energies Article Hygrothermal and Mechanical Behaviors of Fiber Mortar: Comparative Study between Palm and Hemp Fibers Younes Zouaoui 1,2 , Ferhat Benmahiddine 2,3 , Ammar Yahia 1 and Rafik Belarbi 1,2,3, *   Citation: Zouaoui, Y.; Benmahiddine, F.; Yahia, A.; Belarbi, R. Hygrothermal and Mechanical Behaviors of Fiber Mortar: Comparative Study between Palm and Hemp Fibers. Energies 2021, 14, 7110. https://doi.org/10.3390/ en14217110 Academic Editor: Chi-Ming Lai Received: 20 September 2021 Accepted: 21 October 2021 Published: 1 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Civil Engineering, Université de Sherbrooke, 2500 Bd de l’ Université, Sherbrooke, QC J1K 2R1, Canada; younes.zouaoui@usherbrooke.ca (Y.Z.); Ammar.yahia@usherbrooke.ca (A.Y.) 2 Laboratory of Engineering Sciences for the Environment (LaSIE), UMR CNRS 7356, La Rochelle University, Avenue Michel Crépeau, CEDEX 1, 17042 La Rochelle, France; ferhat.benmahiddine1@univ-lr.fr 3 4ev Lab, EDF R&D, CNRS, LaSIE, La Rochelle University, Avenue Michel Crépeau, CEDEX 1, 17042 La Rochelle, France * Correspondence: rbelarbi@univ-lr.fr; Tel.: +33-646-683-453 Abstract: This paper presents an experimental investigation of the hygrothermal and mechanical properties of innovative mortar mixtures reinforced with natural fibers. Fibers extracted from palm stems (PS) and hemp (HF) were evaluated at different percentages. Scanning electron microscope (SEM) observations showed that the PS fibers have rough surfaces and very complex microstructures. Prior to their incorporation into the mortar, the fibers were subjected to different treatments to reduce their hydrophilic character. The employed treatments showed good efficiency in reducing the water absorption of both PS and HF fiber types. Furthermore, the mortar mixtures incorporating these fibers exhibited low thermal conductivity and excellent moisture buffering capacity. Indeed, the moisture buffer value (MBV) of the investigated mixtures ranged between 2.7 [g/(%HR·m 2 )] and 3.1 [g/(%HR·m 2 )], hence providing them excellent moisture regulator character. As expected, the fiber mortar mixtures showed very high porosity and low compressive strength ranging between 0.6 and 0.9 MPa after 28 days of age. The low-environmental footprint materials developed in this study are intended for thermal insulation and building filling. Keywords: natural fibers; morphology; chemical treatments; fiber mortar; moisture buffer value; hygrothermal properties 1. Introduction The building industry is a large materials and energy consumer, highly polluting (emission of CO 2 ), and a residues generator. Therefore, the new global research policy is focused on the development of new innovative bio-based materials with low environmental impact [1]. Nowadays, the demand for bio-based materials is greatly increasing due to their abundance and regenerative capacity. Natural materials are known to develop comparable performance to the standard building ones [2]. Indeed, the date palm wood is shown to be an efficient insulating green material compared to the other materials [3–6], hence becoming attractive in developing good thermal-insulation and low-cost materials. In this context, several types of vegetable fibers, including hemp, flax, bamboo, coir, animal hairs, and cork have been combined with different materials, including Portland cement, clay, sand, and gypsum to produce composite materials [7,8]. The properties of composite materials reinforced by natural fibers are influenced by the shape and size of the fibers, distribution and orientation of fibers in the matrix, bond between the fibers and matrix, mixture design, and the mixing efficiency and processing methods [9]. The various investigations carried out on the composites considered only content, length, and type of fiber to evaluate their performance [10]. Cork–gypsum composites are shown to exhibit good thermal and acoustic insulators and can be used as partitioning materials [11]. Other research has Energies 2021, 14, 7110. https://doi.org/10.3390/en14217110 https://www.mdpi.com/journal/energies