Keywords

1 Introduction

Partially-unzipped multiwalled carbon nanotubes (PUCNTs) are synthesized by longitudinal unzipping of the outer walls of multiwalled carbon nanotubes (MWCNTs) [1, 2], resulting in a portion having a morphology similar to that of graphene nanoplatelets (GNPs). MWCNTs are characterized by a relatively high aspect ratio, which may be beneficial when using these nanoparticles as reinforcement in cement composites [3]. Mechanical enhancements have also been reported for nano-amended cement composites with GNP concentrations between 0.03% [4] and 0.5% [5] in weight of cement (wt%). Related mechanisms may be associated with improved cement hydration through the presence of carboxyl and hydroxyl functional groups on the surface of oxidized MWCNTs and GNPs [6, 7], with a reduction in total porosity [8, 9], pore-size refinement [8, 10], and densification of the cement matrix [11].

PUCNTs have appealing properties compared with their fully-unzipped counterparts; elongated strips of graphene with more functional group-edge sites compared with MWCNTs or GNPs, but also a smaller aspect ratio [12]. Partial unzipping of the outer walls of MWCNTs results in a radical increase in the open surface area [12], and oxidation produces a relatively larger amount of functional groups [1], thus enhancing the chemical affinity with the cement matrix. Through this process, a few outer layers of a given MWCNT are partially unzipped without affecting the inner core walls, resulting in a hybrid graphitic structure together with the inner (intact) carbon nanotubes [13]. Therefore, the resulting PUCNTs exhibit: (a) larger surface area and greater functionality than their precursor MWCNTs, which may enhance dispersibility in aqueous solutions and in the cement matrix, and (b) a relatively high aspect ratio, which may enhance the mechanical properties of the cement composite.

We present a feasibility study of cement paste amended with low concentrations of oxidized PUCNTs, namely, 0.001 and 0.005 wt%, which is one order of magnitude smaller than successful lower-bound concentrations reported in the literature for MWCNTs and GNPs.

2 Methods

2.1 PUCNT Aqueous Suspensions

The PUCNTs were prepared at Savannah River National Laboratory (Aiken, SC, USA) using an oxidative unzipping process modified from Kosynkin et al. [14]. Defects in the pristine MWCNTs were created by soaking in concentrated sulfuric acid and o-phosphoric acid, with potassium permanganate serving as the oxidant. By controlling the amount of oxidant, heating time, and soaking time, the PUCNTs shown in Fig. 1a were produced, and then suspended in deionized (DI) water. Stock solution of 3 g/L of these PUCNTs in DI water was diluted with additional DI water to prepare 0.02 and 0.1 g/L suspensions, to be used for the manufacturing of cement paste specimens with PUCNT concentrations of 0.001 wt% and 0.005 wt%, respectively. A small amount of NaOH (<0.01 mol/L) was added to the suspensions to reach an approximate pH of 12, thereby enhancing the stability of acid-treated (i.e., low pH) PUCNTs in water. The suspensions were ultrasonicated for 15 min using an ultrasonic bath sonicator (model CPX 2008, Branson Ultrasonics Corp., CT, USA).

Fig. 1
2 images. A scanning electron microscopy image of the morphology of oxidative unzipping of multiwalled carbon nanotubes in a cement paste specimen indicates P U N C Ts. Two photographs of the cement paste specimens manufactured with P U C N Ts in concentrations of 0.001 and 0.005 weight percentages.

Partially-unzipped multiwalled carbon nanotubes (PUCNTs) used in cement paste specimens: a scanning electron microscopy image showing PUCNTs resulting from oxidative unzipping of multiwalled carbon nanotubes; b aqueous suspensions

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2.2 Cement Paste Specimens

Prismatic specimens with dimensions 25 × 25 × 76 mm were prepared per ASTM C305 [15] using Type I ordinary Portland cement (OPC) and PUCNT aqueous suspensions, with a water-to-cement ratio of 0.5 (in weight). Plain cement paste specimens to be used as the benchmark were also cast using Type I OPC and DI water. All specimens were cast in acrylic molds from which they were removed after 24 h, and then placed in saturated lime water for 28 days.

2.3 Procedure

The dispersibility of the functionalized PUCNTs in DI water solutions was quantitatively assessed via dynamic light scattering (DLS) testing using a particle and molecular size analyzer (model Zetasizer nano ZS, Malvern Panalytical Ltd.).

For each PUCNT concentration, a suspension sample of 1 mL was used to measure the zeta potential. Each measurement was repeated 10 times to assess the variability of the results. Representative aqueous suspensions used for the manufacturing of cement paste specimens with PUCNTs in concentrations of 0.001 and 0.005 wt% are shown in Fig. 1b.

Uniaxial compression tests were performed on the 28 day cured prism specimens using a servo-hydraulic loading frame. The load was applied in displacement-control mode at a rate of 0.3 mm/min and was measured using a 20-kip load cell. Thin polytetrafluoroethylene inserts were placed between the specimen surfaces and the loading platens to minimize friction. A representative test setup is shown in Fig. 2a. Four specimens were tested for each PUCNT concentration (0, 0.001 and 0.005 wt%).

Fig. 2
An image and a graph. A photograph of the setup of a cement paste prism specimen on the uniaxial compression testing machine. A histogram with error bars of compressive strength in megapascals for 0, 0.001, and 0.005 weight percentages. The compressive strength of 0.005 weight percentage is high, and 0 weight percentage is low.

Uniaxial compression testing of cement paste prism specimens: a test setup, b summary of results (histogram bars and error bars indicate mean and standard deviation)

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The fractured specimens were used for scanning electron microscopy (SEM) analysis to investigate the incorporation and dispersion of PUCNTs in the cement paste structure. Specifically, SEM micrographs were used to visually assess the presence of undesirable PUCNT agglomerates as a result of ineffective dispersion. To this end, specimens’ fragments <5 mm in length in any direction were used. Following the compression tests, these samples were extracted from the prisms, air-dried for 24 h, and placed in a vacuum-suction chamber for 1 h to remove the excess moisture. The samples were gold-spattered before being examined under a SEM (Ultraplus Thermal Field Emission Scanning Electron Microscope, Zeiss).

3 Results and Discussion

3.1 Dispersibility in Aqueous Suspension

The effective dispersion of PUCNTs in the aqueous suspensions was assessed by zeta potential measurements made as part of the DLS tests. The zeta potential values obtained were –39.5 ± 1.30 mV and –39.6 ± 1.07 mV for the suspensions used to manufacture cement paste specimens with 0.001 wt% and 0.005 wt% of PUCNT concentration, respectively. Because suspensions characterized by a zeta potential smaller than –30 mV are considered stable [16], and also based on visual inspection (Fig. 1b), it was concluded that the oxidized PUCNTs were effectively dispersed in the aqueous solutions used in this research.

3.2 Compressive Strength

The mean and standard deviation of the 28 day compressive strength were 32.0 ± 3.10 MPa, 34.7 ± 1 0.93 MPa, and 41.4 ± 7.4 MPa for cement paste with PUCNT concentrations of 0, 0.001 and 0.005 wt%, as summarized in Fig. 2b.

Our results showed that the incorporation of very small amounts of PUCNTs resulted in significant compressive strength enhancement, namely 29% on average for a concentration of 0.005 wt%, compared with plain cement paste.

Evidence collected through SEM image analysis was used to better understand plausible strengthening mechanisms. In fact, the micrographs shown in Figs. 3, 4 and 5 suggest that the incorporation of PUCNTs resulted in preferential formation of amorphous (C–S–H) and crystalline (calcium hydroxide) cement hydrates (Figs. 4 and 5), with a less porous structure than plain cement paste (Fig. 3).

Fig. 3
A scanning electron microscopy image indicates the morphology of a plain cement paste structure.

Scanning electron microscopy image of plain cement paste structure

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Fig. 4
A scanning electron microscopy image of the formation of C-S-H and incorporated P U N C Ts on the 0.001 weight percentage P U C N T-amended cement paste structure.

Scanning electron microscopy image of 0.001 wt% PUCNT-amended cement paste structure. PUCNTs, partially-unzipped multiwalled carbon nanotubes

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Fig. 5
A scanning electron microscopy image of the formation of C-S-H and incorporated P U N C Ts on the 0.005 weight percentage P U C N T-amended cement paste structure.

Scanning electron microscopy image of 0.005 wt% PUCNT-amended cement paste structure. PUCNTs, partially-unzipped multiwalled carbon nanotubes

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In addition, the lack of visible clusters of PUCNTs suggested that the methodology used will produce cement composites with well-dispersed and chemically-affine PUCNTs, and thus with a meso-scale homogeneity comparable to or better than that of plain cement paste. However, the significantly larger standard deviation of the compressive strength results for the 0.005 wt% concentration suggested that larger concentrations of PUCNTs may be more difficult to disperse in the cement matrix.

4 Conclusions

The following conclusions are presented.

  • It is feasible to incorporate well-dispersed and chemically-affine PUCNTs to enhance the physicomechanical properties of cement paste. The unzipping process is similar in chemical approach (and thus cost) to the well-known Hummer’s method used to generate graphite oxide.

  • Due to the morphology and high graphene-edge content of PUCNTs, dispersibility in aqueous solutions was easily achieved.

  • A significant increase in the compressive strength of cement paste occurred with a PUCNT concentration of 0.005 wt%, which is one order of magnitude smaller than lower-bound concentrations reported in the archival literature for MWCNT- and GNP-amended cement pastes.

  • Microscopic inspection of the cement paste nano- and micro-structures suggests that plausible contributing mechanisms for strength enhancement include the preferential and well-distributed formation of cement hydrates in the vicinity of PUCNTs—also resulting in a less porous structure—compared with plain cement paste.