughness compared to those of the cement-only sample.

For CNF/cement composites, maximum fracture energy and toughness, at a 1.0% content ratio, were 9.22 N/m and 0.81 MPa/m^1/2, respectively. Compared to cement-only samples, the CNF-added samples with a 1.0% dosage showed an increase in fracture energy and fracture toughness of 59.40% and 41.76%, respectively. The fracture energy of CNF/cement composites increased by 30.54%, 50.91%, and 28.55% when the CNF contents were 0.1%, 0.3%, and 0.6%, respectively, compared to that of the cement-only sample. Meanwhile, the fracture toughness of the CNF/cement composites increased by 22.29%, 28.79%, and 20.06% when the CNF contents were 0.1%, 0.3%, and 0.6%, respectively, compared to that of the cement-only sample. The overall improvement in the fractural characteristics of the CNF-added cement composite is likely due to the CNF nanostructure, which consists of a conical-shaped graphite plane that improves the mechanical properties by creating an interfacial bond between the cement and CNF [18,79,80]. The pore-filling ability of CNF also improves the fracture energy by restricting nanocrack propagation in cement composites when well-dispersed [39,81].

Regarding the CNT/cement composite case, the maximum fracture energy and toughness were 7.99 N/m and 0.84 MPa/m^1/2, respectively, achieved at a 0.6% content ratio. Compared to the cement-only sample, adding 0.6% CNT to a cement composite increased fracture energy and toughness by 38.06% and 46.06%; further increasing the CNT content decreased the fractural characteristics. Improper dispersion of 1.0% CNT content on cement matrix may lead to agglomeration, which could cause a decrease in flexural characteristics. The overall improvement compared to the cement-only samples could be attributed to crack bridging and interconnectivity between the CNT and the hydration product, leading to a load transfer [24,25].

Regarding GNP/cement composites, the maximum fracture energy and toughness occurred at a 1.0% content ratio and were 8.48 N/m and 0.71 MPa/m^1/2, respectively. Due to the 1.0% GNP addition, the fracture energy increased by 46.57% and the fracture toughness by 24.44%, compared to the cement-only composite results. The fracture energy of GNP/cement composites increased by 4.42%, 27.19%, and 30.96%, when the GNP content was 0.1%, 0.3%, and 0.6%, respectively, compared to the cement-only samples. The improvement in the fractural characteristics of the GNP-added cement composites can be attributed to GNP’s ability to improve the density of cement composites, which leads to crack bridging [82].

Compared with other CNM-added composites, cement composites created with a graphene content ratio of 1.0% demonstrated the most significant improvement. For graphene/cement composites, the maximum fracture energy and toughness occurred at a 1.0% content ratio and were 12.57 N/m and 0.86 MPa/m^1/2, respectively. Adding 1.0% graphene increased the fracture energy by 117.26% and fracture toughness by 50.09% compared to cement-only sample results. The fracture energy of the graphene/cement composite increased by 41.69%, 29.09%, and 31.49% when the graphene contents were 0.1%, 0.3%, and 0.6%, respectively, compared to that of the cement-only sample. Similarly, graphene/cement composite fracture toughness increased by 32.73%, 23.83%, and 24.4% when the graphene content was 0.1%, 0.3%, and 0.6%, respectively, compared to that of the cement-only sample. Graphene forms bonds in the cement matrix to improve the load-transfer capacity and fracture characteristics [24]. Correlation graphs between load and displacement of CNM-added cement composite are shown in Figure 7.

3.3. Electrical Conductivity

Figure 8 shows the electrical conductivity test results for CNM/cement composites as the CNM content ratio increased from 0 to 1.0%; the results are stated as follows. In the CNT/cement composite samples, the electrical conductivity increased exponentially with an increase in the CNT content ratio, which is a trend similar to that reported in a previous study [71]. A previous investigation concluded that an exponential increase in electrical conductivity indicated an even distribution of MWCNTs in the cement matrix. In this study, a maximum electrical conductivity of 4.79 × 10³ S/cm was obtained after adding a 1.0% dosage of CNT. The electrical conductivity of the CNF/cement composites increased with an increasing CNF content. The maximum electrical conductivity was 0.0002 S/cm at a 1.0% CNF content ratio. The conductivity of cement composites increased when CNF content was increased from 0.3% to 1.0%. Similar to the CNT/cement composite, it can be concluded that there is an even distribution of CNF in the cement matrix.

Regarding CNT/cement composites, it was found that the electrical conductivity increased dramatically in the range of 0.1–0.3% CNT content ratios; a similar trend was also reported in previous studies [27,71]. Moreover, regarding CNF/cement composites, the electrical conductivity rose dramatically in the range of 0.3–1.0% CNF content ratio. When the conductivity drastically increases with an increase in carbon nanomaterial content, the composite sample behavior changes from an insulator to a conductor; this phenomenon is believed to occur when the CNM content is at or above the percolation threshold [83]. The percolation threshold is defined as the average of the CNM content range where electrical conductivity drastically increases [12]. Therefore, it can be concluded that between 0.1% and 0.3% CNT content ratio, the percolation threshold for CNT/cement composites is approximately 0.2%. Moreover, the CNF/cement percolation threshold ranges between 0.3% and 1.0%. Because CNT and CNF exhibit percolation phenomena, it can be concluded that they are well dispersed in the cement composite.

Both the graphene and GNP-incorporated samples showed significantly less improvement in electrical conductivity than that observed with the CNT- and CNF-added cement composites. GNP/cement composite samples exhibited a maximum electrical conductivity of 1.14 × 10⁸ S/cm at a 0.3% GNP content ratio. Similarly, graphene/cement composite samples attained a maximum electrical conductivity of 1.44 × 10⁸ S/cm at a 0.1% graphene content ratio. However, neither the GNP- nor graphene-added cement composites showed the percolation phenomenon in this study. Studies conducted by Le, Du, and Pang, 2014 [61], and Du, Quek, and Pang, 2013 [84], showed that the percolation threshold of GNP/cement composites was between 2.4 to 3.6% of cement volume. Meanwhile, Ming Jin et al., 2023 [85], showed the percolation threshold of graphene/cement composites was between 0.9 to 1.8% of cement weight. Referring to these studies, GNP or graphene content ratio should be increased further to values higher than 1%.

3.4. Porosity

The porosity test results of the CNM/cement composites after 28 days of curing are discussed in this section. The test results are presented in Figure 9. The highest porosity was observed for CNF/cement composites with a CNF content ratio of 0.6%. The porosity of the CNM/cement composite sample increased up to 10.3% for 0.6% CNF content compared to that of the cement-only sample. Considering the CNF-added cement composite porosity results along with its mechanical and fractural characteristics results, it was determined that a high porosity significantly hampered the mechanical and fractural characteristics. Meanwhile, the mechanical properties and fractural characteristics improved when porosity was the lowest, i.e., at 1.0% CNF-content ratio.

For GNP/cement composites, maximum porosity occurred at a GNP dosage of 0.6%. The porosities of the GNP/cement composite samples increased by 7.3%, 6.03%, and 7.6% when GNP dosages were maintained at 0.1%, 0.3%, and 1.0%, respectively. Compared to the porosity results reported in Figure 5 and Figure 6b, the compressive strength and fracture toughness decreased at a GNP content of 0.6%. Meanwhile, the flexural strength fracture energy, and fracture toughness of GNP/cement composites showed maximum improvement when the porosity value was lowest, namely, at 1.0% GNP content.

For graphene/cement composites, the maximum porosity was observed in the 0.3% graphene-added composite. A further increase in the graphene dosage led to a drastic decrease in the porosity of the cement composite. Considering the results of the mechanical and flexural characteristics, it is noteworthy that the compressive strength, flexural strength, fracture energy, and fracture toughness showed the lowest values for cement composites with 0.3% graphene content. In contrast, at 1.0% graphene content, the cement composite showed the lowest porosity but significantly improved compressive, flexural strength, and fractural characteristics.

For CNT/cement composites, a reduction in porosity was observed as the CNT content was increased to 0.6%. Further increasing the CNT dosage to 1.0% significantly increased the porosity. A drastic increase in porosity also led to a decrease in the mechanical and fractural characteristics of 1.0% CNT-added cement composites. In contrast, when porosity was low, namely, at a 0.6% CNT dosage, the flexural strength and fractural characteristics showed the maximum improvements. Hence, it may be that a decrease in porosity is closely related to the mechanical and fractural characteristics.

Apart from reduced porosity, the improvement in the mechanical and fracture characteristics of cement composites containing CNMs can also be attributed to other beneficial characteristics of CNMs, such as their ability to bridge cracks and serve as nucleation sites for cement hydration products. Additionally, factors such as the dispersion quality and aspect ratio of different types of CNMs also influence the mechanical and fracture characteristics of CNM-enhanced cement composites.

4. Conclusions

In this study, CNF/cement, GNP/cement, CNT/cement, and graphene/cement composites with various CNM content ratios were fabricated, and their mechanical (compressive strength, and flexural strength), fractural (fracture energy, fracture toughness, and maximum flexural displacement) and electrical characteristics were examined. The mechanical properties were further analyzed by evaluating their porosity characteristics. The following conclusions were drawn from this study.

• Compared to all other CNM-added cement composites, 0.3% CNT-added cement composites showed the maximum increase in compressive strength. For the CNF-added cement composite, the current study concluded that compressive strength increased with an increase in the CNF content ratio. GNP- and graphene-added cement composites demonstrated that adding a small dosage of 0.1% is ideal for improving the compressive strength.
• Similar to compressive strength results, cement composites with 1.0% CNF ratio exhibited the highest flexural strength among all CNM-added cement composites. Regarding GNP or graphene-added cement composites, the maximum flexural strength was achieved with a 1.0% dosage. The flexural strength of cement composites improved as the CNT dosage increases up to 0.6%.
• The fracture characteristics of the CNM/composites underlined that graphene-added cement composites attained the maximum fracture energy. Accordingly, fracture toughness was improved with a 1.0% content ratio compared to that of other CNM-added composites. Similarly, CNF- and GNP-added cement composites showed maximum improvement in fractural characteristics at 1.0% dosage. The CNT-added cement composites exhibited maximum fractural characteristics at a CNT dosage of 0.6%.
• A CNT content ratio between 0.3% and 0.6% can be the optimal ratio for improving cement composites’ mechanical and fractural characteristics. When CNT content was further increased to 1.0%, a sharp decrease in mechanical properties and fractural characteristics was observed. This phenomenon was due to the non-uniform dispersion of CNT in the cement matrix, which created agglomerations of CNT, thereby decreasing the mechanical properties and flexural characteristics.
• The porosity results of CNF/cement composites showed an inverse relationship with all mechanical properties and flexural characteristics. In the GNP/cement, CNT/cement, and graphene/cement composites, flexural strength, fracture toughness, and fracture energy improved as porosity decreased. However, an increase in compressive strength did not exhibit a clear relationship with the porosity. Besides porosity, factors such as the aspect ratio of CNMs and the degree of dispersion of CNMs in cement composites require further investigation.
• Based on electrical conductivity test results, the CNF/cement composites showed a percolation threshold ranging from 0.3% to 1.0%, and CNT/cement showed a percolation threshold ranging from 0.1% to 0.3%. Meanwhile, the GNP and graphene showed an insignificant change in electrical conductivity with an increased content ratio, and exhibited no percolation threshold phenomena.