This paper develops a kind of molded disc samples to investigate the carbonation and related behaviors of hardened cement pastes under different previous hydration degrees. Weight and length changes of cement pastes over time are monitored during a multistep process including carbonation, drying, rewetting, and redrying. The combination of X-ray diffraction (XRD) and thermogravimetric analysis (TGA) is used to identify and quantify the mineral compositions of carbonated cement pastes. An exponential function between CO2 uptake capacity and hydration time of cement pastes is established, which shows that the CO2 uptake capacity of cement pastes decreases dramatically at the very beginning days of hydration and then remaining relatively stable as hydration time is prolonged. Two reasons for this finding are revealed: i) the equilibrium between the carbonation and the post-carbonation reaction of carbonation product, i.e., silica-alumina gel; ii) refining of pore structures by hydration products which hinders carbonation. A clearer zonation of carbonation areas is proposed, and the spatial distribution equations of CO2 absorption are initially established. By monitoring carbonation and drying behavior of cement pastes with different hydration ages, it is revealed that carbonation reduces drying shrinkage of cement pastes especially for early-age samples, whereas drying increases carbonation shrinkage. By investigating the water changes during the multistep process, it is found that water is little released during the carbonation of C–S–H gels. New insight into mechanism of carbonation shrinkage is provided by a newly proposed model.@en

This paper employs PVA, PE, steel fibers, as well as the hybrids of two of the three fibers to reinforce alkali-activated slag (AAS) material, aiming to prepare strain-hardening and clinker-free composites. The flexural strength, compressive strength, uniaxial tensile performance of the composites and bond behavior between fibers and the matrix were tested to clarify the reinforcement effects of different fibers on the matrix. Strain-hardening AAS materials are obtained with compressive strengths of 116 MPa − 137 MPa (with fibers contributions of 17%−38%) and strain capacities over 0.8% at 60 d. The results indicate that there are several kinds of reinforcement effects of fibers on the matrix, namely bridging effect, lapping effect (for steel fibers), synergetic effect (for hybrid fibers) and static effect (for flexible fibers). Deterioration of PVA and PE fibers are found, indicating that these two fibers have poor adaptability in AAS material with a high alkalinity. This paper specially distinguishes the difference of the crack numbers during the strain-hardening stage only with the ones during the whole period including the following strain-softening stage. A new relationship is established between the crack numbers and the strain-stress curves, which provides a more reasonable way to characterize the strain-hardening property of fiber-reinforced composites.@en