Effect of eggshell powder addition on the properties of cement pastes with early CO2 curing and further water curing

Bio-waste eggshell powder (ESP) has a similar chemical composition to natural limestone, which has attracted interest as a partial cement replacement to reduce the carbon footprint in cement. However, ESP replacement in cement systems would cause a decrease in compressive strength. The compressive strength can be promoted after carbonation and further water curing. Also, the addition of limestone materials can promote the carbonation of cement under early CO2 curing. This study investigated the effects of adding 0%, 5%, 10%, and 20% of ESP on early CO2 curing and further water curing of cement pastes. The CO2 uptake, compressive strength and water absorption of pastes were observed, and the chemical composition and microstructure were evaluated by thermogravimetric analysis (TGA), X-ray powder diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The filling effect of ESP decreased the access of CO2, while the increased nucleation site provided by ESP led to the increase of carbonation products, which mainly resulted by the more calcium ions from calcium silicate hydrate (CSH) participating in carbonation. Thus, the CO2 uptake was increased with more than 5% ESP replacement after 24 h carbonation at 20% CO2. The 3d strength of ESP-mixed cement was increased after carbonation with less than 10% ESP replacement, the increment of the strength of carbonated pastes was decreased with the increase of ESP addition due to the dilute effect, and the strength of 20% ESP-mixed cement paste was decreased after accelerated carbonation. Also, the subsequent hydration rate of cement pastes with ESP replacement was significantly increased during water curing, and the porosity of carbonated pastes was less than standard cured pastes. However, the decreased degree of cementation of hydration products in carbonated pastes caused the 28d strength did not improve compared with standard cured pastes. Therefore, around 10% ESP replacement is beneficial to both CO2 uptake and early strength improvement of cement pastes after accelerated carbonation.