## 1 Introduction
On the other hand, in the framework of a circular economy and waste recycling, the interest in adopting plant-based natural fibers as reinforcement has attracted great interest among researchers. These kinds of fibers possess interesting specific engineering properties, such as mechanical characteristics and sound and thermal insulation, together with interesting specific economical properties, such low raw material cost, low-cost processing and large availability [25]. Furthermore, from a green-economy point of view, these fibers show many interesting properties since they are renewable and biodegradable and show a carbon-dioxide-neutral life-cycle [26]. All of these remarks have led, in the last years, several authors to address research regarding the utilization of natural fibers as reinforcements for geopolymer materials [27]. Among these studies, it is worth noting that [28] reported a preliminary investigation to evaluate the compatibility of metakaolin-based geopolymers with several natural fibers being carried out. The fibers considered in this research were classified as wood and non-wood ones, and the results of the experimental campaign showed that wood fibers are more compatible with the geopolymeric matrix in comparison to non-wood fibers. The research reported in [29] investigated the role of the cotton fiber weight content (in the range 0.3-1.0%) based on the physical, mechanical and fracture behavior of fly-ash-based geopolymers. The results showed that a fiber weight content equal to 0.5% leads to optimal mechanical performances in terms of fracture toughness and flexural strength. The presence of an optimal value of fiber weight replacement is reasonable since the increment in mechanical performance of the geopolymer composite due to the fibers is compensated for by both the porosity and fiber agglomeration increment (leading to a reduction in the density of the geopolymer composites). Among natural fibers, sisal ones are recognized as those with high mechanical performances due to their composition (65-68% cellulose, 10-22% hemicellulose and 10-14% lignin). Sisal fibers also possess further advantages: their length is between 1 and 1.5 m (allowing the use also for high-dimension samples), the diameter is about 100-300 \(\upmu\)m with a smooth diameter profile along the fiber and the productivity is high (about 12 years life, 120 and 240 leaves for each plant and about 1000 fibers for each leaf). For more detailed information about sisal fibers, see [1; 2]. For an overall review regarding the processing and properties of geopolymers reinforced with natural fibers see [27]. In the research reported in [30], the geopolymer composite was produced using a matrix obtained from fired clay brick wastes adopting short sisal and jute fibers as reinforcement. The obtained results evidenced a meaningful increment in the mechanical properties of fiber reinforced geopolymers with respect to unreinforced material showing good fiber-matrix adhesion for both examined natural fibers. Furthermore, in [30], it was also shown that--due to the presence of both natural fibers--the failure mode of the composite changes from a brittle to more ductile one and that the post-peak behavior is strongly influenced. On the other hand, the role played by geopolymers in the construction industry is confirmed by many papers in which the use of geopolymers as the basis of concrete (with or without recycled aggregates) to be adopted in structural members is investigated [31]. To support and develop the adoption of natural-fiber-reinforced geopolymers (mortar and/or concrete) in the construction industry, it is fundamental to define a suitable and affordable numerical model of the mechanical behavior of the relevant material to be introduced in Finite Element Method (FEM) software allowing for an evaluation of the acting stress-strain fields. This aspect is worthy of investigation, and few papers have been devoted to this topic (see, e.g., [32; 33; 34; 35; 36]).