[Objective] To address the difficulty in simultaneously achieving workability retention, early strength, and shrinkage-cracking resistance in thin-walled, highly restrained concrete linings of water conveyance tunnels, this study aimed to develop a high-performance lining concrete by reconstructing the binder composition and particle size distribution using ultra fine ground granulated blast-furnace slag. The effects of slag particle size and replacement level on fresh-state properties, mechanical performance, shrinkage-cracking sensitivity, particle packing, and interfacial transition zone (ITZ) characteristics were systematically quantified. The study also sought to elucidate how the physical filling effect and latent hydraulic reactivity of ultrafine slag jointly influence the macroscopic performance of concrete. [Methods] Concrete containing 15% fly ash, designated FA15, was used as the reference mixture. The binder composition and particle-size distribution were reconstructed by incorporating two ultrafine slags with different particle sizes (SL₁ and SL₂) at different replacement levels, and ultrafine slag-fly ash blends were compared. The optimized mixture SL₂10F10 contained 10% SL₂ and 10% fly ash. At the same initial flow spread, superplasticizer dosage and time-dependent flow-spread loss were measured. The rheological behavior of the corresponding pastes after 4 h of rest was characterized using the Herschel-Bulkley model, with emphasis on yield stress and consistency coefficient. Compressive strength and splitting tensile strength were measured to evaluate mechanical development. Drying shrinkage was monitored, and a cracking risk coefficient was calculated to assess shrinkage-cracking sensitivity. The maximum wet packing density and liquid-film thickness of the binder particles were determined using a centrifugal wet-packing method. Nanoindentation profiles across the aggregate-paste interface were further used to determine ITZ thickness and average microscopic elastic modulus. [Results] Slag particle size exerted distinct effects on strength development and fresh-state performance. At a total mineral-admixture content of 30%, the 28 d compressive strength of SL₁15F15 reached 47.3 MPa, 6.8% higher than that of SL₂15F15. In contrast, the SL₂ mixtures generally require less superplasticizer and exhibited smaller flow-spread losses and lower increases in rheological parameters. Relative to FA15, and at the same initial flow spread, the superplasticizer dosage of SL₂10F10 decreased by 0.10 percentage points, and its 3 h flow-spread loss decreased by 15 mm. After 4 h rest, the increases in paste yield stress and consistency coefficient were reduced by 6.1% and 9.2%, respectively. The 3-day compressive strength and splitting tensile strength increased by 6.9% and 18.0%, respectively. The drying shrinkage of SL₂10F10 was broadly comparable to that of FA15 during the first 7 d and became lower thereafter, while its cracking risk coefficient decreased by more than 9%. Its maximum wet packing density showed no appreciable change, whereas the calculated liquid-film thickness decreased by 6.3%. Nanoindentation results showed that the ITZ thickness decreased from approximately 20 μm in FA15 to no more than 16 μm in SL210F10, a reduction of at least 20%. Meanwhile, the average microscopic elastic modulus of the ITZ increased from approximately 15 GPa to above 20 GPa, representing an increase of at least 33%. [Conclusions] Decreasing slag particle size did not monotonically improve all concrete properties. The finer and more reactive SL₁ was more favorable for strength development, whereas SL₂ provided better workability retention and rheological stability. An appropriate combination of SL₂ and fly ash coordinated fresh-state performance, early mechanical capacity, and shrinkage-cracking resistance. The results for SL₂10F10 indicate that the beneficial effects of reconstructing the binder composition and particle-size distribution cannot be explained by bulk wet packing density alone; they are also associated with local particle filling and ITZ improvement induced by the latent hydraulic reaction of ultrafine slag. The simultaneous increase in early splitting tensile strength and decrease in cracking risk are particularly beneficial for thin-walled tunnel linings under strong restraint. Accordingly, SL₂10F10 is recommended as a promising binder composition for high-performance water-conveyance tunnel lining concrete.