Damage characterization of carbonated cement pastes with a gradient structure

CO₂ curing cementitious materials shows promise as a method to both reduce and sequestrate CO₂, nonetheless, it results in the formation of a gradient structure in them. In this study, the mechanical behavior, damage mode and inhomogeneity of carbonated cement pastes are investigated, aiming to establish the intrinsic link between their damage and inhomogeneity. The results indicated that carbonated cement pastes exhibit pronounced stress instability and brittle damage at low strengths, closely linked to their inhomogeneity. Moreover, carbonated cement paste is an inhomogeneous mass with a gradient structure. It displays a three-layer structure comprising an outermost, intermediate, and innermost layer. The outermost layer primarily comprises calcite, with minor amounts of aragonite and silica gel. Furthermore, its porosity, average micro-hardness, and elastic modulus are 26.81 %, 58.62 HV, and 84.66 GPa, respectively. The intermediate layer consists mainly of calcite, aragonite, calcium hydroxide, C-S-H gel, and silica gel, with porosity, average micro-hardness, and elastic modulus of 28.46 %, 37.21 HV, and 53.74 GPa, respectively. The innermost layer is composed of C-S-H gel, calcium hydroxide, calcite, aragonite, calcium hydroxide, and silica gel, with porosity, average micro-hardness, and elastic modulus values of 29.29 %, 25.73 HV, and 58.87 GPa, respectively. The damage in cement pastes with a low degree of carbonation primarily arises from mixed shear-tensile cracks, whereas in cement pastes with a high degree of carbonation, tensile cracks are the predominant cause of damage.