Evaluation of Solid and Hollow Sand Brick Properties with Partial Replacement of Fine Aggregates by Ground Granulated Blast Furnace Slag

Ground granulated blast furnace slag (GGBFS), an industrial by-product of steel manufacturing, can be utilized as a partial replacement for natural fine aggregate in clay brick production. Although widely used in cementitious systems, its incorporation into sand bricks and its effects on key performance parameters remain insufficiently investigated. To fill in the gap, sand bricks containing 0–35% GGBFS (at 5% intervals) were tested for compressive strength, water absorption, thermal conductivity, and efflorescence. Optimal performance was achieved at 25% replacement. Compressive strength increased from 17.5 MPa (control) to 24 MPa (28.5% improvement). Water absorption decreased from 11.67% to 8.20% (29.7% reduction), and thermal conductivity decreased from 1.08 to 0.85 W/m·°C. No efflorescence was observed at 25% GGBFS, whereas higher replacement levels (30% and 35%) exhibited increased efflorescence. The results confirm that 25% GGBFS replacement enhances mechanical and durability-related properties of clay bricks, demonstrating its technical feasibility as an alternative fine aggregate.