Barium sulphate and zinc oxide as reflective mineral additives in cement-slag composites for urban heat island mitigation: Effects on fresh, optical, thermal, and mechanical properties

Urban heat mitigation requires cementitious materials with high solar reflectance and strong mid-infrared emissivity. This study examines barium sulphate (BaSO₄) and zinc oxide (ZnO) as reflective mineral additives in grey Portland cement and cement-slag composites (0 – 15 wt% of binder, w/b = 0.45). Fresh properties, hydration, spectral reflectance (200 – 2500 nm), MIR emissivity, thermal conductivity and effusivity, Solar Reflectance Index (SRI), steady-state surface temperature, and compressive strength were measured using standardised tests and thermal modelling. BaSO₄ acted as an optically efficient, largely inert filler. Solar reflectance rose from 0.45 (control) to 0.52 at 15 wt% with MIR emissivity remaining above 0.95, and SRI rose from 39 to 53 with a corresponding reduction in modelled steady-state surface temperature, at the cost of only a small loss in workability and a limited strength penalty at higher dosages (peak 28-day strength 49.5 MPa at 1 wt% then 42.0 MPa at 15 wt% versus 45.3 MPa for the control). In contrast, ZnO caused pronounced hydration retardation and coarsening of the pore structure, leading to reduced overall solar reflectance despite some NIR gains, a substantial slump-flow reduction, and a sharp decay in 28-day strength beyond 5 wt% (to 6.2 MPa at 5 wt% and 1.0 MPa at 15 wt%). Mechanistically, BaSO₄ improves reflectance mainly through high-reflectance, inert particle scattering with limited disruption to hydration, whereas ZnO strongly perturbs hydration kinetics and porosity so that its high intrinsic lattice conductivity and NIR reflectance are outweighed by microstructural degradation. The results provide quantitative guidance for mix design of cool cementitious composites, suggesting BaSO₄ is engineering-feasible at 1 – 15 wt% (preferred 5–10 wt% for balanced performance), whereas ZnO should be restricted to ≤ 1 wt% unless retardation can be handled.