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Currently, there is a limited understanding of the material performance effects and hydration mechanisms of solidified waste-based binder filling materials containing municipal solid waste incineration fly ash in the actual underground filling environment. This study utilized the Circulating Fluidized Bed (CFB) and Grate Furnace (GF) techniques to produce quadri-component uncalcined binder fill materials from municipal solid waste incineration fly ash (MSWI FA), blast furnace slag (BFS), flue gas desulfurization (FGD) gypsum, and tailings. The solidified test blocks were then cured in-situ in a void space of a certain iron underground mine in Hebei with conditions (temperature: 25–30℃, humidity: 18–19%, mine water influx: annual average of 150 m3/h). The research results indicate that the mechanical performance and heavy metal solidification outcomes of these blocks surpassed those samples cured under laboratory conditions (temperature: (20±1)℃, humidity: not less than 90%), demonstrating the feasibility of using MSWI FA for underground binding and backfilling in mines. Building upon preliminary research, a binder mix proportion was established: 20 wt% MSWI FA, 70 wt% BFS, and 10 wt% FGD gypsum. Test blocks were fabricated using lead-zinc tailings sand and iron mine tailings as aggregates. All blocks met the 28-day strength requirements for typical mine backfills ranging from 1 to 6.5 MPa, with the strength of the XB-M group reaching 24 MPa (when cured underground). The leaching concentrations of heavy metals were all below the limits set for class III groundwater standards, with Pb, Zn, Cr, and As achieving a solidification rate of over 98% at 28 days. Microscopic hydration mechanisms indicate that the primary hydration products in the solidified body were ettringite, Friedel's salt, and C-S-H gel. The high content of reactive Al 2 O 3 in the CFB fly ash enables the [AlO 4 ]5- to substitute [SiO 4 ]4-, leading to the formation of a long-chain, high polymerization C-A-S-H phase. Additionally, the abundant Cl- in GF fly ash readily reacted with [Al(OH) 6 ]3−, forming Friedel's salt. The hydration products in the blocks cured under mine conditions significantly increased in their degree of polymerization. This provides a practical application approach for the future treatment and reuse of MSWI FA. [Display omitted] • The study achieved non-cement clinkers with MSWI FA comprising 20% of the mix. • The optimal batch, XB, reaching an impressive 12.7 MPa after 28 days. • GF fly ash with higher chloride content promotes the formation of Friedel's salt. • The solidification rates of Pb, Zn, Cr, and As in XB specimen all exceed 98%. [ABSTRACT FROM AUTHOR]

• The LT3 sample optimizes BFS reactivity for enhanced cement clinker substitution after carbonation. • CO 2 activation stimulates the hydroxylation of Si(Al)-O bond of BFS and the dissolution rate of [SiO 4 ]4− and [AlO 4 ]5−. • The ion exchange between Ca2+ on the surface of BFS and H+ derived from H 2 O triggers the pH increase during CO 2 activation. • C-S-H can be generated with a longer chain (4.21) and a higher degree of polymerization (0.25) with carbonation. The current study proposes CO 2 can increase the dissociation degree of blast furnace slag (BFS), thus demonstrate the untapped potential to maximize BFS reactivity at the early stage (3d). This low-carbon cementitious materials employs CO 2 activation to increase the dissociation of [SiO 4 ]4− and [AlO 4 ]5− species of BFS, with a more significant effect on [AlO 4 ]5−. This has been demonstrated by the 27Si and 29Al NMR spectral fitting statistics, a 20.12 % and 23.86 % increase in the degree of dissociation for [SiO 4 ]4− and [AlO 4 ]5−, respectively. The introduction of CO 2 facilitated the dissolution of Ca2+ on the surface of BFS. Additionally, it accelerated the ionization of H 2 O, leading to the hydroxylation of [AlO 4 ]5−, as well as, the increase in pH of the leachate. Furthermore, low carbon cementitious materials with low clinker (LT3-MSWI fly ash:BFS:PC:FGDG = 20:42:28:10) exhibit excellent compressive strength (40.57 MPa at 3d) and stabilization/solidification (S/S) properties due to a significant number of functional products. C-S-H with a lower C/S ratio, C-A-S-H, hydrocalumite, and AFt phase are the main functional products of LT3, with relative contents of 17.30 wt%, 13.60 wt%, 8.40 wt%, and 6.3 wt%, as determined by the TIMA quantitative analysis system. The role of CO 2 induces the formation of longer-chain C-S-H species (MCL = 4.21) with a higher degree of polymerization (Pol = 0.25) for LT3, as contrasted with LH3, where these values stand at MCL = 3.58 and Pol = 0.14, respectively. [ABSTRACT FROM AUTHOR]

[Display omitted] • The clinker-free slag-based cemented backfill materials containing coal fly ash (10–20 wt%) exhibited excellent filling performance. • The leaching risk of FST was controllable except G9II-2, and the diffusion coefficient (Do) ranged 2.13 × 10−11–2.50 × 10−10 cm2/s with a leachability index (LI) of 10.08–10.54. • The hydration products of G9I-2 (28d) accounted for approximately 29 wt%. • G9I-2 showed greater polymerization degree (Al/Si = 0.39, 28d) of C-A-S-H gels. • The CO 2 emissions generated by G9I-2 with better performance of 1 t are only 0.45 kg/t, and the total economic benefit is 2.76 USD/t. Herein, the blast furnace slag (BFS), steel slag (SS), desulphurisation gypsum (DFG) and mechanically modified fly ash (MFA) were used as a coal fly ash–slag-based binder (FSD) for the solidification/stabilisation (S/S) of non-ferrous metal tailings and to prepare a coal fly ash–slag-based backfill materials (FST). The results revealed that the optimal ratio for FST was G9I-2, which consisted of 20% MFA, with an appropriate fluidity (240 mm) and setting time (13 h) that satisfied the backfill pipeline transportation requirements, and compressive strengths with 7 and 28 d curing of 2.43 and 8.47 MPa, respectively. The diffusion coefficient (D o) of FST ranged from 2.13 × 10−11 to 2.50 × 10−10 cm2/s, with a leachability index (LI) of 10.08–10.54. These values indicated that the FSD effectively encapsulated Pb and Zn by S/S, with G9I-2 exhibiting the lowest risk of leaching and the leaching mechanism of Pb and Zn was dissolution. The solidification of lead and zinc primarily occurred through the formation of hydration products such as C–(A)–S–H gel, possibly forming a complex salt phase. The addition of an appropriate amount of MFA (20 wt%) increased the content of functional hydration products (15%, ettringite; 14%, complex salt mineral phase) and the degree of polymerization. An increase in the Al/Si ratio (0.11087 ± 0.00948 ∼ 0.38838 ± 0.01935) in the gel region of G9I-2 indicated the continuous transformation from C–S–H to C–A–S–H phase. Life cycle assessment (LCA) and economic benefit analysis (EBA) demonstrated that FST materials significantly reduced CO 2 emissions (0.45 kg/t) and generated significant economic benefits (2.76 USD/t) for mine backfill applications. These findings highlight the significance of MFA in achieving sustainable and high-performance construction materials, with potential applications in various construction projects. [ABSTRACT FROM AUTHOR]

The use of tailings as aggregate for the preparation of wet shotcrete is one of the most important technical approaches to the sustainable development of mines. In this study, the pyrolysis kinetics of tailings wet shotcrete with different Nano-Al 2 O 3 (NA) dosage and the formation mechanism of hydration products are carried out based on the theory of pyrolysis kinetics and various micro characterization methods. The findings show that 1% NA dosage(NA1) specimen significantly improves uniaxial compressive strength by 48.72% and 34.69% compared to NA0 and NA2, respectively. Pyrolysis of concrete has 3 stages: dehydration of ettringite(AFt) and calcium silicate hydrate(C-S-H), dehydroxylation of calcium hydroxide(CH), and CO 2 release from calcium carbonate decomposition. These stages conform to the S-B model. The average apparent activation energy for NA1 specimen during the first pyrolysis stage is 114.01 kJ/mol, indicating an increased energy barrier for the pyrolysis of hydration product, which diminishes when NA dosage exceeds 1%. Furthermore, NA1 specimen exhibits enhanced kinetic parameters, including a notable increase in the preexponential factor during the first pyrolysis stage. This suggests higher activation and effective collision of molecules, leading to optimal AFt and C-S-H formation at 1% NA dosage. Microscopic and thermal analyses reveal that NA aids TWSC hydration through gap filling, nucleation, bond-breaking recombination on tailings surfaces, and calcium (alumino) silicate hydrate(C-(A)-S-H) formation. This process augments hydration product generation, decreases microstructural porosity, and enhances macroscopic strength. However, exceeding 1% NA dosage inversely affects these properties, highlighting an optimal NA concentration for maximizing TWSC performance. [Display omitted] • TWSC pyrolysis in 3 stages: dehydration, dehydroxylation, and CaCO 3 decomposition. • 1% NA dosage can generate more AFt, C-S-H, increasing E, A, and strength. • Excess NA (>1%) reduces TWSC strength and hydration products due to agglomeration. • NA promotes TWSC hydration and strength through gap filling, nucleation, bond recombination, and C-(A)-S-H formation. [ABSTRACT FROM AUTHOR]

The durability of concrete is significantly affected by the chloride solidification property of the binder. This study investigated the chloride-binding capacity and penetration process of metallurgical slag (MS)-based binder and mortar. It also analysed the influence of the ratio of steel slag (SS) to granulated blast furnace slag (GBFS). The results showed that the C–S–H gels and Friedel's salt (Fs) bound more chloride ions in high- and low-concentration NaCl solutions, respectively. In a 0.1 mol/L NaCl solution, Fs captured 0.01–0.04 mmol/g Cl–. However, in a 5 mol/L NaCl solution, the chloride bound by the C–S–H gels was 0.07–0.20 mmol/g. In addition, the increase of the SS to GBFS ratio (SGR) promoted the generation of Fs through the activity promoting of GBFS and reduced the Cl content captured by the C–S–H gels by improving calcium ions and their unstable connections with silicate chain oxygen. The increasing SGR decreased the percentage of pores smaller than 10 nm, reaching a minimum of 22.6 % at R3. However, the chloride resistance of R3 also exhibited the highest C s of 1.01 % and the lowest D a of 8.54 × 10–12 m2/s among MS-based mortars due to a 0.10 % decrease in porosity. Furthermore, the chloride-binding capacity and resistance performance of MS-based binders were all better than those of ordinary Portland cement. • The MS binder of 1:9 SGR had highest compressive strength and bound chloride content. • At a 0.1 mol/L NaCl solution, Fs captured 0.01–0.04 mmol/g Cl–. • At a 5 mol/L NaCl solution, C–S–H gels bound 0.07–0.20 mmol/g Cl–. • The SGR increasing caused the decrease of <10 nm pores percentage and mortar porosity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Effects of gypsum on carbonation of MAS was investigated. • Carbonation curing significantly improves strength of MAS–gypsum samples. • Gypsum promotes both the strength and CO 2 uptake of carbonation-cured MAS samples. • It is feasible to make building materials through MAS–gypsum CO 2 mineralization. Medium Al ladle furnace refining slag (MAS) can potentially absorb CO 2 owing to its high percentages of active Ca, Mg and Al. However, it was hardly focused on. Here, the effects of gypsum on the compressive strength and CO 2 uptake of carbonation-cured MAS blocks were evaluated. Carbonation curing improved the strength of MAS–gypsum samples by 1.57–3.64 times. Moreover, 8.33% gypsum promoted the strength and CO 2 uptake of 24-h carbonation curing MAS samples by 97.82% and 24.69%, respectively. Microanalysis indicates that Gypsum stimulates reactivity of aluminates and shifts the hydration products of aluminates from C 4 AH x to ettringite and hemicarboaluminate. They were then carbonated into calcite and monocarboaluminate in CO 2 -rich environments. Adding gypsum contributes to an increase in carbonation products amount. The carbonation products filled the pores and bound the MAS particles, thereby enhancing the strength of MAS. This study preliminarily proves the feasibility of CO 2 mineralization of MAS–gypsum for building materials. [ABSTRACT FROM AUTHOR]

• Ultra-fine tailing cementitious activity index increased from 0.49 to 0.71 by grinding. • Mine backfilling requirements were met with a slurry concentration of 78%. • The hydration products formed in the G3 sample greatly improved its strength. • C − S − H and C−(A)S − H gel augmentation increased less harmful pores from 17.4 to 66.4%. • Extended curing time (3 to 28 days) improved strength due to C − S − H and C−(A)S − H gels. This study explores the influence of mechanical activation on the cementitious activity of ultra-fine tailing and the filling performance and hydration mechanisms of a clinker-free backfilling body. A binder with 50% granulated blast furnace slag, 35% steel slag and 15% desulfurisation gypsum, a binder to tailing ratio of 1:4, and a slurry concentration of 78% resulted in strength values of 6.69 MPa, 12.05 MPa, 16.36 MPa and 18.37 MPa after 3, 7, 28 and 180 days, respectively. Ettringite formed during metallurgical slag hydration and C-(A)S-H gels formed by superfine tailings under alkali activation resulted in the high compressive strength. [ABSTRACT FROM AUTHOR]

• The replacement of FG has little effect on the trend of strength than DG. • Mortars containing DA have a high long-term strength. • The binder containing DA generates less ettringite in the early hydration stages. • C–S–H gels in pastes with DA present a low Ca/Si and high polymerisation degree. • The SO 3 2− in DA may convert to SO 4 2− during the hydration process at 20 ℃. Desulphurisation gypsum (DG), desulphurisation ash (DA) and fluor gypsum (FG) are three types of industrial by-product gypsum. The production and stockpiling of these types of gypsum may cause waste of resources and environmental problems. In this study, we focus on the activation mechanisms of these materials on steel slag (SS)–granulated blast furnace slag (GBFS)-based binders and their hydration mechanisms. Results showed that the binder with DA had a lower early strength and higher long-term strength than the other two binders. DG and FG provided Ca2+ and SO 4 2− to generate C–S–H gels with SiO 4 4− and ettringite with AlO 4 5−. The attached spherical C–S–H gel particles and aciculate ettringite formed a reticular structure to achieve improvements in strength. The portlandite in DA promoted the dissolution of AlO 4 5− and SiO 4 4− in GBFS during the production C–S–H gels. However, the generation of ettringite was resisted by the less gypsum content in DA, thereby providing space for C–S–H gels to generate the network structure. The C–S–H gels in the binders containing DA exhibit a low Ca/Si atom ratio and enhanced mechanical properties. Further, the SO 2 − could be observed after 28 days, and hannebachite was consumed and gypsum was observed to increase. The conversion of SO 3 2− to SO 4 2− in binder with DA may occur at 20 ℃. DA and FG have a similar activation effect to that of DG on SS–GBFS-based binders, offering an alternative utilisation for solid wastes. [ABSTRACT FROM AUTHOR]

• The raw materials of concrete are solid wastes from iron and mining industries. • Curing temperature strongly affects the strength of SS–GBFS-based concrete. • The optimum proportion could raise the SS proportion to 29.3%. • Higher curing temperature could accelerate the hydration progress. • The utilisation of SS, GBFS and DG could reduce cost of prefabricated concrete. Steel slag and blast furnace slag-based clinker-free concrete has very low early strength. This paper focuses on the effect of different curing temperatures on the compressive strength of clinker-free concrete and on optimising the mix proportion by orthogonal tests based on the perspective of prefabricated concrete, and then discusses the hydration procedures. The results showed that the early strength of concrete could be improved by 13.43 or 22.15 MPa when cured at 30 °C or 45 °C, respectively. The proportion of steel slag could be increased to 29.3% under the optimised conditions of an SS:GBFS ratio of 3:6, desulfurisation gypsum composition of 12%, sand ratio of 0.43, and water consumption of 140 kg/m3. The compressive strength of the concrete prepared with the optimised mix proportion and cured at 45 °C meets the requirement for C40 concrete. During the hydration, the generation of calcium hydroxide through the reaction of tricalcium silicate in the steel slag could promote the breaking of Si–O–Al and O–Si–O bands. The dissolving alumina tetrahedra could react with SO 4 2− and Ca2+ ions to form ettringite, and the dissolving silica tetrahedra could participate in the generation of calcium silicate hydrate gels. The network formed by aciculate ettringite crystals was the main source of early strength. With increasing duration of curing, the growth of ettringite crystals, accompanied by the calcium silicate hydrate gels filling their pore spaces, could be observed by field-emission scanning electron microscopy; these products were responsible for the improvement in strength of the concrete. [ABSTRACT FROM AUTHOR]