Compositional Characterization of Ultrafine Composite Powder as a Novel Supplementary Cementitious Material

To investigate the application potential of ultrafine composite powder (UCP) as a novel supplementary cementitious material to replace ground granulated blast-furnace slag (GBFS) in cement-based materials and its underlying mechanism, this study first compared the activity differences between UCP and GBFS and their effects on mortar workability. Subsequently, multiple characterization techniques including XRF, XRD, TG/DTG, FTIR, mapping, SEM-EDS, and BET were employed to systematically examine the morphology, composition, particle size distribution, and pore structure characteristics of the two powders. Results show that UCP exhibits slightly higher 3 d and 28 d strength activity indices than GBFS, but contributes less to strength progression between 3 and 28 days. In terms of chemical composition, UCP contains lower combined CaO + MgO + Al2O3 content but significantly higher C and Fe levels and alkalinity than GBFS. Phase and microstructural analyses further reveal that UCP is predominantly composed of GBFS, fly ash (FA), steel slag, limestone powder, gypsum, superplasticizer, and alkaline activator, and is characterized as a mesoporous material with pores arising from fragmented FA, unburned carbon residues, and grinding-induced cracks. Quantitatively, the BET specific surface area, Blaine specific surface area, and total pore volume of UCP are 2.47, 1.59, and 3.31 times those of GBFS, respectively. Therefore, the early-age activity advantage of UCP is mainly attributed to the filling effect, the additional nucleation sites provided by its larger specific surface area, and the chemical activation induced by alkali and gypsum.