PSI - Issue 13

Zoi S. Metaxa et al. / Procedia Structural Integrity 13 (2018) 2011–2016 Z.S. Metaxa and S.K. Kourkoulis / Structural Integrity Procedia 00 (2018) 000 – 000

2013

3

Table 1. Properties of graphene nanoplatelets. Thickness (nm) Carbon content (wt.%)

Young’s Modulus (GPa)

Density (g/cm 3 )

Thermal conductivity (W/mK)

<100

99.5

2.25

~ 3000

~ 1060

2.2. Nanocomposites preparation

The main goal of this work is to identify an efficient method to uniformly distribute the aforementioned GnPs into a cementitious matrix made of Type II cement paste. As with all carbon-based nanomaterials, GnPs agglomerate due to Van der Waals forces and dispersing them homogeneously is a challenging task. Due to their hydrophobic nature their dispersion in aqueous suspensions is a challenge. Another parameter that needs to be taken into account is the compatibility of the dispersant agent with the cementitious matrix. Preliminary research has shown that typical dis persing agents, used for the carbon nanomaterials ’ distribution in polymers, downgrade the mechanical performance of the cementitious matrix. However, to achieve effective reinforcement, improving the mechanical and electrical performance of Type II cement paste, the GnPs have to be homogenously dispersed within the matrix. To achieve this, a method, similar to the one used for the dispersion of carbon nanofibers (CNFs) and multi-walled carbon nano tubes (MWCNTs) in Type I cement paste (Konsta-Gdoutos et al. (2010), Metaxa et al. (2012), Metaxa et al. (2013)), was employed. Treatment with a superplasticizer in conjunction with the application of ultrasonic energy were used to effectively disperse the GnPs in the mixing water. The facilitation of the superplasticizer as the GnPs dispersing agent, besides its common usage to improve the workability of cementitious materials, was examined. The super plasticizer used was a 3rd generation admixture having a chemical composition based on polycarboxylate ether. Initially, several solutions were prepared containing the mixing water and the superplasticizer at eight different concentrations (namely 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0 % by weight of cement). Then the GnP suspensions were prepared by mixing the 8 μm G nPs at a constant concentration of 0.1% by weight of cement into the aforementioned solutions. A tip ultrasonicator (running at 35% amplitude at room temperature for 15 s and stopping for 30 s so as to prevent the GnP suspensions from overheating) was used for ultrasonic processing of the GnP suspensions. The nanocomposites were prepared by mixing the cement (CEM Type II 32,5) with the dispersed GnP suspensions following the ASTM C305 using a standard mixer. All the nanocomposites prepared had a water to cement ratio of w/c=0.3 by weight. After mixing, the Type II cement paste nanocomposites were placed into prismatic molds and they were demoulded after one day. Then they were cured (28 days) in a water bath saturated with lime until testing. 2.3. Nanocomposites testing The electrical properties of the nanocomposites, and specifically the electrical resistivity, were evaluated using both the 2-wire and the 4-wire Ohms methods. For the 2-wire Ohms method, two surface contact points were placed at the bottom of the nanocomposites. The wires were placed using silver paste and silver paint following the method described by Metaxa (2016). For the 4-wire Ohms method, mesh electrodes covering the whole cross section were introduced inside the samples immediately after casting (Metaxa (2016)). Using an Agilent Data Acquisition / Data Logger Switch Unit both the electrical resistance and the time were recorded during the test. The mechanical performance of the nanocomposites was accessed by three-point bending tests with prismatic notched beam specimens at the age of 28 days, as shown in Fig. 1. The artificial notch had a depth of 6 mm and was

Fig. 1. Three-point bending test set up.