Potencial de utilização do endocarpo do fruto da seringueira como elemento de reforço em compósitos cimentícios

Abstract

Concrete is widely used in civil construction due to its versatility and compressive strength. However, its low tensile and flexural strength represent a limitation that can compromise its application and durability. Thus, the incorporation of reinforcing elements, such as synthetic or natural metal fibers, in the cement matrix has been evaluated as an effective solution to improve the mechanical properties and crack resistance of this material. The rubber tree is a well-known tree for its latex production. Its fruit is composed of a woody and apparently resistant endocarp, which may have potential to be used in the production of cementitious composites. Therefore, this study aimed to investigate the potential use of the endocarp of the rubber tree fruit as a reinforcing element in cementitious composites. The residue used in this study was collected on the campus of the Federal University of Lavras (UFLA). To quantify the contents of total extractives, lignin, ash and holocellulose present in the material, the chemical characterization of the endocarps in their natural form was performed. In order to improve chemical compatibility with the cement matrix, the endocarps were subjected to specific treatments, involving washing in hot water at 80°C and immersion in an alkaline solution of calcium hydroxide (Ca(OH)2). Additionally, in order to improve endocarp adhesion to the cement matrix, part of the endocarps were coated with the XSBR polymer. To evaluate the efficiency of the treatments, semi-adiabatic calorimetry analysis was performed. The water absorption capacity, moisture content and density of the residue were evaluated through an adaptation of the ASTM D2395–17 standard. The adhesion of the endocarps to the cement matrix was evaluated through flexural tests performed on cementitious composites containing treated and natural endocarps. To analyze the potential of the endocarp as a reinforcing element in the cement matrix, concretes were produced with the incorporation of 1% of endocarps coated with the XSBR polymer, steel fibers and polypropylene macrofibers, which were subsequently subjected to flexural and compression tests. The results of the chemical analysis indicated that the material presented on average 5.72% extractives, 23.74% lignin, 1.37% ash and 69.17% holocellulose. The results obtained in the calorimetry test showed that the treatments applied to the endocarps were efficient in improving the cement hydration. The water absorption capacity, moisture content and density of the natural endocarp were 74.51%, 5.24% and 0.80 g/cm³, respectively. The high water absorption capacity of the endocarp negatively impacted the cement hydration at the matrix-endocarp interface of the composites. The results of the flexural tests of the cementitious composites indicated that the coating with the XSBR polymer improved the adhesion of the endocarp to the cementitious matrix. The compression and flexural tests performed on the concrete showed that, even at low concentrations, the endocarp showed potential to be used as a reinforcement element in the cementitious matrix.