PSI - Issue 28

Angeliki-Eirini Dimou et al. / Procedia Structural Integrity 28 (2020) 1694–1701 A.-E. Dimou et al. / Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Several types of nanomaterials have been studied for their potential application in construction materials, e.g. Jafariesfad et al. (2017). Carbon-based nanomaterials (CBNs), particularly carbon nanotubes (CNTs) and graphene (G), have been extensively studied for such applications. CBNs are composed by distinct carbon structures ranging from sp3 hybridized diamond to sp bonded graphite and can be categorised based on their dimensions. CNTs belong to one-dimensional CBNs (tubes), while graphene and its products are two-dimensional CBNs (platelets). CBNs have been studied due to their unique combinations of physical and chemical properties, including thermal and electrical conductivity, high mechanical strength, and optical properties, e.g. Li et al. (2019). Recent studies, as for example the one by Sun et al. (2017), have shown that small amounts of CBNs lead to improvement of mechanical properties of cement composites and they are linked to the development of self-sensing properties in cement pastes and mortars. The incorporation of CBNs in construction materials and the enhancement of their mechanical properties mostly refer to cementitious composites, indicating that nano-reinforced composites have improved mechanical properties in comparison to the blank matrix. For example, Danoglidis et al. (2016) found that the addition of MWCNTs in mortars increases the flexural strength by 87 % and the elasticity modulus by 92 %. Xu and Zhang (2017) added graphene nanoplatelets (GNPs) at different concentrations (up to 2 wt%) and concluded that such addition improves the compressive strength at lower contents. Alves e Silva et al. (2017) used multilayer graphene nanoparticles in cement mortars at very low concentrations, namely 0.015 and up to 0.033 wt% by weight of cement. Their results indicate that the optimal tensile strength is achieved by multilayer graphene at concentration of 0.033 wt%, while the maximum compressive strength is achieved at multilayer graphene concentration of 0.021 wt% concentration. Du and Pang (2015) studied the effect of very high contents of GNPs (up to 7.5 wt%). Their results showed that compressive strength shows insignificant changes, while flexural strength decreases for concentrations larger than 2.5 wt%. In more recent studies, raw CBNs are subjected to chemical modification. The modification of CBNs by incorporating oxygen groups leads to a more hydrophilic material that can be better dispersed in water, e.g. by Meirinho et al. (2020). Roy et al. (2018) found that the addition of graphene oxide (GO) particles at an optimal content of 0.05 wt % leads to an increase compressive strength of pozzolanic cement mortars, while the respective content for flexural and tensile strength improvement is 0.10 wt %. Those results are similar to those of Mokhtar et al. (2017), which concluded that the optimal concentration for mechanical enhancement was 0.03 wt% GO. Any concentration higher than 0.03 wt% resulted in a decrease in strength. However, Peng et al. (2019) found that a combination of a w/b ratio of 0.35 and a GO content concentration of 0.05 wt% led exhibited simultaneously to both the highest flexural and compressive strength. Apart from mechanical characteristics, another property of cement-based composites with CBNs that gained attention is that of the electrical conductivity. One of the first researchers to conduct such experiments was Banthia et al. (1992), which used micro-fibres far before the invention of nanofibers. Due to the fine size of carbon fibres, highly conductive composites have been produced. More recently, the effect of CNFs in the electrical properties was examined by Wang et al. (2018), who found that the electrical resistivity decreases as the content of CNFs increases. Yoo et al. (2019) concluded that CNFs was the most effective nanomaterial at improving the conductivity at low volume fraction, while the addition of CNT was most at high volume fractions. Regarding graphene nanostructures, the addition of graphene oxide leads to cement pastes with reduced electrical resistivity, as also shown by Li et al. (2018). The above review indicates the beneficial role of carbon nanostructures to the reinforcement of cement matrix, as well as the potential of these nano-composites to be used both as repair materials and real-time sensors for the health monitoring of built structures. These properties are particularly interesting for cultural heritage applications, since the early prediction of weathering effects may prevent the extensive damage of architectural monuments, decrease the maintenance cost and preserve the values of Cultural Heritage monuments. However, in order to support the transition from cement structures to Cultural Heritage monuments, the compatibility and the performance of the new matrices must be studied, in order to provide safe and reliable solutions. In the previous decades, the use of cement as the main restoration material was strongly criticised, since it resulted in incompatible interventions and acceleration of the deterioration process, e.g. Fang et al. (2015). Velosa and Veiga (2007) suggested that the application of lime-based mortar mixtures is necessary in order to achieve the required compatibility. The superiority of lime-based materials for such mixtures lies in their microstructure and pore space properties, as well as in the low modulus of elasticity. Faria et al. (2017) investigated the incorporation of GO in natural hydraulic lime