**4. Conclusions**

This paper presents the effect of SWCNTs (Rhenofit® CNT-3) on the strength properties of cement composites. In addition, the effect of the addition of TX10 (SAA) on the strength properties of cement composites is also discussed. The main results of this investigation are summarized as follows:

1. Directly using Rhenofit® CNT-3/CNT-2 (without addition of TX10) in cement matrix reduced the compressive strength of cement composites, resulting from the agglomeration of CNTs. The compressive and flexural strengths decreased as a function of the concentration of CNTs. Plain mortars showed the highest strength at this stage.

2. When TX10 was not added, MWCNTs (Rhenofit® CNT-2) added to cement-based specimens showed better strength properties than SWCNTs, resulting from the higher mechanical properties of MWCNTs. When TX10 was applied, the strength properties of SWCNT–cement composites were higher than those of MWCNT–cement composites, because TX10 was not suitable in dispersing MWCNTs.

3. When TX10 was applied, the compressive and flexural strengths of cement composites were greatly reduced compared to the case where no TX10 was used. In addition, the bulk density and mass of specimens were also reduced, from 2.102–2.141 to 1.884–1.998 g/cm3. The reduction of strength and bulk density could be attributed to the formation of foam during the specimen preparation process and the retardation of hydration caused by TX10.

4. Even though strength reduction of cement composites occurred when TX10 was used to disperse SWCNTs, when the effect of SWCNTs on strength properties of cement composites was studied at this stage, it was found that the general variation trend of compressive and flexural strengths was upward; the strength increased with the increase of the concentration of SWCNTs. Conversely to when no TX10 was added to specimens, the maximum compressive and flexural strengths occurred when the concentration of SWCNTs was 0.06 wt%.

5. SWCNTs affected the early hydration progress of cement. SWCNTs could produce high hydration rates at early age: at seven days of curing, the compressive and flexural strengths could reach over 90% of the 28-day strength.

6. The variation of compressive and flexural strengths was related to the mass (bulk density) of mortar specimens. They shared similar variation patterns: the strength of specimens increased or decreased as a function of the mass of specimens.

7. The flexural strength/mass of cubes ratio was close to compressive strength/mass of prisms ratio, with difference less than 10%. This relation could be used to estimate the strength of specimens.

For future work, TEM should be applied to study the failure modes of specimens and the effect mechanism of CNTs on hydration products of cement/CNTs composites.

**Author Contributions:** Conceptualization, J.K. and S.A.-S.; methodology, J.K. and S.A.-S.; software, J.K. and R.T.; validation, J.K. and S.A.-S.; formal analysis, J.K. and R.T.; investigation, J.K.; resources, S.A.-S.; data curation, J.K.; writing—original draft preparation, J.K.; writing—review and editing, J.K. and S.A.-S.; supervision, S.A.-S.; project administration, S.A.-S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** I thank Salam Al-Sabah for all his support throughout this project. I would also like to thank Derek Holmes for his assistance and for providing materials needed in the concrete laboratory in UCD. I must also mention Roger Théo who worked with me and helped me greatly in the laboratory.

**Conflicts of Interest:** The authors declare no conflict of interest.
