Non-Destructive Screening of Fly Ash and Slag Replacement Ratios in Hardened Cement Paste Using SWIR Reflectance Spectroscopy

Verifying the type and initial proportion of supplementary cementitious materials in hardened binders is relevant to the quality assessment of low-carbon construction materials. This study examined whether short-wave infrared (SWIR) reflectance spectroscopy can identify fly ash (FA) and slag (SL) and estimate their initial replacement ratios in ordinary Portland cement (OPC) paste. Spectra were acquired from 28-day-cured OPC–FA and OPC–SL series spanning 10–80% replacement, together with a pure-OPC reference. A two-stage framework combined nearest-centroid material-group classification with material-specific partial least-squares regression. The two series showed contrasting reflectance and derivative responses, with prominent shape differences near 1.9–2.1 μm. The selected regression ranges were 1350–2350 nm for FA and 1850–2050 nm for SL. Calibration mean absolute errors were 1.88 and 1.12 percentage points for FA and SL, respectively. Six separately prepared validation mixtures containing SCMs from different suppliers were all classified correctly; overall mean absolute error and root mean square error were 1.59 and 1.89 percentage points, respectively, with R2 = 0.9866. TG/DTG supplied complementary quantitative context. These results demonstrate initial composition screening in the investigated paste systems and provide a basis for extending spectral binder assessment to mortar and concrete.