Corrosion of reinforcement In alkali-activated materials

Abstract

The increasing demand for sustainable concretes has promoted the development of a category of alternative binders called alkali-activated binders, including materials described as geopolymers, which are produced by the reaction of solid precursors such as fly ash, slag, and metakaolin with alkaline solutions. As buildings and infrastructures are aging nowadays, the durability aspects of concrete have become more important than in the past, and despite the acceptable mechanical properties of alkali-activated concretes, there are some concerns over their durability and service life as they were brought into service very recently. This dissertation aims to investigate various influential parameters involved in the initiation and propagation phases of corrosion of reinforcements in alkali-activated materials, as one of the main causes of deterioration of the concrete structures. Moisture transport, which influences several other corrosion-related properties such as the electrical resistivity and the oxygen diffusivity after depassivation of the reinforcement, is investigated for a range of representative samples. Water-vapour-sorption-isotherms were investigated and valuable insights into the pore structure of alkali-activated binders were obtained. The sorption kinetics was studied and the contribution of the nonFickian sorption process was assessed for both aluminosilicate-dominated and calcium-rich binders. A systematic study of the effect of alkali, calcium, and silicate content on the apparent chloride diffusion coefficients and chloride threshold values of a wide range of alkali-activated mixes were conducted and the results were interpreted in light of the nano and microstructural properties. Applicability of the electrochemical test methods conventionally developed and used for Portland cement-based concretes was assessed for low calcium alkali-activated concretes, and the chloride-induced corrosion propagation phase was studied. The effect of alkali concentration in the activator, and the carbon dioxide concentration on the pH drop and passivity of the reinforcement were investigated by modeling the adsorption of carbon dioxide into an aqueous NaOH solution, as representative of the pore solution, and the results were validated against the experimental observations. Finally, the alkali cation leaching which can lead to the loss of alkalinity was also evaluated for a range of alkali-activated samples.