Optimizing dispersion techniques for enhanced electrical and piezoresistive properties in cement composites with carbon fibers, multi-walled carbon nanotubes, and nano nickel powder

Abstract

The dispersion techniques used for conductive materials in cement matrices significantly influence the piezoresistive sensitivity of smart concrete. Consequently, researchers have explored various methods to enhance the sensing efficiency of concrete structures. Micro and nanomaterials, including carbon fibers (CF), multi-walled carbon nanotubes (MWCNT), and nano nickel powder (NNi), have been utilized to produce smart concrete. This study presents an in-depth analysis of various dispersion techniques reported in the literature, aiming to identify the most effective method for each material. The evaluation of each mixing technique was based on electrical resistivity (ER), piezoresistive properties, and compressive strength of conductive cement composites. The content of conductive materials was fixed at 0.5% by volume in all mixes to assess the relative effectiveness of the dispersion methods. ER was investigated using a two-electrode method. The findings indicate that CF, when mixed using a dry mixing method, achieved superior dispersion compared to other techniques. The ER of CF composites decreased by up to 99% compared to plain cement composites after 28 d. For MWCNT, mechanical mixing with water and a superplasticizer provided better dispersion than ultrasonic mixing, with a reduction in ER of up to 89% compared to other methods. Appreciable dispersion of NNi was achieved by directly adding NNi powders to the fresh cement mixture. However, the ER reduction for NNi composites was relatively low at approximately 28%, compared to CF and MWCNT composites. The dispersion methods that improved ER also demonstrated excellent piezoresistive sensitivity for all materials. However, enhancing the electrical properties through improved dispersion methods did not necessarily lead to better compressive strength outcomes. Compressive strength tests were conducted for all dispersion methods, and the results underline the trade-off between electrical performance and mechanical properties in conductive cement composites.

Graphical abstract

Keywords

dispersion methods / conductive materials / electrical impedance / piezoresistive response / compressive strength

1 Introduction

Portland cement (PC) is widely regarded as the most extensively used material for construction owing to its desirable characteristics [1]. PC has been used to manufacture a wide range of conventional and specialized cement pastes, mortars, and concrete over time. In addition to the variety of conventional types, specific types of cement have evolved over time. Over the last decade, a new generation of smart cement composites has emerged as part of various specialized composites [2]. Dunuweera and Rajapakse [3] noted that with the rapid development in the construction sector, various sustainable and intelligent materials have garnered significant attention in the production of smart cement composites. The electrical characteristics of smart cement composites, including self-sensing, self-heating, and self-healing capabilities, were investigated [4]. Accordingly, Kashif Ur Rehman et al. [5] stated that the primary objective of developing smart cement composites involve incorporating conductive or semi-conductive components with cementitious materials to establish a conductive network within the cement matrix. An efficient dispersion approach is required to enhance the electrical characteristics of cement composites using conductive elements, such as micro and nanoscale components. Whereas, Li et al. [6] stated that the electrical conductivity of a material is primarily determined by the effectiveness of dispersion methods, which, in turn, significantly impacts the formation of the conductive network in the cement matrix. However, there are several factors that influence the efficiency of the conductive elements, in terms of dispersion effectiveness. This includes its specific gravity, surface area, density, and particle sizes, among other characteristics. The forms of functional conductive materials are complex; accordingly, they have high surface energy. Because of this, mixing and dispersion are difficult processes. Therefore, effective mixing procedures are necessary to enhance cement composite self-sensitivity.

As proposed in the previous studies, the efficient mixing/dispersion of conductive materials includes physical or chemical methods. The physical methods include mechanical mixing or ultra-sonication, whereas mechanical mixing includes high shear and ball milling techniques. Some researchers stated that complex uniform dispersion could be an unfavorable issue in the mechanical mixing of graphene nanoplatelets (GNPs) into cement-based composites [7]. Simultaneously, they found that the dispersion method is influenced by various factors, such as sonication amplitude and duration, surfactant type, sample volume, and the surfactant-to-GNP ratio. Chemical mixing involves the surface modification method to improve the surface structure of material components. Han et al. [8] confirmed that this method improves the wettability of material surfaces. Chemical treatment, on the other hand, necessitates additional conditions, such as high temperatures or high levels of humidity. Characteristics that have a negative effect on the mechanical and electrical properties of cement composite.

In accordance with the findings of Refs. [9,10], the mechanical and electrical properties, of conductive composites were altered, by the incorporation of micro or nanoscale structures. Furthermore, it was demonstrated by Li et al. [11] that the orientation and arrangement of micromaterials had an effect on the electrical and mechanical properties of composites. It was due to the excellent electrical properties of carbon fibers (CF) that researchers decided to include them into a variety of conductive cement composites. It has been demonstrated by Zhao et al. [12], that the contribution of fibers in controlling crack propagation depends on many factors including but not limited to their size. Microfibers may help mitigate crack propagation once cracks reach a microscopic scale but cannot prevent the initiation of cracks (nano-cracks) [13]. On the other hand, nanofibers/tubes such as carbon nanotubes (CNT) have the potential to prevent crack initiation in cement composites [14].

Mixing methods that are either dry or wet are utilized in the distribution of CF. In the dry mixing process, cement or fine aggregates are mixed with CF and stirred together continuously. In addition, other components are mixed together at a later stage in the process. Chung [15] conducted research on both dry and wet methods in his study. The wet technique involves combining CF with water and different dispersants. Silica fume (SF), has been used, as a dispersant in CF composites, due to its tiny particle size and ability to reduce concrete porosity [16,17]. They found that adding 10% of SF by weight of cement improved the microstructure and electrical properties of the composites. On the other hand, ozone and silane [18], and oxygen plasma and nitric acid, increased the mechanical characteristics of CF composites [19].

Nanoparticles, improved cement composite mechanical, chemical, thermal, and electrical properties [20]. Strong van der Waals, interactions cause nanomaterials to agglomerate [21]. To improve nanomaterial distribution, in cement composites, an effective dispersion approach is needed [22]. Surface treatment, ultrasonic, and centrifuge processing, helped Nemours researchers to solve nanomaterial agglomeration [23–25]. By reducing water surface tension, and purifying nanomaterial surfaces, chemical surface treatment helped nanomaterials disperse [22,26,27]. To disperse nanoparticles in solvent, ultrasonication turns electrical voltage into millions of waves [28]. Studies show that adding, surfactants to water or solvents improves sonication [29,30]. Sonication’s only drawback is nanomaterial, re-agglomeration before cement mixing [31]. The type of nanomaterials, water or solvent, surfactant characteristics, sonication power, temperature, and amplitude setting affect this issue. Kim et al. [14] used the thermal chemical vapor deposition (CVD) growth method to mix 0.6% of CNT in cement matrix. CVD method improved thermal, electrical and mechanical properties of the composites. While Mardani et al. [32] found that using sodium dodecyl sulfate (SDS) as a surfactant to disperse the CNT showed best electrical properties and high sensitivity.

Van der Waals forces are eliminated by high-speed mechanical mixing with surfactants, enabling nanomaterial dispersion. Mechanical mixing has improved nanomaterial dispersion, in composites for many studies [33,34]. However, mechanical mixing requires two considerations. First, appropriate mixing is needed to disperse materials evenly, regardless of energy and time [31]. Second, adequate material breakup, with high mixing power, is essential [35]. However, there are little or no previous comprehensive studies that investigated the effect of different dispersion methods on electrical and piezoresistive properties of micro and nano materials. This study provides comprehensive investigation into using various methods from the existing literature to investigate the excellent method for every material used. Micro and nano Conductive materials, such as CF, multi-walled carbon nanotubes (MWCNT), and nano nickel powder (NNi), were utilized to examine the effects of dispersing materials in cement composites. All conductive materials were used at 0.5% by volume to determine the variability in dispersion techniques. Six dispersion methods are used to disperse MWCNT or NNi, while three methods were used for dispersion of CF in cement composites. This study investigates the electrical, piezoresistive, and compressive strength properties of cement composites incorporating various conductive fillers, namely CF, MWCNT, and NNi, using different dispersion techniques. The novelty of this work lies in its comparative analysis of three dispersion methods: mechanical stirring, ultrasonication, and surfactant-assisted mixing, aimed at enhancing the uniform distribution of each filler type. Electrical resistivity (ER) measurements were conducted at multiple curing ages without loading to assess baseline conductivity. Under applied unit loads, the time-dependent piezoresistive responses of all composite mixes were evaluated. The most effective dispersion method for each material was then identified and analyzed. Additionally, compressive strength tests were performed to evaluate the mechanical performance of the optimized composites.

2 Experimental program

2.1 Raw materials and mixture proportions

Three conductive materials, namely CF, MWCNT, and NNi, are used in the experiment. Chopped CF with a length of 12 mm and an aspect ratio of 1600 was incorporated. The diameter of CF is 7 µm and had a unit weight of 1700 kg/m³, where the tensile strength and tensile modulus were 4150 MPa and 252 GPa, respectively. The nanomaterials utilized in the present study included hydroxyl-functionalized multi-walled carbon nanotubes (MWCNT-OH) and NNi (99.95%, 65 nm). Table 1 presents the characteristics of MWCNT-OH and NNi, while Table 2 presents the element analysis of NNi. It is important to mention that the electrical conductivity of CF, MWCNT, and NNi materials are 10⁴, 10⁵, and 1.4 × 10⁷ S/m, respectively. Scanning electron microscope (SEM) images of all electrically conductive materials are shown in Fig. 1. Ordinary PC type CEM I 42.5R was used as a cementitious material in all mixes. SF was used to improve cement composites’ mechanical properties and enhance the distribution of conductive materials. The chemical and physical properties of PC and SF are detailed in Table 3. The particle size distribution PC and SF were measured using Mastersizer 2000 (Malvern Instruments, UK) based on laser diffraction technology. This allowed for accurate evaluation of the size range and dispersion quality of the materials. Figure 2 illustrates the particle size distributions of PC and SF. Besides SF, SDS-based dispersion agents were used in mixing methods to enhance the distribution of conductive materials. The particle size of SDS is −325 mesh, and the purity is +95.5%, whereas other properties as shown in Table 4. Deformer material (Triisobutyl phosphate (TIBP)) is utilized to remove or minimize the unwanted air bubbles in mixes. The technical properties of TIBP are detailed in Table 4. A high-range water-reducing (HRWR) hyper plasticizer (MasterGlenium 51) was used to improve the flowability of fresh cement paste mixtures.

Several mixing methods were employed to address the challenges associated with mixing and dispersing micro and nano conductive materials. In all mixes, the CF, CNT, and NNi content was maintained at a volume ratio of 0.5% due to the variation in the density of these materials. The dosage of 5% by volume was selected to all conductive materials to enhance workability and effective electrical conductivity of cement composites. Additionally, a preliminary trial mix was conducted to compare 0.25%, 0.5%, and 0.75% dosages. The 0.5% volume ratio consistently yielded optimal electrical performance while preserving the compressive strength and workability within acceptable limits. This made it a practical and reliable choice for ensuring consistent comparisons across the different dispersion techniques used in this study. In all mixing methods, SF was as 5% by cement weight, as advised by prior research [34,37]. Each mixture contained 500 kg/m³ of total binder, composed of PC and SF in a ratio of 95:5 by weight. All mixtures had a 0.35 water-to-cementitious (W/B) materials (PC + SF) binder ratio. A defoamer (TIBP) dosage of 0.25% per cementitious materials (PC + SF) mass was utilized in certain mixes as proposed by Refs. [38,39]. SDS-based dispersion agent was used as 0.25% per cementitious materials (PC + SF) mass in all nanomaterial mixes. The HRWR ratio was 0.25% by total weight of (PC + SF), except in specific mixing procedures where the HRWR ratio is changed until good workability is attained based on cement flow table testing.

2.2 Dispersion of micro and nanoscale conductive materials

2.2.1 Dispersion methods of micro scaled materials (carbon fibers)

In previous studies, researchers used various methods to disperse CF in the cement matrix. Building on this prior work, the most effective dispersion methods were applied in this study to investigate the most efficient techniques. Therefore, three dispersion methods were used to investigate the dispersion efficiency based on the results of carbon-cement composites’ electrical and piezoresistive properties. In addition, the mixing method of plain cement paste mixture was also presented.

1) Control method: first, PC, and SF were mixed in a mortar mixer at a speed of 100 r/min for 10 min. Later, two-thirds of the required water was added to the mixture and mixed for 2 min. Then, HRWR was mixed with one-third of the rest of the water and mixed with the mixture for another 2 min. A flow test was conducted for all fresh mixes before fabricating the samples, as noted in Ref. [40].

2) 1st method (CF-1): in this method, the solution contained TIBP + SDS was stirred with the required water for 2 min. CF was added to the solution and mixed for 3 min with a kitchen blender. The resulting mixture was combined and mixed with cementitious materials for 5 min in a 7-L mortar mixer at 100 r/min. Chen et al. [41] propose this strategy.

3) 2nd method (CF-2): PC + SF, were stirred for 5 min, in a mortar mixer. After progressively adding the CF to the PC + SF mixture at 100 r/min, the mixer was increased to 300 r/min for 5 min. Water-HRWR solution then added and stirred for 5 min as suggested by Ref. [42].

4) 3rd method (CF-3): the dispersant SDS was stirred with 30% of the required water and left for 20 min to ensure the dispersant dissolved. CF were added to that solution and stirred for 2 min. Then, the remaining water, HRWR, and SF were added and mixed for approximately 20 s at low speed. Later, the PC was added and mixed for the 30 s at a low speed. Afterward, the speed gradually increased to the medium level, and the mixture was stirred for an additional 3 min, as described by Ref. [43].

2.2.2 Dispersion method of nano scaled materials

The dispersion of nanomaterials is more complex than that of micromaterials. Therefore, effective dispersion methods should be used to ensure excellent dispersion of nanomaterials in the cement matrix. According to the previous studies, six methods were employed to mix the CNT, and six methods for mixing the NNi. These methods are explained below.

2.2.2.1 Dispersion methods of carbon nanotubes

1) 1st method (CNT-1): The PC and SF were premixed in a mortar mixer for 5 min. Then, the nanomaterials, water, HRWR, and defoamer were mixed, preparing to sonicate the solution using an ultrasonicator (Vibra-Cell^TM, VCX750) for 10 min. The setting of the ultrasonic sonicator was controlled as (frequency 20 kHz, energy 2200 J, temperature 50 °C, amplitude 80%). Then, the sonicated solution was added to the PC and SF and mixed in a mortar mixer for 5 min [37,44,45].

2) 2nd method (CNT-2): the HRWR and water were mixed for 1 min. Then, CNT was added to the solution and sonicated for 30 min using an ultrasonic sonicator. The setting of the ultrasonic sonicator was controlled as (frequency 20 kHz, energy 2200 J, temperature 20 °C, amplitude 50%). Finally, a mortar mixer was used to mix this suspension and cement for about 5 min, as suggested by Ref. [46].

3) 3rd method (CNT-3): stirring SDS, in water for 2 min, was suggested by Ref. [47]. One hour of sonicating the solution with CNT, provided a homogeneous suspension. Although, the temperature and amplitude were 10 °C and 50%, the sonicator was 20 kHz, energy 2200 J. For 15 min, a cement mortar mixer blended this suspension with PC and SF. The defoamer last added and mixed for 5 min.

4) 4th method (CNT-4): a Vibra-Cell^TM VCX750 sonicator, was used to sonicate CNT with water for 12 min at 20 kHz, 2200 J, 50 °C, and 80% amplitude. Next, cementitious materials PC and SF, and HRWR were mixed. A mortar mixer was used to mix the mixture for 3 min. Later, adding a defoamer and stirring for 5 min as followed in Ref. [48].

5) 5th method (CNT-5): first, the surfactant and water were stirred for 3 min in a kitchen blender. Then, the CNT added to the surfactant solution and ultrasonicate, for 12 min at 20 kHz, 2200 J, 50 °C, 80% amplitude to create a homogenous suspension. Following ultrasonic mixing, a cement mixer mixed with dry PC and SF for 10 min. Mixture mixed for 3 min after adding suspension to cementitious components. Finally, a defoamer added to the mixture and stirred for 3 min [39].

6) 6th method (CNT-6): first, the entire amounts of water, HRWR, and CNT were mixed with a kitchen blender for 15 min at 3000 r/min. At the same time, the dry materials PC and SF were mixed in a mortar mixer for 10 min at 100 r/min. Next, the nano solution gradually added to the mixer over 10 s, and the mixture was mixed for more than 10 min at 300 r/min [34].

2.2.2.2 Dispersion methods of nano nickel powder

1) 1st method (NNi-1): the first mixing method, which is used to mix (CNT-1), was also used here to ensure the efficiency of dispersion of nano nickel particles by using ultrasonic sonication. This method was also used in previous studies by Refs. [37,45].

2) 2nd method (NNi-2): first, water, NNi, and HRWR were mixed for 30 s in a kitchen blender at low speed to ensure filler dispersion into the solution. At the same time, (PC + SF) were mixed in a mixer for 10 min. Next, the solution was added slowly to the dry mixture at 100 r/min; the speed increased to 500 r/min and continued mixing for 5 min in a cement mortar mixer [37].

3) 3rd method (NNi-3): first, NNi and water, are stirred in a kitchen blender, at high speed (1600 r/min) for about 10 min. Subsequently, the solution was sonicated for 10 min in an ultrasonic sonicator (frequency 20 kHz, energy 2200 J, temperature 20 °C, amplitude 50%) to guarantee dispersion. The (PC + SF) mixed in the mortar mixer for 10 min at 100 r/min. Then, the sonicated solution was added slowly to the mixture and mixed for another 5 min [49].

4) 4th method (NNi-4): in this method, the NNi was dissolved in water, and sonicated within an ultrasonic sonicator (frequency 20 kHz, energy 2200 J, temperature 10 °C, amplitude 50%) for 1 h, to achieve a good dispersion. The (PC + SF) mixed in the mortar mixer for 10 min at 100 r/min. Then, after the finishing sonication time, the solution and cement mixture were mixed in a mortar mixer for 3 min at 300 r/min [50,51].

5) 5th method (NNi-5): in this method, (PC + SF) were mixed in a 7-L cement mortar, for 10 min at low speed (100 r/min). Next, the required amount of water and superplasticizer (HRWR), was added slowly to the mixture and mixed for 3 min at 300 r/min speed. Then, NNi was added to the mixture and continued mixing for 6 min at 300 r/min [52,53].

6) 6th method (NNi-6): the 6th method of dispersion (CNT-6) was used in this method.

2.2.3 Stability of nanomaterials suspensions

The stability of nanomaterial suspensions is a challenge and a critical factor to be considered. In this work, a unique technique has been used to ensure the stability of nanomaterial suspensions. Regardless of the dispersion method, whatever mechanical or ultrasonic sample of nanomaterial suspension was used after every preparation process. Five milliliters of each suspension were put into a graduated cylinder. The sedimentation rates were measured at different times by looking at the moving phase interface at the initial level of five milliliters, as described in Ref. [54].

Figure 3 shows the change in the settlement of nanomaterials during that time. It can be seen from Fig. 3 that the suspension of nano nickel particles settled after one hour. In contrast, the CNT suspension kept its stability even after a very long time (about one month or more), regardless of the difference in dispersion method. The instability of nano nickel suspension was expected due to the high density of nickel (8.9 g/cm³) compared with that of CNT (2.4 g/cm³). The high specific surface area and the chemical properties make CNT suspensions more stable than nano-nickel. This simple stability test may give us the impression that the dispersion of nano nickel into the water may not enhance the excellent distribution of cementitious composites. This fact was observed depending on the results of ER in the next section. In contrast, CNT dispersion into water shows the best suspension stability, regardless of the dispersing methods. This fact is also supported by the results of ER and piezoresistive properties, which are explained in the next section.

2.3 Sample preparation

50 mm × 50 mm × 50 mm specimens were prepared to determine each testing method’s electrical and mechanical properties (Fig. 4). A flow test of the fresh cement pastes was conducted before fabricating the specimens according to Ref. [40] (Fig. 5). The results of flow tests for all mixes are shown in Table 5. In some mixing methods, the content of HRWR% increased by more than 0.25% to enhance the workability of cement paste, as shown in Table 5. Later, the fresh mixture was casted into oiled cube molds with 50 mm × 50 mm × 50 mm dimensions and compacted using a table vibrator. Two copper electrodes (2 cm × 6.5 cm) with a thickness of 0.3 mm and 3 cm apart were inserted into the specimen to measure the electrical properties of the samples. Copperplate has been chosen as an electrode due to its high electrical conductivity, as suggested in previous studies [20,45]. However, a translucent plastic plate cut off with a specific dimension was used to ensure the straight embedding of electrodes (Fig. 4). Later, the top surface of the specimen was arranged and smoothed. The cubic specimens were left in the laboratory at (23 ± 2) °C for 24 h, then removed and kept in isolated plastic bags at (95% ± 5%) relative humidity, (23 ± 2) °C preparing for the tests at certain ages. To ensure the mixing method and material compositions, samples were labeled based on martials name and the number of mixing methods. For example, CNT-3 refers to the 3rd mixing method of CNT composites, and the same procedure was used to label all the samples.

The reason for keeping the specimens in isolated plastic bags is to enhance more accurate electrical properties results by allowing the hydration process inside the specimens using their mixing water. The water content inside the specimens will inversely affect the electrical properties of cement composites. The wet specimens cured in water will show low electrical properties, while the dry specimens will show very high electrical properties and the mechanical properties will be affected negatively [36,55].

2.4 Mechanical and electrical testing procedures

Three cubic specimens were utilized for each mixing method to determine the average compressive strength after 28 d of curing. The electrodes were not inserted in the specimens for the compressive strength tests. The specimens were loaded at 0.7 kN/s using a 1000 kN capacity compression testing machine. The maximum load and maximum compressive stress were then recorded.

Three cubic samples with embedded electrodes were used to measure the electrical properties for each mixing method. The electrical impedance measurements for each mixing method were recorded at the end of 1, 3, 7, 14, 28, 60, 90, and 180 d using the RCON2^TM concrete resistivity meter-GIATEC, as shown in Fig. 6. RCON2 m employed an alternative current (AC) impedance technique, with a 1–30 kHz frequency variation and (0°–180°) phase angle. However, the meter frequency was set at the max (30 kHz) to decrease the unwanted polarization, similar to Refs. [36,45]. The resistivity meter used in this study results in impedance and phase angle values, which can be easily converted into resistance values using the equation (R=Z\cdot \cos \theta), where R, Z and θ are the magnitude of resistance (Ω), impedance (Ω), and phase angle (° ), respectively.

After recording the impedance results with various curing ages, all specimens were tested under monotonic loading at the age of 180 d. For the piezoresistive tests, specimens were loaded at 0.7 kN/s using the compression testing machine (Fig. 7). The change of electrical impedance under loading was collected using the Giatic RCON2 device, and the results were recorded on the connected computer. The test method was AC-2 probes, and the frequency was fixed to the value (30 kHz), which is the highest provided value in the device.

The fractional change of impedance (FCI) during the monotonic loading was calculated according to Eq. (1)

(1)FCI=(I_x-I_0)/I_0,

where I_0 is the initial electrical impedance of the composite (Ω), I_x is the electrical impedance of the composite during the loading (Ω).

3 Results and discussions

3.1 Electrical resistivity (ER) results

Table 6 shows the electrical impedance of cement composites containing CNT, NNi, and CF. In addition to the results presented in Table 6, the results are illustrated in Fig. 8 for better comprehension. As seen from Table 6, the electrical impedance increased with the curing age, regardless of the mixing method, especially for the CNT and NNi cement composites. Many factors, such as the microstructure, pore structure, porosity, and pore network, can affect the composites’ electrical properties [56]. The continuous increment of ER with the curing age depends on the pore structure and the water content inside the pore, which evaporates with the cement hydration process over time. Adding the SF to the cement paste will support the dispersion of conductive fillers, and the pore solution will be reduced for further hydration.

The CF cement composites did not show much electrical impedance increment with the curing age, regardless of the mixing methods. On the contrary, the electrical impedance of CF composites has been stabled somehow after the age of 60 d. This is because the well-dispersed CF inside the cement matrix offer a high electrically conductive network, enough to decrease the electrical impedance even with the progress of the curing age. On the other hand, the high aspect ratio (length to diameter) of fibers guaranteed the continuous stable decrease of the electrical impedance inside the cement composites. Tian et al. [57] found that the resistivity change is consistent with internal change of CF composites, while the fractional change of resistivity was low under compression in the long-age condition. This is because the little amount of CF (0.05–0.1 vol.%) that is used in the mixture faces difficulty in forming the conductive network in long-age conditions.

In this study, CF composites showed an excellent electrical impedance reduction compared to control composites, regardless of the mixing methods. Among the three mixing methods, the 2nd produced a better electrical impedance reduction than the other methods. For example, the 2nd method exhibited a 99.5% reduction in electrical impedance compared to the control specimen at 28 d of curing age. Lower electrical impedance means better CF dispersion capability, which was in the order of CF-2 > CF-3 > CF-1 as shown in Table 6. The high conductivity and aspect ratio (1600) of CF lead to a massive reduction in the electrical impedance results of the CF composites. In addition, the dry cementitious materials (PC + SF) play a good role in the dispersion of CF and reduce or remove the accumulation of CF to each other as much as possible. Vu et al. [58] also investigated that the dry mixing of PC and SF, incorporated with 5 wt.% SF and 3 wt.% super plasticizer, enhanced fiber dispersion and significantly improved electrical conductivity. This finding aligns with the observed trends in our study.

Recent research supports these findings. Kim et al. [59] demonstrated that boron doping in CF-reinforced cement composites can significantly enhance thermoelectric and electrical performance by improving electron mobility and filler distribution. Similarly, Lawongkerd et al. [60] compared graphite powder and steel fibers for thermoelectric harvesting in cement composites and emphasized the importance of uniform dispersion for performance optimization. These studies align with our conclusions regarding the critical role of filler distribution in achieving improved electrical characteristics. Adding the conductive materials to the cement matrix reduces the electrical impedance, and this reduction varies based on the mixing methods used [61,62]. The CNT cement composites exhibited a reduction in the electrical impedance with the different mixing methods. Nevertheless, the (1, 2, 3, 4, and 5) CNT mixing methods were successful in previous studies but needed to show better electrical conductivity in our preliminary works. It can be seen from Table 6 or Fig. 8 that the sixth mixing method has been shown to have the best electrical impedance reduction compared with other methods. For example, the electrical impedance of CNT cement paste based on the sixth mixing method was 413 Ω, less than 89% of the impedance of the control specimen (3626.67 Ω) at the 28 d-curing age. This is because the high surface area of the CNT (510 m²/g) filled the spaces of hydration progress and decreased the porosity, which led to a decrease in the electrical impedance. The effective CNT dispersion methods were in order of CNT-6 > CNT-4 > CNT-3 > CNT-2 > CNT-1 > CNT-5 as shown in Table 6. The uniform dispersion of CNT mixed based on the six mixing methods forms an effective electrically conductive network. Similar findings were also improved with the sixth mixing method by the researcher [34]. It is important to note that the specimens of the sixth method did not display a high increment of electrical impedance with the progress of curing age compared with other mixing methods. The other CNT mixing methods did not exhibit an efficient reduction in the electrical impedance, which could be because of the aggregation and clustering of CNT particles inside the cement composites [63]. The sonication dispersion method did not show any effective reduction in the electrical impedance of CNT composites, even with the increasing sonication times. This could be because of the re-aggregation of CNT in a highly alkaline medium of the cement matrix, crosslinking of CNT particles, and porosity of the paste, as discussed in Ref. [63]. Mardani et al. [32] found that dispersion CNT using magnetic stirrer for 10 min and sonicating the solution for 30 min improved the piezoelectric properties of CNT composites. Recent studies have introduced novel materials such as carbon dots (CDs) in cement composites, which exhibit excellent water dispersibility, surface reactivity, and size-controlled functionality. CDs can act as nano-bridging agents, improving the interfacial transition zone and the conductivity of cement matrix [64]. While other nano materials such as nanoclays, GNPs and doped quantum materials significantly improved crack healing and mechanical reinforcement of cement composites [65,66]. For example, Prasittisopin et al. [67] demonstrated that the high surface energy and homogeneous dispersion behavior properties of factionalized CDs enhanced the piezoresistive sensitivity of cement composites.

Introducing NNi in the cement paste has reduced the electrical impedance compared to the control composites. The 5th mixing method presents the lowest electrical impedance results compared with other methods. The high density of nano nickel particles makes difficulties to disperse well in water or mix well with dry raw cementitious materials (PC + SF). Therefore, the nano nickel particles are mixed and dispersed better with fresh cement using the 5th mixing method. The fresh wet cement matrix can carry heavy nickel particles better than the dispersion in water or dry mixing. For example, the reduction in electrical impedance based on the 5th method of Nano nickel composites was about 28% compared to control composites (curing age 28 d). Based on the presented results in Table 6, the effective dispersion methods of NNi composites were in order of NNi-5 > NNi-2 > NNi-6 > NNi-1 > NNi-3 > NNi-4. Generally, the nano nickel composites did not display a reasonable reduction in the electrical impedance compared to CNT or CF composites. The main reason is the high density of Nano nickel (8.9 g/cm³) compared to that of CNT (2.4 g/cm³), or CF (1.7 g/cm³) makes a small size of nano nickel while the size of CNT or CF is more significant with the same mixing content ratio (0.5% by vol.). Nickel particles are heavier than CF and CNT and need extensive dispersion methods to guarantee the uniform distribution of NNi. In recent years, some researchers used the magnetic field (MF) to improve the dispersion of Ni particles in cement matrix. Tian et al. [68] found that the MF improved the electrical conductivity of composites contains 15 vol.% of Ni better than those of non-MF. In addition, the amount of Ni should exceed 10 vol.% to show better electrical conductivity as recommended in Refs. [69,70].

As a result, it is clearly shown from Table 6 or Fig. 8 that the 6th, 5th, and 2nd mixing methods showed the best results for the composites of CNT, NNi, and CF, respectively. These well-established methods have been used in the following experiments to study the electrical and piezoresistive properties of the conductive cementitious composites.

3.2 Piezoresistive response results

The piezoresistive properties of conductive cement composites have been investigated for all conductive materials based on the mixing methods. Figure 9 shows the FCI for the control composite. The impedance (red dotted line) decreased as the load (blue continuance line) increased, but the average impedance change for the control composite was small and irregular. Since the control composite had no conductive materials, the electrical response is expected to be irregular.

The piezoresistive response of the conductive cement composites based on the type of materials and the mixing methods are discussed separately.

1) CNT cement composites

Figure 10 shows the piezoresistive (self-sensing) response of the cement composites incorporating CNT based on different mixing methods. Regardless of the mixing method, all the proposed methods showed a significant self-sensing response under monotonic loading, with varying rates of electrical impedance change. As the load increased, the electrical impedance decreased due to the close distance between the electrodes and the conductive materials being closer to each other. This allows the electric current to pass more efficiently through the conductive network and decreases the electrical impedance. The 6th mixing method of CNT showed better results for the piezoresistive property (Fig. 10). For example, the loading time of 130 s was taken as a reference to determine the average rate of electrical impedance change. It can be seen from Fig. 11(a) that the 6th mixing method exhibits the best change rate of impedance, which means the better-sensing property. This finding aligns with the electrical impedance results mentioned in Subsection 3.1. Since the 6th displayed an excellent result in the electrical impedance and the piezoresistive property, it is considered a preferable method for evaluating the electrical and piezoresistive performance of CNT-composites.

It is important to mention that the sensing property of smart CNT composites is affected by some parameters. These parameters are material proportion, max fractional change of resistivity, force sensitive coefficient, and strain sensitivity coefficient (strain gauge factor) [71]. Konsta-Gdoutos and Aza [72] discovered excellent sensitivity exceeding 45% for composites containing 0.1 wt.% of CNT. While Azhari and Banthia [73] fabricated composites containing 15 wt.% CF and 1 wt.% MWCNT and found that these composites exhibited good sensitivity response under the monotonic and cyclic loading trends.

2) NNi cement composites

Figure 12 shows the piezoresistive property of NNi cement composites under monotonic loading based on different mixing methods. The composites showed a piezoresistive response with varying mixing methods. The impedance decreased regularly with increasing loads at various rates depending on the mixing methods. In line with the electrical impedance results discussed in Subsection 3.1, the 5th mixing method displayed an excellent response compared to the other NNi mixing methods. This means the dispersion of nano nickel particles mixed based on the 5th method was excellent for forming a conductive network path.

Figure 11(b) indicates the average rate of impedance changing under the loading at the time 130 s. It is seen that the piezoresistive response was higher in the 5th method than in other methods. However, the sensing response of NNi (5th method) under loading is not as high or sensitive as that of CNT (6th) because of the difference in materials properties, pore structure, and mixing methods. The 5th NNi mixing method is the best for the following NNi experiments due to its better results according to the electrical impedance and self-sensing property. The study in Ref. [68] showed that the magnetically aligned nickel particles (15 vol.%) exhibited better sensing response under the cyclic loading. The fractional change of resistivity reached up to 31.5% under cyclic loading which means good sensitivity has been achieved.

3) CF cement composites

The piezoresistive responses of CF cement composites under monotonic loading are presented in Fig. 13. Although all CF composites displayed a noisy response, the 2nd mixing showed a better piezoresistive response than other mixing methods. The noisy response means that the piezoresistive behavior was not as steady as in other composites. The CF in this method may have dispersed more uniformly than in other methods. This regular dispersion of CF forms a conductive network, presenting a decreasing impedance under monotonic loading. The noisy response is caused by the accumulation of CF within the cement matrix and the pore structures they form. Figure 11(c) illustrates the average impedance change of the composites. The 2nd CF mixing method presented a better response than other mixing methods, which agrees with the electrical impedance results in Subsection 3.1. Based on those results, the 2nd is considered the preferable mixing method for the CF composites. The results of this study were in line with the experiments of Ref. [74]. However, the researcher used the MF to control the orientation of the nickel-coated CF. They found that the MF supported the uniform distribution of CF and thus improved the sensing sensitivity under various conditions of cyclic loadings.

3.3 Compressive strengths results

The compressive strengths for all conductive cement composites based on the mixing methods are presented in Fig. 14. The compressive strength of CNT composites varied between 60.6 and 73.8 MPa, while the compressive strength of the control specimen was about 72.3 MPa (Fig. 14(a)). The 2nd and 3rd mixing methods showed a slight increase in compressive strength compared to the control composites. The other mixing methods showed a slight decrease in compressive strength compared to the control composites. The slight variation in the compressive strength, regardless of increment or decrement, was due to many factors such as variations in mixing methods, sonication time, pore size, pozzolanic reactivity, SF, and the superplasticizer HRWR content, as well as the non-uniform distribution of the conductive materials in the cement matrix [75,76].

The compressive strength results of NNi composites are shown in Fig. 14(b). The compressive strength of NNi composites based on the mixing methods was somewhat near that of control composites, except that the 5th and 6th methods were less. The amount of added nano nickel particles was just 0.5% by volume of cementitious materials (PC + SF), which is minimal compared to the high density of nano nickel (8.9 g/cm³). This can be an essential reason to understand the slight variation in the compressive strength of composites based on the mixing methods. In addition, dissolving the heavy nano nickel particles in the water did not produce any effectiveness in cement composites for the electrical impedance or the compressive strength.

Regardless of the mixing method type, the CF showed a noticeable decrease in the compressive strength of the cement composites. CF are known to be more effective in the tensile than the compressive. The long 12-mm CF strengthen the composites against the tension, especially in the initial cracking phase, while not very useful in post-peak micro-cracking, which results in a sudden loss in load-carrying capacity, and the compressive strength has been decreased [34]. In addition, CF produced voids inside the cement matrix, which affected the pore structure of the composites. It can be seen from Fig. 14(c) that the 2nd mixing method displayed a better compressive strength compared to the other mixing methods. However, this study focuses on the electrical properties of conductive cement composites. Therefore, the results of electrical impedance and the piezoresistive properties are considered more crucial than the compressive strength results to achieve our aims in this work.

4 Conclusions

This work discusses various mixing techniques to guarantee better distribution of conductive materials based on the best obtained electrical impedance and piezoresistive property results. Different micro and nanomaterials were used as conductive materials in the cement matrix. Many factors were evaluated, such as material type, mixing method, curing age, ER, and piezoresistive property. The following conclusions were obtained.

1) The electrical impedance of CNT and NNi composites increased with the curing age, regardless of the mixing method. Meanwhile, CF composites did not show much electrical impedance increment with the curing age, irrespective of the mixing methods.

2) The mechanical mixing of CNT showed a significant electrical impedance reduction up to 89% compared to other mixing techniques.

3) The optimal dispersion of NNi was achieved by directly incorporating the NNi powders into the fresh cement mixture.

4) The dry mixing of CF with cementitious materials showed an excellent decrease in electrical impedance up to 99% compared to that of plain cement composites at the age of 28 d.

5) The effective mixing method in reducing electrical impedance also showed better piezoresistive properties of all conductive composites. The best mixing methods also exhibit the highest rate of change of electrical impedance.

6) The 2nd and 3rd CNT mixing methods showed a slight increase in compressive strength compared to the control composites. The other mixing methods showed a decrease in the small value of the compressive strength compared to the control composites. Thus, the best mixing method regarding electrical properties does not need to be conditionally a significant mixing technique in compressive strength improvement.

The outcomes of this research align well with the evolving requirements of smart cities, particularly in enhancing infrastructure intelligence. By improving the dispersion of conductive fillers like CF, MWCNT, and NNi, the cement composites developed in this study demonstrated reliable piezoresistive performance. These materials can be integrated into concrete elements to support real-time monitoring systems, helping detect structural issues early and improve maintenance efficiency. Such smart materials represent a key advancement toward resilience and responsive urban infrastructure.

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