Research Article Effect of Nanotube Geometry on the Strength and Dispersion of CNT-Cement Composites Mohamed O. Mohsen, 1 Ramzi Taha, 1 Ala Abu Taqa, 1 Nasser Al-Nuaimi, 1 Rashid Abu Al-Rub, 2 and Khaldoon A. Bani-Hani 3 1 Civil and Architectural Engineering Department, Qatar University, P.O. Box 2713, Doha, Qatar 2 Mechanical and Materials Engineering Department, Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, UAE 3 Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, Jordan Correspondence should be addressed to Mohamed O. Mohsen; 200202128@student.qu.edu.qa Received 2 April 2017; Revised 12 May 2017; Accepted 21 June 2017; Published 27 July 2017 Academic Editor: Andrew R. Barron Copyright © 2017 Mohamed O. Mohsen et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Tis study investigated the efect of multiwalled carbon nanotubes’ geometry on the dispersion and strength of cement composites. Mixes with CNTs contents ranging from 0.03 to 0.25% were prepared and tested using CNTs’ diameters of 10–20, 20–30, and 30–50 nm and lengths of 0.5–2 and 10–30 m. Te samples’ microstructures were then examined using a scanning electron microscope. Afer that, Response Surface Methodologies techniques were implemented to determine the strength functions’ response to the aspect ratio and weight fraction variables. Te results showed that CNTs of smaller diameters and longer lengths resulted in the highest strength values in batches having CNT contents of up to 0.15%. At 0.25% CNTs, the results showed that those CNTs of shorter lengths attained higher strengths than those of longer ones. SEM images showed better dispersion properties at lower CNTs’ contents regardless of their geometry. Te analysis also indicated that short CNTs of 0.5–2 m length have better dispersion properties compared to long ones of 10–30 m length. Te RSM model confrmed this fnding. Furthermore, the model showed that the CNTs with the lowest aspect ratio of 83 and highest weight fraction of 0.25% provided the highest strength response among all batches. 1. Introduction Carbon nanotubes (CNTs) are molecular-scale shells of graphene placed in hexagonal arrays. Tey can be comprised of single, double, or multiple shells. CNTs have extraordinary properties such as high strength and thermal and electrical conductivity [1–4]. Tese properties raised the interest of many concrete and cementitious composites researchers to incorporate the nanoflaments into cement pastes and mor- tars to develop their properties [5–19]. However, up to date, it was certainly shown that fndings reported in the literature were contradictory. Some results confrmed an optimization of the tested composites mechanical properties, while others showed no improvement or, in some cases, there was a reduction in the properties. Most studies related the efects of CNTs’ addition on the mechanical properties of cement materials to the diferent selections of carbon nanoflaments’ aspect ratio and weight fraction, which in return will afect the dispersion of the flaments within the solution and the matrix. Te unique high aspect ratio (length to diameter ratio) property of CNTs provides a high surface area to volume ratio that allows for additional contact between the tubes and the adjoining hydration products. However, CNTs of very high aspect ratios are harder to disperse than CNTs of low aspect ratios [20]. Te efect of CNTs’ aspect ratio on the mechanical properties of cement paste was rarely inves- tigated. Konsta-Gdoutos et al. [21] conducted a research on the efects of MWCNTs’ content and aspect ratio on both the mechanical properties and fresh paste properties of cement. Te study samples were tested on the 3rd, 7th, and 28th days. Te CNTs used were long CNTs with an aspect ratio of 1600 and short CNTs with an aspect ratio of 700. Te results showed that low quantities of long CNTs between 0.025 and 0.048% of cement improved the mechanical properties of Hindawi Journal of Nanomaterials Volume 2017, Article ID 6927416, 15 pages https://doi.org/10.1155/2017/6927416