**1. Introduction**

Natural fibers, whether of plant, animal or mineral origin, are widely available throughout the world. The diversity and abundance of plant fibers make them a highly renewable resource. And while some plant resources need to be preserved to ensure a sustainable global food supply, a very large quantity of plant fibers remains available. This concerns invasive plants that disrupt natural ecosystems and threaten biodiversity in the long term, as well as waste fibers. Waste fibers are mostly agricultural by-products or residues from industrial manufacturing processes. They constitute a renewable resource that amounts to millions of tons per year, of which only a small part is presently recovered.

The recycling of waste fibers is a part of an environmental strategy for sustainable waste management and implements the three Rs – reduce, reuse and recycle. It aims to reduce waste, preserve natural resources, save space for disposal and/or landfill and prevent the burning and incineration commonly applied to these waste fibers thus limiting CO2 emissions. In many cases, the voluntary incineration of waste fibers results in the production of ash which, due to its chemical properties, can be used

as a binding material as an addition to the components of cement or as a substitute for it [1]. However, the ecological impact of this disposal process is quite negative. It has been observed that for some fibers, waste or not, it is possible to design innovative products with high added value. However, their recovery has a cost and requires energy and the use of other raw materials: bio-based composite materials for the automobile and other modes of transport, furniture, packaging, nanofibers, building materials [2]. However, it is the most basic building materials, i.e., bricks, blocks and tiles, that appear to be best suited to recycle natural fibers, whether short or long, intact or crushed. The incorporation of waste fibers in the manufacturing of these basic materials has little effect on the production process while improving some of the properties and eliminating the waste without additional greenhouse gas emissions. Furthermore, if the brick is unfired, as is the case with fiber-reinforced bricks or blocks made from cementitious products, it is important to minimize the use of ordinary Portland cement with a high clinker content, i.e., OPC CEM I cement. The use of cement made from industrial by-products up to low-carbon binders allows to limit or even drop the carbon footprint. And for these bricks and binders, the economic cost can be reduced by eliminating one or more waste products, fibers and industrial byproducts. This is how the concept of co-valorization was developed [3, 4], which is both economic and ecological: eliminating waste, saving natural resources and limiting the carbon footprint. The crude brick reinforced with waste fibers makes it a perfectly ecological construction element without firing and without the use of binders. It is based on the principle of eco-valorization, which is founded on the integration of the circular economy, sustainable development, the conservation and the renewability of natural resources, and ultimately the limitation of greenhouse gas emissions. This is illustrated in **Figure 1**. Eco-valorization is intended to be more environmentally friendly. The soft material of these crude bricks that bind the fibers most often comes from clay-loam soils, but the introduction of waste soils is preferable, such as sediments or dredged sludge.

The waste fibers that can be recycled into eco-friendly building materials are numerous and diversified. Their quantity is closely linked to the world production of agricultural plants. Some of the fibers are consumed almost entirely by livestock (food such as straw and flour) and industry (textiles such as flax), but the rest are considered as waste, such as palm oil or coconut fibers. In the last decade, there has

**Figure 1.** *Circular economy, eco-valorization, sediment and fiber waste, earth reinforced bricks [5].* been a disproportionate growth in the agro-industry, which has resulted in an expansion of crops and consequently the production of waste fibers, as shown in **Table 1** for oil palm fibers. In the same **Table 1**, it can be seen that natural fibers of tropical origin alone constitute a huge potential of fibrous materials for recycling.

The recycling of waste fibers into building materials implies an industrial process to use a sufficient quantity of fibers over time i.e., renewability of the resource, which is why natural tropical fibers are of great interest. To ensure and maintain a quality manufacturing process, a methodology must be followed. It can be simply illustrated as in **Figure 2**.

This chapter demonstrates the importance of natural fibers in renewable and environmentally friendly building materials and also, the availability of fibers (introduction). Section 2 discusses the variability of shape, i.e., aggregates or fibers, structure (internal and external), intrinsic properties and applications of natural fibers. Section 3 gives background information on the process of fiber extraction, processing and methodologies for determining the main characteristics of fibers useful for use in building materials. Two applications are thoroughly described, one for a fiber-reinforced mortar (Section 4) and the other for fiber-reinforced raw earth, a truly ecological material (Section 5). The chapter concludes with a discussion on the advantages and shortcomings of tropical natural fibers as reinforcement materials.

In detail and accordance with **Figure 2**, the identification of the resource is necessary before any action of recycling waste fibers, this is the focus of Section 2 of the chapter "natural fibers and tropical fibers". This identification must be more complete with the knowledge of the properties of the waste fiber and its intrinsic characteristics useful for its future material recovery. These characteristics are obtained from specific tests carried out on these fibers and in particular, on natural tropical fibers such as oil palm and coconut fibers, see Section 3. The material recovery considered for these tropical waste fibers concerns the production of eco-materials for applications that are primarily local, i.e.,


**Table 1.**

*Production of main agricultural products as a potential natural fiber resource in Mt. [6].*

**Figure 2.**

*A certain methodology for recycling waste natural fibers in building materials.*

close to the sources of waste fiber collected. A case study of a mortar based on coconut fibers is reported in Section 4. In particular, this mortar uses calcium sulfoaluminate cement with a 37% smaller carbon footprint than Portland cement. The development of mud bricks based on oil palm waste fibers incorporated into dredged river sediment is an example of a possible eco-valorization in Section 5. These two studies demonstrate that the recycling of waste fibers into building components is potentially possible and beneficial for sustainable development.
