###### Abstract  
The object of this study is to investigate the feasibility of ball-milled graphene oxide nanoflakes (GONFs) produced by a mechanochemical process as an additive in Ordinary Portland Cement (OPC)-based paste and concrete. Different percentages of GONFs (0.01-1 wt. %) were added in OPC paste mix to find the optimum content of GONF in GONF-combined cement composites. To investigate the effect of the changes in the mix design on OPC paste, two mix design methods were employed: (1) Dry-mix, where GONFs and OPC powders are mixed prior to paste formation and (2) Wet-mix, where sonicated GONFs solution is used as the pour water with OPC paste mix. Data from mechanical tests (compressive and flexural tests) showed that OPC paste incorporating 0.05% GONF resulted in the highest strength for OPC paste. The results of strength tests indicated that Dry-mix is competitive as compared with Wet-mix. Scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) was used to identify GONFs inside products of cement hydration. To explore the practicality of using GONF in concrete, concrete specimens were made, cured and tested with different percentages of dry-mixed GONFs (0.01-0.1 wt. %). The result of strength test showed that GONF improves the compressive and flexural strengths of concrete around 15% with 0.01 wt. % GONF. ball-milled graphene oxide; cement hydrates; petrographic analysis; dry-mix design 201  
(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]. [PERSON] 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. [PERSON] 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. [PERSON] 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%. [PERSON] 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 [PERSON]'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 [PERSON]'s method, depend on strong and somewhat harmful oxidizing agents and acids (H\({}_{2}\)O\({}_{2}\), H\({}_{2}\)SO\({}_{4}\) 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].