**1. Introduction**

Concrete is known to have high compression strength but weak tensile capacity. Many scholars use steel fiber, polyvinyl alcohol (PVA) fiber, polypropylene (PP) fiber, carbon fiber, etc. as micro-reinforcement paste in concrete to prepare composite materials with better bearing capacity [1–8]. The results show that the mix of fiber can effectively enhance the tensile strength and toughness. The problem is that the strength of toughening fibers used in concrete are relatively low, and may cause larger porosity, imposing a negative effect on the mechanical properties of concrete. High strength fibers that are smaller than microfibers for applications of high-performance fiber reinforced concrete composite still has potential. Thus, the feasibility of applying carbon nanotubes (CNTs), as potential nanofibers, into cementitious composites has been widely studied due to CNTs' unique properties.

CNTs are known to have spectacular properties, such as high specific conductivity, high Young's modulus, high yield strength, good thermal properties, etc. [9–21]. CNTs' thermal conductivity is at least twice that of diamond [22], and its electricity conductivity is 1000 folds that of copper [12]. Specifically for mechanical properties, CNTs have an average Young's modulus around 1 TPa, which is approximately five times greater than steel, and the tensile strength is up to 63 GPa which is approximately 50 times greater than steel [12–17,19]. CNTs have great potential for reinforcing cementitious composites.

However, when comparing CNTs to traditional microfibers, it is evident that using CNTs in a cement matrix is challenging. One of the main obstacles is the method to disperse CNTs

**<sup>\*</sup>** Correspondence: salam.al-sabah@ucd.ie

homogeneously in the cement matrix due to their strong van der Waals force (VDW), which originates from their polarizable extended π-electron systems [20], causing CNTs to have a tendency to agglomerate [23–25]. This behavior can cause negative effects to the mechanical properties of the cement composites [25]. A homogeneous morphology possesses an improved mechanical resistance, as mentioned by Gavallaro et al. [26] and Lisuzzo et al. [27]. According to Cota et al. [28], the mixing process using a Hobart mixer, commonly used to prepare mortar paste, cannot ensure proper dispersion of CNTs within the cementitious matrix, resulting in large CNT clusters within the hydrated paste. Since the CNTs are not well dispersed in the cement matrix, their spectacular physical properties cannot be fully revealed. The agglomerates and bundles could lead to a decrease in mechanical performance of cementitious materials [25]. Therefore, finding dispersion methods is an important aspect for further study.

Dispersion performance is a key for improving the interface bonding force among CNTs and cement matrix. In previous studies [24,29–34], it was found that surfactant (SAA), ultrasonic dispersion, chemical covalent modification, and electricity field induced method are the main methods of dispersing CNT, among which SAA and sonication are the most widely used methods. SAA, as the most widely used method, has many options, such as Sodium dodecyl benzene sulfonate (SDBS), sodium deoxycholate (NaDC), polyethylene glycol octylphenol ether (TX10), Gum Arabic (GA), cetyl trimethyl ammonium bromide (CTAB), and dodecyltrimethyl-ammonium bromide (DTAB), which can make carbon nanotubes disperse and stay stable for a long time.

Many scholars have investigated the possibility and feasibility of introducing CNTs into the field of civil engineering, and have found positive effects of CNTs on cementitious composites. Wang et al. [35] found that the addition of multi-walled carbon nanotubes (MWCNTs), with surfactant GA and sonication treatment, improved the mechanical properties of cement composites. The flexural toughness and fracture energy of cement-based composites were increased with 0.08 wt% addition of MWCNTs. Sindu et al. [36] also found that the combination of GA and sonication can improve the mechanical properties of cement composites. Wang et al. [35] found that, when the concentration of MWCNTs was increased to a certain amount, a negative effect would occur. Wu et al. [37] found that the addition of MWCNTs could significantly relieve the mechanical properties of cement-based composite materials. When the amount of MWCNTs reached its optimal value, i.e., 0.1%, the improvement of the mechanical properties of cement-based composite materials was the best. Fatemi and Foroutan [38] concluded that TX10 was the best at dispersing MWCNTs. Luo et al. [39] found that, by using SDBS and TX10 with sonication, the MWCNT–cement composites saw 29.10% and 20.8% increase in flexural and compressive strength, respectively. Mohamed et al. [40] found that, when sonication was applied to CNT/SAA solution to disperse MWCNTs, higher strength of specimens was obtained. Ana et al. [41] found that the addition of MWCNTs treated with superplasticizer (SP) and sonication improves the compressive strength of cement composites. Similarly, Li et al. [42] concluded that, by dispersing SWCNTs using sonication and SP, a 26.3% increase in strength of cement composites was obtained. Parveen et al. [43] concluded that the compressive strength and flexural strength of mortars saw 19% and 7%, respectively, increase when 1% SWCNTs were dispersed by sonication and Pluronic F-127, a novel dispersing agent. Based on the research presented above, it can be concluded that, when SAA and sonication are combined, a positive effect on the mechanical properties on cement composites can be obtained within a certain range of concentrations of CNTs.

However, some studies show a slight increase or even a reduction in mechanical properties of CNT–cement composites when SAA and sonication are used. Camacho et al. [44] concluded that the addition of CNT to Portland cement mortars shows little effect on bending strength and apparent density of mortars. Collins et al. [33] found the compressive strength was decreased to four times less compared to a plain cement composite. Similarly, Shao et al. [45] concluded that, when surfactants, namely SDS (Sodium dodecyl sulfate), TX10, and Polyethylene glycol sorbitan monolaurate (Tween-20), and sonication were combined to disperse CNTs in cement matrix, the compressive strength decreased compared to plain cement composites. Sobolkina et al. [46] also found that the use of SDS led to a drop

in the strength of cement composites due to the formation of foam. Kim and Chung [47] found that less yield stress was obtained when SWCNTs were treated by applying sonication and SDS or Sodium deoxycholate (DOC). These results conflict with the conclusions obtained in the above studies.

It is worth mentioning the research of Bharj et al. [48], who found that, by using sonication only (without SAA) to disperse CNTs, it was possible to improve the mechanical properties of cement composite paste. Therefore, sonication probably always has a positive effect on strength for cement composites by dispersing CNTs in matrix, but it should be noted that ultrasonication can result in fragmentation of CNTs and poor stability of dispersion [49]. Therefore, SAA plays an important role in producing a stable CNT dispersion. Different types of SAA can produce either positive or negative effects on cement composites, regardless of their CNT dispersion performance. It has also been found that adding some types of surfactants into cement matrix can cause formation of foam or prevent hydration reaction among cement [45,46,50]. However, few studies have mentioned the formation of foam in sample preparation and the change of rheological behavior of slurry. According to Parveen et al. [43], the surfactant Pluronic F-127 works similarly to polycarboxylate-based superplasticizers, which can disperse cement particles and modify the fluidity of mortar [51]. It is known that plasticizers can keep the slump and cement dosage unchanged while significantly reducing water consumption of mixing additives. However, few studies have considered this and changed the water/cement ratio to keep the slump result constant. Moreover, there are few studies on the effect of using surfactant only to modify the cement composites.

In addition, most studies use MWCNTs as reinforcement materials in cement-based composites [52]. As mentioned above, CNTs have spectacular properties. However, SWCNTs and MWCNTs have many different properties such as Young's modulus [53,54], electrical conductivity, thermoelectric properties, and optical properties [55]. Table 1 shows the differences between SWCNTs and MWCNTs [53,54].


**Table 1.** Differences between SWCNTs and MWCNTs.

Table 1 shows that the mechanical properties of MWCNTs are generally better than SWCNTs. This is why most studies use MWCNTs to reinforce cementitious composites, as well as their relative lower cost. Most literature reports are related to mechanical properties. However, in recent years, the needs of different functions of cementitious composites are proposed. Heeyoung et al. [56] investigated the heating characteristics of CNT–cement mortars by using SWCNTs and MWCNTs. They concluded that cement mortars mixed with SWCNTs are more effective for modifying the heating characteristics compared to MWCNTs mixed mortars. However, Luigi et al. [57] found that MWCNT–cement composites showed better electrical properties, which can be applied for future work of monitoring the stress level of concrete element. Heeyooung et al. [58] found that adding CNTs to cement-based materials enhances their electrical and thermal characteristics, and they found that adding SWCNTs to concrete grout is more suitable than MWCNT for identifying voids in the duct through electrical resistance analysis. Jin et al. [59] concluded that extremely small amounts of SWCNTs can be used as optical strain sensors if the SWCNTs can be homogeneously dispersed in cement composites. These studies aimed to fully use the better electrical properties and thermal properties of SWCNTs; however, the mechanical properties of CNTs added to cement-based composites were not considered. For future application, on the premise of meeting the above cementitious composite functions, it is worth studying their mechanical properties.

This study investigated the effect of different concentrations of SWCNTs on the strength properties of cement composites when SAA dispersion method was used. TX10 was used as SAA because it has shown a higheer dispersing power than SDBS, CTAB, and DTAB for SWCNT suspensions [49]. In addition, the effect of TX10 on strength properties of cement composites was investigated and a comparison of strength of adding SWCNTs and MWCNTs to cement composites after 28 days of curing was also conducted.
