## 1 Introduction  
In the last decades, the increasing problem of waste management all over the world has led to the definition and implementation of different strategies to achieve sustainable environments in the framework of the so-called circular economy. In this framework, specific attention is paid to the construction industry, which is regarded as more responsible for the high usage of raw materials than any other industry, mainly due to the manufacturing of ordinary Portland cement (OPC). To mitigate this impact, several alternative materials are being developed and used with the aim of reducing CO\({}_{2}\) emissions and improving the overall sustainability of the construction industry. In recent years, geopolymers have gained popularity as possible substitutes for OPC due to some of their characteristics, such as low energy consumption, low carbon dioxide emissions, high compressive strength, fire resistance and good durability. However, geopolymers are fragile ceramic materials, with relatively low tensile strength and are sensitive to micro-fissure propagation. To address these limitations, they can be used as packaged matrices by adding fibers, limiting crack propagation and improving ductility, toughness and tensile strength. With respect to otherentitious media in which the use of natural fibers possesses many drawbacks (see, e.g., [1, 2]), geopolymers have the additional advantage of using very little water for their preparation resulting in a very compact media enveloping the fibers. A further advantage is their low environmental impact since they can be realized with industrial by-products, such as coal fly-ash, high-temperature glass slag mine tailings or construction wastes, which are usually accumulated as waste. On the other hand, nowadays, geopolymers possess some disadvantages mainly related to their production costs and difficulty in handling due to high alkalinity. Anyway, these disadvantages do not invalidate their use and the interest for the development of further detailed scientific studies, and it is reasonable to expect that these disadvantages can be overcome in the future with the development of related technology and the increment of grants from public and private institutions. When interventions with existing buildings are performed (e.g., restoration and/or updating), the construction industry is also responsible for construction wastes, which are actually treated as special wastes outside the virtuous paths of waste recycling. Due to these remarks, the international research has focused on the definition of eco-friendly alternatives to OPC (see, e.g., [3; 4; 5]), and growing attention has addressed geopolymers [6] as construction materials [7]. Geopolymer production is mainly obtained from raw materials based on aluminosilicate (referred to as precursors), alkaline activators and other elements [8]. Theses specific features lead the geopolymers to be considered as \"cement-free\" materials for which production can be achieved by using by-products [9; 10; 11]. Another remark on behalf of geopolymers as eco-friendly materials [12] is that the CO\({}_{2}\) emissions and energy savings required in the manufacturing process of geopolymers are lower than for OPCs [8]. From a mechanical point of view geopolymers show characteristics common to concrete-like materials and are attracting the interest of civil engineering industries [13; 14]: good compressive behavior (both in strength and in stiffness), brittle behavior in tension, characterized by low tensile strength and energy fracture values [15], and sensitivity to early-age shrinkage [16]. As it has been already performed in concrete engineering, to overcome the drawbacks in tensile behavior and to enhance the ductility, the reinforcement of geopolymers with different kind of fibers has been investigated (see, e.g., [12; 17] for steel fibers, [18; 19; 20] for glass fibers, [21; 22] for carbon fibers and [23; 24] for basalt fibers).