Multi-Scale Characterization of Carbonation Reaction in Alkali-Activated Materials

Abstract

The concept of capturing and storing CO2 within alkali-activated materials (AAMs) represents an environmentally friendly solution in the construction industry. With the added advantage of utilizing industrial byproducts as raw materials and possessing high mechanical and durability properties, AAMs offer the potential to transform concrete into a carbon-negative product. This study aims to understand the relationship between processing, microstructure, and properties in carbonation of AAMs. Toward that end, this study investigates the effects of carbonation on AAMs by subjecting them to CO2 curing at different concentrations (1% and 5%) and durations (up to 84 days) and conducting experimental characterization crossing multiple length scales. To synthesize the AAM binders, sodium metasilicate was employed as the alkaline activator, as well as raw precursors of slag and class F fly ash. Based on a comprehensive literature review, three primary processing parameters were selected for investigation: MgO content, pH level and the saturation level of pore solution with respect to calcium and carbonate ions. Key findings are as follows: (1) Dissolution of CO2 in the pore solution lowered the pH level, but acidification enabled calcium leaching from the solid matrix, resulting in a pH rebound. Binders containing higher levels of calcium exhibited increased initial alkalinity and CO2 dissolution, leading to a more intense pH level recovery following the process of carbonation. (2) Carbonation resulted in the precipitation of calcium carbonate within the solid matrix with two primary polymorphs of vaterite and calcite, while higher dissolution of carbonate ions in the pore solution and addition of MgO led to formation of aragonite in some cases. (3) Effect of carbonation on compressive strength of AAM binders was highly dependent on the CO2 curing conditions. When subjected to 1% CO2 curing for 28, 56, and 84 days, strength gain was observed. However, with 5% CO2 curing, the strength gain was only significant up to 14 days, beyond which the strength started to decrease. (4) Calcium leaching from the solid matrix resulted in an increase in the gel pores' volume. In summary, this dissertation explored the CO2 curing of AAM binders and revealed its high potential as a carbon-negative product.