*3.2. Compressive Strength Test Results*

To get compressive strength of mortars, three specimens were prepared. According to BS 1881-119:1983 and ASTM C 116-90, the average of the three values was taken as the representative compressive strength of the concrete. Table 11 shows the failure compressive strength of all specimens. The differences of SWCNTXN and SWCNTXS groups (where X equals 0, 2, 4, and 6) are water/cement ratio and TX10 content. Although the two groups had similar slump results, which indicates similar workability, the values of compressive strength shows great difference. Non-TX10-treated samples showed higher compressive strength than SAA treated samples. The cause might be the decrease of mass of cubes and prisms when TX10 was added. TX10 results in the formation of foam during the stir process, which retards or prevents the hydration of cement. The relationship between strength and mass of specimens are discussed in Section 3.3. In this section, the effects of SWCNTs and TX10 on sample strength are discussed.


**Table 11.** Compressive strength test results of SWCNT-added specimens.

#### 3.2.1. Specimens without Treatment of TX10

The compressive strength of specimens with no TX10 added is plotted against time in Figure 2. The effect of SWCNTs on these specimens is discussed in this section.

**Figure 2.** Compressive Strength of No TX10 contained mortar cubes.

Figure 2 shows variation of the compressive strength of specimens under four different SWCNT concentrations at different curing times. With no treatment of TX10, the higher was the concentration of the SWCNTs, the lower was the compressive strength of mortar cubes, which is similar to the results of Suprompituk et al. [66]. For plain cementitious mortar, the compressive strength could reach 30.15 Mpa after 28 days of curing, which decreased to 25.58 Mpa when the concentration of CNTs was increased to 0.06 wt%. The reason might be that, when SWCNTs were used directly, they easily tangled together, thus were not well dispersed in the cement matrix and formed agglomerates. This reduced the bond between the hydration products, and in turn the compressive strength of mortar was reduced.

Moreover, at early age, the differences in compressive strength were small. At three days of curing, they shared similar compressive strength, all around 20 MPa. Thus, at early age, SWCNTs showed little effect on the mechanical properties of mortars. In addition, t the compressive strength of SWCNT0N and SWCNT2N after three and seven days of curing were similar, although the strength of SWCNT2N was slightly higher. However, at 28 days of curing, the strength of SWCNT2N was 6.43% lower than SWCNT0N. The reason the strength of 0.02 wt% SWCNT-containing specimens was higher at early stage might be the positive effect of SWCNTs on early hydration reaction of cement. This phenomenon that CNTs may affect the early hydration progress of cement, producing higher hydration rates, was also found by Markar and Beaudoin [67] and Markar et al. [68]. In general, the reduction of 28-day strength with the increase of concentration of SWCNTs was related to the agglomerates of CNTs.

To analyze the early hydration rate, the evolution of the strength of mortars is plotted in Figure 3. Adding CNTs improved the evolution of strength of mortar in gaining strength more quickly. At three days of curing, plain mortar gained around 66% strength. With the increase of the quantity of SWCNTs, the hydration rate was improved. Obvious differences occurred at seven days of curing;

the compressive strength could reach around 90–94% of 28-day strength, which was higher than the 83.4% for SWCNT0N. However, when the concentration of CNTs reached 0.06 wt%, the percentage of gained strength at seven days of curing was around 87%, between plain mortars and SWCNT2N. This might be the negative effect of agglomerates. In general, adding SWCNTs can make cement composites gain strength more quickly at an early age. In addition, it was also found that, when the concentration of SWCNTs was 0.04 wt%, the evolution of compressive strength of samples was the highest.

**Figure 3.** The evolution of the strength of mortars (no TX10) as a function of curing time.

Figure 4 shows the effect of concentration of SWCNTs on compressive strength at different curing times. It clearly shows that the higher was the concentration, the lower was the compressive strength. After 28 days of curing, specimens showed a higher compressive strength than seven- or three-day cured specimens. At three days of curing, the compressive strengths of specimens at the four concentrations showed similar values, around 20 MPa.

**Figure 4.** Compressive strength as a function of concentration of SWCNTs (No TX10).

#### 3.2.2. Specimens with Treatment of TX10

When TX10 was added and mixed, the variation pattern of strength changed. The most obvious change shown in Table 11 is that the compressive strength of SWCNTXS (where X equals 0, 2, 4, and 6) was much lower than that of SWCNTXN (where X equals 0, 2, 4, and 6) at each curing time. The reasons might be the change of water/cement ratio and the effect of TX10 on the specimens. For samples with the addition of TX10, their masses were less than those of non-TX10-treated specimens because, when TX10 was added, much foam was generated in the cement paste, as mentioned in Section 3.2, causing high porosity in mortars, and in turn the decrease of the specimens' bulk density. Moreover, TX10 might have partly decreased the bond of cement reaction. Based on this situation, the effect of

SWCNTs and variation pattern on strength properties of mortars under the dispersion method of TX10 were studied.

Figure 5 shows that the compressive strength increased with the increase of concentration of SWCNTs; the higher was the concentration, the higher was the compressive strength. After 28 days of curing, the compressive strength of SWCNT6S increased 21% compared to SWCNT0S. The reason might be that TX10 changed the surface energy of SWCNT suspensions; getting SWCNTs well dispersed in water allowed the unique properties of CNTs to show up. Moreover, it should be noted that, at three days of curing, different from SWCNTXN (where X equals 0, 2, 4, and 6), which had no TX10 added and shared similar compressive strength of 20 MPa, the strength of TX10-treated mortars increased with the increase of concentration of CNTs, although the strength of SWCNT2S was slightly lower than that of SWCNT0S. Furthermore, Figure 6 shows that, with the addition of SWCNTs, the rapid hardening phenomenon of mortar was more obvious, even though CEM |, a type of rapid hardening cement, was used in experiments. At seven days of curing, when the concentration of SWCNTs was 0.04 wt%, the compressive strength could reach over 95% of the 28-day compressive strength. Compared to Figure 3, when the concentration of SWCNTs was 0.06 wt%, the percentage of strength gained could reach approximately 94%, which was higher than the 87% of non-TX10-treated specimens. Moreover, Figures 3 and 6 show that, for plain mortars with no CNTs added, TX10 decreased the hydration rate to some extent. The strength gained percentage decreased from 83.4% to 80% when TX10 was added.

**Figure 5.** Compressive Strength of TX10 added mortar cubes.

**Figure 6.** The evolution of the strength of mortars (SAA) as a function of curing time.

Figure 7 shows the compressive strength as a function of concentration of SWCNTs. The compressive strength was reduced initially, but then, with the increase of the concentration of SWCNTs, the compressive strength increased, reaching the peak when the concentration was 0.06 wt%. The reduction of strength might be due to the change of fluidity when CNTs were added, which

requires more research. Thus, TX10 as a SAA worked well on dispersing SWCNTs in cement matrix, and SWCNTs helped enhance the interface bond force among aggregates and cement matrix. The agglomerates that might obstruct the cement pastes were reduced. Therefore, SAA is a useful dispersion method regardless of the negative effect on mechanical properties of cement composites. However, it should be noted that the addition of TX10 produced foam in the cement, resulting in the high porosity of mortars, causing a decrease in the bulk density of samples, and in turn the overall decrease of strength. Therefore, in future works, defoamer should be considered to study the possibility of improving the reduction of strength when TX10, or another SAA is added.

**Figure 7.** Compressive strength as a function of concentration of SWCNTs.
