Radiative-thermal optimisation of solar-reflective lightweight cement-slag composites using hollow glass microspheres, perlite and crumb rubber

To develop thermally optimised, solar-reflective lightweight cement-slag composites, fine sand was volumetrically replaced by hollow glass microspheres (HGM), crushed expanded perlite, or crumb rubber (0–100 vol%) while maintaining 60% slag replacement (by mass of binder), w/b = 0.45 and 15 wt% TiO 2 . Spectral reflectance (280–2500 nm), mid-infrared emissivity (4000–400 cm −1 ), solar reflectance index (SRI), transient plane source thermal properties and 28-day compressive strength were evaluated. Overall, HGM delivered the best radiative-thermal performance with adequate strength, achieving a substantial reduction in density and thermal conductivity (to 1461 kg/m 3 and 0.55 W/m∙K at full replacement) while improving solar reflectance (0.61) and maintaining high emissivity (0.96), resulting in SRI of 75. Crushed perlite primarily enhanced mechanical performance, likely due to its fine particle size (D 50 = 19 µm), which allowed it to act as a micro-filler and possible supplementary cementitious material, while maintaining high reflectance (0.62), although its thermal benefit was comparatively smaller (0.83 W/m∙K at full replacement).Crumb rubber provided strong insulation potential (0.57 W/m∙K) but markedly reduced compressive strength (down to 6.5 MPa), limiting structural feasibility to ≤ 25 vol% replacement ( f c ≥ 20 MPa). Within the adopted multi-criteria decision framework, H-100 was identified as the most favourable overall option, not because it was superior in every individual property, but because it provided the best balance of low density, low thermal conductivity, high solar reflectance, high emissivity, and adequate compressive strength among the feasible mixes.