Performance insights of cementless UHPC incorporating treated waste concrete powder and metakaolin under different curing regimes

Ultra-high performance concrete (UHPC) is increasingly recognized for its exceptional strength and durability, but its reliance on silica fume raises concerns regarding availability and sustainability. In this study, metakaolin (MK) and thermally treated waste concrete powder (WCP) were independently investigated as partial substitutes for silica fume in a CaO-activated ground granulated blast furnace slag (GGBFS) based cementless UHPC system. Steam curing at 90 °C accelerated hydration, promoting the formation of stable hydrates such as C-A-S-H, hydrotalcite, and wollastonite, and resulting in compressive strengths exceeding 160 MPa. In contrast, normal curing led to slower hydration, characterized by residual portlandite and ettringite alongside sharper C-A-S-H decomposition, producing compressive strengths below 100 MPa. Low levels of WCP replacement provided beneficial filler effects and improved performance, whereas higher replacement levels of both MK and WCP caused strength reductions. Life cycle assessment demonstrated substantial reductions in CO 2 emissions and mineral resource consumption compared with cement-based UHPC.