Ambient pressure carbonation curing of cold-bonded fly ash lightweight aggregate using coal-fired flue gas for properties enhancement and CO2 sequestration

Abstract

In the current CO₂ curing process, pure CO₂ gas with a concentration exceeding 99 % is primarily used. However, flue gas, which typically contains 10–30 % CO₂, can also be utilized for carbonization. This study sought to explore the viability of employing flue gas for carbonation and assessed the impact of impurity gases such as SO₂. Two typical industrial solid wastes (fly ash and coal gangue) were used to substitute a portion of the cement to prepare light aggregates, which were carbonized under varying concentrations of CO₂ and SO₂. The porosity and water absorption of the samples decreased after carbonation. A higher degree of carbonation was observed at increasing CO₂ concentration. Aggregates carbonated with 15 % CO₂ improved the CO₂ absorption by 48 %. The actual CO₂ uptake reached up to 58.3 % of the theoretical value. The presence of SO₂ has been found to impact the uptake of CO₂. The CO₂ uptake initially declined and then increased as the SO₂ concentration increased. The existence of SO₂ led to varied increases in the leaching concentrations of the aggregates following the process of carbonation, and some even exceed standard limits. In the presence of both CO₂ and SO₂, SO₂ reacted with the aggregates, resulting in the creation of calcium sulfate. This reaction disrupted the structure of the aggregate, facilitating the diffusion of CO₂ into the samples.