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

Injecting CO2 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 CO2-injecting caused negative impact. The effects of CO2-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 CO2-mixing dose (0.3 and 0.6 %), the formation of CaCO3 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 CO2-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 CaCO3 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 % CO2 dose can be mixed in the FA blended cement pastes for the production of environmentally-friendly CO2-sequestrating construction materials.