Mix Optimization of Polymer-Modified Cementitious Composites: Combined Effects of FA-PFDS, Cellulose Nanofibers and Synthetic Fibers on Mechanical Properties and Water Absorption

Partially replacing cement with superhydrophobic fly ash (FA-PFDS), cellulose nanofibers (CNF) and synthetic fibers (SYN) is a promising route to combine mechanical performance and water resistance in polymer-modified cementitious composites. However, current studies rarely quantify the dosage sensitivity under the complex interactions of multiple polymers, which often results in trade-offs between mechanical strength and impermeability. This study employs an L36(63) orthogonal experimental design combined with least absolute shrinkage and selection operator (LASSO) regression to analyze macroscopic performance data, supported by X-ray diffraction (XRD), scanning electron microscopy (SEM) and a simplified hydro-mechanical slope model to discuss possible mechanisms and structural responses. The LASSO models predicted that a mixture of 20% FA-PFDS, 1% SYN and 0.4% CNF by mass of cement replaced would reach a 7-day compressive strength of 18.20 MPa with a water absorption of 8.15% and a consistency of 16.3 mm while reducing cement consumption by 21.4%. XRD and SEM observations are consistent with capillary pore filling by CNF and crack bridging by SYN, but the proposed mechanism remains to be verified. The slope model suggests that a modified surface layer with reduced permeability would delay rainfall breakthrough of a thin surface layer and keep stresses well below the assumed 20 MPa load. Overall, this study provides empirical evidence on how the dosage of multiple polymers affects the trade-off of strength and impermeability, and a data-driven starting point for mix design in slope-protection applications, pending experimental validation.