Early-Age Mechanical and Shrinkage Performance of CaO-MgO Expansive Agent Self-Compacting Concrete

Material properties and construction processes can cause structural issues like shrinkage and debonding in steel tube concrete. This study examines the effect of mix proportions on the mechanical and shrinkage properties of CaO-MgO expansive self-compacting concrete. Using X-ray diffraction, low-field nuclear magnetic resonance, scanning electron microscopy, and mechanical and shrinkage tests, this study examines how microstructural changes, structural characteristics, and pore evolution influence early-age strength and shrinkage. CaO-MgO expansive agents densify concrete microstructure, enhancing mechanical properties. At 28 days, a CaO 63 wt.%, MgO 37 wt.% mix increased cubic compressive strength (74.8 MPa) and splitting tensile strength (4.74 MPa) by 8.6% and 25.2%, respectively. Expansive agents reduced flowability from 50 mm to 27 mm but effectively suppressed shrinkage. Drying shrinkage decreased by 19.8%–99.9%, autogenous shrinkage by 20.0%–99.2%, with a CaO 75 wt.%, MgO 25 wt.% mix showing positive deformation. By analyzing the development of early-age mechanical strength, an expansive agent dosage model was proposed. Further research was conducted on early-age strength development based on various models, resulting in a predictive model for early-age mechanical strength in relation to the expansive agent dosage. This study demonstrates that CaO-MgO expansive agents optimize microstructure, enhance strength, and mitigate early shrinkage, providing a theoretical basis and practical guidance for improving concrete durability.