Non-monotonic synergy between ultra-fine cement and basalt fiber in cement-stabilized sand: Mechanism and quantitative criterion

: Cement-stabilized soil is widely used in geotechnical engineering but still exhibits limited tensile strength and inherent brittleness. Basalt fiber (BF) and ultra-fine cement (UFC) can improve these properties individually, yet their combined effect and the mechanism governing their interaction remain unclear. This study investigates the UFC–BF interaction in cement-stabilized fine sand through mechanical testing, scanning electron microscopy, and porosity measurement. The results reveal that the effect of UFC on the tensile strength of BF-reinforced specimens is non-monotonic: splitting tensile strength first rises to 1.024 MPa at 10% UFC, then declines to 0.821 MPa at 90% UFC. To quantify this interaction, a synergy factor β is defined as the ratio of measured tensile strength to the theoretical strength from linear superposition of the individual UFC and BF contributions. The β –UFC curve identifies a positive synergy window ( β > 1) between 10% and 30% UFC, with a peak β of 1.107 at 10% UFC. Microstructural analysis shows that this behavior is governed by a transition in the fiber–matrix interface: moderate UFC densification activates fiber reinforcement through stronger anchorage, whereas excessive UFC causes matrix embrittlement that suppresses fiber pull-out. The optimal formulation achieves a splitting tensile strength of 1.024 MPa with balanced strength and deformability. These findings offer a quantitative criterion for designing UFC–BF composites and clarify the interfacial mechanism underlying the non-monotonic synergy between matrix densification and fiber reinforcement.