Influence of carbon nanofiber clustering on the chemo-mechanical behavior of cement pastes
Introduction
Carbon nanotubes/nanofibers (CNTs/CNFs) possess a number of unique properties, including exceptional mechanical properties and high aspect ratios, that make them attractive for reinforcement of cement-based materials at the nano-level [1], [2], [3]. The contribution of CNTs/CNFs to the reinforcement is strongly dependent upon their distribution and arrangement in the cement matrix – CNT/CNF clustering has proven to be a critical factor in influencing the material mechanical and electrical properties [4]. Despite much effort to homogeneously disperse CNTs/CNFs in the cement matrix [5], [6], [7], there is still evidence of the presence of localized sub-micro and micro-scale clusters and inhomogeneous distribution of CNTs/CNFs [5], [7], [8], [9], [10], [11], [12]. While the occurrence of CNT/CNF clustering may constitute a defect causing a loss of mechanical properties, it may not always be undesirable. For example, it has been shown for polymer composites to enhance certain mechanical properties [13] and to favor the formation of a percolating network for electrical conductivity [14], [15], [16]. During their service life, cement-based materials are subjected to a wide variety of weathering induced degradation. Given that perfect dispersion at the individual fiber level may not be achievable and that controlled clustering may even be desirable for certain applications, it is therefore critical to understand the effect of CNT/CNF clustering on the durability of cement-based materials exposed to aggressive environments.
Decalcification is closely associated with various types of concrete degradation [17], including during sulfate attack and leaching by exposure to neutral or acidic waters. It is a complex dissolution-diffusion process that involves the removal of calcium from the cement paste and results in both chemical and physical degradation. A loss of cohesion of the cement paste, an increase in porosity, and a decrease in the mechanical properties are all common manifestations of decalcification [17], [18]. While the addition of CNFs is anticipated to improve the cement paste durability through pore refinement [19], [20] and by increasing the cohesion of the paste, the influence of a non-uniform dispersion of the CNFs, specifically CNF clustering on the cement paste during exposure to a decalcifying environment, is unclear. Yet, CNF clustering is expected to introduce a secondary porosity into the cement paste, which could significantly impact the chemical and mechanical property evolution during decalcification. This paper focuses on CNFs in Portland cement paste and reports on the effects of CNF clustering on the chemo-mechanical behavior of cement pastes subjected to a decalcifying environment. Portland cement pastes with and without CNFs were exposed to a concentrated solution of ammonium nitrate to accelerate decalcification. Microstructural changes and evolution of the porosity were examined as a function of exposure duration. Changes in the flexural response of the paste with CNFs were studied and reviewed in relation to CNF clustering and microstructural evolution.
Section snippets
Cement paste preparation
Commercially available, vapor grown, Pyrograf® –III PR-19-LHT CNFs (Applied Sciences, Inc., Cedarville, Ohio) were used as received from the manufacturer. Type I/II Portland cement (Lafarge, Nashville, Tennessee) was used as the cementitious material, and a polycarboxylate-based high range water reducer (HRWR), Glenium 7500 (BASF, Ludwigshafen, Germany), was used to promote the dispersion of the CNFs in the cement paste [5], [7], [21]. Two types of paste were prepared: a plain Portland cement
CNF dispersion state and cluster characteristics
Despite a uniform dispersion of the CNFs in solution before mixing, a large number of clusters of varied size and shape were found throughout the cement paste (Fig. 1). Statistical analysis of the clusters in a 2D cross-section indicated that the cluster size ranged from 125 μm up to 1500 μm (max Feret diameter). Greater areal coverage (13.0%) and larger size clusters (Fig. 2a) were seen in the first 2 mm from the top edge (as poured) of the paste compared to the interior (1.3% areal coverage),
Conclusions
The chemo-mechanical behavior of cement pastes with and without CNFs exposed to a decalcifying environment was investigated. Results showed a strong coupling between decalcification, CNF clustering, microstructural evolution, and the flexural properties of the cement paste. The following conclusions could be drawn:
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Before decalcification, the CNF clusters showed a strong presence of CH plates intermixed with the CNFs and were surrounded by an interphase region of thickness independent of the
Acknowledgments
This research was supported by the National Science Foundation under NSF CAREER CMMI 0547024 and NSF GRFP DGE-0946822.
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