Effect of the Cement–Tailing Ratio on the Hydration Products and Microstructure Characteristics of Cemented Paste Backfill

Preliminary Study of Preheated Decarburized Activated Coal Gangue-Based Cemented Paste Backfill Material

This study proposes a novel idea of the use of coal gangue (CG) activation and preheated decarburized activated coal CG-based cemented paste backfill material (PCCPB) to realize green mining. PCCPB was prepared with preheated decarburized coal CG (PCG), FA, activator, low-dose cement, and water. This idea realized scale disposal and resource utilization of coal CG solid waste. Decarbonization and activation of CG crushed the material to less than 8 mm by preheated combustion technology at a combustion temperature of 900 °C and a decarbonization activation time of 4 min. The mechanism of the effect of different Na₂SO₄ dosages on the performance of PCCPB was investigated using comprehensive tests (including mechanical property tests, microscopic tests, and leaching toxicity tests). The results show that the uniaxial compressive strength (UCS) of C-S2, C-S3, and C-S4 can meet the requirements of backfill mining, among which the UCS of C-S3 with a curing time of 3 d and 28 d were 0.545 MPa and 4.312 MPa, respectively. Na₂SO₄ excites PCCPB at different curing time, and the UCS of PCCPB increases and then decreases with the increase in Na₂SO₄ dosage, and 3% of Na₂SO₄ had the best excitation effect on the late strength (28 d) of PCCPB. All groups' (control and CS1-CS4 groups) leachate heavy metal ions met the requirements of groundwater class III standard, and PCCPB had a positive effect on the stabilization/coagulation of heavy metal ions (Mn, Zn, As, Cd, Hg, Pb, Cr, Ba, Se, Mo, and Co). Finally, the microstructure of PCCPB was analyzed using FTIR, TG/DTG, XRD, and SEM. The research is of great significance to promote the resource utilization of coal CG residual carbon and realize the sustainable consumption of coal CG activation on a large scale.

Preparation of controlled low-strength materials from alkali-excited red mud-slag-iron tailings sand and a study of the reaction mechanism

To address the low utilization of fines in iron tailings sand (IOTs), a controlled low-strength material (CLSM) was prepared from a combination of fine IOTs and red mud (RM) slag. The 7-day unconfined compressive strength (7-d UCS), slump and cost were used as evaluation indicators, and 16 sets of tests were designed with the Box-Behnken design (BBD) response surface method. X-ray diffraction (XRD), Fourier transform infrared (FTIR), and scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS) were used to study the microscopic morphology and reaction mechanism of the CLSM samples made with the optimal ratios. The results show that the best matching ratio for the alkali-activated RM-slag-IOTs CLSM was a sand ratio of 0.797, an NaOH dose of 3.667% and a mass concentration of 80.657%, and the 7d-UCS, slump and cost indicators verified the feasibility of applying the CLSM to the base course of pavement. Alkali activation of the CLSM also showed that the RM-slag cementation system produced new substances. Internal calcium-silicate-hydrogel (C-S-H) and calcium-aluminosilicate-hydrogel (C-A-S-H) agglomerates were the main sources of strength, and hydration products were interwoven to form a dense structure with crystals as the framework and gels as fillers.

A preliminary study of aeolian sand-cement-modified gasification slag-paste backfill: Fluidity, microstructure, and leaching risks

To realize low-cost green backfill mining, this paper proposes a novel model of aeolian sand-cement-modified gasification slag-paste backfill (ACGPB). This model realizes the safe disposal and resource utilization of hazardous solid wastes. A comprehensive experiment (including slump test, uniaxial compressive strength tests, microscopic test, and leaching toxicity tests) was conducted to explore how the mechanism of ACGPB depends on activator type and dosage. The results showed that fresh ACGPB slurry can be expressed by the Herschel-Bulkley model (R² ≥ 0.965 in all recipes). With Na₂SO₄ as activator type, the yield stress, apparent viscosity, thixotropy, and slump of ACGPB slurry increased with increasing activator dosage. With CaO as activator type, the yield stress, apparent viscosity, thixotropy, and slump of ACGPB slurry fluctuated with increasing activator dosage. The mechanical properties of all recipes (not including Control group and C-C1) met the mechanical requirement (3 d ≥ 0.5 MPa and 28 d ≥ 1.0 MPa). In addition, the concentrations of all heavy metals remained within the range specified by the national standard. Specifically, the activator exerted a positive effect on the stabilization/solidification of heavy metal ions (Cu, Cd, Ba, Ni, Cr, Se, and As). Finally, FTIR, TG-DTG, SEM, and hydration heat were used to analyze the microstructure of ACGPB. The research results provide a creative way for the resource utilization of solid waste.

A Support Vector Machine and Particle Swarm Optimization Based Model for Cemented Tailings Backfill Materials Strength Prediction

This study aimed to investigate the feasibility of using a model based on particle swarm optimization (PSO) and support vector machine (SVM) to predict the unconfined compressive strength (UCS) of cemented paste backfill (CTB). The dataset was built based on the experimental UCS values. Results revealed that the categorized randomly segmentation was a suitable approach to establish the training set. The PSO performed well in the SVM hyperparameters tuning; the optimal hyperparameters for the SVM to predict the UCS of CTB in this study were C = 71.923, ε = 0.0625, and γ = 0.195. The established model showed a high accuracy and efficiency on the prediction work. The R² value was 0.97 and the MSE value was 0.0044. It was concluded that the model was feasible to predict the UCS of CTB with high accuracy and efficiency. In the future, the accuracy and robustness of the prediction model will be further improved as the size of the dataset continues to grow.

The Recent Progress China Has Made in High-Concentration Backfill

With the development of science and technology, backfill technology has made continuous progress, and the traditional backfill method is no longer suitable for various complicated practical engineering situations. Therefore, researchers in the field of backfill mining have gradually shifted their research focus to the study of high-concentration backfill, and Chinese researchers are no exception. In order to solve the problems caused by the traditional backfill method, China began to vigorously develop high-concentration backfill in recent years, and achieved a lot of results. In this paper, some important achievements made by Chinese researchers on high-concentration backfill in recent years are reviewed; it also presents a summary report of the latest research results from several key laboratories across China. Therefore, this paper reviews the development progress of high-concentration backfill China has made, of which the main contents include: (1) research progress of the high-concentration backfill theory in China; and (2) research progress of high-concentration backfill equipment in China. Finally, we claim that this paper serves just as a guide to start a conversation, and we hope many more experts and scholars will be interested and engage in the research of this field.

Performance Analysis of a Combined Solar-Assisted Heat Pump Heating System in Xi’an, China

This study proposed a combined solar-assisted heat pump (SAHP) system that could operate in the serial mode or parallel mode. For this proposed system, a stable year-round operation could be achieved without the assistance of electric heating or low-temperature heat pump. By analyzing the heat balance equations, a correlation of the combined SAHP system for the two modes switched was obtained, which provided a theoretical basis for the optimal operation of this system. In addition, the performance of the proposed system applied in a university bathroom in Xi’an district was investigated using TRNSYS. The results illustrated that compared to the serial and parallel systems, the proposed system exhibited a good performance on energy efficiency. The annual average coefficient of performance (COP) of the proposed system was 5.7, obviously higher than those of the serial system and the parallel system, which were 3.3 and 4.3, respectively. Therefore, the results in this study could provide the theoretical guidance and reference for practical engineering design.