Characterizing the effects of magnesium content and carbonation on the pore structure of alkali-activated slags

Abstract

This thesis is focused on characterizing the pore structure of alkali-activated slag paste (AAS, a sustainable cementitious material) and how its morphology changes with magnesium content and carbonation, a common concrete degradation process. Currently, ordinary Portland cement (OPC) is the most commonly used binder for concrete, but AAS offers a promising, sustainable alternative. However, little is known about its long-term durability, including its resistance to carbonation and interaction with CO2. Recently, studies have shown that increasing the magnesium content in AAS, via different blast furnace slag sources, plays a role in resisting accelerated carbonation through the formation of a Mg-stabilized amorphous calcium carbonate (ACC). However, the effects of this ACC phase on AAS pore structure has yet to be elucidated. My thesis examines the pore size distribution and diffusion tortuosity of low- and high-magnesium content silicate-activated AAS through the techniques of nitrogen sorption, mercury intrusion porosimetry (MIP), and X-ray microtomography (μCT). Additionally, a random walker method is implemented in the reconstructed pore-space from μCT to calculate the diffusion tortuosity of the pastes. AAS is shown to be more resistant to pore structural degradation following carbonation than OPC, and increased Mg content in AAS is shown to further increase resistance to capillary pore formation and gel pore loss from decalcification due to carbonation.