Caracterização avançada e in situ de concretos refratários aluminosos ligados com geopolímeros

Abstract

Monolithic refractory castables are widely used as linings in equipment subjected to high temperatures, chemically aggressive atmospheres, and severe mechanical loads, requiring high thermal stability, mechanical strength, and long-term service durability. Traditionally, calcium aluminate cement (CAC) has been employed as the binder due to its rapid development of green strength and ease of processing. However, its hydraulic nature and the transformation associated with the decomposition of its hydrates during heating may compromise the thermomechanical behavior and microstructural stability of the castables, particularly at intermediate and high temperatures. In this context, the present work investigated the thermomechanical behavior and microstructural evolution of geopolymer-bonded refractory castables synthesized from different precursors – metakaolin, calcined kaolin, and aluminum sulfate production residue –, comparing them with conventional CAC-bonded systems through advanced in situ characterization. The methodology involved formulating castables bonded with CAC and/or geopolymers, followed by physical-mechanical and microstructural characterization at different temperatures, employing in situ elastic modulus, optical dilatometry, and mechanical testing coupled with digital image correlation (DIC). The results demonstrated that binder chemistry directly influences the microstructural evolution and thermomechanical performance of the castables. Geopolymer-bonded systems progressively developed a more efficient ceramic bond through viscous sintering mechanisms, resulting in gains in mechanical strength and elastic modulus after thermal treatments. Among the formulations evaluated, the castable bonded exclusively with metakaolin showed the best overall performance, evidenced by the highest levels of densification, mechanical strength, and stiffness after firing, as well as quasi-ductile behavior at high temperatures. The integrated in situ approach proved effective in identifying phase transformations, liquid phase formation, and damage mechanisms during heating, evidencing the potential of geopolymers as alternative binders to CAC for refractories subjected to high-temperature service conditions.