Carbonation-induced properties of alkali-activated cement exposed to saturated and supercritical CO2

Abstract

Carbonation of well cement under high-temperature and high-pressure conditions is a significant concern in geological sequestration projects. This study evaluates the carbonation-induced properties of low to high calcium-based one-part alkali-activated cements (AACs) as an alternative to Portland oil well cement under reservoir conditions. Fly ash/slag-based AACs exposed to saturated (SAT) and supercritical (SUP) CO2 for 28 days were characterised using compressive strength, XRD and SEM tests. The strength properties of AACs increased with increasing slag content from 40% (low calcium AACs) to 100% (high calcium AACs) by weight of the binder under both SAT and SUP conditions. Aragonite is the major polymorph of CaCO3 in all AACs, irrespective of the type of CO2 exposure, and the highest aragonite content was observed in 100% slag AAC. The amorphous phase content decreased while the crystalline CaCO3 increased in carbonated AACs with increasing slag content, resulting in a dense microstructure in high calcium AACs. Sealing of microcracks and pores was observed in high calcium AACs with the accelerated precipitation of CaCO3 by reducing total porosity. Low calcium AACs resulted in a more highly porous carbonated microstructure than high calcium AACs under SAT and SUP conditions.