Density Reduction and Mechanical Performance of Metakaolin-Based Alkali-Activated Composites Containing EPS Beads

Abstract

Lightweight alkali-activated composites offer a route to reducing component mass while retaining useful mechanical performance. This study examines how the volume fraction and particle size of expanded polystyrene (EPS) control the physical, microstructural, and mechanical behavior of metakaolin-based alkali-activated composites. A full 3² factorial design was adopted using EPS contents of 18, 35, and 53 vol.% and average bead diameters of 0.85, 2.16, and 6.00 mm. The metakaolin was activated with an 8 M NaOH solution combined with sodium silicate, and the composites were evaluated after 28 days by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, water absorption, apparent porosity, density, and compressive strength. The alkali-activation reaction produced an aluminosilicate binder without detectable interference from EPS incorporation. Increasing the EPS content progressively reduced density, with the largest reduction obtained for the coarsest beads. Composites containing 53 vol.% EPS reached densities as low as 0.80 g cm⁻³. This reduction was accompanied by a decrease in compressive strength, from approximately 47.5 MPa for the reference matrix to 14 MPa for composites containing 53 vol.% of 0.85 mm EPS and to 1.5 MPa when the same volume fraction of 6.00 mm beads was used. SEM observations associated the greater strength loss of coarse-EPS composites with wider interfacial discontinuities and crack formation around the polymeric inclusions. Despite this trade-off, selected formulations combined densities of 0.8–1.2 g cm⁻³ with compressive strengths of 15–25 MPa, indicating potential for lightweight non-structural building components.