O conceito de materiais com gradação funcional aplicado ao desenvolvimento de gesso de alto desempenho

Abstract

This work shows the feasibility of using the concept of functionally graded materials (FGM) to develop pressure-formed plaster with high mechanical performance and improved resistance to water. Controlled porosity variation was achieved using the multiple optimiza-tion methodology. Firstly, this work seeks to understand the forming parameters that con-trol the physical, mechanical and water properties of pressure-formed gypsum (PCG). Subsequently, it uses the concept of materials with a functional gradation of porosity to achieve a plaster with high mechanical performance and lower susceptibility to water. The experimental part of this work was made up of four studies: (i) Pressure-formed gypsum, the effects of forming parameters on physical and mechanical properties; (ii) The effects of the retarder additive and forming parameters on the properties of pressure-formed gypsum; (iii) Optimization of forming parameters for assessing the mechanical and water-resistant behaviour of pressure-formed gypsum; (iv) Development of pressure-formed gypsum with a functional gradation of porosity. The experimental design was carried out using the 2k full factorial design and the central composite design (CCD), according to the study carried out. The studies showed that the level of pressure applied and the time of pressing, in relation to the setting time of the gypsum, interfere with the packing and ad-hesion between the hemihydrate crystals to form the dihydrate, with interference in porosity, mechanical strength and behavior against the action of water. The presence of the retarder additive acted as a lubricant, favoring the mobility of the gypsum granules during compaction, ensuring that pressure was applied before the gypsum hydration reactions began. This helped to increase the compactness and strength of the final piece. The combination of the parameters studied, pressure, mass, water/gypsum ratio and the presence of a retarding additive, enabled the maximum particle packing of (2,00 g/cm³), a maximum dynamic modulus of elasticity of (24,9 GPa) and maximum flexural tensile strength (27,8 MPa) and compressive strength (57,4 MPa) to be achieved. The results showed a reduction in capillary absorption of GCP (0,28 g/cm³) of more than 95% compared to GCC (5,51 g/cm³). During the wetting and drying cycles, GCP showed a reduction in immersion water absorption of around 88% compared to GCC. The average water absorption per immer-sion of the GCP, at 7 cycles (3,55 ± 0,13) % and 14 cycles (3,52 ± 0.28) %, indicated hydrophobic characteristics according to the Brazilian standard for water repellent gypsum blocks (≤ 5,0 %) and the European standard for plasterboard with a low water absorption coefficient (≤ 10,0 %). It was possible to obtain thin pieces with thicknesses of (6,88 ± 0,32) mm, low porosity (22,90 ± 1,72) %, high dynamic modulus of elasticity (17,39 ± 1,88) GPa and high flexural tensile strength (10,66 ± 1,20) MPa. Applying the concept of functional gradation made it possible to achieve a pressure-formed plaster with a functional gradation of porosity with a high flexural tensile strength (15,0 MPa) in the less porous layer and (9,7 MPa) in the more porous layer. The functionally graded plaster reached contact angles of (110,48° > 90°) on the less porous side and (99,12° > 90°) on the more porous side, being classified as hydrophobic (θ > 90°). The results achieved in this work indicate a possible application in the production of prefabricated blocks and components for structural use and in wet and wettable areas.