1. Introduction
The strength and durability of concrete are the two most significant characteristics that govern structural efficiency for load capacity and service life purposes [
1]. Strength is a function of different aspects such as mix design, structural design, and the curing process [
1]. Durability of concrete, which controls the concrete’s service life, may be defined as the capability to maintain a minimum performance level over a specific time when exposed to a degrading environment [
2]. Concrete durability has been covered by many studies and researchers are still trying to achieve more durable structures.
With more recent developments in nanotechnology, cement composites performance is being improved with the utilization of nanomaterial as an engineering material [
3,
4]. Carbon nanotubes (CNTs) along with other carbon-based nanomaterials have been used to enhance the mechanical properties of cement composites by controlling the cracks at the nanoscale level [
5,
6,
7]. Musso et al. [
5] discovered that compression resistance and the modulus of rupture in plain cement paste is able to produce an improvement of around 10%–20% and 14%–34%, respectively, by adding 0.5% in weight of pristine and annealed multi-wall carbon nanotubes (MWCNTs). Konsta-Gdoutos et al. [
6] also noted that adding 0.08% weight of cement CNTs to cement paste is able to increase the flexural strength by 25% in comparison to plan cement paste. Cwirzen et al. found that the addition of CNTs into plain cement paste is able to increase the compressive strength by about 50% [
7].
Graphene oxide (GO) is currently attracting interest because of its unique properties that enable it to effectively improve the properties of cement-based materials. Researchers have revealed that GO with oxygen-containing functional groups is capable of improving the performance of cement composites [
8,
9,
10,
11,
12,
13]. Lv et al. [
8] investigated the effects of GO nanosheets on the shapes and formation process of cement hydration crystals. They also observed that adding low dosages of GO (<0.03%) forms flower-like crystals and polyhedral or lamellar crystals at high dosages (>0.03%). Moreover, it was noted that compressive strength increased 34.3% and 38.1% by adding 0.03% and 0.05% of cement weight GO to plain cement paste, respectively. The flexural strength showed higher improvement and increased by 52.4% and 52.3%, respectively. Wang et al. [
9] reported that incorporating 0.05% weight of GO into cement paste is able to increase the 28th-day compressive and flexural strength by 40.4% and 90.5%, respectively; the corresponding increases of 24.4% and 70.5% for hardened mortar. An improvement in compressive strength between 15% and 33% and flexural strength between 41% and 59% over ordinary Portland cement paste was reported after the addition of 0.05% weight GO [
10]. Investigation into the effects of graphene oxide nanoplates (GONPs) revealing the properties of cementitious materials was accomplished by Tong et al. [
11]. They found that GONPs reshaped the microstructure of cement paste and showed better interfacial bonds between GONPs and C–S–H gels. The compressive strength of mortar samples also improved because of the role of functional groups of graphene oxide. Gong et al. worked to investigate the effect of GO on Portland cement paste and found that adding 0.03% by weight GO sheets to the plain cement paste was able to enhance the tensile and compressive strength by more than 40%. However, a reduction in workability was observed [
12]. Lv et al. [
13] revealed that the compressive, flexural, and tensile strength of cement paste increased significantly by around 40%, 60%, and 79%, respectively, when 0.03% weight GO was added.
It has been hypothesized that the carbon-based material is able to improve concrete permeability by improving the pore structure, which results in improved resistance to fluid ingress and chemical attacks. There is a direct relationship that exists between concrete durability and the mobility of fluids with concrete [
1]. The durability is related to the ease with which liquids and gasses are able to enter the concrete [
2,
14], which are denied as transport properties. Transport properties highly depend on pore size distribution, total porosity, as well as pore connectivity and its tortuosity [
15,
16]. Recently, it was recognized that sorptivity is a significant index of concrete durability [
17]. The durability of concrete can be improved if the resistance to water penetration is increased [
18]. It is vital to analyze the porosity of nano-reinforced cement because of the close relation to the mechanical properties. Many techniques can be utilized to quantify the porous nature of cement such as water sorptivity, nanoindentation, and mercury intrusion porosimetry (MIP). Nanomaterials play an instrumental role in refining the pore structure. For instance, CNTs are able to densify the cement microstructure by filling the pores between hydration products of cement. CNTs also reduce the porosity of the cement composite by reducing the number of mesopores, which are pores less than 50 nm in diameter [
19]. Nochaiya et al. [
20] revealed that the total porosity and total surface area diminish as MWCNT content is increased by up to 1.0% of the cement weight. The cement composites microstructure and transport properties (i.e., water sorptivity and water permeability) are improved with the addition of a small amount of MWCNTs to the composite as revealed by Han et al. [
21]. Kim et al. [
22] utilized 0.15% by cement weight of CNTs dispersed with 10% silica fume and concluded that compressive strength is affected by total porosity and by CNT dispersion. GO has been shown to have a profound impact on the pore structure and surface area at the nanoscale. The increased surface area directly corresponds to the development of a highly porous phase. Small pores, measuring between 1 and 10 nm (also called gel pores), are made up of a pore system in C–S–H gel. The porosity of GO-cement described in a pore size distribution was characterized by using an alternative method, which was in fact MIP [
12]. The inclusion of GO was able to successfully refine the microstructure of cement composite by lowering the number of capillary pores (between 10 nm and 10 µm) by 27.7%, and this is related to the accelerated hydration owing to the 2D shape of GO.
Previous studies focused on the effect of GO sheets, prepared with common chemical processes (e.g., Hummer’s method) to produce GO sheets with full oxidations (edges, top, and bottom). These methods make the price of GO very high (approximately
$100/g), which is one of the major challenges in promoting GOs in large-scale constructions. The high price of this GO limits its practical usage within the construction industry. The graphene oxide used in this research is produced by a ball milling process along with common reactants. Graphite powder was subjected to milling with non-toxic oxidizing agents. The result was edge-oxidized graphene oxide (EOGO) with a few layers. The direct milling process could achieve a dramatic reduction in cost in the manufacture of EOGO by eliminating hazardous waste disposal [
23]. Therefore, this innovative mechanochemical process can reduce the price of graphene oxide to under
$1.0/g. Consequently, this low cost alternative nanomaterial can be used in construction fields due to the economic advantages. In addition, EOGO can be used to improve the electrical and thermal conductivity of polymers, coating, and composites [
24].
The other challenge to introduce GO into large scale constructions is the dispersion method of GO. Using a sonication method to disperse GO in water is well known as the ideal method [
4,
25]. However, this method may not be practical for high quantities of cement paste or concrete. Therefore, in this study, EOGO is dispersed as powder in cement before mixing with water to investigate the feasibility of using EOGO as an additive material. The interaction of cement particles with the conventional GO will be higher than EOGO because the oxygen-containing functional groups in GO are higher compared to those in EOGO. This may cause higher agglomeration of cement particles with GO when compared with EOGO. The Van der Walls force between GO layers can be weakened with the use of the edge of the oxygen-containing functional groups [
26,
27], and this will assist in giving a better dispersion of EOGO as powder in cement composites compared to GO. Therefore, EOGO and its mixing methods should be carefully examined to observe the effects that can be had on cement composite properties along with the feasibility of EOGO as an additive material.
In this paper, two mix design methods are used: (1) Dry-mix design, where EOGO and cement powders are mixed before cement paste and mortar formation and (2) Wet-mix design where a sonicator is used for 60 minutes to disperse EOGO into water while using that as the mixing water for cement paste and mortar mixes. To quantify the difference between dry and wet-mix design methods, this research investigates the effect of different mixing methods of EOGO in the cement paste and mortar, including mechanical properties, total porosity, and sorptivity. Five percentages of EOGO between 0.01% and 1.0% by cement weight for both mix design methods were used in this study.
4. Discussion
The results of the mechanical properties of EOGO-cement composites with dry and wet-mix design methods show significant improvements compared to control cement composites. Previous studies conclude that a small amount of GO provides significant improvements in the mechanical properties of cement composites [
13,
36,
37,
38,
39,
40]. Most of the previous studies used Hummers-produced GO for GO-cement composites. Also, the optimum GO content for the strength improvement ranges from 0.01% to 0.05%. The mechanical properties of EOGO-cement composites exhibited the greatest improvements when EOGO contents were 0.05% and 0.1%. There are two possible reasons why the EOGO optimum content is higher than the optimum GO content. First, the total amount of oxygen groups for EOGO are lower than for GO. EOGO has most of the oxygen groups along the edges while GO has oxygen groups over the entire surface. Second, GO is a single layer of graphite with oxygen groups. On the other hand, EOGO is composed of several layers of graphene. Despite these demerits of EOGO in comparison with GO, from a practical application perspective, EOGO is actually worthwhile to use. The innovative mechanochemical process which produces EOGO can reduce the price of graphene oxide nanoflakes to under
$1.0/g; therefore, the use of EOGO in large scale production of concrete becomes applicable.
The EOGO functionality has an impact on the crystal seed growth of the calcium silica hydrate (C–S–H) gels and other hydration products similar to GO [
13,
41,
42,
43,
44]. Oxygen groups along the edges of the EOGO flakes are able to provide nucleation sites through coordinating with the calcium ions and improving the crystal growth. Thus, EOGO-cement composites were able to achieve higher mechanical properties. In addition, for both mix designs, the significant improvement in mechanical properties on incorporating 0.05% and 0.1% of EOGO can be explained by the significant reduction in capillary pores when the same percentages are added to cement paste and mortar.
The total porosity of cement composites containing EOGO for both dry and wet-mix design methods cured at 28 days are lower than those cured at 7 days. This is because the increase of curing time provides a more suitable environment for cement hydration, reducing the total porosity of the cement based materials [
45,
46]. A slight reduction in the total porosity of cement composite specimens was noticed after the incorporation of EOGO for both mix designs compared to the control samples. A similar result was reported in cement composites incorporating graphene oxide [
12,
47]. The possible mechanism is that the EOGO as a nano-scale material can fill the nano and micro-pores of the cement matrix. Additionally, the addition of the 2D shape of graphene oxide accelerates the hydration of cement composites, refining the microstructure by reducing the total porosity [
4]. It is interesting to note that increasing the EOGO content to 0.5% and more has less effect in reducing the porosity of the cement composites for both mix designs. This could be because of poor workability caused by the high surface area of EOGO that introduces large pores [
47].
The results of the water sorptivity tests for both dry and wet-mix design show a significant effect of EOGO on the water sorptivity of cement composites. The water absorption rate is mostly influenced by the capillary pores. The capillary pore size can be classified as: large capillaries or macropores (50–10,000 nm), large mesopores (10–50 nm), and small mesopores (2.5–10 nm) [
47,
48]. According to Li et al. [
47], the addition of graphene oxide has no significant effect on pores larger than 50 µm (50,000 nm), but it significantly reduces the large capillary pores (50–10,000 nm) and can refine the pore structure. Therefore, the pore structure of EOGO-cement composite can be changed but the total porosity of EOGO-cement composite may not be changed much compared to the control specimen. It was found that there is no proportional relation between the water absorption rate and the total porosity of EOGO-cement composite. In other words, the total porosity of cement pastes and mortars was not significantly affected by the addition of EOGO. However, a significant reduction in large capillary pores was detected based on the results of the water sorptivity test. This finding agrees with relevant research results [
1,
12,
47]. One of the possible reasons for the reduction in the water sorptivity is due to the seeding and filling effects on EOGO incorporation in the cement matrix. The second reason for this phenomenon can be attributed to the fact that EOGO refines the capillary pore system and increases the tortuosity of cement composites and this leads to improved resistance of cement composites to water absorption. The large capillary pores of cement composites showed a significant reduction when 0.05% and 0.1% of EOGO were incorporated on the basis of the water sorptivity result. The results also show that further addition of EOGO beyond 0.1% did not further reduce the water absorption rate. This phenomenon may be explained by the introduction of larger pores due to the poor workability of cement composites with 0.5% and more of EOGO [
47]. In addition, incorporating 0.5% or more of EOGO would cause agglomeration of EOGO into porous clusters and this leads to more pathways for water to be sucked into by the specimens.
On comparison of the dry and wet-mix designs, the results show that the wet-mix design has a better effect on the compressive and flexural strengths, total porosity, and water absorption rate of cement composites compared to the dry-mix design. This is attributed to the better exfoliation and dispersion of EOGO in the cement matrix with the wet-mix design. The well-dispersed EOGO in the cement matrix may increase the filling and interlocking effect of EOGO, inducing a structure with less pores. Moreover, the well-dispersed EOGO flakes in water with the wet-mix design have a larger surface area compared to the dry powder of the EOGO flakes in the dry-mix design because of lower agglomeration. The larger surface area of EOGO flakes absorbs more free water in the mixes and has more oxygen-containing functional groups, which equates to more nucleation sites. These groups can act as a binder between graphene oxide nanoflakes, and cement paste to obtain greater uniformity of the cement matrix [
49,
50,
51], leading to lower capillary pores and higher strengths.
However, the compressive and flexural strengths gained by the dry-mix design method are sufficiently high as structural materials. In addition, using the dry-mix design showed significant improvements in reducing the capillary pores of cement composites. The use of the edge for the oxygen-containing functional groups is able to make the Van der Walls force between the EOGO layers weaker. This results in good dispersion of the dry powder of EOGO in cement matrix [
26,
27]. Moreover, by applying the dry-mix design method, the use of EOGO in large scale production of concrete becomes applicable. The study results support the fact that the dry-mix design is economical, feasible, and practical for EOGO-cement composites and can be implemented in the concrete industry.