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To investigate the synergistic effects of steel slag (SS) and granulated blast furnace slag (GBFS) on cementitious systems during carbon dioxide (CO 2) mineralization curing. Ternary paste specimens containing SS, GBFS, and Portland cement (PC) were prepared to analyze the influence of drying pre-treatment time and SS–GBFS mixing ratios on mechanical properties, CO 2 fixation rate, and microstructure evolution. Results emphasize the significance of appropriate drying pre-treatment in enhancing mineralization degree and compressive strength. At 60 °C drying pre-treatment temperature, extending drying pre-treatment time up to 12 h enhances both compressive strength and CO 2 fixation rate. However, beyond 12 h, a decline is observed in both parameters. Moreover, under the optimal drying pre-treatment time, the compressive strength of ternary paste specimens post-mineralization increases with higher SS content, with SS exerting a more pronounced positive effect on CO 2 fixation rate and compressive strength compared to GBFS. After 4 h of mineralization curing, specimens with 50% SS and 10% GBFS demonstrate a 13.16% increase in compressive strength and a 9.16% increase in CO 2 fixation rate compared to specimens with 10% SS and 50% GBFS. Microscopic test results reveal that the primary crystalline product in ternary paste specimens after mineralization is calcite, with traces of aragonite. The SS–GBFS mixing ratio has minimal effect on the type of reaction products but has a predominant influence on the total content and crystallinity of calcarea carbonica. • The increase of steel slag content can improve the degree of mineralization curing. • Increasing blast furnace slag reduces calcite crystallinity. • Steel slag boosts carbon fixation more than blast furnace slag. [ABSTRACT FROM AUTHOR]

• Carbonation of steel slag results in the formation of CaCO 3 barrier which blocks the reaction of silica gel. • The incorporation of metakaolin enhances the reactivity of steel slag by promoting the reaction of CaCO 3 and formed silica gel. • The coupled substitution of carbonated steel slag and metakaolin leads to significant improvement of strength and water absorption. This study aims to promote the reactivity of basic oxygen furnace slag (BOFS) through accelerated carbonation and the combined use of metakaolin (MK). Influences of the incorporation of the combination of carbonated BOFS and MK at various cement substitution levels on strength development, water absorption, phase composition, and microstructure are investigated. It is revealed that the coupled substitution with carbonated BOFS and MK improved the formation of carboaluminate phases and stabilized ettringite owing to the synergic reaction between metakaolin and CaCO 3 derived from the carbonation of BOFS; and the consumption of CaCO 3 facilitates the pozzolanic reaction of the amorphous SiO 2 gel formed during carbonation, generating more hydration products. All these reactions contribute to improved impermeability and enhanced strength development; 45% of ordinary Portland cement (OPC) can be replaced while achieving a comparable strength to sample consisting of 100% OPC. In addition, compared to the sample incorporated with uncarbonated BOFS and MK, the compressive strength of the sample blended with 30% carbonated BOFS and 15% MK was improved by 32.5% at the age of 56d; meanwhile, the water absorption during 72 h was lowered by 67.7%. In terms of the environmental impact, the CO 2 emission and intensity for sample incorporated with 30% carbonated BOFS and 15% MK only account for 59% and 62% of those in sample comprised of 100% OPC. The study proposes an approach to improve the reactivity of BOFS efficiently and sequestrate a significant amount of CO 2 simultaneously. [ABSTRACT FROM AUTHOR]

• Application of steel slag in cementitious materials based on dense packing can achieve high utilization efficiency of both steel slag and cement. • Early age hydration kinetics, fluidity of blends, and properties including strength and water absorption are investigated on paste specimens. • The appropriate amount of gypsum addition can enhance long-term strength through improving the early-age hydration kinetics SS-SPC cementitious systems. • The findings of this research help to valorize steel slag utilization in cement-based materials without sacrificing mechanical property and durability. Steel slag is an industrial solid waste with latent hydration nature, possessing a high potential use in cement and concrete but has not been fully utilized yet. This study reveals an implication approach based on dense packing to achieve high utilization efficiency of both steel slag and cement. Steel slag-based reverse filling approach utilizes higher volume of relatively coarser steel slag particles as micro fillers and a smaller quantity of fine cement grains as the major binding component. Thus, giving an initial compact packing structure and an enhanced strength of the resulting materials. Based on the packing density of various mixes, steel slag-based reverse filling cementitious materials are formulated. Early age hydration kinetics, fluidity of fresh pastes, and properties including mechanical strength and water absorption are further investigated on paste specimens. Results indicated that a maximum packing density of 0.6956 was attained for the blend containing 85 % steel slag with 15 % superfine Portland cement. The compressive strength of formulated mixes reached 96.4 MPa, 105.6 MPa and 123.8 MPa at 180 days for pastes containing 15 %, 25 % and 35 % of superfine Portland cement, respectively. The addition of appropriate amount of gypsum can enhance long-term strength by 8 ∼ 18 MPa through improving the early-age hydration kinetics of the steel slag based cementitious systems. The findings of this research help to valorize steel slag utilization in cement-based materials without sacrificing mechanical property and durability. [ABSTRACT FROM AUTHOR]

The incorporation of steel slag usually leads to retardation on the early-age hydration of cement, which limits its use as supplementary cementitious material in cement or concrete. Aiming to a better understanding of the retarding mechanism, the influences of two representative basic oxygen furnace steel slags on setting behaviour, hydration kinetics, aqueous and solid composition during early age were investigated. Different from the existing mechanisms, this study highlights the role of aluminate phase contained in steel slags in the retardation of hydration. The incorporation of steel slag with high aluminate phases content reduces C 3 S/aluminate phase ratio significantly. This reduction in C 3 S/aluminate phase ratio leads to undersulfation in the system, where aluminate phases react prior to C 3 S, resulting in flash setting, reduced C 3 S reaction, and thus the hindered early-age strength gain. This study also proposes an efficient way to alleviate the retarding effect of steel slag on early-age hydration by increasing the sulfate supply, which allows the system to react in a properly sulfated manner. [ABSTRACT FROM AUTHOR]