Os efeitos do cristalizante e da diatomita super-hidrofóbica sobre as propriedades físicas e mecânicas de pastas de gesso

Abstract

The construction gypsum is a recyclable binder with low energy consumption and low CO2 emissions; however, after its hardening, it exhibits low resistance to water action, compromising its integrity and durability. As a result, several studies have been carried out with the aim of addressing this issue. The use of additives with the potential to promote hydrophobic characteristics in gypsum emerges as one of the alternatives to reduce its susceptibility to water action. The present study proposes the evaluation of the effects of the crystallizing agent and super-hydrophobic diatomite (DT) with silane modifier (OTS) on the mechanical properties, porosity, and contact angle of gypsum pastes. For this purpose, various statistical mixture designs were developed using the Design-Expert® software. The formulations generated for the studies on the additions of the crystallizing agent and treated diatomite were molded following the NBR 12129:2019 procedure and evaluated for dynamic modulus of elasticity (MOE), hardness (D), flexural tensile strength (RT), compressive strength (RC), porosity (), and contact angle (C). Some formulations with low water content were pressed as an alternative to enhance the miscibility between the super-hydrophobic diatomite and the gypsum paste. The pressed specimens were evaluated for MOE, RT, RC, and C determination. The optimized formulations from the statistical designs of the crystallizing agent and DT paste with ethanol were subjected to natural aging for durability evaluation. The results show that the use of hydrated lime, employed as a source of calcium hydroxide to react with the compounds of the crystallizing agent, led to an increase in D and reduced  in pastes containing the crystallizing agent. There was a significant increase in C with the increase of the crystallizing agent; however, with the maximum content used (5%), the pastes are still classified as hydrophilic (C < 90°). The application of a 30% content of the crystallizing agent in the mixture statistical design allowed obtaining hydrophobic pastes (90° < C < 150°). The treatment of diatomite with OTS conferred super-hydrophobic properties to diatomite (C > 150°) and modified its morphology. However, increasing the mass fraction of super-hydrophobic diatomite paste and ethanol in the formulations did not promote the expected increase in C and reduced MOE, RT, and RC, due to the use of ethanol in the pastes, resulting in changes in crystal structure and increased porosity. On the other hand, pressing provided an increase in C with the increase of DT in the pastes. The optimized formulations with the addition of the crystallizing agent and DT paste with ethanol, after exposure to natural aging, presented reductions in the contact angle and mechanical properties (D, RT, and RC) due to the increase in porosity as the matrix ages, wear, and surface pores observed, in addition to factors related to the additives employed. Furthermore, the additives showed promising results in increasing the contact angle and reducing gypsum's susceptibility to water action.