Influence of carbonized fly ash on the early-age shrinkage of blended cement paste with different water-to-binder ratios

The research investigates the carbonation of fly ash-carbide slag by wet carbonation method to obtain carbonized fly ash loaded with nano-calcium carbonate. The carbonized fly ash is utilized as a supplementary cementitious material (SCM) to partially replace cement. Its effect and mechanism of action on the early shrinkage of blended cements with different water-binder ratios were studied. The results indicate that the specific surface area of fly ash increases after carbonation modification. With the increase in the water-to-binder ratio, the incorporation of carbonized fly ash significantly influences the setting time, non-evaporable water content, autogenous shrinkage, and chemical shrinkage of the blended cement compared to the blended cement with uncarbonized fly ash. At the same water-to-binder ratio, the incorporation of carbonized fly ash shortens the setting time of the blended cement, increases the non-evaporable water content, and reduces both chemical shrinkage and autogenous shrinkage. The nanoscale calcium carbonate particles on its surface can act as nucleation sites and react with aluminates to produce Mc and Hc, increasing the volume of hydration products. This further prevents the transformation of ettringite into Ms, thereby reducing the early shrinkage of the blended cement. This study aims to reduce the durability reduction caused by early shrinkage of cement-based materials by replacing some cement with carbonized fly ash carbide slag.