Compósitos geopoliméricos obtidos pela ativação alcalina de metacaulim e aluminossilicatos de minerais suscetíveis a reações do tipo álcali-agregado

Abstract

Alkali-activated matrices with Si+Al based precursors or geopolymers are cementitious composites obtained from the alkaline activation of amorphous aluminosilicates and present physical-mechanical properties similar to hydrated Portland cement matrices. The activating solutions are made from alkaline hydroxides and silicates, being capable of dissolving the precursor materials. For geopolymer precursors, any material with high amounts of silicon and aluminum and a disordered molecular structure can be used to produce these matrices, as long it obeys the appropriate Si/Al molar ratios. This includes materials such as metakaolin, fly ash, red mud and some mineral aluminosilicates. At the same time, it is well-established knowledge that the use of amorphous silica-based mineral phases in Na/K rich cementitious matrices can lead to a degradation mechanism known as the alkali-silica reaction (ASR). This phenomenon is likely to occur in Portland cement concretes and alkali-activated matrices with Si+Ca based precursors, where the pathological manifestations of ASR are severe. However, this is not observed in geopolymers, and given the similarity between their precursors and the mineral phases that cause ASR, it is possible that the reason for this is that during the consolidation of geopolymers, the mineral phases are an active phase of the reaction. In order to verify this hypothesis, the aim of this work was producing geopolymers made of metakaolin and mineral aggregates susceptible to ASR at different levels and to analyze the physical, chemical, mechanical and morphological changes that the products underwent. The methodological procedure was divided into 5 main phases: precursors and alkaline activators statement and preparation; characterization of these materials and definition of their proportions; geopolymer samples test moldings; definitive moldings and physical-mechanical characterization; then finally, chemical and microstructural characterization of the products. Two pulverized and sieved mineral aggregates with metakaolin were used as precursors, and sodium hydroxide and sodium silicate as activators. These precursors were chemically characterized by XRF, XRD, FTIR and Raman spectroscopy. After the initial test and validation of the defined materials ratio, all the remaining specimens were molded to be physical and mechanical characterized by their immersion water absorption and axial and diametrical rupture. Finally, chemical characterization was carried out and microstructural characterization by scanning electron microscopy. The results indicate that the presence of mineral aggregates in geopolymers reduces or increases their mechanical strength depending on the percentage of metakaolin replacement, age and curing temperature. Water absorption is drastically reduced in geopolymers composed of aggregates, indicating that there is a reduction in capillary pores in the matrix when metakaolin is replaced. Chemical and molecular characterization indicates that some of the mineral phases of the aggregates have the same chemical bonds as crystalline geopolymers, and the replacement of metakaolin with aggregates leads to the formation of zeolites in the matrices when cured at 80 oC. Microstructural characterization showed that the aggregates act as nucleation centers for the geopolymer product, or dissolve in the alkaline medium to produce the geopolymer. It was concluded that aggregates susceptible to ASR can be used as geopolymer precursors, but in-depth studies to ensure efficient mechanical properties are still required.