Concrete is the most used construction and building material which is a combination of aggregates and binders. The binding materials are cement (Portland or pozzolanic materials), and water. However, because of its poor tensile strength & strain, and quasi-brittle nature, the use of conventional concrete is limited in the different construction areas. At present, nano-materials such as nano-silica, and carbon nanotubes (CNTs) have drawn much attention from the scientific community because they can effectively control the size of cracks [1].
It was reported that the introduction of nano-silica in cement matrix improved the mechanical performance [2]. Nano-silica has a high specific surface area of 300 m²/g and a spherical shape (low aspect ratio) with less than 30 nm diameter [3, 4]. The incorporation of nano-silica in cement composite and the improvement of mechanical properties are generally attributed to two mechanisms [5]. Firstly, as nuclei for the cement phase, hydration of cement composites can be promoted due to the optimum specific surface area of nano-silica. Secondly, as filler, nano-silica can densify the microstructure since the size of nano-silica is comparable to that of gel pores within the cement matrix. It was concluded that the nano SiO2 leads to better interfacial bond strength between cement paste and aggregates [6]. In a study, it was found that the addition of 5.0% of nano-TiO2 into cement accelerated the cement hydration process in the first two hours. Additionally, its addition showed improvements in the solidified cement paste's volume stability and water retention abilities [7]. After 28 days of curing, the authors stated in another experimental investigation (Liu et al. 2021a) that the addition of 2.5% nano-silica with 20% admixture raised the compressive strength and tensile strength by around 6.6% and 15.15%. It was also reported that the impermeability, durability, and shrinkage performance of cement matrix is increased with the incorporation of nano-silica.
Recent studies on CNTs in a cement matrix reported that the inclusion of a small quantity of CNTs in cement composite enhanced the mechanical behavior [7–9]. It not only increased the mechanical behavior but also reduced the porosity [10, 11] and early-aged shrinkage [12]. It was reported that the fracture energy and fracture toughness of cement composite increased with the presence of well-dispersing CNTs [12–14]. A study found that after 28 days, adding a small quantity of CNTs to cement mortar increased the compressive strength by up to 23% and the flexural strength by up to 17% [15]. Moreover, the presence of well-dispersed CNTs in mesoporous cement composite contributed to refinement and produced a denser matrix [16, 17]. It is considered that this pore refinement of cement composite led to the enhancement of the mechanical strength and reduced the total porosity [18].
Recently, carbon-based, single-layered, tow dimensional (2D), nanomaterial such as Graphene oxide (GO) has attracted research concertation and have been widely used to enhance the mechanical properties of cement composite due to its high specific surface area, and exceptional mechanical properties [19–21]. GO consists of different types of oxygen-containing groups such as hydroxyl, carbonyl, epoxy, and carboxylic groups [22–24]. It is well known that the carbon atomic structure of GO is hexagonal. The connection in between of carbon atom of GO is very strong σ-bond and π-bond and it is accepted that the σ atomic bonding is responsible for its ultra-high mechanical properties [24]. GO can promote chemical and physical interaction with host materials such as cement composite due to its own high specific surface area [25]. The enhancement of mechanical properties of cement composite with GO is caused by its remarkable dispersibility after sonication [26, 27]. It was concluded that GO can conveniently form composite and leads to producing the toughened and crystal micro-structure in ceramic and polymer materials [28]. However, GO can be synthesized in large quantities from inexpensive graphite powder by strong chemical oxidation [29].
From previous studies, it has been concluded that such a small amount of GO addition to OPC based cement composite can reduce the fluidity and enhance the physical strength. Not only increasing the mechanical properties but introducing GO with cement composite also can upgrade the microstructure [28, 30–32]. It was accepted that the incorporation of 0.03% of GO by weight of cement reduced the fluidity of OPC based mortar by around 23%. For mechanical properties, with the introduction of 0.05% GO the compressive strength and flexural strength of OPC cement-sand mortar increased by around 20% and 26% after 28 days, respectively. The primary and secondary water absorption rate of GO-based OPC cement-sand mortar was also low compared to the control. A morphological study of cement composite concluded that the presence of GO produced denser and upgraded micro-structure [28]. It was determined by Long et al. [33] that the fluidity of OPC cement composite decreased by about 18% when 0.2% of GO was added. Additionally, after 28 days of curing, the compressive strength and flexural strength showed increases of 41% and 16%, respectively. It was reported by Abdullah and co-worker [34] reported that the compressive strength and flexural strength increased by 31% and 18% with the addition of 0.03% of GO with silica fume. From another experimental study [35] it was indicated that the addition of 0.03% of GO into cement composite increased the compressive strength, flexural strength, and tensile strength by 38.9%, 60.7%, and 78.6% respectively. It was shown that the incorporation of GO into cement composite takes an important role in effectively forming the microstructure of hydration crystal, reduced brittleness, and increased toughness significantly.
As per our extensive literature review, most of the researchers investigated the effect of Graphene oxide on Ordinary Portland Cement (OPC) but sufficient research has not been done on fly ash-based cement, yet. The present work illustrates the effect of incorporating GO at different dosages (0%, 0.03%, 0.04%, 0.05%, and 0.06%) by weight of cement in Portland Pozzolana Cement (PPC) based mortar in terms of mechanical properties, fluidity behaviour, and microstructural analysis. Also, durability properties based on acid resistance, rapid chloride permeability test (RCPT), and sorptivity tests were conducted with/without GO-based PPC cement-sand mortar. The hardened GO-based cement-sand mortar morphological property was accessed by using Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray Spectroscopy (EDS), and X-ray Diffraction (XRD) analysis. In this present study, no other chemicals or minerals were used while preparing the GO-modified cement-sand mortar specimens.