2.1.2. Cement and Sand

Ordinary Portland cement CEM I 42.5 R (rapid hardening) manufactured by Irish Cement (Ireland) was used in the following experiments. Rapid hardening Portland cement (RHPC) is a special purpose cement used in concrete to achieve a higher rate of early strength development, compared to using normal cement. Its main particle size is 5–30 μm, it has a relative density of 2.75–3.20 g/cm3, and its apparent density is 0.9–1.5 g/cm3.

Standard silica sand from Glenview (Ireland) was used in the experiments. Sand was dried by placing into oven for 24 h at a temperature of 105–110 ◦C. The particle size of the sand was variable. To prepare cement mortars, the sand was sieved through a 600-μm sieve. This was carried out to reduce the voids formed when preparing specimens.

## 2.1.3. Surfactant (SAA)

SAAs, such as TX10, SDS, GA, etc., are amphiphilic in nature and can be used as surface active agents. Structurally, they have a hydrophilic polar head and hydrophobic long tail [61]. They can be used to lower the surface tension between two liquids, between a gas and a liquid, or between a liquid and a solid.

TX10 from Carl Roth GmbH + Co KG (Karlsruhe, Germany) was used in experiments. TX10 is known for having good dispersing performance on SWCNTs [38,49]. Furthermore, it was found by Carriçot et al. [62] that, for different surfactants, the optimum mass ratio of CNTs to dispersant are different. Therefore, the optimum dosage of TX10 to treat SWCNTs should be researched. Wang et al. [63] measured that the saturation adsorption ratio of surfactant vs. SWNT is ca. 0.004 mol/g. With the increase of the concentration of TX10, no SWCNTs in suspension liquid will react with extra TX10.

TX10 has an average molecular mass of 625, a density of 1.065 g/cm3, and a critical micelle concentration (CMC) of 0.22–0.24 <sup>×</sup> 10−<sup>3</sup> moles/liter in water at 25 ◦C. One mole of TX10 is 625 g on average. The linear molecular formula is (C2H4O)nC14H22O. It is a nonionic surfactant.

Therefore, 1 g of SWCNTs needs the addition of 0.004 mol TX10 (2.5 g) to reach saturation dispersion [63]. In this experimental set, the dosage of TX10 should be determined. The maximum concentration of SWCNTs is 0.06 wt% by the mass of cement. In each mix group, the amount of cement was 1833 g; thus, the amount of added SWCNTs could be calculated as around 1.0998 g. Thus, the dosage of TX10 was around 2.75 g. Considering TX10 may negatively affect the cementitious matrix, 2.75 g TX10 was used for all concentrations of SWCNTs (0.02 wt%, 0.04 wt%, and 0.06 wt%), and, for all concentration, SWCNTs were well dispersed. In general, the mass ratio of TX10 was 0.15 wt% by the mass of cement.

#### *2.2. Specimens Preparation*

## 2.2.1. Mortar Mix Ratio

The Mix Ratio 1 proportion is shown in Table 3. Normal mortar specimens, consisting of cement, sand, and water, were prepared following Mix Ratio 1. However, for mortars treated by TX10, Mix Ratio 2 was adjusted, as shown in Table 4. This is discussed in Section 2.2.4.


**Table 3.** Mix Ratio 1 used for mortars (no TX10).

Slump tests were conducted to determine the workability of cement mortar mixed according to the mix ratios. The slump test was carried out as prescribed by ASTM C 143-10 and BS 1881:103:1993. According to Jensen et al. [64], surfactant TX100 acts as a plasticizer to some extent. In the following

tests, the slump test was carried out to ensure that the mortar mix has similar workability. Mortar was divided into two groups: untreated plain mortars and TX10-treated mortars. For plain mortars, the slump test result was 52 mm.

According to BS EN 206-1:2000, Table 5 gives the European classification of slump test. It can be concluded that the workability of mortar with Mix Ratio 1 belongs to S2 classification.

**Table 5.** Classification of Workability and Magnitude of Slump.

