Influence of physico-chemical characteristics on the carbonation of cement paste at high replacement rates of metakaolin

- Cement paste with fluidized bed and flash metakaolin was tested at replacement levels of up to 50%.
- Carbonation of the cement paste was evaluated in high CO2 and humidity levels.
- Physico-chemical characterization using water porosity, absorption, TGA and XRD.
- Flash metakaolin was less susceptible to carbonation than fluidized bed.

The study detailed in this paper examined the influence of two types of metakaolin (MK1 and MK2) at high replacement rates on the carbonation rate of cement paste. “Flash” calcination produced MK1, which had a high amount of quartz mixed in, while “fluidized bed” calcination produced MK2, which was relatively pure. The metakaolins were incorporated into the cement paste at rates of up to 50% by weight of cement and cured for 90 days. With regard to durability, MK replacement increased water porosity and susceptibility to carbonation, while water adsorption and primary sorptivity decreased. Thermogravimetric analysis (TGA) and X-ray Diffraction (XRD) analysis of the hydration phases showed a decrease in portlandite (Ca(OH)2) content with MK replacement, increasing carbonation susceptibility. Decreased portlandite showed a strong correlation with an increased carbonation rate, indicating that these are primary factors in metakaolin cement paste carbonation. With TGA deconvolution, MK1 significantly increased CSH content up to a replacement level of 20%, while replacement levels over 30% appeared to reduce CSH formation. MK2 replacement did not have as significant an effect on CSH formation, corresponding with the higher primary sorptivity and greater susceptibility to carbonation penetration of MK2.