Cement, among the constituents of concrete, mortar, and other cementitious composites, is the second most widely used material in the world after water [
1]. In the last decade, a rough estimate of about 3 billion tonnes of Portland cement was recorded to have been manufactured. Due to global industrialisation, there is a prospect for continuous and increased usage of this material [
2]. The current use of cement is estimated at four tonnes per capita, and the embodied energy responsible for concrete production ranks among the highest across energy-consuming industries in the world [
2,
3]. One tonne of Portland cement production is responsible for one tonne of CO
2 emission, resulting in 5–8% of CO
2 emissions globally [
1,
4,
5].
With the continuous reliance on Ordinary Portland Cement (OPC), there will be an increment in the amount of CO
2 and other harmful gases such as nitrogen oxides (NO
x) and sulphur trioxide (SO
3) released into the atmosphere. In the next 40 years, the emission will be twice the current record, contributing largely to global warming and acid rain [
6,
7]. Furthermore, besides the increased greenhouse gas emissions, non-renewable resources such as limestone are consumed exhaustively in cement production [
8]. These resources deplete by the day due to uncontrolled and non-regulated mining in several countries [
9].
The undesirable impacts of cement production and usage have prompted the need to investigate and develop construction materials that will serve as alternatives to cement. These materials are expected to be produced with less energy and reduce global carbon footprint and cost while ensuring the performance is comparable to or higher than that of OPC [
10,
11]. The advancement in this respect has resulted in the development of some alternative materials. These materials range from supplementary cementitious materials such as fly ash, palm oil fuel ash, and ground granulated blast furnace slag used to partially replace cement or improve laterite soil [
12,
13] to entirely cementless binders known as geopolymers [
14]. The dire need for durable, mechanically efficient, and environmentally friendly construction materials and the potential of geopolymer composite to satisfy these needs have brought increased attention to geopolymer composite [
15].
Geopolymers are alternative cementitious materials proposed by Davidovits in 1978. This name resulted from the binder’s formation through alumina-silicate (source) materials’ polymerisation with alkaline solutions [
16]. The geopolymer composite poses the overall environmental benefit of an 80% decrease in CO
2 emission and a 60% decrease in embodied energy in its production when compared to cement concrete [
7]. The abundance of industrial by-products have also contributed to the advantages of geopolymers over cement. Overall, geopolymer composite has demonstrated lower shrinkage and creep, improved freeze–thaw resistance, improved resistance to chlorides, acid, and sulphate attacks, improved fire resistance, greater thermal insulation qualities, and excellent bonding properties [
7,
16,
17,
18,
19].
Fly ash-based geopolymer composite (GC) is the most common and oldest among geopolymer’s broadly researched source materials. Its qualities have been investigated more than any other type of geopolymer composite [
20]. It is affirmed that the fly ash-based GC, compared to the OPC composite, is less susceptible to the Alkali-Silica Reaction (ASR) between the OH
- within the pores of the composite matrix. This reaction is responsible for the strength loss, cracks, and expansion of concrete structures [
8]. Furthermore, fly ash-based GC, compared to OPC composite, has been seen to possess a denser microstructure, lower chloride diffusion, and lower porosity [
8]. Despite these outstanding qualities, GC possesses a quasi-brittle behaviour resulting from its ceramic-like properties. It is low in flexural and tensile strength and characterised by catastrophic failure under loading. These drawbacks have limited its application in safety-based structural designs [
3,
21].
Defects in pure geopolymer may arise from cracks existing inside the geopolymer matrix and its inherent porosity due to the inorganic bond formation during geopolymerization [
15]. These defects have made it inherent to improve the fracture properties of the geopolymer mix, thereby necessitating the improvement using secondary reinforcing particles [
15,
22]. Among the several additives used to enhance the performance of concrete, the majority, according to their morphology, are either zero dimension (nano-SiO
2, nano-Al
2O
3, and nano-TiO
2) or one dimension (carbon nanotubes and nanofibers). The zero-dimensional additives with low aspect ratio and the one-dimensional additives’ lack of interfacial areas between the nanomaterial and the GC matrix limit their performance in bonding and arresting cracks, resulting from the nanoscale at the macroscale [
23,
24,
25]. They are unable to efficiently enhance the reinforcement.
1.1. Nanoparticles in Geopolymer Composite
According to a recent study on the various types of nanomaterials, the effects of the zero- and one-dimensional nanoparticles on geopolymer composite have been extensively researched [
26]. For example, nanosilica, the most researched nanoparticle in geopolymer application [
26], has been seen to reduce workability in geopolymer composites. According to [
27], the reduction in slump flow was about 16% when a 3% dosage of nanosilica was added to the geopolymer. A similar result was noticed by [
28] at the same dosage, where the reduction in slump flow was 8.5%. The influence of nanosilica on the composite was linked to the high surface area of the nanosilica, which has many unsaturated Si-O bonds. These bonds absorb water from the alkali solution to form a silanol group (Si-OH). The formation of the silanol group Si-OH will result in a stiffer geopolymer [
29].
In another study that incorporated nano-TiO
2, the fluidity was affected as the increase in the nanoparticle led to a decrease in the flow of the geopolymer up to about 31% reduction when 5% of nano-TiO
2 was added to the composite [
30]. In a study where one-dimensional carbon nanotubes were used, adding the nanoparticle up to 0.2% only slightly reduced the mini-slump diameter. This result contrasts the reduced fluidity recorded in cement composite because of the carbon nanotubes’ high specific surface area. In geopolymer, the reduction in the flow observed by adding carbon nanotubes was statistically insignificant in reducing the flow of the composite. The result of this effect was assumed to have been the small quantity of carbon nanotubes in the composite [
31].
Considering the compressive strength at 28 days, the use of nanosilica had increased compressive strength up to 11% compared to the control when the nanosilica dosage was capped at 1.5% in the composite [
26,
32]. Another study reported an optimum dosage of 0.5% nanosilica dosage to improve the metakaolin-loaded geopolymer concrete’s dry and wet compressive strengths by 12% and 17%, respectively [
33]. The effect of nanosilica on the increased strength values was attributed to the nanosilica’s ability to fill pores, creating a denser and more compact matrix. Moreover, the geopolymerization reaction is accelerated, resulting in a stronger binder. Excess nanosilica leads to the agglomeration of the particle, causing the strength reduction of the geopolymer composite [
32].
In a study involving carbon nanotubes in geopolymer composites, the compressive strength at 28 days increased by 1.5%, 13.6%, and 1.3% when the carbon nanotube dosage was 2, 5, and 10%, respectively [
34]. In another study, a lower carbon nanotube content, 0.02%, resulted in a dramatic 81% increase in compressive strength at 28 days [
26]. The tremendous increase in compressive strength can be linked to the influence of sodium hydroxide on the dispersion of carbon nanotubes. The sodium hydroxide helps to ensure that the carbon nanotubes are adequately dispersed in the mix [
35]. Furthermore, several studies involving the use of nanoparticles have shown improved splitting tensile [
28,
36] and flexural strength [
37,
38] values for both zero- [
28,
38] and one-dimensional nanoparticles [
34,
39].
Irrespective of the advantages that the zero- and one-dimensional nanomaterials offer, as previously discussed, the low aspect ratios of the zero-dimensional nanoparticles hinder their ability to arrest cracks propagated from the nanoscale. Therefore, its enhancement of reinforcement efficiency is hindered [
25]. Furthermore, one-dimensional materials such as carbon nanotubes have been limited by the inability to generate full bonding with cementitious materials as a result of an absence of interfacial area between them [
23,
40]. Moreover, much has been done covering the properties of various zero- and one-dimensional nanomaterials in the literature. Among the various nanoparticles, nanosilica is the most researched, having a frequency percentage of 63.4%, while nanomaterial, such as graphene, falls within the other group (2.4%), which comprises the least used nanoparticles in geopolymer composites [
26].
Graphene is different from zero- and one-dimensional nanomaterials. It is a two-dimensional nanomaterial consisting of carbon atoms with a honeycomb lattice arrangement [
3,
41]. It is an allotrope of carbon with a large specific surface area [
22,
42]. Graphene has a planar shape that allows both sides of its atomic lattice to be in close contact with the matrix generating a stronger bond between graphene and the composite [
21,
43]. It requires a small amount to boost the performance of the composite because of its large surface area [
3], and it is added as a percentage of the source material. The potential of graphene in improving composites has made it imminent to seek various variants, particularly those with fewer defects, to enhance the composite’s mechanical properties. Hence, studies are expected to cover the grey areas in graphene application and seek the best means to improve the properties of cementitious composites such as geopolymer.
1.2. Graphene in Geopolymer Composite
The different derivatives of graphene, pristine graphene (PG), graphene oxide (GO), reduced graphene oxide (rGO), and graphene Nanoplatelets (GNPs), have been explored in the literature. These derivatives have proven to hold the potential capable of opening doors in interdisciplinary research and enhancing strength properties, particularly in concrete [
15]. However, the van der Waals force between graphene sheets may make dispersion difficult. The graphene, if not adequately dispersed, may agglomerate, affecting the reinforcing action of graphene in the composite material [
44]. Thankfully, the techniques for dispersing graphene have been widely explored [
8,
45,
46,
47,
48,
49]; nonetheless, it is essential to consider them to ensure the best technique is employed for the application. Furthermore, as [
15] acknowledged, the state of the dispersion needs to be reported in studies as there is a limited report in literature covering this.
Graphene is produced either through a bottom-up or top-down approach. Various means of synthesising graphene have proven to either be defective, non-economical, or non-scalable [
50]. Moreover, the graphene derivatives prepared before now on a laboratory scale have varying quality and properties that may not be reproducible [
40]. For instance, GO has been the most sought-after form of graphene in the research on graphene-reinforced geopolymer composites because it possesses oxygen-embodied functional groups, such as epoxy, hydroxyl, and carboxyl, that foster easy dispersibility. However, Hummer’s method, one of the several modified chemical exfoliation methods commonly used to synthesise GO, is not environmentally conducive [
51]. The method releases obnoxious gases and causes explosion risk, limiting its large-scale production [
15]. The GO formed is also impure due to the deposition of the cations on the GO sheet [
52].
GO-reinforced geopolymer composites confirmed optimal dosages ranging from 0.03 to 3% [
45,
46,
47,
49] to improve the mechanical properties of geopolymer composites up to 61.9% [
46]. However, GO is limited by chemical, thermal, or mechanical instability [
15]. The rGO formed by reducing GO has also proven to be effective in producing a crystal structure similar to that of pristine graphene [
53]. However, reducing the GO to rGO results in structural defects [
15].
The GNPs have been found to improve the mechanical properties of geopolymer composites, with some studies affirming 1% [
21,
54] addition as the optimum, while other studies confirmed 0.5% [
3,
55] to improve the mechanical properties. Nonetheless, the appearance of the structural defects also reflects in the graphene Nanoplatelets. The GNPs have a small specific surface area [
56] and layers greater than 10. According to [
57], the difference in the percentage of the fracture stresses of graphene with layers less than 10 is not sensitive. Numerous layers and small specific surface areas of GNPs will result in a considerable reduction in fracture properties and reinforcing efficiency of the graphene [
56,
57]. Hence, the confidence in using graphene with layers less than 10, considered to be pristine graphene [
58], is ascertained, provided its proper dispersion in the geopolymer composite, which has not been considered yet in literature.
Considering the planar sizes of graphene used in geopolymer composite, graphene of 25 µm or less has been used over time [
45,
54,
59,
60]. There is a paucity of studies on incorporating larger sizes in geopolymer composite. GO, widely employed in geopolymer research, has improved mechanical properties using smaller sizes because smaller sizes have more oxygen-containing function groups [
61]. These functional groups exhibit a stronger interfacial adhesion with the cement composites [
56]. Therefore, the more oxygen-containing functional groups, the better the adhesion.
On the other hand, the oxygen groups located at the edges of pristine graphene are extremely few, and various mechanisms ensuring the adhesion of PG with the composite are coordinated at these points. The fewer oxygen-functional groups at the edges of PG indicate the possibility of a different mechanism of PG enhancement of cementitious composite. As a result, the mechanisms at play are linked to friction–adhesion forces between the PG sheets and the composite matrix [
40,
62]. Therefore, [
40,
62] proposed and confirmed that the larger sizes of PG have a better enhancement of the mechanical properties of the cement matrix. However, there is no study on using large-size PG in the mechanical property enhancement of geopolymer composite.
From the thorough review of literature, as reported in the previous sections, on the use of nanomaterials in geopolymer composites, it is evident that the studies on the use of graphene are limited compared to other nanomaterials. Moreover, the use of pristine graphene and its effect on the properties of geopolymer composite has not been encountered in previous studies. In addition, it was revealed from a study that future studies are expected to cover flexural strength tests [
3]. Furthermore, [
15] confirmed that the use of PG in geopolymer composite was not encountered in the literature because of the problem of dispersion that it faces. A call for this study was thereby reinforced by the recommendation made by [
24] that, in future studies, the effect of various PG dosages needs to be investigated to provide a relevant understanding of the enhancement mechanism of PG to improve the mechanical properties of Alkaline Activated Binders (geopolymer), after the researchers’ successful attempt in improving the cement mortar with PG.
To address the observed gaps, this study investigated the effect of using an electrochemically exfoliated, industrially produced large-size (50 µm) pristine graphene on the mechanical properties of geopolymer composite. To achieve this aim, a unique procedure for preparing the geopolymer composite was determined. The dispersion techniques were explored, and the state of PG dispersion was ascertained prior to incorporation into the geopolymer. The effect of the PG’s various dosages (0% to 0.3%) on the mechanical properties of the geopolymer mortar was determined. This study further tested the hypothesis that a large-size PG, greater than 25 µm, will significantly improve the mechanical properties of the geopolymer composite. The results of this study chart a new curve for the practical applications of pristine graphene in construction materials, specifically the industrially manufactured PG variant.