Mechanical and microstructural performances of fly ash blended cement pastes with mixing CO2 during fresh stage.

[Display omitted] • CO 2 -mixing dose decreased the flowability but enhanced the early cement hydration. • The presence of fly ash can offset the flowability loss. • Low CO 2 -mixing dose (<0.6 %) slightly improved the early compressive strength. • Micro cracks formed in CO 2 -mixing pastes degraded the 28 days strength. Injecting CO 2 in the fresh cement-based mixture during mixing could considerably lower the workability. In this study, fly ash (FA) was used as a cement replacement at 10, 15, and 20 wt.% to overcome the CO 2 -injecting caused negative impact. The effects of CO 2 -mixing dose (injected during fresh stage) on the fresh, mechanical and microstructural performances of cement pastes were examined. The experimental results showed that FA resulted in a lower hydration heat and improved the blended cement paste workability. At a fixed FA content of 20 % and with the presence of a relatively lower CO 2 -mixing dose (0.3 and 0.6 %), the formation of CaCO 3 on the clinkers can act as a nucleation site to promote the hydration of cement and fill the pores in the matrix to reduce the pores, resulting in a 3.3–8.2 % improvement of the early compressive strength at 1 and 3 days. However, at a relatively higher CO 2 -mixing dose (0.9 and 1.2 %), prolonging the curing ages to 7–28 days resulted in a slightly decrease in the compressive strength due to the excessive CaCO 3 covering on the clinker surface and the formation of micro-pores and cracks between the clinker and the new hydration products. It can be concluded that below 0.3 % CO 2 dose can be mixed in the FA blended cement pastes for the production of environmentally-friendly CO 2 -sequestrating construction materials. [ABSTRACT FROM AUTHOR]