Microstructure of Carbonation-Activated Steel Slag Binder

A steel slag material demonstrated rapid hardening and a considerable gain in compressive strength upon reacting with carbon dioxide. Two-hour carbonated paste compacts achieved an average compressive strength of 80 MPa, warranting the slag’s consideration as a cement-like binder for building applications. The microstructure of this CO2-activated binder system was examined. The reaction was found to engage the di-calcium-silicate component of the slag to generate a hardened matrix consisting of CaCO3 and a low-lime calcium-silicate-hydrate (C─ S─ H) phase. The latter differed in composition and structure from C─ S─ H variants generated from normal portland cement hydration. Raman spectroscopy confirmed bands (330, 510–550, 600–630, and 1,005 cm−1) consistent with C─ S─ H species having low-lime compositions. High-resolution transmission electron microscope (TEM) resolved lamellar features for C─ S─ H with short basal spacings (averaging 0.73 nm), correlatively indicating superior interlayer cohesion. Moreover, abundant nano-CaCO3 crystals were found interlocked within the C─ S─ H phase, forming a dense nanoscale composite matrix. Such a proposed binder system is completely by-product-sourced, thus presenting the potential of eliminating or significantly reducing the carbon footprint of building products.