Dynamic Response and Pore Evolution Mechanism of Composite Improved Loess Using an Eco-Friendly Curing Agent and Cement: A Macroscopic and Microscopic Experimental Study

To address the loose structure and insufficient dynamic stability of collapsible loess subgrades in Gangu, Gansu Province, as well as the engineering and environmental concerns associated with conventional stabilizers (high energy consumption and carbon emissions), this study proposes a composite stabilization strategy using an eco-friendly curing agent (EFCA) and P·O 42.5 Portland cement. Unconfined compressive strength (UCS) tests and dynamic triaxial tests were performed, and scanning electron microscopy (SEM) image processing together with fractal theory was employed to systematically elucidate the macroscopic dynamic response and the microscopic pore-reconstruction and evolution mechanisms of the composite-improved loess. The results indicate that the optimal mix proportion determined by an orthogonal design is “6% cement + 0.02% curing agent”, yielding a 28-day UCS of 2.51 MPa, which is 483% higher than that of the untreated loess. The dynamic resilient modulus ( E _d ) increases markedly and reaches 826.49 MPa under a confining pressure of 60 kPa and a dynamic stress of 30 kPa (an 8.07-fold increase). Nonlinear regression analysis confirms that the Ni model, by jointly accounting for the coupled effects of mean and deviatoric stresses, provides exceptionally high predictive accuracy for E _d of the composite-improved loess, with an average relative error of only 0.026. Quantitative microstructural analysis reveals that the synergistic effects of chemical cementation and hydration products promote the transformation of loess particles into dense aggregates, resulting in a decrease in the pore fractal dimension ( D ) from 1.323 to 1.249. This topological reconstruction from connected macropores to discrete micropores fundamentally reduces the structural complexity of the soil. The study clarifies a cross-scale physical-mechanical mechanism whereby “microstructural pore-fractal dimensionality reduction” drives a “macroscopic surge in dynamic stiffness”, providing theoretical and data support for green, low-carbon subgrade construction and long-term dynamic stability evaluation in loess regions.