Recycling coal gasification slag to produce eco-friendly ultra-high performance Concrete: working properties, mechanical properties and microstructure

This study investigates the feasibility and underlying mechanisms of utilizing coal gasification slag (CGS), an industrial byproduct, as aggregate replacement for river sand (RS) in ultra-high performance concrete (UHPC), aiming for sustainable material development. UHPC with 0 %, 50 %, and 100 % CGS substitution (by mass) for RS were prepared and systematically evaluated. As CGS content increases, UHPC fluidity improves due to its higher sphericity and lower water absorption. Notably, optimal overall performance was achieved at 50 % CGS substitution, which exhibited the highest packing density, lowest porosity, and superior later-age mechanical properties compared to the control (0 % CGS) and full replacement (100 % CGS) mixtures. Microstructural analyses (TG-DTG, SEM, LF-NMR, Nanoindentation) revealed a dual role for CGS: its surface reactivity contributed to additional hydration product formation (C-A-S-H), enhancing matrix density, particularly in the C50 group. However, detrimental effects, including weakened interfacial transition zones (ITZ) around CGS particles (evidenced by increased thickness and potentially higher porosity) and the negative impact of residual carbon, became dominant at 100 % substitution, limiting performance. The findings demonstrate that partial replacement (50 %) of RS with CGS is a viable strategy for producing eco-friendlier UHPC with optimized properties, highlighting a promising avenue for high-value CGS utilization.