Objective: Self-protected underwater concrete (SPUC), characterized by strong anti-washout performance and adaptability to underwater construction, offers a promising solution for scour protection of offshore wind turbine monopile foundations. Conventional riprap protection systems rely on loose granular materials and are therefore susceptible to stone displacement, progressive scour, and structural instability under long-term wave-current interactions, leading to limited durability and high maintenance costs. Cementing riprap using underwater concrete has been proposed to improve the integrity and stability of the protection layer; however, the formation mechanisms of cemented rockfill under underwater conditions remain insufficiently understood. In particular, the effects of key construction and material parameters on the filling behavior and final morphology of cemented rockfill have not been systematically quantified. Methods: Systematic physical model experiments were conducted to investigate the free filling and cementation behavior of SPUC in rockfill under unconfined conditions. Rockfill with particle sizes of 10—15 cm, representative of offshore wind engineering practice, was used. Three key parameters-pouring height, concrete slump-flow diameter, and aggregate particle size-were varied to examine their effects on cemented rockfill morphology and cementation characteristics under static water conditions. Nine static-water test cases were designed. After curing, uncemented surrounding stones were removed to expose the cemented rockfill bodies. A layered measurement method based on equivalent diameters was applied to quantitatively characterize the spatial distribution, slope angle, and fully cemented height. In addition, large-scale flume experiments were performed to assess SPUC feasibility under dynamic water conditions representative of offshore construction environments. Results: Pouring height was identified as a critical construction parameter controlling concrete discharge behavior and penetration depth. At a pouring height of 15 cm, outlet blockage occurred, causing concrete accumulation near the rockfill surface and resulting in hourglass-shaped or inverted conical cemented structures. When the pouring height was increased to 25 cm or greater, concrete flowed freely into rockfill pores, forming typical pyramidal cemented bodies with wider bases. Increasing pouring height enhanced shear-induced mixing between concrete and water, increasing the apparent water-cement ratio and reducing viscosity and yield stress, thereby improving penetration capacity. The slump-flow diameter governed cemented rockfill morphology through the competition between surface spreading and downward penetration. A slump-flow diameter of approximately 500 mm achieved an optimal balance between low yield stress and moderate plastic viscosity, promoting deeper penetration and producing the largest cemented base area. In contrast, excessively large or small slump-flow diameters intensified surface spreading and inhibited penetration, resulting in steeper cemented slopes. Aggregate particle size controlled the filling mechanism through a particle-to-throat size ratio effect. Concrete with smaller aggregates (5—10 mm) readily passed through rockfill pore channels and achieved integral cementation, whereas larger aggregates (10—20 mm) induced pronounced particle blocking, leading to surface retention and inverted conical cemented structures. Dynamic water experiments demonstrated that SPUC application remains feasible at a flow velocity of approximately 0.63 m/s. Although flowing water enhanced surface spreading and reduced penetration depth relative to static conditions, stable cemented rockfill structures were still achieved through appropriate aggregate selection and continuous anti-washout admixture supply. Conclusions: This study advances the mechanistic understanding of cemented rockfill formation by quantitatively elucidating penetration-diffusion competition and particle exclusion thresholds. Pouring height, slump-flow diameter, and aggregate size were identified as key design parameters governing SPUC performance. Large-scale flume experiments confirmed the feasibility of SPUC under representative offshore flow conditions. The results provide practical guidance for material design, construction parameter optimization, and offshore engineering applications.