Multi-scale mechanical and fracture characteristics and early-age strain capacity of high performance carbon nanotube/cement nanocomposites
Introduction
Advanced technological aspects of cement based materials have recently focused on developing high-performance cementitious composites, which exhibit high compressive strengths. Such composites however, exhibit also extremely brittle failure, low tensile capacity and appear sensitive to early age microcracking as a result of volumetric changes due to high autogenous shrinkage stresses. These characteristics of cement based materials are serious shortcomings that not only impose constrains in structural design, but also affect the long term durability of structures. To overcome the aforementioned disadvantages reinforcement of cementitious materials is typically provided at the millimeter and/or the micro scale using macrofibers and microfibers, respectively. Cementitious matrices however, exhibit flaws at the nanoscale, where traditional reinforcement is not effective.
Carbon nanotubes (CNTs) present several distinct advantages as a reinforcing material for high strength/performance cementitious composites as compared to more traditional fibers. First, they exhibit significant greater strength and stiffness [1], [2] than conventional fibers, which should improve overall mechanical behavior. Second, their higher aspect ratio is expected to effectively arrest the nanocracks and demand significantly higher energy for crack propagation. Thirdly, provided that CNTs are uniformly dispersed, and due to their nanoscale diameter, fiber spacing is reduced.
Few attempts have been made to add CNTs as reinforcement in cementitious matrices. Makar et al. [3], [4] investigated the reinforcing effect of 2.0 wt.% CNTs in cement using SEM and Vickers hardness measurements. The results obtained indicated that CNTs may affect the early hydration progress, producing higher hydration rates. Li et al. [5], [6] employed a carboxylation procedure to improve the bonding between 0.5 wt.% MWCNTs and cement matrix and obtained a 25% increase in flexural strength and a 19% increase in compressive strength. Saez de Ibarra et al. [7] measured the stiffness of cement samples reinforced with MWCNTs and SWCNTs using an AFM nanoindentation technique and reported modest gains in the Young’s modulus. More recently, Cwirzen et al. [8], [9] investigated the mechanical properties of cement matrices reinforced with different concentrations of MWCNTs. The results showed no increase in the flexural strength and a slight increase in compressive strength of the cement paste with the addition of CNTs. More recently, research on the reinforcing effect of MWCNTs in cement matrix (w/c = 0.5) indicated that CNTs can strongly reinforce the cement paste matrix by increasing the flexural strength and the Young’s modulus of plain cement paste by 25% and 50%, respectively [10], [11].
In this study, the development of high-performance nanocomposites reinforced with multiwall carbon nanotubes was investigated. The two major drawbacks associated with the incorporation of CNTs in cement based materials are poor dispersion and cost. To achieve good reinforcement, it is critical to have uniform dispersion of CNTs within the matrix [12]. However, since CNTs tend to adhere together due to Van der Waals forces, are particularly difficult to separate [13]. In this experimental work, effective dispersion of MWCNTs in the mixing water was achieved by using a simple, one step technique involving the application of ultrasonic energy and the use of a commercially available surfactant, commonly used in the development of advanced high performance cement based materials [10], [11], [14]. Fracture mechanics tests were performed to investigate the effect of MWCNTs aspect ratio in conjunction with the effect of the concentration of MWCNTs on the fracture characteristics of the nanocomposites. A determination of the nanomechanical properties and the porosity of the composites was carried out through nanoindentation experiments. Finally, since nanoindentation results implied significant changes in the nanostructure of the composites, autogenous shrinkage experiments were conducted to determine the effect of the MWCNTs on the early strain capacity of the cementitious matrix.
Section snippets
Preparation of the MWCNTs nanocomposites
Two types of commercially available purified multiwall carbon nanotubes (MWCNTs), designated as short and long, were used. The MWCNTs were produced by catalytic chemical vapor deposition (CCVD) of carbon and were used untreated as received. The MWCNTs had the same diameter, but different aspect ratios, close to 700 for the short and 1600 for the long MWCNTs. The characteristic properties of the MWCNTs used are shown in Table 1. The cementitious material used was Type I ordinary Portland cement
Mechanical performance
The fracture mechanics test results of the average flexural strength of cement paste samples reinforced with short MWCNTs at amounts of 0.048 wt.%, 0.08 wt.% and 0.10 wt.% by weight of cement at the age of 3, 7 and 28 days are presented in Fig. 4. In all cases, the samples reinforced with MWCNTs exhibit higher flexural strength than plain cement paste. Samples reinforced with 0.08 wt.% short MWCNTs outperformed all other mixes, exhibiting the largest increase in flexural strength. Generally, the
Conclusions
The development of high-performance cementitious nanocomposites reinforced with multiwall carbon nanotubes was studied. It was found that small amounts of effectively dispersed MWCNTs (0.025–0.08 wt.% of cement) can significantly increase the strength and the stiffness of the cementitious matrix. In particular, lower amounts of long MWCNTs (0.025–0.048 wt.%) provide effective reinforcement, while higher amounts (close to 0.08 wt.%) of short MWCNTs are required to achieve the same level of
Acknowledgements
The authors would like to acknowledge the financial support from the Infrastructure Technology Institute at Northwestern University under Grant DTRT06-G-0015/M1. The nanoindentation experiments were carried out in the NIFTI facility of NUANCE center at Northwestern University.
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