This paper investigates the influence of mine tailings particle size and curing temperature on the rheological properties and strength development of cemented paste backfill(CPB).Laboratory experiments were conducted on CPB to evaluate their physical and mechanical behavior.A series of CPB samples with different mixture designs were prepared and cured for 28 days.The effect of curing temperature and particle size on mechanical properties was assessed using uniaxial compressive strength(UCS)tests.In addition,the influence of curing temperature(ranging from 5°C to 50°C)on CPB samples was studied through UCS testing.The microstructure of selected specimens was further analyzed using the mercury intrusion porosimetry(MIP)technique.The results indicate that:(i)the mechanical performance of CPB can be enhanced by reducing the proportion of fine particles in the tailings;(ii)the rate of strength gain increases with decreasing particle size,samples exhibiting faster strength development than CPB samples over the 28-day curing period;(iii)curing temperature has a significant impact on both the mechanical and microstructural properties of CPB;(iv)microstructural analysis revealed that reducing fine particles modifies pore size distribution and decreases overall porosity,thereby improving the mechanical behavior of CPB;and(v)MIP results showed that the reduction of particle size altered the microstructure,leading to a higher proportion of fine pores in CPB.

To address the persistent challenge of accurately predicting the unconfined compressive strength(UCS)of cemented paste backfill(CPB)in underground mining,this study developed a hybrid intelligent model integrating quantum-behaved particle swarm optimization(QPSO)with a fully connected neural network(FCNN).A comprehensive database comprising 383 laboratory test samples was compiled,encompassing eight input variables:coefficient of uniformity(C_u),chemical composition(CaO and MgO),cement type(CT),tailings-to-cement(T/C)ratio,slurry mass concentration(MC),curing temperature(Temp),and curing time(Time).The QPSO algorithm was employed to perform a global optimization of critical hyperparameters,including hidden nodes,learning rate,regularization coefficient,dropout rate,and batch size.Subsequently,the Adam optimizer was applied for deterministic fine-tuning of network weights,yielding concurrent enhancements in convergence efficiency and predictive accuracy.Furthermore,SHAP,±10%perturbation analysis,and partial dependence plots(PDPs)were utilized to quantitatively evaluate the global importance,local marginal effects,and interactive sensitivities of input parameters.The proposed QPSO-FCNN model exhibited outstanding predictive performance,achieving an R²of 0.969,RMSE of 0.198 MPa,mean absolute erron(MAE)of 0.139 MPa,and a composite performance score(CPS)of 0.930,surpassing both conventional FCNN and QPSO-BPNN hybrid models.Sensitivity analysis revealed that T/C and Time were the most influential factors,while pronounced coupling effects occurred between Cu and MC,as well as between curing conditions and T/C.These results provide robust quantitative insights for optimizing the mix design and curing regimes of CPB,thereby enhancing its mechanical reliability in underground mining operations.

At Olympias Underground Mine,drift-and-fill mining under the cemented paste backfill is required for the safe and effective recovery of sill pillars within complex geology and variable ground conditions.Cured cemented paste backfill provides critical confinement to the surrounding rock,enabling controlled blasting and reducing the risk of instability during mining.This case study reviews operational practices and design considerations specific to Olympias Mine,including geomechanics and geotechnical aspects of mining with paste backfill,paste fill strength requirements and quality control,curing times,blast sequencing,and the integration of ground support systems.

Waste rock and tailings represent the primary solid wastes generated in metal mining.Underground co-backfilling with waste rock and cemented tailings offers an effective approach for the large-scale utilization of these materials.However,quantitative methods for characterizing the permeability and compactness of such composite backfill systems remain limited.In this study,semi-industrial surface experiments were conducted using a self-developed experimental device combined with infrared imaging technology.Three dumping sequences—waste rock followed by slurry,slurry followed by waste rock,and simultaneous dumping—and three accumulation angles of waste rock(90°,60°,and 30°)were investigated.The compactness of the co-backfill and the permeability of cemented tailings slurry within waste rock voids were quantitatively analyzed.Results show that simultaneous dumping yields the highest compaction,although practical constraints may limit its application.Therefore,a sequence of slurry dumping followed by waste rock,at a volume ratio of 3:2,is recommended.Moreover,the inclination angle of waste rock piles was found to be positively correlated with slurry penetration.A quantitative relationship between slurry permeability and the contact surface area was established,expressed as the penetration volume per unit area,with values of 0.185 m³/m²and 0.107 m³/m²depending on whether the pile base was in contact with the stope boundary.Based on these findings,a co-backfilling strategy for high stopes in iron mines is proposed,providing a technical basis for the efficient utilization of mine waste through integrated waste rock–tailings backfilling.

In underground mining with backfill,it is essential to evaluate the minimum required strength of side-exposed backfill.An analytical solution proposed by Mitchell and coworkers in 1982 is largely used.Since 2012,Li and coworkers have published several improved solutions,accounting for several limitations in the Mitchell 1982 model.However,the improved solutions have not fully been validated by experimental results.Experimental work is necessary.As such,box instability tests were performed using an uncemented paste backfill under undrained conditions with a high aspect ratio stope.The unconfined compressive strength of the backfill was measured.Comparisons between experimental results and analytical results were made.A part of the experimental results was used to obtain the missing parameters through calibration.The other part of experimental results was then used to test the predictive capability of the calibrated analytical solutions.The results show that the improved analytical solutions provide a better agreement with the experimental results than the original Mitchell solution,at least for short-term stability of side-exposed backfill submitted to quasi-unconsolidated and undrained conditions.More experimental works are necessary to test the validity of the improved solutions under wider conditions.

Aiming at the prominent problems of high cost of cementitious materials and low CO₂sequestration efficiency in mine filling,this study systematically explored the influence mechanism of the mineral activator carbide slag(CS)and its composite activation with Na₂SiO₃(sodium silicate,SS)on the properties of low-activity electric arc furnace slag(EAFS)and granulated blast furnace slag(GBFS)-based cementitious materials.By preparing carbon-sequestering backfill materials with CS and CS–S_Sas activators,mineralization tests were conducted under controlled CO₂curing parameters(pressure:0.2–0.4 MPa,concentration:20%–60%,curing duration:1–4 h).Comprehensive characterization via X-ray diffraction(XRD),Fourier-transform infrared spectroscopy(FTIR),thermogravimetry-differential scanning calorimetry(TG-DSC),and scanning electron microscopy(SEM)was performed to elucidate the evolution of hydration products and microstructures.It was revealed from the experimental results that,for the CS single-activation system,the 3-day and 7-day uniaxial compressive strengths(UCS)of the cementitious materials decreased with an increasing EAFS replacement ratio.The UCS peaked at 6.81 MPa(3-day)and 10.96 MPa(7-day)when EAFS replacement was 20%(with 20%or 30%CS,respectively).However,excessive CS reduced strength due to insufficient GBFS participation.The CS–S_Ssynergistic activation system exhibited superior early-age strength(17.66 MPa at 3-day or 22.01 MPa at 7-day),confirming the dominant role of SS in early hydration.Mineralization significantly enhanced the performance of the CS single-activation system:the 3-day strength increased by 1.90–2.77 times(reaching 11.60 MPa)compared to the control group(4.20 MPa),but it weakened the CS–SS synergistic activation system.It was identified via orthogonal analysis that CO₂concentration(40%)was the primary factor influencing the 3-day strength of the CS single-activation system,followed by pressure(0.4 MPa)and curing duration(1 h).This validates that mineralization can be achieved under non-pure CO₂and non-high-pressure conditions.Microstructural analyses showed that the CS single-activation system promoted the formation of C–(A)–S–H gel and CaCO₃,whereas composite systems underwent decalcification of C–(A)–S–H gels during mineralization.This study uncovers the synergistic advantages of CS under CO₂mineralization conditions and the incompatibility between the CS–S_Ssynergistic activation system and CO₂mineralization curing,providing theoretical foundations and technical pathways for developing low-cost mine backfill materials with high CO₂sequestration capacity.

A reasonable evaluation of the required strength of side-exposed backfill is essential for safe and economic mining in open stoping with subsequent backfill.The backfill stress distributions and contact properties between backfill and rock should be considered in the evaluation method.In this study,analytical models for arching stresses of uncemented backfill in secondary stopes and side-exposed cemented backfill in primary stopes have been proposed,respectively.Additionally,the lateral pressure exerted by uncemented backfill and the frictional resistance along the rock-backfill interfaces were investigated to reveal the contact mechanisms affecting the stability of side-exposed backfill.An analytical model was then proposed to evaluate the required strength of side-exposed backfill with a potential sliding plane.Numerical simulations across varying stope sizes and backfill parameters were conducted to validate the analytical solution.The results demonstrate a high level of consistency between the analytical solutions and numerical results when the direction angle of the sliding plane is α = 45° + ϕ_c/2 and the direction angle of frictional resistances is β = 45°-ϕ_c/2.The optimized three-dimensional analytical model effectively evaluates the strength requirements of side-exposed backfill subjected to lateral pressure from the adjacent uncemented backfill.At last,a method for translating theoretical strength requirements into practical design strengths of backfill using a floating factor of safety is discussed,which can provide a framework for cost-effective and safe backfill strength design in open stoping mining.

Increasingly stringent environmental regulations,both in Germany and across the European Union(EU),are limiting or prohibiting the above-ground disposal of certain industrial(non-mining)chemotoxic wastes.As a result,alternative,sustainable disposal routes are gaining importance.Over the past three decades,K-UTEC AG(K-UTEC)has supported mining operations in utilizing underground cavities not only for conventional backfilling but also for the controlled and legally compliant disposal of chemotoxic non-mining waste.In many cases,these wastes contribute to cavity stabilization and are kept safe in terms of long-term environmental safety,making such projects technically and economically viable.The legal framework in Germany and the EU is outlined,distinguishing between pure disposal and waste utilization as a substitute for primary raw materials in backfill applications.The advanced and well-established European standards could serve as a model for international adoption.Feasibility assessments must always be site-specific,considering factors such as hydrogeology,cavity geometry,logistics,and the availability of suitable wastes.Current practices have identified promising waste streams,e.g.,from incineration,steel production,cement manufacturing,and waste treatment,that are both suitable for underground use and economically attractive due to limited disposal capacities.A forward-looking approach to mine backfilling is presented,showing how underground cavities can become valuable assets in industrial waste management.K-UTEC anticipates growing international relevance for this dual-purpose strategy.

The filling mining method is a core mining approach for deep and high-risk ore bodies.However,the filling slurry often fails to effectively reach the roof due to issues such as water seepage and settlement,and an insufficient roof contact rate can lead to safety hazards such as roof collapse.The existing forced roof contact technologies have issues such as complex processes and limited effectiveness.Therefore,a new "active roof-contact" filling concept is proposed.The core principle of this concept is to introduce special chemical reaction-based solid phase expansion components,which induce controlled volume expansion of the filling material through hydration reactions.This will drive the filling body in the local uncontacted areas to actively "fill in",thereby significantly improving the roof contact effect.During this process,the continuously developing expansion force can also act on the rock interface to form a "pre-stress field",improving the secondary stress concentration of the surrounding rock.This study takes the cemented filling body of all tailings and waste rocks as the main research object,analyzes the expansion mechanisms of three different types of chemical reaction-based solid phase expansion agents.Then,through the grey target decision-making method,with strength as the benefit type index,slump,final vertical expansion rate and lateral expansion stress as the cost type index,an optimization decision is made to ultimately select the expansion agent suitable for mine filling and its optimal level,to provide a useful reference for future research on active roof-contact backfill(ARCB) materials.

In open-stope subsequent filling mining,the impact load from second-step orebody blasting threatens the stability of the first-step backfill,and rock thickness(RT) is a key factor regulating the mechanical behavior of rock-backfill composites(RBC).However,existing damage constitutive models fail to accurately capture the three-stage damage evolution of RBC under impact loading,especially the post-peak softening characteristics,which limits the scientific design of pillar thickness and the safety of mining operations.To address this engineering and academic gap,this study adopted the split Hopkinson pressure bar(SHPB) system to conduct impact tests on RBC specimens with four RT levels.A novel three-stage damage constitutive model(TS model) was established,with innovative consideration of residual strength in boundary conditions to improve prediction accuracy.Additionally,the TS model was systematically compared with the Weibull distribution-based model(WD model) and energy-based model(EM model) via error analysis and regression analysis.This study clarifies the regulatory effect of RT on the anti-impact performance of RBC and provides a reliable constitutive model for predicting its damage behavior,offering a theoretical basis and engineering reference for the stability control of filling mining systems.