Keywords

1 Introduction

Concrete is the most widely used construction material, but its quasi-brittle nature and low durability can affect its performance. Recently, polymer-modified concrete has become popular in response to the durability issue, because redispersible polymers such as ethylene–vinyl acetate (EVA) can modify and enhance flexural strength, adhesion, flexibility and resistance against water penetration [1, 2]. However, EVA shows poor interaction with the highly alkaline cement matrix, which adversely affects the material scale performance, such as the compressive strength of the resulting composite [3, 4].

Polymer additives can cluster in a highly alkaline cement composite due to their poor interaction with the cement matrix [5]. The resultant weak microstructure deteriorates engineering performance, such as the compressive strength of the cement composite [6, 7]. The underlying mechanism is the hydrophobic groups present in the macro-molecular long chain polymer [8]. Additionally, there is variation in the adsorption rate and molecular diffusion to the interface, where they cluster together and interact poorly with the cementitious environment [8, 9].

In this regard, a two-dimensional (2D) nanomaterial such as graphene oxide (GO) can potentially modify the nano- and microscale characteristics by its unique physical and chemical properties and larger surface area [10, 11]. In addition, outstanding mechanical properties of GO have been widely reported to enhance the compressive and tensile properties of resulting cement composites [12, 13]. In addition, the abundant oxygenated groups of GO can strongly interact with cement particles and modify their microstructure [14, 15]. As reported previously, adding a low dosage of 0.05% GO can significantly enhance the engineering performance of cement composites [16]. Muhammad et al. also reported that GO incorporation could alter the microstructure and enhance the transport properties of cement composites [17].

In the present work, we investigated the effect of 2D GO sheets on the nano- and microscale characteristics of the EVA polymer. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were utilized to determine the effects of GO on the polymer’s structural features. Engineering performance, such as compressive strength, was evaluated using the Instron 4204 50KN. Our experimental results revealed that the addition of GO altered the nano- and microscale characteristics of EVA, resulting in uniform dispersion of the EVA polymer and enhanced performance in the alkaline cement environment.

2 Methods

2.1 Nano- and Microscale Structural Characterization

Nanoscale structural characterization of samples was performed with TEM using a FEI Tecnai T20 instrument operated at 200 kV. Comprehensive nanoscale characterization of EVA polymer, as well as evaluation of the effect of the 2D GO sheets on the polymer, was performed in comparison with the reference EVA polymer. The microscale surface characteristics were investigated by SEM (FEI Nova NanoSEM 450 FEG SEM) under an accelerating voltage of 5 kV and the effect of incorporating GO was compared with reference EVA polymer.

2.2 Specimen Preparation for Performance Evaluation in Alkaline Cement Matrix

EVA polymer powder was first dispersed in water, followed by the addition of GO solution before ultrasonication for 10 min. Next, the solution was mixed with ordinary Portland cement (OPC) according to the procedures specified in ASTM C1738-11a to prepare cube-shaped specimens that were vibrated for the 30 s, covered with polyethylene sheets and demolded after 24 h, followed by the curing method. The engineering performance was specifically investigated as the compressive strengths of the cubic specimens. A universal loading machine, the Instron 4204 50KN, was used to test the cement specimens at 28 days of age. For each batch, a minimum of five specimens was tested and the average values were taken as the compressive strength.

3 Results and Discussion

3.1 Modification of Nano- and Microstructural Characteristics by GO

The nanostructure of the EVA polymer as examined by TEM is shown in Fig. 1. EVA polymer exhibited a clustered structure of polymer particles (Fig. 1a), which could be attributed to the rate of change of adsorption and diffusion, causing aggregation of the polymer particles. One of the critical reasons for their poor interaction with the highly alkaline cementitious environment is their aggregation, which deteriorates the material scale performance of the resultant cement composites. Remarkably, as presented in Fig. 1 (b, c), the incorporation of GO sheets disperses the polymer particles uniformly compared with the reference EVA polymer. The underlying reason could be the unique physical and chemical properties and larger surface area of GO, which alters the EVA polymer’s nanostructure. In addition, effective electrostatic and steric interactions between the GO sheets and polymer molecules could hamper their aggregation [18].

Fig. 1
3 T E M images. They are labeled a, b, and c. They indicate clustered structures of polymer particles and uniformly dispersed polymer particles.

Nanoscale structural characteristics by TEM. a EVA polymer shows clustering of molecules and aggregation, which causes poor interaction with the cement matrix. (b, c) GO dispersion of clustered polymer structure through electrostatic and steric interactions and alteration of the nanostructure

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Furthermore, SEM also showed the microscale characteristics, which confirmed the changes in the the nanoscale characteristics. Aggregated surface features were present in the reference EVA polymer (Fig. 2a). The addition of GO altered the microscale structure characteristics of the polymer (Fig. 2b), which was consistent with TEM results and confirmed the potential of GO to modify the EVA polymer characteristics at the nano- and microscale due to its unique physical and chemical properties and abundant functional groups on the basal plane that improved the interaction and material scale performance of the resultant cement composites [10].

Fig. 2
2 S E M images. They are labeled a and b. They indicate aggregated surface features and altered microscale structure characteristics of the polymer.

Microscale surface features by SEM. a EVA polymer shows aggregated surface features at the microscale. b Altered aggregated surface features of the polymer with GO addition compared with the reference EVA polymer in (a)

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3.2 Engineering Performance in the Alkaline Cement Environment

The material scale performance of the EVA polymer with and without GO in the highly alkaline cementitious environment was further assessed. Specifically, the compressive strength of the prepared cement composite specimens was investigated at 28 days of hydration age. As shown in Fig. 3, the EVA-incorporated cement composite (PMC) exhibited a lower compressive strength than OPC composites. The underlying reason is the clustered polymer structure and the poor interaction with the highly alkaline cement matrix, resulting in a deteriorated performance of the PMC. Remarkably, the addition of GO improved and enhanced the compressive strength by ~40% higher than the reference PMC samples. The underlying phenomena could be attributed to altered nano- and microscale characteristics of EVA by the GO sheets. In addition, the presence of GO can disperse the polymer particles through electrostatic and steric interactions and hamper their aggregation in the alkaline cementitious environment. As a result, significantly enhanced material scale performance was observed in the resultant cement composites (GO-PMC).

Fig. 3
A vertical bar graph with error bars depicts compressive strength in megapascals versus composite specimens. The compressive strengths of composite specimens O P C, P M C, and G O-P M C are 90, 68, and 88, respectively. The values are approximate.

Compressive strength of cement samples showing significantly enhanced compressive strength compared with the reference PMC sample

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4 Conclusion

In this study, we used the nanomaterial GO as a novel approach to altering and improving the structural characteristics and the material scale performance of polymers in cementitious environments. TEM showed that GO altered the aggregated surface characteristics of EVA at the nanoscale, which was further confirmed by SEM displaying the modified surface features compared with the reference EVA polymer. The compressive strength of the composite specimens was quantified to signify their engineering performance at the material scale. Compared with the reference EVA composite, the GO–polymer-modified composite (GO-PMC) achieved ~ 40% higher compressive strength. This improvements in engineering performance is attributed to the altered nano- and microscale characteristics of the polymer by GO causing electrostatic and steric interactions and hampering particle aggregation in the cement matrix.