1. Introduction
In recent decades, carbon-based nanoscale materials such as graphene, carbon nanofiber (CNF), graphene oxide nanoflake (GONF), and carbon nanotubes (CNT) have been studied and applied as reinforcing materials of cement-based mixtures, because of their unique combination of properties (high tensile strength and modulus, surface area and electrical conductivity). Many researchers reported that carbon-based nanoscale materials improve the mechanical properties of Ordinary Portland Cement (OPC) composites [
1,
2,
3]. Nano-reinforcements in cementitious matrix materials differ from conventional reinforcements at the millimeter scale and larger because they can control nano-size cracks before they develop into micro- and meso-size cracks [
4,
5,
6,
7,
8].
GONF, in particular, has received attention from civil engineering researchers among carbon-based nanoscale materials due to the oxygen groups on the surface of GONFs. These oxygen groups are thought to coordinate ions to initiate the nucleation of cement hydrates, such as calcium silicate hydrate (C-S-H), calcium hydroxide (Ca(OH)
2), AFt (ettringite) and AFm (monosulphate), etc. [
2,
3,
6,
9,
10], thereby making GONF a seeding material. Moreover, GONF’s nano-filling and nano-bridging mechanism in the cement matrix has been introduced by other researchers [
3,
9,
11,
12]. Mohammed et al. [
5] reported that cement composite with the addition of very low fraction of graphene oxide (GO) (0.01%) can hinder the penetration of chloride ions. Tong et al. [
13] discovered that incorporation of GO in cementitious materials improves not only the strength but also corrosion resistance and freeze-and-thaw performance. Lu et al. [
6] found that the incorporation of 0.08% GO in Strain Hardening Cementitious Composites (SHCCs) increases 24.8% of compressive strength and 37.7% of flexural strength. Moreover, flexural toughness was increased up to 105%. Gong et al. revealed that the introduction of 0.03% of GONF into cement paste leads to increment of compressive and flexural strengths of GO-cement composite by more than 40%, because of the reduction of porosity of cement paste [
12]. In addition, it was mentioned that the total amount of nonevaporable water and calcium hydroxide in cement paste was increased. Furthermore, other researchers also reported that the incorporation of GO improves the mechanical properties and microstructure of cement-based composite [
3,
9,
11,
14].
GONFs used in previous studies have been made using Hummer’s method [
2,
3,
5,
6,
7,
8,
9,
11,
12], a chemical process of oxidizing graphite, which is untenable as an additive in cement and concrete because of cost and scalability. Traditional methods for producing GONFs, such as Hummer’s method, depend on strong and somewhat harmful oxidizing agents and acids (H
2O
2, H
2SO
4 and KMnO
4) and produces significant quantities of acidic byproduct [
1].
Figure 1 shows the typical manufacturing process of GONFs through the chemical process.
GONFs used in this study were produced using an innovative mechanochemical process which uses milling technology like those in the mining industry with common reactants yielding GONFs more cost-effectively and scalably (see
Figure 2). This innovative technology directly mills graphite power with a non-toxic oxidizing agent using conditions that minimize collision forces and optimize shearing forces thus graphite powder is simultaneously oxidized and delaminated with a few layers suitable for the manifold purpose. These proprietary achievements eliminate hazardous waste disposal costs and deliver a product suitable for large-scale production at commodity-type prices [
16,
17]. This promising alternative method to produce large-scale production of GONFs is a requirement for introducing GONFs into the concrete industry. One of the major hindrances encountered by previous researches which prevented the introduction of GONFs into concrete (relatively large scale) is the small production of GONFs. This major hindrance can be overcome by addressing this innovative technology.
Figure 2 shows the manufacturing process of ball-milling GONF.
The other hindrance to introduce GONFs into concrete is the dispersion issue of GONFs in concrete mix. It is well known that the best method for dispersing of GONFs in a solvent such as water is by applying sonication [
1,
11]. All previous researchers have used GONF-solution to make GONF-combined cement composites [
3,
5,
6,
9,
12]. Originality, the uniqueness of this study, is to disperse GONFs as powder in OPC prior to mixing with water to investigate the feasibility of using GONFs as dry-addition. Some of previous studies have used either surfactant or dispersant with water to improve the dispersion of GONFs [
18,
19,
20]. However, none of the previous studies used dry GONF powder for the cement composite mix without applying sonication.
In general, concrete mix requires relatively large amounts of water when compared with cement paste mix due to larger scale specimens and structure. Thus, for the design of GONF-combined cement paste mix (relatively small scale), most of the previous studies used GONFs dispersed in water by applying sonication.
Figure 3 shows dry GONF powder and GONF dispersed solution. For concrete mix, however, it requires a relatively large quantity of water. Therefore, applying sonication for the dispersion of GONFs into water can be a major issue. The construction industry typically uses ready mixed concrete. Each batch of ready mixed concrete is tailor-made and normally consumes a high volume of water. Even though the sonication method is an effective method to disperse GONFs in water, it has some imitations, especially for high volume process. One of limitations is the difficulty of reproducibility of sonication results. The transmitted energy is highly depending on the location of the probe. In other words, the sonicator has a limited range of transmitted energy around the probe. In this study, two different mix designs were employed to investigate the effect of mix design method on the mechanical properties of OPC based specimens. The applied two mix designs are called Dry- and Wet-mix designs. Dry-mix design indicates the GONF is mixed with cement as a dry power while Wet-mix design signifies that the GONF as a solution (after sonication of GONF with water) is mixed with cement. Dry-mix is more practical and effective for concrete mix due to the ease of preparation, mixing and placing of concrete mix. Wet-mix is useful as a more direct comparison to published literature as a baseline for GONF efficacy in cement and concrete.