Hydration, fresh and mechanical properties of clinker-free ultra-high performance concrete (UHPC) incorporating thermo-mechanically activated waste concrete powder

Abstract

Waste concrete powder (WCP), a fine residue from demolished structures, is typically landfilled due to its low reactivity and organic impurities. To mitigate environmental impact and address resource scarcity, this study investigates thermo-mechanically treated WCP (TMW) as a sustainable alternative to GGBFS, silica powder, and silica sand in clinker-free ultra-high performance concrete (UHPC). TMW was classified into coarse (C-TMW, retained on 106 mu m), fine (FTMW, retained on 75 mu m sieve size), and ultra-fine (UF-TMW, passing sieve size below 75 mu m) fractions, and selected from the lowest to highest specific surface area, to replace silica sand I, GGBFS, and silica powder, respectively. The higher surface area of TMW increased water demand, reducing mixture flowability. Substituting 50% of GGBFS with F-TMW decreased compressive strength from 159 MPa to 137 MPa due to a dilution effect, whereas complete replacement of silica powder caused only around 5% reduction, and replacing silica sand improved strength to about 170 MPa. Hydration kinetics and mineralogical analyses confirmed enhanced formation of C-S-H, C-(A)-S-H gels, hydrotalcite, and minor phases, contributing to strength development. These results demonstrate that TMW is a promising, eco-friendly alternative to scarce cementitious materials, enabling sustainable production of clinker-free UHPC.