Sustainable lightweight alkali-activated FA-OPC concrete incorporating recycled EPS foam and seawater: Mechanical, thermal, and acoustic performances

This study develops sustainable, ambient-cured lightweight alkali-activated concrete by upcycling recycled expanded polystyrene (EPS) foam and fully replacing freshwater with raw seawater. An FA-OPC blended precursor was activated using 5 M and 10 M seawater-based NaOH solutions. A counter-intuitive microstructural mechanism was discovered: the 5 M seawater activator outperformed the conventional 10 M baseline. The lower alkalinity prevented the quick setting typically induced by seawater ions, allowing for a controlled, dense gel network. The optimum mixture (0.80% EPS, 5 M NaOH) achieved a 28-day structural-grade compressive strength of 26.8 MPa and a low unit weight of 1,498 kg/m³. Furthermore, the cellular EPS inclusions effectively reduced thermal conductivity to 0.718 W/m·K and improved acoustic damping, reaching a peak sound absorption coefficient of 0.358 at 500 Hz. These findings present a highly viable, zero-freshwater approach for manufacturing functional, load-bearing lightweight concrete blocks.