Effect of ground granulated blast furnace slag and cement chemistry on the reflectance and strength of cementitious composites

Urban heat mitigation and sustainable construction increasingly demand concrete materials with improved solar reflectance and maintained structural performance. This study investigates the influence of cement chemistry and slag incorporation (0–60 wt%) on the solar reflectance and mechanical properties of cementitious composites. Grey Portland cement (GPC), white Portland cement (WPC), creme cement, slag, and sand were evaluated. Solar reflectance was measured across wavelengths of 200–2500 nm. Material characterisation included X-ray fluorescence, X-ray diffraction, thermogravimetric analysis, colourimetry, and 28-day compressive strength testing. The key findings show that increasing slag content in GPC mixes substantially enhances reflectance: at 60 % slag, VIS reflectance rose by 58 % (from 0.33 to 0.52), NIR by 46 % (from 0.37 to 0.54), and solar reflectance by 49 % (from 0.35 to 0.52). In WPC mixes, slag substitution reduced reflectance, with solar reflectance falling from 0.79 to 0.67 at 60 wt% slag. Strong negative correlations were observed between reflectance and transition metal oxides such as Fe 2 O 3 , MnO, and Cr 2 O 3 , R = −0.84 to −0.99). Hydration degree decreased as slag content increased, from 42.6 % to 31.9 % in GPC and from 61.1 % to 44.9 % in WPC, and was strongly correlated with VIS, NIR, and solar reflectance (R 2 > 0.75). Rietveld refinement revealed that higher C 3 S, C 2 S, C 4 AF, and C 3 A contents corresponded to lower reflectance, e.g. GPC (71.1 % C 3 S, 9.4 % C 4 AF) showed a solar reflectance of 0.35, whereas WPC (0.8 % C 4 AF, 1.8 % C 3 A) reached 0.79. Colour space coordinates showed strong predictive power for reflectance (R 2 > 0.90). Compressive strength of GPC mixes decreased from 47 MPa (0 % slag) to 29 MPa (60 % slag), with a strong inverse relationship to solar reflectance (R = −0.93). • Slag in GPC boosts VIS, NIR, and solar reflectance by up to 58 %. • WPC reflectance stays higher than GPC even with 60 % slag substitution. • Reflectance negatively correlates with Fe 2 O 3 , MnO, and Cr 2 O 3 (R = −0.84–0.99). • Higher hydration and cement phase content reduce reflectance. • GPC compressive strength drops with slag, inversely linked to reflectance (R = −0.93).