New sustainable alkali-activated composite materials reinforced with natural fiber: preliminary experimental investigations

A new class of materials known as alkali-activated materials (AAM) has rapidly grown in interest in the last two decades in order to reduce the CO2 emissions for cement and ceramic materials productions. This new class of materials is based on alkali-activation of precursors able to consolidate at room or slightly higher temperatures. One of the main advantages of AAM is the possibility to use waste-based powders as for example coal fly ashes, thus promoting a circular economy approach. Although being characterized by a lower environmental impact, AAM suffer from some drawbacks, mainly the extreme brittleness and the low dimensional stability. The most frequently used solution to these problems is the addition of fibers, such as polypropylene, steel or basalt ones. The use of natural fibers with the scope to obtain a similar improvement could provide a huge environmental benefit. In this work, the effect of natural fibers (i.e., bamboo, Arundo, hemp and kenaf) on the properties of mortars formulated with fly ashes activated by alkaline solutions has been investigated. The length of the fibers was varied in order to obtain the best properties of the composite materials. A certain amount of water has been added in all the alkali-activated mixes to obtain a good workability. A reference mortar without fibers and the same liquid/binder and binder/sand ratio has been prepared for comparison. No heat curing has been used in all the mortars. The consistency at the fresh state, as well as the microstructure, physical and mechanical properties at the hardened state of the obtained materials have been studied. Higher flexural strength and increased toughness have been obtained. These preliminary results foster the use of natural fibers to improve the performance of low carbon footprint building materials such as alkali-activated composites in the civil engineering applications. Further studies are necessary to better explain these preliminary results