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To address the environmental pollution and resource wastage caused by the massive stockpiling of coal gasification slag (CGS), in this study, chlorine salt (CaCl 2 , CC) was adopted to stimulate the potential volcanic ash activity of CGS and to prepare a new modified CGS-based cementitious backfill material (MC-CPB). The optimum CC admixture was determined by comparing the compressive strength, and the effect of different admixtures on the strength of the MC-CPB was investigated. The effects of the CC content on the hydration properties and the composition and the micro-morphology of the hydration products of the MC-CPB were analyzed using hydration heat, X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermogravimetry (TG) analyses. It was found that the addition of CC promoted the early and mid-strength of the MC-CPB, but an excessive amount of CC (>1%) inhibited the development of the strength in the later stages. A concentration of 1% CC had the best effect on the strength of the MC-CPB in the entire age range, increasing the compressive strength from 3 to 90 days by 65.8%, 56.1%, 80.6%, 30.9%, and 31% compared to the control group. The CC accelerated the hydration reaction process of the MC-CPB by shortening the durations of the induction and acceleration periods, and it enhanced the hydration reaction intensity of the MC-CPB by increasing the second release heat peak and forming a third release heat peak. The MC-CPB sample containing CC generated a new hydration product, Friedel's salt, which increased the compactness of the matrix and thus improved the compressive strength of the sample. However, excessive addition of CC reduced the amount of Ettringite (AFt) generated in the later stage, leading to a decrease in the strength of the high CC content samples in the later stage. The results of this study promote the large-scale and resourceful use of CGS in mine backfill and provide a theoretical basis for the field proportioning design and industrial application of MC-CPB for backfill. [Display omitted] [ABSTRACT FROM AUTHOR]

With the mass production of coal-based solid waste, coal mine filling can effectively consume it. The coal gasification slag is modified and prepared as coal mine filling material to meet the relevant technical requirements, which can realize the recycling of coal mine → coal chemical industry → coal mine. In this paper, in order to explore the evolution law of the mechanical properties and pore structure characteristics of the modified coal gasification slag-cement cemented paste backfill (MCGS-CPB) prepared by sodium sulfate excitation coal gasification slag, a combined macro-meso-micro testing method is used. MCGS-CPB with different sodium sulfate contents (1~5%) were prepared and tested for uniaxial compressive strength (UCS), mercury intrusion (MIP) and microscopic tests. The results show that sodium sulfate has a significant effect on the UCS and pore structure characteristics of MCGS-CPB. The mechanical properties and pore structure characteristics of MCGS-CPB were best when sodium sulfate was doped at 3%; the mechanical properties and pore structure characteristics of MCGS-CPB were deteriorated when the addition of sodium sulfate is higher than 3%. On the meso-scale, when sodium sulfate was doped with 3%, the more harmful pores of MCGS-CPB gradually changed into harmless, less harmful, and harmful pores, and the macroscopic mechanical properties were gradually improved; when the addition of sodium sulfate is higher than 3%, the harmless, less harmful, and harmful pores of MCGS-CPB gradually changed into more harmful pores, and the macroscopic mechanical properties were deteriorated. On a microscopic scale, sodium sulfate can cause MCGS-CPB to form hydration products with expansion properties. The presence of a reasonable amount of sodium sulfate in the pores of MCGS-CPB is beneficial to the development of mechanical properties. However, excessive presence will lead to the formation of expansion stress, gradual formation of micro-expansion cracks, and deteriorate the macroscopic mechanical properties. Hence, the volcanic ash activity of coal gasification slag excited by external addition of sodium sulfate should not exceed 3%. This study provides a reference value for application ratio of sodium sulfate–stimulated MCGS-CPB used in coal mine filling design. [ABSTRACT FROM AUTHOR]

The strength of the backfill body is an important aspect of safe production in backfill mining technology. In order to quickly and accurately obtain the strength of coal gasification slag -based cemented backfill materials with different ratios, the orthogonal design of 4 factors and 5 levels and variance analysis method were used to study the mass concentration (A), coal gasification slag (CGS)/cement (B), bone glue ratio (C), activator content (D) on the compressive strength of backfill body and a genetic algorithm with 28-day compressive strength of CGS-based backfill material as the response value was constructed, and verify the applicability of the model. The results show that: the primary and secondary factors affecting the 28-day compressive strength are C > A > B > D, the optimal ratio is 80% mass concentration, CGS/cement ratio 4:1, bone glue ratio 3:7, and activator 3%, according to orthogonal variance analysis; the activity of CGS is gradually stimulated as the curing age grows, the hydration products Ettringite and C-S-H are continually generated, and the compressive strength of the backfill material increases in tandem; In the GA-ELM model test set, the intensity correlation coefficient between the prediction and the measured value reached an average of 0.99, and the prediction accuracy was 71.8% and 43.5% higher than that of ELM and PSO-ELM respectively; Laboratory studies have shown that the GA-ELM prediction model has a prediction accuracy of 98.2% and that it can accurately estimate the strength of CGS cemented backfill. [ABSTRACT FROM AUTHOR]

Based on the test results of laser particle size analyzer, specific surface area analyzer and infrared spectrometer, the grinding kinetics of coal gasification slag (CGS) was systematically described by using Divas–Aliavden grinding kinetics, Rosin–Rammler–Bennet (RRB) distribution model and particle size fractal theory. The influence of grinding time and particle group of CGS on the strength activity index of mortar was studied by using the strength activity index of mortar and grey correlation analysis. The results show that the particles are gradually refined before mechanical grinding of CGS for 75 min. When the mechanical grinding time is greater than 75 min, the "agglomeration phenomenon" of fine CGS particles led to the decrease in various properties. Divas–Aliavden grinding kinetics, the RRB model and fractal dimension can characterize the change of CGS particle size in the grinding process quantitatively. The strength activity index of CGS at different curing ages is positively correlated with grinding time, and the influence on the later strength activity index is the most obvious. The relationship between CGS particle size distribution and strength activity index were probed using grey correlation analysis. The CGS particle groups with the particle size of 20~30 μm and 10~20 μm have the greatest impact on the early and late strength activity index, respectively. Therefore, the optimal grinding time of CGS as auxiliary cementing material is 75 min, considering factors, such as economy and performance, and the specific surface area (SSA) is 4.4874 m2·g−1. [ABSTRACT FROM AUTHOR]

In order to promote the resource utilization of bulk solid waste, it is an effective way to improve the comprehensive utilization level of solid waste and alleviate the environmental pressure caused by storage by using all solid waste as raw material for backfill cementing. In this paper, a magnesium coal based backfill material (MCB) with modified magnesium slag (MMS) and fly ash (FA) as cementing agents, coal gasification coarse slag (CGCS) and coal gangue (CG) as aggregates was proposed. First, the mechanical properties of MCB were studied and then the hydration effect was verified microscopically. Then, the leaching characteristics of raw materials CGCS, CG, and backfill material MCB were characterized. Furthermore, the mechanism of heavy metal leaching and solidifying of MCB was revealed by studying As and Pb. The results show that: (1) The addition of a suitable amount of CGCS with potential pozzolanic activity produces more MCB hydration products such as C-S-H and AFt with a high CG-CGCS ratio. The compressive strength reaches the maximum value of 10.21 MPa when the curing time is 28 days and the CG-CGCS ratio is 3:1. (2) The leaching levels of the target elements (As, Pb, Cr, Mn, Zn, Cu, Cd, Hg, Ag) of CG meet the limits of the class III groundwater quality standard (III-GQSL) in GB/T 14848–2017. The concentration of Pb in CGCS is 12 μg/L, which exceeds the III-GQSL limit of 10 μg/L. The overall leaching risk is CGCS>CG. (3) MCB has a good stabilization/solidification effect on the heavy metal elements Cd, As, and Pb in raw materials. The heavy metal leaching results of MCB with a low CG-CGCS ratio meet the limit of III-GQSL, and the environmental leaching risk is low. (4) In addition to physical encapsulation, the main curing mechanisms of MCB include chemical bonding, alkaline environment, and adsorption. MCB can be used as a potential consolidation agent for the solidification of heavy metal elements. While having good mechanical properties, it can also treat solid waste and effectively stabilize/solidify toxic elements in materials and has the potential for safe application in backfill. [ABSTRACT FROM AUTHOR]

• Salt modification is the best excitation idea for the hydration activity of CGS. • The salt modification of CGS had a significant effect on the early hydration process of MCGS-CPB. • The effect of salt modification of CGS on the hydration mechanism of MCGS-CPB was clarified. To realize the green mining and clean utilization of coal resources, environmental issues such as coal mine subsidence and coal gasification slag (CGS) solid waste storage require urgent resolution. High residual carbon and low activity serve as bottlenecks for CGS to achieve safe, efficient and resourceful consumption. Therefore, in this study, we proposed the concept of graded utilization of CGS and conducted a study on physical–chemical coupling to stimulate the hydration activity of CGS. We prepared a modified coal gasification slag-based cement paste backfill material (MCGS-CPB) to meet the technical requirements of coal mine backfilling. Through various experimental tests and microscopic characterization methods (uniaxial compressive strength, hydration heat and microscopic tests, etc.), we elucidated the modification mechanism of CGS, and the synergistic reaction law of MCGS-CPB was clarified. The following results were obtained. (1) The best excitation approach for CGS hydration activity was salt modification, and the early strength of MCGS-CPB prepared with CGS excited by sodium sulfate (SS) developed rapidly, while the later strengths showed expansion and deterioration. The strength development of MCGS-CPB prepared by CGS activated with calcium chloride (CC) was relatively stable, and the excitation effect was ideal. (2) The salt modification of CGS had a significant effect on the early hydration process of MCGS-CPB. The durations of the initial dissolution, acceleration, deceleration and slow reaction stages were prolonged, and the duration of the induction stage was shortened, while the hydration heat release rate increased. (3) The salt modification of CGS had a significant effect on the microstructure of MCGS-CPB, primarily due to the difference in excitation effect of salt modification on CGS on pozzolanic activity. This study provided theoretical guidance for the safe, efficient and resourceful utilization of CGS. [ABSTRACT FROM AUTHOR]

[Display omitted] • The hydration exothermic characteristics of MGCAPB by isothermal microcalorimetry. • Hydration kinetic of MGCAPB was analysed by Krstulović-Dabić model. • Mechanical properties of MGCAPB was investigated by UCS and elastic modulus. • Microstructure of MGCAPB was studied by TG-DTG and SEM. A clear understanding of the hydration characteristics of cemented paste backfill (CPB) is helpful to analyze the reasons for the defects of low strength, large deformation and high cost in the early curing period of CPB. This is conducive to realizing the sustainable utilization of solid waste. In this paper, the hydration exothermic characteristics of MGCAPB slurry was tested. Moreover, uniaxial compressive strength(UCS) tests were carried out on the MGCAPB with curing times of 3 and 28 d. In addition, microstructure tests such as TG-DTG and SEM were performed on the MGCAPB with curing time of 3d. The hydration exothermic characteristics of MGCAPB were emphatically studied, the hydration kinetic parameters of MGCAPB were analyzed exclusively by using Krstulović-Dabić model. In addition, the mechanical properties and microscopic characteristics of MGCAPB were studied. The results shows that the hydration kinetic process of MGCAPB slurry can be described by nucleation and crystal growth (NG), phase boundary reaction (I) and diffusion (D). All the formulations of MGCAPB went through the NG-I-D stage. The process of hydration reaction was controlled by NG at the initial hydration stage, and gradually changed to process I and process D with the increase of hydration degree. Compared to the Control group, The α 1 of the MGCAPB activated by the activator was high, indicating that the NG process changed to the I process only when the hydration degree was high, which was related to the effective water-cement ratio of the slurry and nucleation effect. In addition, according to the requirement(3d ≥ 0.5 MPa and 28d ≥ 1.0 MPa) that CPB can provide good roof support in a typical underground mine. In this experiment, the mechanical propertiesof all formulations met the requirements, except for the 3d UCS of the Control group(0.37 MPa) and the C-C1(0.46 MPa). Finally, TG-DTG and SEM were used to analyze the microstructures of MGCAPB with activator type and dosage. The research results can provide a new approach for intensive and comprehensive utilization of solid waste resources. [ABSTRACT FROM AUTHOR]