Ultra-high performance concrete (UHPC) has emerged as a critical material for achieving structural lightweight design and enhanced longevity in civil engineering due to its exceptional mechanical properties and durability. The optimization of UHPC performance relies on particle gradation design for constructing dense packing systems and interfacial transition zone (ITZ) regulation to enhance matrix continuity. However, traditional UHPC formulations depend on resource-intensive materials such as river sand and silica fume, while existing particle packing theories predominantly assume ideal spherical particles, lacking systematic understanding of the coupling mechanisms associated with natural aggregate complex morphological characteristics. Volcanic rock, characterized by rich active substances and irregular angular morphology, offers a novel material pathway for UHPC performance regulation through multi-scale morphological control when processed into aggregates (replacing standard sand) and mineral admixtures (volcanic rock powder replacing cement).

The study systematically investigated the physicochemical synergistic enhancement mechanisms of volcanic rock materials in UHPC through comprehensive experimental analysis. Volcanic rock aggregates (VRA) with varying replacement ratios (0%-100%) of standard sand and volcanic rock powder (VRP) with different substitution levels (0%-30%) of cement were incorporated into UHPC mixtures. The modified Andreasen-Andersen model guided the mix design optimization. Fresh properties were evaluated through flow spread tests and wet packing density measurements according to the established method. Mechanical performance was assessed via compressive and flexural strength tests at 3, 7 d, and 28 d. Accelerated ASR expansion was measured using mortar bar method. Three-dimensional structural analysis using X-ray computed tomography (CT) characterized macropore structure and fiber orientation distribution. Hydration processes were monitored using isothermal calorimetry, while microstructural features were examined through scanning electron microscopy (SEM) and nanoindentation testing.

The workability and wet packing density of fresh UHPC exhibited an initial increase followed by a decrease with increasing VRA replacement ratios, achieving optimal values at 50% replacement. Flow spread increased from 122 mm to 133.5 mm before declining to 128.5 mm, while wet packing density improved from 0.803 to 0.819 before decreasing to 0.808. VRP substitution demonstrated a monotonic increasing trend in flowability, with wet packing density peaking at 10% replacement. This phenomenon was attributed to the friction network regulation mechanism of "angular-rounded" particle interactions and particle gradation optimization by volcanic rock powder.

Mechanical performance analysis revealed that 75% VRA replacement maximized flexural strength across all curing ages, while 20% VRP substitution yielded optimal 28 d compressive strength. X-ray CT analysis demonstrated that 50% VRA replacement minimized total porosity, with large pores (10⁷-10⁸ μm³) systematically decreasing as VRA content increased. The angular morphology of volcanic rock aggregates promoted preferential horizontal alignment of steel fibers, with significant clustering in low-angle regions (0°-30°), thereby substantially enhancing UHPC flexural performance. VRP optimized pore distribution structure through pozzolanic reactions filling microscopic pores at appropriate replacement levels, achieving a dynamic balance between secondary hydration benefits and cement dilution effects. Nanoindentation results confirmed that VRA-matrix interfacial transition zones exhibited superior mechanical properties compared to those of standard sand interfaces, with elastic modulus improvements of approximately 49.7%.

Calorimetric analysis revealed that VRA incorporation increased 3 d cumulative heat release compared to standard sand groups, attributed to surface active substances and enhanced interfacial contact promoting early cement hydration. VRP exhibited delayed hydration kinetics with peak heat release occurring at later ages, with 20% replacement achieving the highest 3 d cumulative heat release due to optimized pozzolanic reaction contributions, while higher dosages (30%) inhibited hydration processes due to lower reactivity of VRP and dilution effect.

This study established that volcanic rock materials effectively enhance UHPC performance through physicochemical synergistic mechanisms, with optimal performance achieved at 50%-75% VRA replacement and 20% VRP substitution. The angular morphology of volcanic rock aggregates improved fiber orientation distribution and mechanical interlocking with the cement matrix, while volcanic rock powder optimized particle packing efficiency and promoted secondary hydration reactions. The enhancement mechanisms originate from three synergistic factors: superior intrinsic mechanical properties comparable to standard sand, angular particle morphology creating three-dimensional mechanical interlocking structures that optimize packing efficiency, and surface active components promoting pozzolanic reactions that densify interfacial microstructure. These findings provide experimental evidence for extending particle packing theory beyond conventional spherical particle assumptions, establishing critical relationships between “morphological characteristics-packing efficiency-interface performance” and macroscopic UHPC properties for complex-shaped aggregates design.