Competitive mechanisms of hydration and carbonation in hydraulic lime under natural and accelerated carbonation (3% CO2)

The performance of hydraulic lime primarily depends on the hydration and carbonation reactions, which occur simultaneously and mutually inhibit each other, creating a competitive relationship. This study investigates the competitive mechanisms of hydration and carbonation in hydraulic lime containing 0%, 10%, 20%, and 30% metakaolin (AL, MK10AL, MK20AL, MK30AL) under natural curing (NC) and accelerated carbonation (AC, 3% CO2). The results show that under NC curing, increasing the metakaolin content accelerates hydration, leading to the formation of C2SAH8, C3AĈH11, C3AH6, and C-S-H gels. These products form a dense network with the remaining calcium hydroxide (CH), reducing pore volume, average pore size, and porosity, thereby improving compressive strength. MK30AL reached 14.5 MPa at 28 days, compared to AL’s 2.7 MPa. Under AC curing, carbonation predominates, consuming CH and slowing hydration. This decomposes hydration products, creating a microstructure primarily composed of loosely packed calcite. While AC enhances AL's pore structure, it negatively affects MK-AL’s hydration phases, increasing capillary pores and reducing gel pores and mesopores. As a result, MK-AL’s compressive strength under AC at 28 days is approximately half of that under NC.