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

2012

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properties they are one of the most promising carbon nanomaterials to develop multifunctional nanocomposites with improved mechanical and electrical properties. They consist of few layers of single graphene sheets that are placed on top of each other forming a platelet morphology and demonstrating high stiffness (up to 1 TPa), high ultimate strength (~130 GPa), high electrical conductivity (~6000 S/cm), large specific surface area and, compared to carbon nanotubes, their electrical properties are independent of their chirality (Du et al. (2008), Lee et al. (2008), Pumera (2010)). A crucial factor that strongly influences the reinforcing efficiency of the GnPs and the ultimate performance of the resulting nanocomposites is their dispersion state. Because of the van der Waals interactions the as-produced GnPs have the tendency to form agglomerates. To successfully take advantage of the GnPs ’ exceptional properties and produce nanocomposites with multifunctional characteristics, they need to be homogeneously dispersed within the matrix. 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. One main approach, that has been successfully applied to uniformly disperse CNTs and carbon nanofibers (CNFs) in Type I cement paste is based on the use of a dispersing agent, i.e. a superplasticizer (SP) compatible with the cement-based matrix, and the application of ultrasonic energy (Konsta-Gdoutos et al. (2010), Metaxa et al. (2012), Metaxa et al. (2013)). The aim of this study is to investigate whether a similar method can be facilitated to homogeneously distribute graphene nanoplatelets in a Type II cement paste. Initially, dispersion of the GnPs was attempted though functionalization by facilitating ammonium persulfate, (NH 4 ) 2 S 2 O 8 and sulfuric acid, H 2 SO 4 (Zohhadi et al. (2012)) as well as nitric acid (Alkhateb et al. 2013, Tong et al. (2016)). The goal was to lessen the GnPs hydrophobic nature and aid their distribution in the water. It is well known, however, that the treatment- functionalization with acids introduces defects on the carbon nanomaterials’ surface down grading their mechanical and electrical response (Zohhadi et al. (2012), Coleman et al. (2008)), questioning whether this method is suitable for use in cementitious matrices. Following, a naphthalene sulfonate-based superplasticizer, that operates based on the mechanism of electrostatic repulsion, was employed in conjunction with ultra-sonication for exfoliation and dispersion of the GnPs (Le et al. (2014), Du and Pang (2015)). The mechanism of electrostatic repulsion is known to be less effective compared to other water reducing agents that disperse the cement particles through steric hindrance (Metaxa (2015)). Therefore, treatment with polycarboxylate based superplasticizers was pro posed (Liu et al. (2016), Metaxa (2015), Pisello et al. (2017), Tragazikis et al. (2018)). It was found that even if all the polycarboxylate based superplasticizers work based on the steric hindrance mechanism different types have different effect on the dispersibility of the GnPs (Metaxa (2015)). A newest study has shown that, optimum dispersion of GnPs can be achieved by employing a polycarboxylate based superplasticizer at a concentration of 15% by weight of GnPs, without affecting the workability of cement paste (Du and Pang (2018)). In this study, the applicability of a method, involving the facilitation of a dispersing agent compatible with ce mentitious materials and ultrasonic processing, to uniformly disperse graphene nanoplatelets (GnPs) in the mixing water, was investigated. Initially, the effect of dispersing agent (superplasticizer) concentration on the dispersion of graphene nanoplatelets reinforcing Type II cement paste was experimentally investigated. Eight different super plasticizer concentrations were examined. GnPs with a lateral size of ~8 μm were used. The amount of GnPs were kept constant at 0.1% by weight of cement. In order to optimize further the GnPs dispersion in Type II cementitious matrix, the effect of ultrasonic energy was investigated. In this direction, five different sonication energies (i.e., 100, 300, 400, 500, 600 kJ) were applied to the GnP aqueous suspensions. The electrical resistivity of the nanocomposites produced was examined as an indirect method to quantify the dispersion state of the GnPs. Three-point bending tests were performed at prismatic beam specimens with an artificial notch at the age of 28 days to study the mechanical performance of the nanocomposites and the reinforcing efficiency of the GnPs.

2. Methodology 2.1. Graphene nanoplatelets

In this study, commercially available non-exfoliated graphene nanoplatelets (GnPs) having a lateral size (width) of 8 μm , were used. GnPs were manufactured by Knano© with the brand name KNG 180©. According to the manu facturer, the GnPs demonstrate a thickness of around 100 nm, comprising of several multi-layered graphene sheets forming stacks. The properties of the GnPs are shown in Table 1.