Cement Reinforcement by Nanotubes Roey Nadiv, Michael Shtein, Alva Peled, and Oren Regev
Abstract Loading a matrix with nano-sized particles such as nanotubes (carbon or tungsten di-sulfide) is expected to improve the mechanical properties of composite materials better than traditional (macroscopic) fillers due to extra-ordinary mechanical properties accompanied by high surface area. One of the major challenges towards achieving this goal is an effective dispersion of the as-produced aggregated nanotubes. In this work we demonstrate a novel dispersion method, facilitating the integration of individual nanotubes in cement paste matrix. We demonstrate the effectiveness of our nanotubes dispersion method by enhancing both flexural strength and compressive strength of cement paste using carbon and tungsten di-sulfide nanotubes. Finally, a comprehensive fractography study indicates that both types of nanotubes fail via pull-out mechanism with an intermediate state of bridging mechanism. Key words Nanotubes • Composite • Dispersion • Cement • Matrix
R. Nadiv (*) Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel e-mail:
[email protected] M. Shtein • O. Regev Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Department of Chemical Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel A. Peled Department of Structural Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel © Springer International Publishing Switzerland 2015 K. Sobolev, S.P. Shah (eds.), Nanotechnology in Construction, DOI 10.1007/978-3-319-17088-6_29
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Abbreviations CNT NT PC SEM TEM WS2NT
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Carbon nanotube Nanotube Plain cement paste Scanning electron microscope Transmission electron microscope Tungsten di-sulfide nanotube
Introduction
Cement, is characterized by a high compressive strength on one hand and a low tensile and flexural properties on the other [1, 2]. The latter properties are expected to be improved by loading nanofiller into the cement paste matrix. For cement nanocomposite (CNC) reinforcement purposes, carbon nanotube (CNT) is the most widely used nanofiller because of its extra-ordinary mechanical properties [3] and high surface area (~100–700 m2/g) [4]. Tungsten di-sulfide nanotube (WS2NT) is also a promising cement nanofiller [5], characterized by high aspect ratio (e.g., high surface area) along with attractive mechanical properties [6, 7], as manifested in epoxy composites [8]. The utilization of nanotubes (NT) (both CNT and WS2NT) in composite materials is often hindered by the NT's tendency to form aggregates. NT aggregates due to strong interfacial van der Waals interactions, hence reducing their effective surface area, which consequently leads to a decrease in the stress transfer between the matrix and the filler. Furthermore, NT aggregates act as stress concentrator and could initiate crack propagation. Therefore, developing a method to effectively disperse NTs is essential. In this study, we demonstrate a novel dispersion method of NTs in cement paste matrix. The research aims at reinforcing plain cement paste (PC) by individually dispersed NTs in order to enhance the flexural properties; while preserving or even improving other mechanical properties (e.g., compressive strength). A fractographic study reveals the reinforcing and failure mechanisms of NTs in CNC.
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Experimental
Multi-wall CNT (diameter = 10–20 nm, length = 10–30 μm, Cheaptubes), WS2NT (diameter = 30–100 nm, length = 1–10 μm, NanoMaterials), β-Lactoglobulin (CAS 9045-23-2, Sigma–Aldrich) and Pluronic F-127 (CAS 9003-11-6, Sigma–Aldrich) were used as received. NT dispersions in aqueous solution are prepared by sonication in the presence of dispersants (Table 1).
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Cement Reinforcement by Nanotubes Table 1 NT dispersion parameters
NT CNT
Dispersant Pluronic F-127
WS2NT
β-Lactoglobulin
Dispersant concentration (mg/ mL) 1.5 2.0
NT concentration (mg/mL) 2.0 6.0
Sonication (energy) (J) Tip sonication (5,040) Bath sonication (540)
Two sonication methods are used: Bath sonication (30 W, 32 kHz, Elma sonic model S10, Elma) and Tip sonication (500 W – 20 kHz, 20 % amplitude μtip, Q500, Qsonica) for CNT and WS2NT, respectively. Temperature is kept at 0 °C. Following sonication, to allow the precipitation of large aggregates, a phase separation by decantation is conducted and the NT's concentration in the supernatant is calculated [9]. The supernatant (exfoliated NTs) is freeze-dried (Lobanco Freezone 2.5) yielding a cotton-like powder of NTs wrapped with dispersant. The freeze-dried NTs are then mixed in water (water:cement ratio of 0.4) and bath-sonicated for 2 min. The cement (Portland cement CEM I 52.5 R, Nesher Israel cement enterprises) is gradually added and mechanically mixed for 4 min. The mixture is then casted and placed for 4 min inside a vibration machine to allow degassing. The CNC specimens are de-molded 24 h after casting and cured by immersion in water vessel at room temperature for 14 d. The NT dispersion quality is examined by Transmission electron microscope (TEM) (FEI Tecnai 12 G2 TWIN TEM). The flexural strength is determined by three point bending test (ASTM D790), using a prism-shaped specimens with dimensions of 8 × 8 × 60 mm3. The measurements are performed by LRX, LLOYD (capacity of 5 kN) using a constant extension rate of 0.5 mm/min. The compressive strength is determined by ASTM C109, using a 12 mm cube specimens. The measurements are performed by Instron 5982 (capacity of 100kN) using a constant extension rate of 2 mm/ min. Finally, the fractographic study is conducted on the specimen’s fractured surfaces using a high resolution scanning electron microscopy (SEM) (JEOL, JSM-7400 F).
3 3.1
Results and Discussion Dispersion Method
Achieving maximal mechanical properties enhancement depends on the dispersion quality of the as-received NT aggregates. Therefore, liquid dispersions of exfoliated NTs in water by sonication are stabilized by dispersants, which sterically prevents NT re-aggregation. Electron microscopy was used to monitor the dispersion quality throughout the preparation. The as-received NTs (Fig. 1a, b) are clearly aggregated. Following sonication and decantation, the supernatant phase contains individual NTs (Fig. 1c, d). Finally, their integration in the cement results in a uniformly dispersed NTs in the CNC (Fig. 1d, f).
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Fig. 1 SEM images of as-received aggregated (a) CNT and (b) WS2NT; TEM images of individually dispersed (c) CNT and (d) WS2NT following sonication and decantation (see text); SEM image of (e) 0.15 vol% CNT-based CNC and (f) 0.063 vol% WS2NT-based CNC fractured specimens, indicating well dispersed NTs (white arrows)
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Fig. 2 (a) Flexural strength and its enhancements (over PC) of CNC following 14 d curing as a function of NT concentration. The connecting lines are only guidance to the eyes. (b) Compressive strength of PC, 0.063 vol% WS2NT and 0.15 vol% CNT-based CNCs (enhancement over PC is indicated (red) for each CNC)
3.2
Mechanical Properties
The efficiency of NT reinforcement is also affected by the filler content. Previous studies indicate that the performance of composite materials is strongly affected by NT concentration [8, 10]. Therefore, we prepared CNCs with various NT vol% (CNT or WS2NT) and tested their flexural strength (Fig. 2a). At low NT vol%, the flexural strength enhancement is relatively mild, and the flexural strength increases with increasing NT vol% until it reaches the optimal concentration (0.15 and 0.063 %, for the CNT and WS2NT, respectively). At the optimal concentration, a significant flexural strength enhancement of 85 and 70 % is obtained for WS2NT and CNT, respectively. At these concentrations the compressive strength is also improved by 23 and 38 % for WS2NT and the CNT, respectively (Fig. 2b). These high mechanical properties enhancements are attributed to the efficiency of dispersion method yielding individually dispersed NT (Fig. 1e, f).
3.3
Fractographic Study
The NTs in the composite materials serve as crack propagation inhibitors (Bridging mechanism) [8, 11]. However, when the CNC experiences failure, the NT could fails either via its pull-out from the matrix or through its fracture. The former generally occurs in the case of relatively weak NT-matrix interfacial adhesion and the latter in the case of strong one [12]. By observing the CNC’s fractured surfaces, we indicate that the NTs indeed reinforce the CNC by bridging mechanism (Fig. 3a, b).
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Fig. 3 SEM images of NT-based CNC fractured surfaces at optimal NT concentration, indicating bridging mechanism of (a) CNT and (b) WS2NT; pull-out mechanism of (c) CNT and (d) WS2NT
Finally, at complete failure, the NTs undergo pullout mechanism, manifested by a substantial part of the NT protruding from the specimen surface (Fig. 3c, d). The pullout mechanism is expected considering the porous structure of cement paste, i.e., low interfacial area and hence weak interfacial bonding. Given these results and based on prior knowledge regarding NTs failure mechanism [8, 12], it is clear that in order to further increase the flexural strength it is vital to increase the NT-cement interfacial adhesion.
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Conclusions
We efficiently disperse two types NTs, namely, CNT and WS2NT. The effective dispersion method yields substantial flexural strength enhancement following 14 d. The compressive strength is also enhanced by the NT loading. Our results show that loading cement by CNTs offer higher compressive strength while WS2NT loading provides better flexural strength. An in-depth SEM fractography study indicates that the NTs reinforce the matrix by inhibiting crack propagation via bridging, and fail via pullout mechanism.
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