**3. Results and Discussion**

#### *3.1. Comparison of Strength Properties of SWCNTs and MWCNTs Added Specimens*

It is known that MWCNTs show better mechanical properties than SWCNTs, and many studies have been conducted to study the effect of MWCNTs on mechanical properties of cement composites. However, SWCNTs are more advantaged in enhancing electrical properties and thermal properties of cementitious materials, which give SWCNTs great potential for functional cement composites. This section compares the difference of strength properties of SWCNT- and MWCNT-added specimens when they were prepared by the same method. Table 10 shows the 28-day compressive strength and flexural strength of the two types of cement-based specimens with or without the treatment of TX10.


**Table 10.** Strength properties of SWCNTs and MWCNT-added specimens after 28 days of curing.

Table 10 shows that, for both specimens with no addition of CNTs and TX10, their strengths were around 30.5 MPa, resulting from the same mix ratio. With the addition of CNTs, the strength of both SWCNT–cement composites and MWCNT–cement composites decreased with the increase of concentration of CNTs when CNTs were directly used. This could be attributed to the increase of agglomerates of CNTs when the concentration of CNTs was increased. It can be observed that the compressive strength and flexural strength of MWCNT–cement composites were higher than those with SWCNTs. For instance, when the concentration of CNTs was 0.04 wt%, the compressive strength of MWCNTs was 27.46 MPa and the flexural strength was 6.96 MPa, 6.5% and 12.3% higher than those of SWCNT–cement composites. Thus, MWCNTs show better mechanical properties than SWCNTs, but the difference is relatively small. However, when CNTs were dispersed by TX10, SWCNT–cement composites show slightly better strength properties. When the concentration of CNTs was 0.04 wt%, the compressive and flexural strengths were 18.94 and 4.95 MPa, respectively, 2.6% and 3.2% higher than MWCNT–cement composites. This might be due to TX10, which shows the highest dispersing power when dispersing SWCNTs [49]. However, for MWCNTs, according to Luo et al. [39], TX10 was not suitable for dispersing MWCNTs. The reduction of strength after treatment of TX10 is discussed in following sections. Generally, MWCNTs show better mechanical properties, but the increase of strength properties is relatively small. There is potential to study the effect of SWCNTs on mechanical properties of cement-based composites. The following sections mainly discuss the strength properties of SWCNT-added cement composites.
