The cordierite–belite core-shell lightweight aggregate (CSLWA) effectively improves the performance of the interfacial transition zone (ITZ) in lightweight aggregate concrete; however, the underlying fracture behavior still needs to be revealed. This study characterized the reinforced mechanical properties of ITZ by CSLWA through nanoindentation and interfacial fracture tests and the reinforcement mechanism was further explained using finite element analysis. The results showed that the continuous hydration of the belite shell of CSLWA enhanced the yield stress and improved the failure strain during fracture. Energy dissipation was observed during the fracture of CSLWA concrete, improving the brittleness of the lightweight aggregate concrete. Finite element analysis showed that the core strength of CSLWA changed the energy dissipation model and affected the toughness of CSLWA concrete, and a suitable composition of 6:4 was proposed for the “cordierite–anorthite” core constitution in CSLWA depending on the strength order between the cement matrix and core of the CSLWA.

Due to low temperature environments and other factors, the performance of cement slurry is affected by deep-water shallow formation cementing, which makes it difficult to ensure the interface bonding quality. The objective of this study is to investigate the effect of fly ash (FA), metakaolin (MK), and microsilica (MS) as supplementary cementitious materials (SCMs) by replacing 5–20% of the cement on the bonding property of cement with casing in the shallow formations of deep-water environments. This is based on the cementing conditions for the 20″ surface casing in the deep-water shallow formation of the LS18-1 well area. Under the 30 °C experimental conditions, the results showed that the compressive and bonding strength of cement with FA and MS decreased gradually as their dosage increased. In contrast, compared with the control group (BG0), the compressive strength of the MK group increased by 25.6%, 32.1%, and 24.2% under the optimal dosages (MK15, MK15, MK20) at 3, 7, and 14 days (d), and the bonding strength increased by 73.6% at 3 d (MK15) and 34.9% at 7 d (MK15). The test analysis showed that MK can promote cement hydration and generate more hydration products, which are conducive to cement-casing interface cementation. Additionally, it can improve the pore structure, reduce the fractal dimension of the pore volume, and ensure a more reasonable distribution of cement stone crack openings.

To clarify how corrosion-driven degradation of cement stone properties affects crack propagation in CCUS well cement sheaths, this study investigated the microstructural evolution and mechanical response of cement stone adjacent to cracks after varying periods of corrosion exposure. An XFEM-based composite model consisting of casing, a microcracked cement sheath, and formation was developed to evaluate crack propagation under different corrosion degrees and to reveal the associated mechanisms. The main findings are as follows: (1) the corroded zone of cement stone exposed to supercritical CO ₂ exhibits a multilayered structure, and its thickness, composition, and mechanical properties vary with increasing corrosion duration; (2) supercritical CO ₂ corrosion promotes crack propagation in the cement sheath, with crack growth becoming increasingly pronounced as the corrosion duration increases. After 10 days of corrosion, the crack length on the inner side of the cement sheath reaches 153 mm, while the crack area increases by only 0.02% relative to the initial area. After 60 days of corrosion, the crack length reaches 158 mm, and the crack area increases by 1.3% compared with the initial area. These findings can provide support for evaluating cement sheath sealing performance and developing technologies to ensure the long-term effectiveness of zonal isolation.