A Review on Nanomaterial Dispersion, Microstructure, and Mechanical Properties of Carbon Nanotube and Nanofiber Reinforced Cementitious Composites

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2013, Article ID 710175, 19 pages http://dx.doi.org/10.1155/2013/710175 Review Article A Review on Nanomaterial Dispersion, Microstructure, and Mechanical Properties of Carbon Nanotube and Nanofiber Reinforced Cementitious Composites Shama Parveen,1 Sohel Rana,1 and Raul Fangueiro1,2 1 2 Fibrous Materials Research Group (FMRG), School of Engineering, University of Minho, 4800-058 Guimaraes, Portugal Department of Civil Engineering, University of Minho, 4800-058 Guimaraes, Portugal Correspondence should be addressed to Sohel Rana; Received 11 March 2013; Accepted 28 May 2013 Academic Editor: Tianxi Liu Copyright © 2013 Shama Parveen et al. This 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. Excellent mechanical, thermal, and electrical properties of carbon nanotubes (CNTs) and nanofibers (CNFs) have motivated the development of advanced nanocomposites with outstanding and multifunctional properties. After achieving a considerable success in utilizing these unique materials in various polymeric matrices, recently tremendous interest is also being noticed on developing CNT and CNF reinforced cement-based composites. However, the problems related to nanomaterial dispersion also exist in case of cementitious composites, impairing successful transfer of nanomaterials’ properties into the composites. Performance of cementitious composites also depends on their microstructure which is again strongly influenced by the presence of nanomaterials. In this context, the present paper reports a critical review of recent literature on the various strategies for dispersing CNTs and CNFs within cementitious matrices and the microstructure and mechanical properties of resulting nanocomposites. 1. Introduction Civil infrastructures are the building blocks of any country’s highway structures, bridges, pavements, runways for airport, and so forth, and concrete is the primary material for their construction. Concrete generally consists of Ordinary Portland Cement (OPC, which is known as the principal binding agent), coarse aggregates, and fillers such as sand, admixtures, and water. Cementitious materials are characterized by quasi-brittle behaviour and are susceptible to cracking. The cracking process within concrete begins with isolated nanocracks, which then conjoin to form microcracks and in turn macrocracks. Reinforcement is required because of this brittle nature of concrete, and as reinforcements, polymeric fibers as well as glass and carbon fibers were used during the 1970s, 80s, and 90s, respectively [1]. Recently, the use of microfiber reinforcements has led to significant improvement in the mechanical properties of cement-based materials by delaying the transformation of microcracks into macroforms, but they could not stop the crack growth. This fact encouraged the use of nanosize fibers or particles for concrete reinforcement in order to prevent the transformation of nanocracks into microcracks [2, 3]. Nanoparticle addition to cement paste was found to improve mechanical, chemical, and thermal properties of cementitious matrix. There are various types of nanoparticles, especially SiO2 and Fe2 O3 , which when incorporated into cement led to considerable improvement in the compressive strength [4– 9]. Nanosized TiO2 has been added to accelerate the rate of hydration and increase the degree of hydration [10]. Moreover, the photocatalytic characteristic of TiO2 helped to remove the organic pollutants from concrete surfaces, which were directly exposed to UV radiation [11]. Carbon nanomaterials present a large group of functional materials with exceptional physical properties. Extensive research endeavors over the last few years demonstrated the application potential of various carbon nanomaterials, mainly carbon nanofiber (CNF) and carbon nanotube (CNT), in polymeric matrices. This fact has motivated the scientists and researchers worldwide to use these nanomaterials in concrete as well, in order to utilize their extraordinary mechanical, electrical, and thermal properties [12, 13]. In addition to 2 Journal of Nanomaterials that, in nanometer length scale, CNFs and CNTs offer the possibility to restrict the formation as well as growth of nanocracks within concrete, thus creating a new generation of crack-free materials. So, concrete reinforcement using carbon nanomaterials is a rapidly growing research area in recent times. However, there exists a large difference in the structure and chemistry between a polymeric and a cementitious, matrix, and, therefore, a great deal of research activities is being directed towards understanding the interaction between these nanomaterials and cementitious matrices for their successful application. 2. Structure of Cement A dry portion of Portland cement is composed of 63% calcium oxide, 20% silica, 6% alumina, 3% iron (III) oxide, and small amount of other materials including some impurities. These materials when react with water cause an exothermic reaction forming a mineral glue (known as “C-S-H” gel), calcium hydroxide, ettringite, monosulfate, unhydrated particles, and air voids. Molecular structure of CS-H gel was not fully understood till recent past, but some researchers in Massachusetts Institute of Technology (MIT, USA) [14] recently proposed a structure, and according to that, cement hydrate consists of a long chain silica tetrahedral and calcium oxide in long range distances, where water causes an intralayer distortion in otherwise regular geometry (Figure 1). The distortion in the structure due to addition of water makes the cement hydrate robust. The density of C-S-H has been determined as 2.6 g/cc [15], and the elastic modulus of different cementitious phases were determined as follows [16]: 35 MPa for the Ca(OH)2 phase, 26 and 16 MPa for high and low stiffness C-S-H, respectively, and 10 MPa for the porous phase. One of the major drawbacks of cement structure is its proneness towards crack formation and degradation. The amorphous phase of cement, that is, C-S-H gel, is itself a nanomaterial, and, therefore, the degradation mechanisms within concrete start at nanoscale, spreading then to micro- and macroscales. Degradation of concrete can be due to physical reasons such as abrasion and erosion, freeze thaw cycles, leaching and efflorescence, drying shrinkage, and so forth or chemical reasons such as aggregate-paste reaction, sulfate and acid attack, carbonation, and so forth [17–22]. 3. Carbon Nanomaterials After the discovery of buckyball (a ball-like molecule made of pure carbon atoms) in 1985 by Kroto et al. [23], a tubular form of carbon was reported by Iijima [24] in 1991 and named carbon nanotubes (CNTs). These nanotubes (called multiwalled carbon nanotubes or MWCNTs) consisted of up to several (...truncated)