Enrijecimento de geopolímeros a base de betacaulim: relações entre propriedades reológicas e parâmetros de síntese

Abstract

Geopolymers are a sustainable alternative to Ordinary Portland Cement (OPC). These binders with a lower CO2 footprint are produced by alkaline activation of a source of amorphous aluminosilicates. The geopolymers can exhibit better performance depending on the synthesis conditions than OPC. However, there are still barriers to its large-scale adoption, as the need for a deeper understanding about its behavior in the fresh state, including controlling setting times and alternatives to adjust the workability. The organic admixtures widely used to improve the rheology and workability of OPC-based systems show lower efficiency in geopolymers due to the high alkalinity of the activator solution. In this scenario, the geopolymers behavior in fresh state can only be controlled by its composition, expressed by the synthesis/activation conditions. Here, fresh state behavior was evaluated to understand the effects generated by different synthesis conditions in geopolymers produced with metakaolin. The effect of the water/binder ratio (a/l: 0.70, 0.75 and 0.80), alkali content (15% and 20% of M2O) and alkali type (sodium or potassium) were assessed. The analysis was based on monitoring the time-dependent behavior of the samples, using oscillatory tests to quantify the storage (G’) and loss (G”) moduli. In addition, the results of oscillatory rheometry were correlated with reactions kinetics by isothermal calorimetry. It was also determined the setting times of the samples. The results showed that alkali type has the greatest impact on the geopolymer’s behavior in fresh state, given that the systems activated with potassium had less pronounced hardening, and more time to be handle when compared to those systems activated with. The increase in alkali content (% M2O) was also effective to extend the workability during a longer period. Increasing a/l ratio was not effective to modify the rheological parameters evaluated, mainly in systems with higher alkali content.