Síntese e avaliação de ligante geopolimérico “one-part” para aplicação em cerâmicas refratárias

Abstract

Refractory ceramics play a pivotal role in the development and operation of strategic industrial processes worldwide. Due to their high versatility and excellent performance in extreme service conditions, refractory concretes containing calcium aluminate cement (CAC) as a binder are widely used. However, the search for alternative materials with a reduced environmental impact has been actively promoted in recent years, with geopolymers or inorganic polymers being potential substitutes for CAC in such ceramics. This study aimed to assess the feasibility of partially or completely replacing this cement with a "one-part" geopolymeric binder in the production of refractory ceramics. This binder was produced by mixing colloidal silica and NaOH solution, followed by drying at 200°C. The solid activator and precursor were chemically and physically characterized to understand their effect on the final ceramic properties. In a subsequent stage, a composition of the reference concrete matrix (containing only CAC as the binder) was prepared and analyzed, along with others containing different CAC contents and/or geopolymeric binders. The evolution of the physical and mechanical properties of the samples was studied after curing at 40°C and after firing at 800, 1000, and 1250°C to comprehend the impact of composition and thermal treatment on the final properties of the refractories. To achieve this, tests were conducted to determine porosity, apparent density, elastic modulus, dimensional variation, mineralogical composition, and mechanical strength of the test specimens produced. The results indicated limited geopolymerization during curing, resulting in low mechanical performance (<1 MPa) in the geopolymeric matrices, compared to matrices containing only CAC (7.2 MPa). Unlike the cement-containing system, the presence of hydrated phases was not identified in the geopolymer-bound compositions, with unreacted raw materials remaining in the microstructure. After firing, the geopolymeric compositions achieved a significant increase in the modulus of rupture, reaching values of around 41 MPa, surpassing the performance of the reference concrete. The effective sintering via a liquid phase in the microstructure, coupled with the formation of nepheline in the geopolymeric matrix, justifies this improved behavior of the produced ceramics. Therefore, the addition of the "one-part" geopolymer as an alternative binder to CAC in refractory formulations has proven to be an advantageous option in terms of the thermo-mechanical properties of the matrices produced. However, additional investigations are required to optimize its performance at low temperatures to ensure its broad applicability.