1. Introduction
Ordinary Portland cement (OPC) is a crucial component of concrete, which is the most popular cementitious material for architectural structures. However, OPC paste is a typically brittle material due to low tensile strength, poor flexural strength and multiple initial cracks. Traditionally, various fibers or steel bars have been used to restrict the propagation of microcracks to improve the mechanical and electrical properties of plain cement materials [
1,
2,
3]. For the last decade, nanomaterials including nanoparticles or nanofibres have been widely applied to cement-based materials as nanofillers because of advancements in nanotechnology [
4,
5]. These nanosized materials could control the formation and development of nanosized cracks. Moreover, many studies have been undertaken to create carbon nanomaterial-based cement composites, including carbon nanotubes (CNTs) and carbon nanofibers (CNFs) [
6,
7,
8,
9].
As an extensively attractive carbon nanomaterial, ideally, graphene is also able to remarkably reinforce cement-based materials due to its excellent mechanical properties. The average tensile strength and Young’s modulus of graphene are 125 GPa and 1.1 TPa [
10,
11]. In addition, the special surface area of graphene can theoretically reach up to 2630 m
2/g [
12], which provides more potential sites for surface adsorption or other interactions between graphene and cement. In the last several years, graphene has been applied to polymers, ceramics or rubbers [
13,
14,
15] as a reinforcing material. Meanwhile, the introduction of graphene into cementitious materials has attracted the comprehensive attention of many researchers and engineers. Gong et al. [
16] found the incorporation of 0.03% graphene oxide (GO) increased the tensile and compressive strength of the cement paste by more than 40%, and the degree of hydration of GO–cement composites was promoted. Saafi et al. [
17] observed that a 134% and 56% enhancement in flexural strength and flexural toughness of graphene–geopolymer composites were obtained for the addition of 0.35 wt % reduced graphene oxide (rGO). Other researchers have incorporated graphene or graphene oxide into cement-based composites as a filler material to enhance the flexural strength by 40%–60% and the compressive strength by more than 10% [
18,
19,
20].
Graphene nanoplatelets (GNPs) consist of several graphene layers with a thickness in range of 3–100 nm [
21]. Compared with single layer graphene, GNPs are not only a remarkable reinforcing material due to their morphological structure like monolayer graphene, but also low-cost, which further expands their application prospects. To date, GNPs have been extensively applied in polymeric or ceramic composites [
22,
23,
24,
25], whereas their use in cementitious materials has remained limited. Recently, Ranjbar et al. [
26] reported that the compressive and flexural strength of a fly ash-based geopolymer were improved by 1.44 and 2.16 times with the help of GNPs. Peyvandi et al. [
27] found that the reinforcement efficiency of different graphite nanomaterials in cementitious paste. The flexural strength of cement had various gains ranging from 27% to 73% with the addition of 0.13 wt % different GNP type and their oxide. Owing to the planer geometry and good chemical bonding with the matrix, GNPs have the ability to transfer the stress to the other positions and relieve the stress concentration in the matrix. Furthermore, GNPs may provide a larger thermal and electrical contact area due to its unique flat morphology, and thus Sedaghat et al. [
28] introduced various quantities of graphene to measure the thermal diffusivity and electrical conductivity of graphene–cement composites. The electrical conductivity and thermal diffusivity of the composites were enhanced by the incorporation of graphene, and the content of needle or rod-shaped ettringite was reduced. Meanwhile, Du et al. [
29] found that the GNPs significantly decreased the water penetration depth and aggressive ions ingress of cement mortar, because the layered structure of GNPs could increase tortuous paths. In a word, utilizing the outstanding properties of GNPs, the GNP-reinforced building materials can be endowed with high mechanical, electrical, thermal properties and excellent durability. However, it is a very formidable challenge to resolve the dispersion of GNPs in water and cement-based materials arising from their surface hydrophobicity and strong interlayer van der Waals forces [
30].
As a traditional surfactant, methylcellulose (MC) is always applied for the dispersion of CNTs and CNFs because of its wettability, dispersibility and adhesion properties [
31,
32,
33]. In this paper, methylcellulose (MC) was employed to dispersion GNPs in aqueous solution with the help of sonication. The ultraviolet visible spectroscopy (UV-Vis) was used to determine the optimum concentration ratio for dispersing. Moreover, the dispersing performance of GNP suspension was evaluated using optical microscope and transmission electron microscopy (TEM). Consequently, the homogenous GNP suspensions were incorporated into cement to investigate the mechanical properties of GNP–cement composites. In addition, the X-ray diffraction (XRD) and thermal analysis (TG/DTG) were used to explore the effect of GNPs on the degree of cement hydration. The pore size distribution and microstructure of GNP–cement composites were studied using mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM).
3. Discussion
In recent years, some studies have demonstrated that the introduction of graphene can remarkably enhance the mechanical properties of ceramic and cement composites [
36,
37,
38]. It is a key precondition for higher reinforcement of GNP–cement composite to disperse GNPs into cement matrix uniformly. In this work, the GNPs were dispersed in aqueous solution using MC as dispersant with the help of ultrasonic, and
Figure 1 indicated that the optimum weight ratio of MC to GNPs was 7:1. Meanwhile, TEM test suggested a fine dispersibility of GNPs in cement matrix using the dispersing method. The workability of the cement paste presented an apparent reduction with the addition of GNPs. A lower flowability and weaker viscosity of fresh paste could affect the compactibility and the mechanical strength of GNP–cement composite. In previous reports, it was reasonable to incorporate superplasticizer to improve the workability of GNP–cement paste when a high content of GNPs is added [
34]. However, the further study is needed to improve the workability of GNP–cement composite.
In addition, the hydration process of cement paste can impact flexural and compressive strength significantly. Previous research showed the effect of carbon nanomaterials on the hydration degree and products of cement paste [
39,
40,
41]. In our work, the XRD and TG/DTG results indicate that the corporation of GNPs has a positive effect on the hydration process and does not transform the hydration products drastically. Furthermore, lower porosity and finer pore size distribution were observed in GNP–cement composite, enhancing the mechanical properties of the composite further. As a porous material, the high porosity indicates lower strength. Due to the addition of GNPs, the cement matrix becomes compacted.
The interaction between the additives and the cement matrix also has a vital impact on the properties of cement composites. In the work, there is more obvious enhancement in the flexural strength than that of the compressive strength. Some studies found that the high improvement in the flexural strength of GNP composites was due to various toughening mechanisms, including stress dispersing, crack deflection, crack bridging, and cracking branching [
26,
42]. As shown in SEM images, the GNPs are wrapped by hydration products, and the large aspect ratio is propitious to enhance the interfacial bond strength between GNPs and cement matrix. The strong bondability can increase the load-transfer efficiency and eliminate the destruction of stress concentration. In addition, the GNP particles provide a higher resistance to crack propagation. For the three-point bending test, the first crack form at the middle of sample. When the crack reaches the surface of GNPs, the crack develops along the interface between the GNP and cement matrix. Thus, the presence of GNPs causes crack branching or deflection, as shown in
Figure 11c,d. The crack branching or deflection mechanism could increase the path of crack development, which improves the mechanical strength of cement matrix. Moreover, the crack bridging mechanism of GNPs could absorb the more energy effectively, when the composite is under external load. The whole mechanical map is presented in
Figure 12. In a word, due to the high tensile strength, high Young’s modulus, and unique two-dimension morphology, GNPs could reduce the stress concentration and prevent the development of the cracks, thus enhancing the mechanical strength of the cement matrix. These results are helpful in understanding the reinforcing effect of nanofillers used for cement-based materials.
5. Conclusions
In this paper, GNPs were treated using MC as a dispersant to improve their dispersibility in aqueous solution. The optimum MC to GNPs ratio of 7:1 by concentration was confirmed using UV-Vis absorbency, and the dispersing performance of GNPs was characterized by optical microscope and TEM. Consequently, the uniform GNP suspensions were used to reinforce the cement matrix as mixing water. The effect of GNPs on the mechanical properties and microstructure of cement based materials were investigated with the help of XRD patterns, thermal analysis, MIP, and SEM analysis.
The incorporation of 0.05% GNPs by weight of cement can enhance the flexural strength by 15%–24% and the compressive strength by 3%–8%. The XRD pattern and thermal analysis demonstrate that the degree of cement hydration has been promoted by GNPs, especially at an early age. Moreover, a more compact microstructure and finer pore size were detected in the GNP–cement composite using MIP and SEM analysis. However, more research is needed to improve the dispersibility of GNPs in cement matrix and the interfacial interaction between GNPs and the hydration products of cement. This study can provide a suitable method to investigate the two-dimensional nanomaterial for cement composites.