Driven by circular economy goals, upcycling dredged marine deposits and waste tire rubber remains a critical challenge. Conventional eco-bricks rely on natural or recycled aggregates to form a rigid skeleton, introducing heterogeneous interfaces and micro-mechanical defects while increasing energy consumption. This study introduces a novel matrix-dominated, aggregate-free eco-brick fabricated from fine-grained dredged marine deposits, recycled granular tire rubber, and a cement-free, lime-activated ground granulated blast furnace slag (GGBS) binder. A comprehensive experimental program evaluated thirteen formulations, varying rubber content from 0% to 20% and particle size from 0.1 to 1.0 mm, to determine how rubber inclusion characteristics affect the composite’s macroscopic performance and microstructural evolution. Results show that rubber particle size fundamentally governs mechanical and functional behavior. While rubber incorporation reduces structural stiffness and increases porosity, optimizing the inclusion size (e.g., 0.1 mm at 10% content) effectively controls interfacial defect formation. This selected formulation ensures a sufficient compressive capacity of 19.5 MPa, alongside a 16% weight reduction and a superior thermal conductivity of 0.73 W/mK. Microstructural analyses reveal that fine rubber grains enhance spatial entrapment and uniform dispersion. This size-dependent spatial restriction limits the widening of the interfacial transition zone (ITZ), thereby mitigating the impact of localized modulus mismatches. Furthermore, utilizing the GGBS binder and eliminating high-temperature firing yields an estimated 74–80% reduction in embodied carbon emissions while ensuring safe heavy metal immobilization. This research establishes a fundamental paradigm for matrix-dominated eco-bricks, demonstrating that tailoring the physical size of waste inclusions and their interfacial compatibility can convert problematic wastes into low-carbon, high-performance building materials.
