Elsevier

Construction and Building Materials

Volume 98, 15 November 2015, Pages 112-118
Construction and Building Materials

WS2 nanotube – Reinforced cement: Dispersion matters

https://doi.org/10.1016/j.conbuildmat.2015.08.085Get rights and content

Highlights

  • Tungsten di-Sulfide NanoTubes (WS2NTs) enhance the cement flexural strength by 74%.

  • We developed a method to disperse WS2NT individually in cement paste.

  • Optimal enhancement occurs at extremely low WS2NT concentration (0.15 wt%).

  • WS2NTs inhibit crack propagation by bridging, and fail via pullout mechanism.

Abstract

Nanotubes are considered as promising nano-reinforcement in cement-based materials. The main challenge towards achieving a significant enhancement in cement properties is an effective dispersion of the agglomerated nanotubes. In this paper, we demonstrate a novel dispersion method of Tungsten di-Sulfide NanoTubes (WS2NTs) that results in substantial flexural and compressive strength enhancements at optimal nanotube concentration as low as 0.15 wt%. The reinforcement by WS2NTs remains significant after a variety of curing processes, suggesting a genuine nanoscale reinforcing effect. Finally, by employing a comprehensive fractography we found that the WS2NTs inhibit crack propagation by bridging with a pullout failure mechanism.

Introduction

Cement, one of the most widely used composite materials, is characterized by high compressive strength on the one hand and by low tensile, flexural and fracture toughness properties on the other [1], [2]. The latter properties are expected to improve by loading appropriate Nano-Materials (NMs) into the cement paste matrix [3].

A wide range of NMs were used in Cement Nano-Composites (CNC), including nano-silica [4], [5], [6], nano-titanium dioxide [4], [7], [8], carbon nanofiber [9], [10] and Carbon NanoTube (CNT) [10], [11], [12], [13], [14], [15], [16], [17], [18]. Tungsten di-Sulfide NanoTube (WS2NT) [19], [20], which possesses attractive mechanical and geometrical properties (Table 1) [21], [22], may be a promising alternative for cement nano-reinforcement.

NMs tend to agglomerate due to strong interfacial van der Waals interactions [23], [24], [25], or to entangele due to their waved structure (e.g. CNT) [26], [27]. Therefore, their effective surface area is reduced, leading to a decrease in stress transfer between the matrix and the NMs. Furthermore, NM agglomerate act as stress concentrators and may initiate crack propagation [28]. As such, it is essential to develop a dispersion method yielding individual NMs. WS2NTs also tend to form agglomerates, however, their straight shape does not allow entanglements (in contrast to CNT) and, consequently, simplifies the dispersion process.

NMs may serve as nucleation sites in the cement hydration reaction and accelerate its kinetics [29], [30], [31]. Such acceleration insinuates a genuine, although short-term, mechanical properties enhancement in the early stages of the hydration. However, after a longer hydration time (>10 d), this increase vanishes almost completely [32]. As such, in order to observe signs of long-term nano-reinforcement, the mechanical properties should be evaluated after a longer period. Alternatively, hydration processes may be accelerated by heating [1] and the outcomes of this process predict the long-term mechanical properties of the CNC.

Recently, we have shown that WS2NTs can potentially serve as nano-reinforcement in cement systems [33]. We now aim at expanding the study on the benefits of incorporating this nano-reinforcement. In this paper, we demonstrate a novel dispersion method, facilitating the integration of only individually dispersed WS2NTs. The mechanical properties of the produced CNCs are measured and compared to the Plain Cement paste (PC) in a variety of curing ages, hence evaluating both WS2NT’s short and long term reinforcing effects. Concerning our methodology, we distinguish between structurally similar cement ettringites and the WS2NTs by employing both quantitative and qualitative techniques to allow an accurate evaluation of both the toughening and failure mechanisms of WS2NT-based CNCs.

Section snippets

Materials

Portland cement CEM I 52.5 R (Nesher Israel cement enterprises Ltd.), protein-based dispersant β-lactoglobulin (⩾90%, Sigma–Aldrich) and WS2NT (batch no. TWPO-MB023, received as a gift from NanoMaterials Ltd.), were used as received.

WS2NT dispersion [34]

WS2NTs are mixed with deionized water (6.0 mg/ml) containing β-lactoglobulin (2.0 mg/ml). The solution is bath-sonicated (Elma, model S10; 30 W, 37 kHz, Singen) for 30 min (540 J). The vial (20 ml) is placed at the center of the sonicator and kept at 0 °C during the whole

Dispersion method and characterization

As stated above, the major challenge in achieving a significant properties enhancement of CNC by means of nanotubes (NTs) nano-reinforcement is their efficient dispersion. Therefore, a novel dispersion method, based on sonication, decantation and lyophilization is implemented (Section 2.2), yielding only individually dispersed WS2NTs in the cement matrix.

The WS2NTs’ structural integrity and dispersion quality were monitored by electron microscopy throughout the dispersion procedure to verify

Conclusions

In this paper, we employed WS2NTs to reinforce cement systems. Using a novel dispersion method, we were able to integrate only individually dispersed WS2NTs, achieving substantial mechanical properties enhancements (over PC) following a variety of hydration processes, suggesting genuine nanoscale reinforcement by WS2NTs. The loading of WS2NTs also enhances the compressive strength of the CNC. These properties enhancements were obtained by adding only a very small concentration of WS2NT (0.15 

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