<b> ABSTRACT </b> </br>

Four high-workability (pumpable and self-compacting) concrete mix designs are presented that incorporate steelmaking slags with additions of both metallic and polymeric fibers. Electric arc furnace slag (EAFS) as aggregate, and ladle furnace slag (LFS) and ground granulated blast furnace slag (GGBFS) as supplementary cementitious material (SCM) are applied to optimize the sustainability of the mix design. The main variables in the microstructural analysis, the porosity and the pore structure of the hardened mixes, were assessed with mercury intrusion porosimetry (MIP), X-ray computed tomography (XCT) and water capillary penetration analysis. Moreover, shrinkage was observed to decrease when adding metallic fibers and LFS. In general, scanning electron microscopy (SEM) observations revealed good quality concrete microstructures. Accelerated aging tests at a moderate temperature (72°C) produced a slight lengthening, which affected the dimensional stability of all the mixtures, which was also conditioned by their micro-porosity. The internal damage induced by this test decreased the brittle fracture strength of the concrete mixes, although the use of GGBFS and LFS moderated that damage, due to the increased compliance of the cementitious matrix.

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<b> ACKNOLEDGEMENTS </b> </br>

The authors wish to express their gratitude for funding this research work to the Spanish Ministry of Universities, MInisterio de Ciencia e INNovación (MICINN), Agencia Estatal de Investigación (AEI), European Union (EU), and European Regional Development Fund (ERDF) (PID2020-113837RB-I00, PID2021-124203OB-I00, RTI2018-097079-B-C31, 10.13039/501100011033, FPU17/03374); ERDF and the Junta de Castilla y León (BU119P17; UIC-231); European Social Fund (ESF) and Youth Employment Initiative (JCyL) (UBU05B_1274); Sustainable And Resilient ENvironment (<b>SAREN</b>) research group (IT1619-22, the Basque Government); and the University of Burgos [Y135.GI]. Our thanks also go to the companies Chryso Additives and Hormor-Zestoa for their ongoing collaboration with research group members.