Ettringite-Related Dimensional Stability of CO₂-Cured Portland Cement Mortars

Carbonation curing, where cement is cured in a CO2-rich environment, is an emerging technology in the precast industry that promotes CO2 sequestration in construction materials. It demonstrates good promise for enhancing material strength development, chemical resistance, and manufacturing sustainability. A critically important but little studied aspect of carbonation curing is the dimensional stability of the cured materials, which poses a significant impact on the longevity of cement-based materials and structures. In this study, we propose the use of carbonation curing, in place of conventional energy-intensive steam curing and time-consuming moisture curing, and examine the dimensional stability of Portland cement mortars manufactured by different curing approaches. The common pathology in precast concrete that potentially creates undesirable expansions, known as Delayed Ettringite Formation (DEF), is particularly investigated. Four curing approaches were examined including moist, steam (maximum temperature at 85 °C to purposefully induce DEF), combined steam-carbonation, and carbonation curing in which steam curing was used to verify favorable cement chemistry for DEF and to indicate DEF onset timing. It was found that, among the four curing approaches designed in this study, the carbonation-cured mortars demonstrated the lowest expansion (∼0%) whereas the steam-cured mortars showed the highest (>0.6%) after 500 days of storage in saturated limewater. Mortar expansion after the combined steam-carbonation curing (∼0.1%) was lower than that of steam curing but remained higher with respect to the moisture-cured reference. The mineral ettringite was detected at 500 days after all curing approaches. However, larger ettringite crystal size (5–10 μm) with greater dispersion was observed via scanning electron microscopy after steam curing in reference to carbonation curing (<5 μm). Additionally, the calcium carbonate precipitated by CO2 carbonation was found to participate in the postcarbonation hydration to promote formation of monocarboaluminate and hemicarboaluminate, which stabilize ettringite and potentially lead to a secondary densification of the mortar microstructure.