###### Abstract  
With the increasing depth of coal resource extraction, the creep characteristics of gauge backfill in deep backfill mining are crucial for the long-term deformation of rock strata. Existing research predominantly focuses on the instantaneous deformation response of either the backfill alone or the strata movement, lacking systematic studies that reflect the long-term time-dependent deformation characteristics of the strata-backfill system. This study addresses gangue-roof composite specimens with varying gangue particle sizes. Utilizing physical similarity ratio theory, graded loading confined compression creep experiments were designed and conducted to investigate the effects of gangue particle size and moisture content on the creep behavior of the gangue-roof composites. A fractional-order creep constitutive model for the gangue-roof composite was established, and its parameters were identified. The results indicate the following: (1) The creep of the gangue-roof composite exhibits two-stage characteristics (initial and steady-state). Instantaneous strain decreases with increasing particle size but increases with higher moisture content. Specimens reached their maximum instantaneous strain under the fourth-level loading, with values of 0.358 at a gangue particle size of 10 mm and 0.492 at a moisture content of 4.51%. (2) The fractional-order creep model demonstrated a goodness-of-fit exceeding 0.98. The elastic modulus and fractional-order coefficient showed nonlinear growth with increasing particle size, revealing the mechanism of viscoplastic attenuation in the gangue-roof composite. The findings provide theoretical support for predicting the time-dependent deformation of roofs in deep backfill mining. g g  
deep backfill mining, strata deformation is determined not only by the mechanical properties of gangue but more critically by the integrated structure of the gangue-roof composite. The mechanical and creep characteristics of this composite significantly influence rock layer deformation. Investigating the deformation mechanism of the gangue-roof composite is pivotal for elucidating strata deformation patterns. After being injected into the goat, the backfill gradually forms an integrated composite with the roof under overlying strata subsidence, collectively bearing the overburden load [6; 7; 8]. In recent years, significant progress has been made in research on the mechanical properties of rock-backfill composites. Through laboratory experiments and theoretical analyses, scholars have systematically revealed the mechanical response mechanisms of these composite materials. [PERSON] et al. [9] demonstrated via loading rate, gradation, and filling speed tests that small-particle gangue exhibits superior deformation resistance. [PERSON] et al. [10] combined physical compression and particle flow simulations to reveal how stratified movement of small-particle gangue critically influences backfill stiffness, highlighting the need for gradation optimization. [PERSON] et al. [11] studied the mechanical performance of rock-backfill composites under uniaxial compression, noting a sharp increase in acoustic emission (AE) events at \(\approx\)90% peak stress, signaling composite failure. [PERSON] et al. [12] employed digital image correlation and static strain loading to show that enhancing backfill strength directly/indirectly boosts composite strength. [PERSON] et al. [13] analyzed the dynamic response of backfill-surrounding rock composites using Split Hopkinson Pressure Bar (SHPB) tests and fragment screening, clarifying microstructure-dependent behavior under impact loads. [PERSON] et al. [14] identified backfill cohesion as a key factor controlling immediate roof subsidence via orthogonal experiments and ANOVA. [PERSON] et al. [15] established a creep constitutive model incorporating structural parameters and horizontal stress by studying their effects on composite samples. [PERSON] et al. [16] developed a damage constitutive model for cemented backfill under varying moisture conditions.