In recent years, CO₂ curing has attracted increasing attention as it not only enhances the early age strength of cement-based materials but also captures and sequesters CO₂, thus it has been regarded as a new curing method with a broad application prospective for eco-friendly construction materials. Since CO₂ curing generally lasts several hours to several days, there are considerable residual cement clinkers in the cement matrix after CO₂ curing. These un-carbonated cement clinkers will rehydrate once in contact with water. The rehydration products of cement clinkers can recover the pH value of the pore solution, reduce the porosity, and refine the pore size distribution of the cement matrix. Thus, the subsequent hydration of CO₂-cured cementitious materials largely governs the long-term performance of cement-based materials, such as compressive strength and durability. Several researchers have studied the subsequent hydration of CO₂-cured cementitious materials, but their conclusions were contradictory. Some researchers reported that the carbonation products calcite promoted the subsequent hydration of un-carbonated cement clinkers. On the contrary, some researchers reported that CO₂ curing hindered the subsequent hydration of un-carbonated cement clinkers and thus lowered the late compressive strength of cement-based materials. Thus, more studies should be conducted to reveal the subsequent hydration mechanism and strength evolution of early CO₂-cured cement-based materials. In this investigation, the subsequent hydration and compressive strength evolution characteristics of early CO₂-cured cement paste were studied by macro-physical experiments and micro-characterization techniques.

Ordinary Portland cement P·I 42.5 complying with GB8076—2008, and de-ionized water was used to prepare cement paste samples. The water-cement ratios were 0.3, 0.4, and 0.5, respectively. Two groups of cement paste cubes measuring 20 mm were cast. One group of cement paste samples was subjected to 24 h of early CO₂ curing and subsequent standard curing in saturated lime water. The other group of cement paste cubes was subjected to standard curing in saturated lime water, which served as a reference.

After CO₂ curing, the carbonation depths of cement cubes were determined using phenolphthalein indicator, and CO₂ uptake was determined by thermal gravimetric (TG) analysis. The compressive strength of cement paste was measured at the curing ages of 2, 28 d, and 90 d, respectively. The pore solution was extracted with an expression device for pH value measurement. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the phase evolution. The amounts of silicate and aluminate hydrates were assessed using thermal gravimetric and differential thermal gravimetric (TG-DTG) analysis. The microscopic characteristics and pore structure were examined by scanning electron microscopy (SEM) and mercury intrusion porometer (MIP) at the curing ages of 2, 28 d, and 90d, respectively.

Similar to the cement paste subjected to standard curing, the compressive strength of early CO₂-cured cement paste continues increasing as it is cured in lime water after CO₂ curing. Nevertheless, the strength improvement of the latter is lower than the former as the subsequent hydration of the former is less significant. In addition, the strength improvement of early CO₂-cured cement pastes in the lime water decreases with the increase of water-cement ratio. After curing to 28 d in lime water, the pH value of early CO₂-cured cement pastes with water-cement ratios of 0.3, 0.4, and 0.5 recovers to 13.14, 12.41, and 12.33, respectively, which is higher than the threshold of de-passivation.

XRD patterns reveal that the diffraction intensities of tricalcium silicate and dicalcium silicate decrease continuously when the early CO₂-cured cement pastes continue curing to 28 d and 90 d in lime water. However, this decrement is less significant than that of standard-cured cement paste. It indicates that CO₂ curing inhibits the subsequent hydration of un-carbonated cement clinkers. This can be attributed to the fact that the formation of calcium carbonate coating on the surface of cement particles. The FTIR spectra reveal that carbonation products silica gel is reactive towards Ca(OH)₂, and it is completely consumed before the age of 28 d. This is responsible for the absence of Ca(OH)₂ in the XRD patterns of early CO₂-cured cement paste with water-cement ratios of 0.4 and 0.5 at 28 d and 90 d, respectively. TG-DTG results suggest that less silicate and aluminate hydrates are produced in early CO₂-cured cement matrix than in standard-cured cement matrix during subsequent lime water curing, and it decreases with the increase of water-cement ratio as denser calcium carbonate coating on the surface of un-carbonated cement particles.

SEM results show that subsequent lime water curing improves the microstructure of early CO₂-cured cement paste due to precipitation of hydration products, while the improvement is less significant than that of standard-cured cement paste. Subsequent lime water curing decreases the porosity and refines the pore size distribution of early CO₂-cured cement paste. The pore diameter of early CO₂-cured cement paste mainly distributes in the range of 10 nm to 50 nm, both at 28 d and 90 d.

In the course of subsequent lime water curing, the main reactions that occurred in early CO₂-cured cement paste include the hydration of un-carbonated cement particles and the interactions between silica gel, calcium-modified silica and Ca(OH)₂. The subsequent hydration degree of early CO₂-cured cement paste is lower than that of cement paste subjected to standard curing, and it decreases with the increase of water-cement ratio. The compressive strength of CO₂-cured cement pastes continually increases with the prolonging of the subsequent curing period, and the growth rate of late compressive strength decreases with the increase of water-cement ratio.