## 1 Introduction
Polymer nano-composites generally exhibit better properties compared to the plain polymers, such as better mechanical properties, thermal and dimensional stability, fire and chemical resistance, gas permeability, and optical and electrical properties [32; 33; 34; 35]. The superior reinforcing effect obtained with the inclusion of nanoparticle in the polymer matrix compared with that of microscale particles is due to the large nanoparticles' interfacial area. This has a significant impact on the final properties of the polymer nanocomposite, since the polymer at the interface present different properties than the bulk polymer, such as a higher degree of crystallinity [36]. Among the possible nanofillers that can be used for this purpose, nanoclay particles are particularly attractive. Clay is a relatively cheap and easily available material consisting of layered silicate that finds several industrial applications, ranging from cosmetics and food industry to production of high performance materials [37; 35; 38; 39]. Clays are a mineral belonging to the structural family of phyllosilicates and are referred to as layered silicates due to their lattice structure consisting of octahedral sheet of alumina or magnesia fused to one or two external silica tetrahedra forming thin layers that bound together with counter-ions. In the construction field metakaolin, a calcined form of clay with mean particle size of about 3 \(\upmu\)m, has been widely employed as pozzonalic binder in mortar and concrete matrix. The incorporation of metakaolin as a partial cement replacement has been reported to influence the properties of the fresh paste, as well as the mechanical and durability performance of the cured matrix [40]. Particularly, cement and mortars containing a proper amount of metakaolin exhibit higher compressive and flexural strength, as well as increased durability and reduced shrinkage, compared to the same matrix without metakaolin [40]. Matrices containing metakaolin have also been employed for TRC composites, which benefit from the small dimension of the binder, its pozzonalic properties, and its ability to inhibit alkali-silica reactions [41; 42]. Although addition of nanoclay particles in concrete is less common, promising results have been obtained in terms of mechanical properties and durability [43]. Particularly, clay dispersed in concrete matrix has been reported to act as nucleation agents for the formation of Calcium Silicate Hydrate (CSH) [44; 45]. Moreover, it was previously shown that the modification of textile reinforcement with a polymeric coating containing nanoclay particles can promote formation of hydration products at the fiber-matrix interface [29; 30]. Another property of nanoclay polymers that make them particularly suitable for modification of reinforcements used in TRC composites is the ability of nanoclay polymers to act as moisture barrier. This is attributed to the tortuous path that permeating molecules have to traverse moving between the nanoclay plates dispersed in the polymer [35]. When the nanoclay polymer is applied on the surface of glass fibers, it forms a barrier layer which improves their resistance to alkaline corrosion (Figure 1) [46]. However, it should be stressed out that a prerequisite for an effective improvement of the polymer barrier properties is the proper dispersion of the nanoclay in the polymeric matrix. Indeed, any physical mixture of polymer and nanoclays does not necessarily results in the formation of a nanocomposite [35]. This is obtained only if the nanoclay in the polymer presents either an intercalated or exfoliated structure. In the first case, the nanoclay polymer has a multi-layers structure of alternating polymeric and silicate layers. In the second case, the clay layers are completely separated from each other and individually dispersed in the polymeric matrix. According to the chemical physical characteristics of the polymer employed, different techniques can be used for the preparation of a polymer-clay nanocomposite.