Effect of Chemical Treatment on the Mechanical and Hygroscopic Properties of an Innovative Clay-Sand Composite Reinforced with Fibers.

The viability of using fibers as reinforcement material for developing lightweight sustainable non-structural construction materials in compliance with the valorization of local by-products has been investigated in this work. This study aims to investigate the effect of the chemical treatment of fibers on the mechanical and hygric properties of bio-sourced clay-sand- fiber composite. This lightweight specimen has been produced from a mixture of 60% natural clay and 40% sand by mass, as a matrix, and reinforced with different amounts of Juncus fibers. The fibers were used as a partial replacement of sand in the mixture by volume at 0% (control specimen), 5%, 10%, and 20%. In order to enhance interfacial bonding between the fibers and the binder matrix, which seriously limits the strength development of the composite, the fibers have undergone an NaOH alkali treatment with different concentrations of 1 and 2 wt. %. Morphological and elementary chemical component evaluations based on SEM micrographs and EDX analyses revealed that the 1 wt. % NaOH alkali treatment exhibited the most beneficial effect due to the removal of impurity deposits without significant surface damage to the fibers. This finding was highlighted through the tensile tests carried out which showed the tensile stress value of 81.97 MPa compared to those of the treated fibers with 2% NaOH (74.45 MPa) and the untreated fibers (70.24 MPa). However, mechanical test results, carried out according to the European Standard EN 196-1, highlighted the beneficial effect of the fiber alkali treatment on both the compressive and flexural strengths, particularly for the fiber contents of 5% and 10%, which corresponds to a strengthening rate of 25% and 30%, respectively. The examination of the hygroscopic properties of the samples, including capillary water absorption, water diffusivity, and moisture buffering capacity under the dynamic conditions have indicated that the specimen containing treated fibers exhibited a better moisture regulating property than that obtained with untreated fibers. However, the specimens with treated fibers are classified as excellent hygric regulators based on their moisture buffer values (MBV > 2 g/(m.%RH)), according to the NORDTEST classification. The results also indicated that the capillary water absorption and the apparent moisture diffusivity of composites were lowered due to high fiber-matrix interfacial bond after fiber treatment. Consequently, the composite with treated fibers is less diffusive compared to that with untreated fibers, and thus expected to be more durable in a humid environment.