A low-clinker design of ultra-high-performance concrete (UHPC) with high-volume slag activated by flue gas desulfurization (FGD) gypsum

Ultra-high-performance concrete (UHPC) exhibits exceptional mechanical properties and durability, but relies on high-clinker design with dramatic carbon emissions. Conventionally, high-volume supplementary cementitious materials substitution often reduces early-age strength and increases cracking risks. This study develops a low-clinker design of sustainable UHPC incorporating high-volume slag and wasted flue gas desulfurization (FGD) gypsum to achieve higher early-age strength and lower shrinkage, thereby reducing the risk of cracking. The wasted FGD gypsum chemically activates the latent reactivity of slag via the sulfate–aluminate reaction, which promoted ettringite formation, thereby significantly increasing 1-day compressive strength by 127 %. In addition, the 7-day flexural strength and toughness increased by 30 % and 34 %. The optimal UHPC design (i.e., 30C70SL-1FG) achieved satisfying compressive strength (120.9 MPa at 28 days), flexural strength ( f f , 28 d =31.2 MPa at 28 days), toughness (16.5 kN·mm at 28 days), and acceptable autogenous shrinkages (-312.5 με at 14 days). The environmental assessment showed that carbon emissions and embodied energy achieved to 582.8 kg · m - ³ and 6750.1 MJ · m - ³ , reducing by 46 % and 22 %. XRD analysis validated the addition of FGD gypsum increased ettringite peaks at early ages via the chemical activation, in which the needle-like ettringite was observed in SEM images. This promoted ettringite formation explains the observed improvements in early-age strength and toughness. These findings provided the novel pathway for the valorization of FGD gypsum as well as the development of eco-efficient and low-clinker UHPC for sustainable construction. • Low-clinker UHPC designs with slag replacement and FGD gypsum activation. • Wasted FGD gypsum activation promotes ettringite and enhances early strength. • Optimal UHPC reaches 28-d compressive strength at 120.9 MPa and low shrinkage • The low-clinker design reduces CO 2 emissions and embodied energy by 46 % and 22 %.