Gas–Solid Carbonation Behavior of Tricalcium Aluminate: Kinetics, Mineral Evolution, and Strength Contribution

A detailed investigation of tricalcium aluminate (C3A) under CO2 exposure is essential for developing Al-rich materials via carbonation. In this study, the carbonation properties of C3A were investigated, including the carbonation kinetics, the contribution of mineral evolution to strength development, the effects of dissolution properties, and the influence of temperature and prehydration treatment. The results indicate that compared to other Portland cement clinkers, C3A exhibited the highest early carbonation rate (k1 = 59.96 min–1) due to its high dissolution rate but the lowest degree of carbonation (22.9%) due to its low Ca2+ leaching concentration (≤388.3 mg/L). Hydration took precedence over carbonation in C3A, forming hydrogarnet (C3AH6), which exhibited lower carbonation activity and was unfavorable for subsequent carbonation. The elastic modulus of C3A decreased from 4.89 ± 0.34 to 4.14 ± 0.42 GPa after 2 h of prehydration followed by 72 h of carbonation at 25 °C, as determined by three-dimensional digital image correlation. Increasing the temperature from 25 to 60 °C effectively promoted the carbonation of hydration products, primarily C3AH6 and Al-monocarbonate (C4AČH11), facilitating the formation of aragonite fibers and gibbsite. Aragonite fibers, with an average length of 1.914 μm, increased in content from 9.5 to 18.5 wt %, interlacing with each other to form an urchin-like structure that provided a fibrous effect, increasing the flexural strength from 5.85 ± 0.19 to 9.07 ± 0.34 MPa. Gibbsite increased in content from 7.0 to 13.4 wt % and tightly wrapped around the surfaces of unreacted C3A grains, working synergistically with CaCO3 crystals with an average elastic modulus of 59.2 GPa (determined by nanoindentation) to serve as cementing agents, increasing the compressive strength from 28.71 ± 2.45 to 45.14 ± 3.94 MPa and the elastic modulus from 4.89 ± 0.34 to 11.77 ± 0.99 GPa. In comparison, at 25 °C, C3A powder primarily produced nanospherical vaterite with a crystallite size of approximately 27 nm and only trace amounts of gibbsite (0.8–4.0 wt %).