CO 2 Treatment of Hydrated Cement Powder: Characterization and Application Consideration

Residual hydrated cement powder (HCP) produced from recovering recycled concrete is usually restricted from being directly reused in concrete due to its high water absorption and porous structure. This study aims to enhance the properties of HCP via CO2 treatment. The simultaneous impacts of CO2 treatment and water-To-cement (w/c) ratio on physical properties and microstructure of HCP were studied. The findings showed that carbonation can effectively reduce the porosity of HCP owing to the formation of calcite, and the calcite content increases with increasing w/c ratio. Furthermore, there was a reduction in the 28-day compressive strength of paste specimens containing uncarbonated HCP, while paste containing 5%-20% of carbonated HCP (CHCP) achieved higher strength than the control paste. It is believed that the presence of calcite in CHCP leads to the formation of more stable calcium aluminate monocarbonate. The mass ratio of highly crystalline to poorly crystalline CaCO3 increased with an increase in the replacement level of CHCP, and the w/c ratio exhibited no significant influence at a given replacement ratio.