<p>This study investigates the potential of ultra-fine slag (UFS) as a partial replacement for cement and crushed steel slag (CSS) as a substitute for natural sand in foamed concrete, with the aim of enhancing durability and sustainability. A series of foamed concrete mixes were prepared with varying foam densities (20, 40, 60, and 80&#xa0;kg/m³), UFS substitutions (0–50%), and CSS substitutions (0%, 25%, and 50%). Mechanical strength, porosity, sorptivity, chloride diffusivity, electrical resistivity, and microstructure were systematically evaluated. Increasing foam content reduced compressive strength and increased porosity due to the introduction of additional voids. UFS improved compressive strength and reduced porosity, attributed to its high pozzolanic activity and fine particle size, which enhanced hydration product formation and pore refinement. CSS decreased porosity through improved interparticle packing, although it caused a slight reduction in strength due to its coarse texture disrupting matrix uniformity. Durability performance improved with both UFS and CSS, as evidenced by reduced sorptivity and chloride permeability. Chloride diffusivity showed strong correlation with electrical resistivity and porosity. A modified chloride transport model incorporating a Freundlich isotherm effectively captured chloride binding behavior, highlighting UFS as more effective than CSS in enhancing resistance to chloride ingress. Microstructural analyses using scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) confirmed the formation of denser and more homogeneous pore structures in slag-modified foamed concretes. These findings demonstrate that the combined use of UFS and CSS not only reduces the environmental footprint through cement reduction and industrial by-product utilization, but also significantly enhances structural durability, offering a sustainable solution for modern construction.</p>