Carbonation Curing of Precast Fly Ash Concrete

The feasibility of carbonation curing of precast fly ash concrete is studied. If fly ash concrete can be produced by carbonation curing, the carbon footprint of the products can be significantly reduced. In this paper, the relationship between carbonation reaction and pozzolanic reaction was examined. After carbonation curing with different duration and fly ash content, the cement reaction degree was estimated through the equivalent nonevaporable water content, and the fly ash reaction degree was analyzed through a selective acid dissolution test. It was found that the pozzolanic reaction of fly ash in a fly ash–ordinary portland cement (OPC) system was hindered by early carbonation reaction. The higher the early carbonation degree, the lower the pozzolanic reaction of fly ash. In addition, fly ash–OPC paste was more reactive with carbon dioxide than plain cement paste. Therefore, controlled carbonation at an early age is necessary to trade off the carbon emission reduction with performance gain. The study shows that if fly ash content is limited to 20% of cementitious material and carbonation curing duration is not to exceed 12 h, fly ash concrete can be produced with higher early strength, comparable late strength, better durability performance, and a carbon emission reduction of 36%. After subsequent hydration, carbonated fly ash concrete can have a pH value comparable with the hydration reference and can be used in precast concretes with steel reinforcement.