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Low ambient temperature is not conducive to hydration of cement material, resulting in low strength for cemented paste backfill (CPB). Therefore, the aim of this study is to investigate the effect of early-strength agent (ESA) on mechanical strength and hydration exothermic behaviour of CPB under low temperature. Influence factors including the ESA types, superposing patterns and dosages were evaluated on CPB by conducting unconfined compressive strength (UCS), hydration exothermic tests and thermogravimetric analysis. The obtained results show that with exception of sodium chloride (NaCl), the single ESAs of sodium sulphate (Na2SO4), calcium chloride (CaCl2), lithium bromide (LiBr) and triisopropanolamine (TIPA) exert positive effect on UCS of CPB. The optimal dosages of Na2SO4, CaCl2, LiBr and TIPA are 2.0-, 1.0-, 0.3- and 0.08- wt.%. By means of superimposing optimisation, the binary ESA that consists of 0.08 wt.% TIPA and 2.0 wt.% Na2SO4 is obtained. Adding different ESAs at optimum dosages can facilitate hydration reaction of cement. Both hydration exothermic rate, cumulative heat release and internal temperature increase, thereby more amount of hydration products form inside CPB. This contributes CPB to obtain higher initial temperature and UCS under low temperature. [ABSTRACT FROM AUTHOR]

Incorporating mine tailings with industrial waste residues (IWRs) is progressively used to manufacture filling materials, which provides an eco-friendly disposal method for IWRs and alleviate their impact on the environment. In this study, steel slag (5, 10, 15, 20 wt%), fly ash (5, 10 wt%) and blast furnace slag (52, 57, 65, 74 wt%) as the precursor, carbide slag (16, 18, 20 wt%) and desulfurization gypsum (0, 2, 3 wt%) as the activator were reutilized to synthesize the IWRs-based binder for cemented ultrafine tailings backfill (CUFTB). Experimental tests were conducted to determine the setting time, hydration heat release, mechanical behavior, and microstructure characteristics. The hydration kinetics and solidification mechanism were expounded correspondingly. Results elucidate that the initial and final setting time of CUFTB are respectively 12.08–13.92 h and 16.92–20 h <24 h much longer than the transport time in pipeline and meets the operational requirements of mine site. Hydration process of CUFTB includes initial dissolution, induction, acceleration, deceleration, and decline stages. As the hydration proceeds, the initial stage of CUFTB is regulated by nucleation and crystal growth (NG), and gradually shifted to interactions at phase boundaries (I) or diffusion (D). The high content of carbide slag (20 wt%) promotes the hydration reaction, resulting in the increase of heat release. IWRs-based binder hydrates to form calcite, ettringite, hydroaluminite, and gehlenite. Especially, the gel products are mainly C-A-S-H, C-(Al,Na)-S-H, C-(Al,K)-S-H with Ca/Si ratio <1, demonstrating a fundamental distinction from Portland cement. Under the bonding and padding of hydration products, tailings particles are solidified well. After curing of 28 days, CUFTB exhibit high UCS values exceeding 2.0 MPa, presenting an evident improvement of 26.70% ∼ 39.20% compared to those made of Portland cement. These findings are referential for preparation of cemented paste backfill by reutilizing IWRs in mine filling field. [Display omitted] • SS, FA, BFS, CS, DG were reutilized to synthesize IWRs-based binder for CPB. • Hydration kinetics parameters of multisource IWRs-based binder were revealed. • Mechanical and microstructure were investigated on CUFTB with IWRs-based binder. [ABSTRACT FROM AUTHOR]

[Display omitted] • Reusing industrial byproducts prepares the new cementitious material for CPB. • UCS and microstructure of CPB are studied under low temperature-atmospheric pressure. • CPB under 5 ℃ with 50, 75 kPa exhibits lower UCS than those under 20 ℃ with 101 kPa. • Looser microstructure with large porosity forms inside CPB under 5 ℃ with 50 kPa. To promote application of cemented backfill mining in underground mines located at high-altitude regions, there is a necessary to investigate mechanical performance of cemented paste backfill (CPB) under low temperature and low atmospheric pressure. In this study, the orthogonal experiment was first scheduled to synthesize the new cementitious material by combining industrial byproducts of blast furnace slag (BFS), carbide slag (CS), desulfurization gypsum (DG) with cement clinker (CC). Then, CPB samples were prepared with the new cementitious material and cured under different temperatures of 5, 10, 20 ℃ and atmospheric pressures of 50, 75, 101 kPa for 3, 7 14, and 28 days. Unconfined compressive strength (UCS) and microstructural analyses were conducted to evaluate strength and microstructure evolution of CPB. Experimental results indicate that UCS values of CPB increase with BFS, but decrease with CS and DG. The optimal proportion of the new cementitious material is determined as: 70 wt% BFS, 12 wt% CS, 1 wt% DG, and 17 wt% CC. Under curing of low temperature and low atmospheric pressure, CPB samples exhibit lower UCS values than those under 20 ℃ with 101 kPa. Especially, the coupling of low temperature and low atmospheric pressure exerts significantly adverse influence on the later strength of CPB after curing of 14 and 28 days. The declines of 57.99 % and 41.75 % are observed from UCS values of 14-day and 28-day CPB under 5 ℃ with 50 kPa, respectively. Regardless of curing temperature and atmospheric pressure, UCS values increase with the ratio of cementitious material to tailings and curing time. Low curing temperature and low atmospheric pressure reduce the hydration degree of the new cementitious material. The amount of hydration products decreases, the number of micropores (0.1 ∼ 1 μm) and the total porosity increases, as well as more loose microstructure forms among solid particles. This is the underlying reason behind performance degeneration of CPB under low temperature and low atmospheric pressure. [ABSTRACT FROM AUTHOR]

• MCFB exhibits improved mechanical strength when compared to CFB. • MICP technology leads to a notable increment in the particle size of fine tailings in MCFB. • The primary cementation mechanisms differ between MCFB and CFB. The extensive exploitation of metal mines has resulted in the generation of vast quantities of fine tailings, posing significant environmental challenges such as surface subsidence and deposition. The conventional cemented backfill technology, which has been widely used for tailings processing, requires substantial cement consumption and thus leads to escalated economic costs and carbon emissions. In this paper, we propose a pollution-free alternative, Microbial-Induced Calcite Precipitation (MICP) technology, as a promising solution for sustainable mine filling. The study employs microscopic and macroscopic analysis techniques, including laser diffraction particle size (LDPS) analysis, scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), to investigate the inner structure and mechanical properties of the microbial cemented fine tailing backfill (MCFB). Key parameters such as unconfined compressive strength (UCS), CaCO 3 content and porosity are examined, with a focus on the effects of bacterial solution dosage and curing time. The results reveal that the MICP-processed fine tailings exhibit an increase in particle size by approximately 292.1% compared to their original size. Furthermore, MCFB specimens demonstrates significantly enhanced UCS and decreasing porosity under comparable conditions, surpassing the performance of traditional cemented fine tailings backfill (CFB) specimens. Microscopic analysis highlights that the strength development in MCFB predominantly arises from the cementation effects of MICP-induced CaCO 3 , primarily in the form of calcite, rather than the cementation effects resulting from hydration products (C-S-H) in conventional CFB. [ABSTRACT FROM AUTHOR]