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This study aimed to investigate the impact of as-received biomass fly ashes (BFA) and green liquor dregs obtained from a pulp and paper plant in Brazil as substitutes for metakaolin in geopolymeric formulations. The properties of this type of waste material vary widely between different industrial plants. This study refrains from subjecting the waste materials to any form of pretreatment, taking into account their organic matter and particle size heterogeneity, requiring extensive characterization to evaluate their influence on the compressive strength, apparent open porosity, and water absorption of the geopolymeric samples. The objective was to assess their potential for upcycling purposes as an alternative to energy-intensive materials, such as ordinary Portland cement (OPC) and advanced ceramics. This potential arises from the ability of alkali-activated materials (AAM) to undergo curing at ambient temperatures, coupled with the possibility of compositions primarily derived from waste materials. To improve the sustainability of the products, the amorphous content of the raw material, which is more reactive than crystalline phases, was quantified and used as the base for mixture ratios. This approach aimed to reduce the requirement for alkaline activators, which have significant environmental impacts, while also increasing the waste content in the formulation. The incorporation of waste materials into the geopolymer matrix generally led to a reduction in the compressive strength compared to the benchmark metakaolin sample (19.4 MPa) but did not present a trend. The dregs led to values of 4.1 MPa at 25 wt% and 7.1 MPa at 50 wt%, a behavior that is somewhat counterintuitive, and BFA at 10 wt% presented 5.7 MPa. Nevertheless, the apparent open porosity remained at high levels for all the samples, close to 50%, and the compressive strength of most of them was over the values obtained for the metakaolin-only samples with mixture ratios calculated from the total composition instead of the amorphous composition. The decrease in strength and the increase in porosity were attributed to the specific characteristics of the waste materials, such as their high crystallinity, presence of organic matter, heterogeneous particle composition, and size. Overall, this study provides insight into the variations in geopolymerization based on the bulk and amorphous content of the aluminosilicate sources and how the characteristics of the waste materials influence the geopolymer matrix. It also highlights how calculating mixture ratios based on the amorphous composition improves the possibility of waste valorization through alkali activation. Additionally, it suggests that BFA and dregs might be effectively utilized in applications other than OPC substitution, such as adsorption, filtration, and catalysis. [ABSTRACT FROM AUTHOR]

As an inherent property of the accumulation of elastic energy and the sudden instability failure of coal, coal bursting liability (CBL) is the basis of the research on the early warning and prevention of coal burst. To accurately classify the CBL level, the support-vector-machine (SVM) method was introduced in this paper, and the dynamic failure time (DT), elastic energy index (WET), impact energy index (KE) and uniaxial compressive strength (RC) were selected as the classification indexes. An imbalanced sample set, containing 95 groups of measured data of CBL, was established, and eight SVM classification models were constructed, based on different kernel functions and swarm-intelligence-optimization algorithms. Focusing on the problem of sample imbalance, the classification accuracy, A, F1-score and kappa coefficient were used to comprehensively evaluate the classification performance of SVM models, and the grey-wolf-optimizer SVM (GWO-SVM) model was selected as the best model in this paper, reaching the highest accuracy of 98.9%. The GWO-SVM was applied to identify the CBL level of the 4# coal seam in Xiaozhuang Coal Mine and the 1# coal seam in the Wanfeng Coal Mine. The results of the engineering application are consistent with those from the engineering field, and show that the proposed model is scientific and practical, and can be a new method for CBL classification. [ABSTRACT FROM AUTHOR]

Various methods of longwall full mining with partial filling have been extensively researched to satisfy the specific mining needs of pressurized-coal and residual-coal resources. This study introduces three longwall partial-filling-mining techniques: room–pillar filling mining, parallel-strip filling mining, and vertical-strip filling mining. Numerical simulations are employed to evaluate the efficacy of these methods. The findings indicate that vertical-strip filling mining results in minimal surface deformation and a more uniform distribution of displacements. In practical operations, the effectiveness of filling largely depends on the choice of filling technology and materials. The research further includes an optimization analysis of the filling technology, emphasizing the composition of the coal-gangue-paste filling system and the refinement of its components. Additionally, the study aims to explore the optimization analysis of filling materials, specifically focusing on performance-optimization methods. The experimental results illustrate that optimizing the filling materials can enhance the performance of filling paste, improving both early-stage and long-term compressive strength. Moreover, the paper examines the quantitative characterization of paste-filling-mining subsidence at various stages in conjunction with theoretical knowledge. Subsequently, mining-subsidence-control measures are recommended to address the primary deformation factors across different stages. Through an in-depth examination of filling-method designs, enhancements in filling technology, and predictions regarding filling-mining subsidence, this research offers valuable insights for optimizing longwall partial-filling-mining methods. [ABSTRACT FROM AUTHOR]

The empty-space subsequent filling mining method is the main mining scheme for underground metal mines to achieve large-scale mechanized mining. The stage height, one of the main parameters of this method, affects the various production process aspects of the mine and influences the stability of the goaf. In order to determine the stage height scientifically and rationally in the empty-space subsequent filling mining method, a formula for the stabilized critical safe height of a high goaf in an underground metal mine was derived based on Pu's arch equilibrium theory, Bieniawski's pillar strength limit theory, and the Kastner equation and combined with the results of an orthogonal analysis to rank the importance of the main factors in the formula. A copper mine in Jiangxi Province was used as a case study, with the reliability of the formula verified by numerical simulation and industrial testing. The factors in the formula influencing the critical stabilized safe height of the goaf were, in descending order, the compressive strength of the rock body, the width of the two-step mining pillar, the width of the one-step mining room, the mining height, and the depth of mining. Based on the calculation results, the recommended stage heights are 30 m (−378 m middle section) and 25 m (−478 m middle section) in the area of poor rock body stability and 50 m in the area of better rock body stability. The simulation results show that the goaf is significantly affected by the compressive stress under the condition of a certain rock body stability and that the compressive stress increases with increasing goaf height. The minimum recommended values of the sidewall safety coefficients in areas of poor and better rock stability are 1.04 and 1.06, respectively. The volume deviation coefficients of the three industrial test mines were all controlled within 3%, indicating that no obvious collapse and destabilization phenomenon occurred in the goaf. This paper provides some theoretical and applied guidance for the stage height design of similar underground metal mines using the empty-space subsequent filling mining method. [ABSTRACT FROM AUTHOR]

This paper presents a machine learning-based approach to estimating the compressive strength and elastic modulus of rocks. A hybrid model, GWO-ELM, was built based on an extreme learning machine network optimized by the grey wolf algorithm. The proposed model was carried out on 101 experimental datasets, and four commonly used models were used as benchmarks to evaluate the accuracy of the proposed model. The results showed that the proposed hybrid model can accurately achieve the prediction of elastic modulus and compressive strength with high correlation coefficients and small prediction errors. The prediction performance of the hybrid model is significantly better than the other four original models, and it is an alternative model for predicting the compressive strength and elastic modulus of rocks, which is recommended as an auxiliary tool for real-time prediction of rock mechanical properties. [ABSTRACT FROM AUTHOR]

Previous studies have confirmed that for cemented tailings backfill, mechanical properties are improved through the addition of fiber. However, for fiber-reinforced cemented sulfur tailings backfill (FRCSTB), physical and flow properties are still unknown. In this paper, the changes in fluidity, splitting tensile strength (STS) and uniaxial compressive strength (UCS) of cemented sulfur tailings backfill (CSTB) are analyzed in detail. Secondly, regarding the addition of glass fiber and polypropylene fiber, the changes in the fluidity, STS and UCS of the CSTB, resulting from the fiber length, fiber content and method of fiber addition used, were analyzed. Moreover, the relationship between the UCS and fiber content is established. Finally, the mechanism behind the influence of fiber and sulfur content on the mechanical properties of CSTB is revealed. The results indicate that with the increase in sulfur content, the fluidity of the tailings slurry exhibits exponential growth. During the process of increasing sulfur content, the UCS and STS of CSTB initially increase and then decrease, reaching maximum values at 12% sulfur content. Similarly, at a fiber content of 0.6%, the UCS and 28d STS of CSTB reach their maximum values. In terms of enhancing the mechanical properties of CSTB, the effectiveness of glass fibers surpasses that of polypropylene fibers. In addition, regarding the improvement of the UCS of CSTB, the mixed addition of fibers is obviously worse than that of fiber alone. However, in terms of enhancing the STS of CSTB, the mixed addition of fibers outperforms the single addition of polypropylene fiber. From a microscopic perspective, polypropylene and glass fiber are able to form strong cohesion with the cement–tailings matrix and effectively prevent the formation and expansion of pores and cracks. [ABSTRACT FROM AUTHOR]

The utilization rate of steel slag in China is far behind that of developed countries. The annual output of steel slag is still increasing, resulting in a large amount of accumulation, causing environmental pollution. This paper summarizes and analyzes the relevant research on steel slag powder (SSP) as a cementitious material, studies the effect of SSP replaces cement as single or multiple admixtures, with different specific surface areas, and the amount of activator on the compressive strength of cement-based material. The results show that due to the lower content of active substances in SSP compared to cement, the strengths decrease with the increase of the replacement ratio R, which is the ratio of SSP to cement. R = 30% is important for replacing cement with single SSP. When replacing cement with the mixture of SSP and slag/fly ash, the strengths of most groups decrease with the increase of the mix replacement ratio Rc. The decreasing trend is not obvious due to the pozzolanic effect. There is an optimal dosage for using a single activator to activate SSP. The effect of using multiple activators in combination is better than that of single one. The strength increases with the increase of the specific surface area (SSA) of SSP. However, if the SSA is too high, it will not only increase the preparation cost, but also reduce the increase in strength due to the agglomeration effect of SSP. The optimal range of specific surface area SSA is 400 m2/kg~500 m2/kg. With the increase of age t, the compressive strength increases. The effect of the curing methods on the compressive strength is hot and heat curing > standard curing > natural curing. [ABSTRACT FROM AUTHOR]

Mechanical activation is known to greatly influence the reactivity of fly ashes. In this paper, we report a comparative study of the properties of alkali-activated geopolymer materials prepared using both ball-milled and attrition-milled fly ashes. Ball milling was carried out for 30 min and 60 min while attrition milling was carried out continuously in a high-speed attritor. The surface area of the raw fly ash decreased from 4017 cm2/g to 3999 cm2/g and 3912 cm2/g after ball milling for 30 min and 60 min, respectively. By contrast, the surface area of the continuously attrition-milled fly ash increased to 5545 cm2/g. Fly ash processed by continuous attrition milling showed a 50% particle size reduction to 25–38 μm, whereas fly ash ball-milled for 30 and 60 min was reduced in size by 33.4 and 42.9%. The milled fly ash samples were activated with 8 M NaOH solution and cured at 40 °C for 68 h. After curing, the samples were maintained at room temperature, and their 7-, 14-, and 28-day compressive strengths were measured. The compressive strength of the attrition-milled 28-day geopolymer paste was 24.6 MPa; that of the geopolymers ball-milled for 30 and 60 min was 23.37 MPa and 17.58 MPa, respectively; and that of the unmilled control geopolymer fly-ash-based paste was 17 MPa. The improvement in the mechanical properties is attributed to the increased gel formation resulting from the increased surface area (decreased particle size) in the fly ash glass starting material. [ABSTRACT FROM AUTHOR]

Most of the previous studies focused on the mechanical characteristics before the stress peak of solid waste cemented backfill, but in the compression process of a solid waste cemented body, the phenomenon of post-peak stress rebound often occurs. Through the uniaxial compression experiment of a solid waste cemented body composed of coal gangue, fly ash, desulfurization gypsum, gasification slag, and furnace bottom slag, this paper analyzed the compression deformation characteristics of a solid waste cemented body with different mix proportions before and after the stress peak, established the stress–strain curve model of rebound stress in the rising and descending section after the stress peak, and revealed the reasons for the rebound stress and secondary unloading of the cemented body after the stress peak. The results showed that the maximum rebound stress accounts for 40%–80% of the compressive strength, and the changes in the two are positively correlated. The stress–strain curve model is a cubic function in the post-peak stress rising section and a quadratic rational function in the descending section. With the increase in the maximum compressive strength of the cemented body, its maximum rebound stress also increases, but its corresponding compressive strain generally shows a downward trend. There is a positive correlation between the rebound stress increment and strain increment of the cemented body. The change in the supporting structure and the evolution of the failure form of the cemented body before and after the maximum rebound stress indicate that the compression failure of the residual supporting structure caused by the main crack is the main reason for the rebound of the stress after the peak value of the cemented body to the complete unloading. [ABSTRACT FROM AUTHOR]

This paper presents the investigations of selected properties of concrete containing two fractions of fly ash microspheres (FAMs) with grain size up to 200 µm and up to 500 µm. Concrete mixtures with ordinary Portland cement and three substitution rates of cement by FAMs, 1.3%, 2.0%, and 2.6%, were investigated. For all concrete mixes, the constant water–binder ratio (w/b) equal to 0.50 was used. The research program included the determination of chemical composition, particle size distribution, and pozzolanic activity of FAMs. The analysis showed that the microspheres consist of three main elements: silicon, aluminum, and iron, whose oxides constitute about 89% of the material. The total content of air pores, measured in concrete at 28 days, ranges from 1.3 to 3.1% for the grain sizes below 200 µm and from 1.4% to 3.9% for the grain sizes up to 500 µm. The FAMs with a particle diameter < 200 µm have a relatively high level of activity pozzolanic at 28 days. The SEM analysis proved that the use of microspheres reduces gaps and increases the CSH phase. The compression test showed that the addition of fly ash microspheres with grain size below 200 µm increases long-term concrete strength. [ABSTRACT FROM AUTHOR]

The disposal of tailings has always been a focal point in the mining industry. Semi-dry tailings stockpiling, specifically high-concentration tailings stockpiling, has emerged as a potential solution. To enhance the stability of tailings stockpiling and minimize its costs, the incorporation of a low-cost curing agent into high-concentration tailings is essential. Therefore, this study focuses on the development of a curing agent for high-concentration unclassified tailings stockpiling. The composition of a low-cost curing agent system is determined based on theoretical analysis, and the curing reaction mechanisms of each composition are researched. Subsequently, an orthogonal experiment is designed, and the strength of the modified unclassified tailings solidified samples at different curing ages is measured. Furthermore, the rheological properties of the modified unclassified tailings slurries are tested, and the feasibility of industrial transportation of the unclassified tailings slurries modified with the optimized curing agent is analyzed. Lastly, the microscopic morphologies of each material and the modified unclassified tailings solidified samples are characterized, their chemical compositions are tested, and the action mechanism of the curing agent in the curing system is further investigated. The results show that the optimal proportions of each material in the curing agent are as follows: slag, 58%; quicklime, 15%; cement, 8%; gypsum, 9%; and bentonite, 10%. The dominance of industrial waste slag exceeding 50% reflects the low-cost nature of the curing agent. Under this condition, the modified unclassified tailings slurry with a mass concentration of 75% exhibited a yield stress of 43.62 Pa and a viscosity coefficient of 0.67 Pa·s, which is suitable for pipeline transportation. These findings lay a foundation for subsequent decisions regarding stockpiling processes and equipment selection. [ABSTRACT FROM AUTHOR]

Geopolymers are in many applications a perfect alternative to standard cements, especially regarding the sustainable development of green building materials. This experimental study therefore deals with the investigation of different factors, such as the water content and the binder to aggregate ratio, and their influence on the workability of fresh mortar and its mechanical properties and porosity on different size scales. Although increasing the water content improved the workability and flow behaviour of the fresh mortar, at the same time, a reduction in compressive strength in particular and a lesser reduction in flexural strength could be demonstrated. This finding can be attributed to an increase in capillary porosity, as demonstrated by capillary water uptake and mercury intrusion porosimetry measurements. At the same time, the increasing water content led to an improved deaeration effect (low air void content) and to initial segregation (see the µXCT measurements). An alternative approach to enhance the compressive and flexural strengths of the mortar specimens is optimization of the binder to aggregate ratio from 1 to 0.25. This study paves the way for a comprehensive understanding of the underlying chemistry of the geopolymerization reaction and is crucial for the development of sustainable alternatives to cementitious systems. [ABSTRACT FROM AUTHOR]

Backfill is commonly used in underground mines. The quality control of the backfill is a key step to ensure it meets the designed strength requirement. This is done through sample collection from the underground environment, followed by uniaxial compression tests to obtain the Uniaxial Compressive Strength (UCS) in the laboratory. When the cylindrical cemented backfill samples are axially loaded to failure, several failure modes can be observed and mainly classified into diagonal shear failure and axial split failure. To date, the UCS obtained by these two failure modes are considered to be the same with no distinction between them. In this paper, an analysis of the UCS results obtained on a cemented hydraulic backfill made of alluvial sand at a Canadian underground mine over the course of more than three years is presented. The results show that the UCS values obtained by diagonal shear failure are generally higher than those obtained by axial split failure for samples with the same recipe and curing time. This highlights the importance of making a distinction between the UCS values obtained by the two different modes of failure. Their difference in failure mechanism is explained. Further investigations on the sources of the data dispersion tend to indicate that the UCS obtained by laboratory tests following the current practice may not be representative of the in-situ strength distribution in the underground stopes due to segregation in cemented hydraulic backfill. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study performed to investigate the effect of activator modulus (SiO2/Na2O) and slag addition on the fresh and hardened properties of alkali-activated fly ash/slag (AAFS) pastes. Four activator moduli (SiO2/Na2O), i.e., 0.0, 1.0, 1.5, and 2.0, and five slag-to-binder ratios, i.e., 0, 0.3, 0.5, 0.7, 1.0, were used to prepare AAFS mixtures. The setting time, flowability, heat evolution, compressive strength, microstructure, and reaction products of AAFS pastes were studied. The results showed that the activator modulus and slag content had a combined effect on the setting behavior and workability of AAFS mixtures. Both the activator modulus and slag content affected the types of reaction products formed in AAFS. The coexistence of N-A-S-H gel and C-A-S-H gel was identified in AAFS activated with high pH but low SiO2 content (low modulus). C-A-S-H gel had a higher space-filling ability than N-A-S-H gel. Thus, AAFS with higher slag content had a finer pore structure and higher heat release (degree of reaction), corresponding to a higher compressive strength. The dissolution of slag was more pronounced when NaOH (modulus of 0.0) was applied as the activator. The use of Na2SiO3 as activator significantly refined the pores in AAFS by incorporating soluble Si in the activator, while further increasing the modulus from 1.5 to 2.0 prohibited the reaction process of AAFS, resulting in a lower heat release, coarser pore structure, and reduced compressive strength. Therefore, in view of the strength and microstructure, the optimum modulus is 1.5. [ABSTRACT FROM AUTHOR]

This study investigates the time-dependent rheological behavior of cemented paste backfill (CPB) that contains calcined clay as a binder, particularly with LC3 (Limestone Calcined Clay Cement) compositions, using two different PCEs (Polycarboxylate Ether) superplasticizers. Rheological measurements have been conducted on four different mix designs using the Bingham model to describe the CPB mixtures. Both yield stress and plastic viscosity have been reported, and the impact of the admixture on these parameters has been investigated. Unconfined compressive strength (UCS) was measured over 182 days for all mix designs. Both admixtures showed better workability in all cases, with significantly improved yield stress and plastic viscosity compared to the reference, while showing little to no negative impact on strength over time. This study highlights that both from a binder and an admixture point of view, relevant to the industry, these calcined clay systems are ready to be used in a CPB and could make a significant impact on the sustainability of a mining operation in the near future. [ABSTRACT FROM AUTHOR]

In underground mining, the application of mine backfill has evolved into a standard practice. Mine backfill typically consists of tailings, water, and hydraulic binders. However, the high cost of binders has prompted scholars to research alternatives to reduce this cost while maintaining or even improving the properties of the backfill. One potential alternative is leveraging microwave irradiation. In this study, an innovative approach was developed to increase the unconfined compressive strength (UCS) of early age cemented paste backfill (CPB). Microwave treatment was applied to CPB samples at various curing ages for varying durations. The UCS and ultrasonic pulse velocities were measured and analyzed in an experiment with a full factorial design. Moreover, the microstructural properties of the CPB were investigated using mercury intrusion porosimetry. The results indicate a significant potential to increase the UCS of CPB by up to 25% when microwave-treating samples for 8 min after 7 days of curing. This approach could shorten mining cycle times and improve productivity, presenting a promising method to enhance CPB strength. [ABSTRACT FROM AUTHOR]

Due to the lack of clarity in the strength design of underground tailings reservoirs, it is imperative to investigate the interaction between the tailings deposit body (TDB) and surrounding rock. Taking the TDB as the subject of analysis, a differential equation for vertical stress on the TDB is proposed, considering the stresses from the hanging wall of the surrounding rock and physical and structural parameters of the TDB. Considering the similarity between the underground tailings reservoir and one-step subsequent filling, in situ data of the one-step subsequent filling body from a mine was utilized to compare calculated values of the theoretical model. The resulting theoretical prediction error was less than 10%, thus verifying the reliability of the proposed model. According to the theoretical model analysis, the height of the TDB exerts the most significant influence on vertical stress, while the width and length of the TDB have a negligible impact. Moreover, internal friction angle has a more pronounced effect on vertical stress than cohesion force. A case study for a lead–zinc mine in China is presented in this work. Through uniaxial compressive strength and triaxial shear experiments, the key mechanical parameters of TDB at different ratios of cement to tailings are obtained. According to the theoretical model proposed herein, the distribution law of vertical stress in the height direction of TDB is determined for various ratios of cement to tailings. The original technical scheme of the mine has been optimized by using uniaxial compressive strength greater than vertical stress as the evaluation index, achieving both storage safety and cost reduction goals. [ABSTRACT FROM AUTHOR]

The rheological properties (yield stress, flow index and infinite dynamic viscosity) and mechanical properties (unconfined compressive strength, UCS) of different cemented paste backfill (CPB) recipes must be determined during the laboratory optimization phase. However, the influence of the mixing procedure on these properties has scarcely been studied so far. The objective of this paper is to assess to what extent these properties depend on the specific mixing energy (SME) for a given type of mixer. CPB recipes were prepared based on two types of tailing (CPB-T1 and CPB-T2, also referred to as T1 and T2) at a fixed solid percentage for each type of tailing using the Omcan laboratory mixer. A mixture of 80% slag and 20% GU was used as a binder. The mixing time and the rotation speed of the mixer were successively varied. For each recipe prepared, we determined the SME, the rheological properties of fresh CPB (at the end of mixing) and the UCS at 7, 28 and 90 days of curing. The results show that yield stress and infinite viscosity decreased when SME increased in an interval going from 0.3 to 3.8 Wh/kg and 0.6 to 6 Wh/kg for CPB-T1 and CPB-T2, respectively. An increasing trend in UCS with increasing SME was also observed. Empirical equations describing the change of the rheological properties with the SME are used to estimate the change in rheological properties of CPB along the distribution system, considering the specific energy dissipation during CPB transportation. A mixing procedure for obtaining CPB mixtures that are representative of CPB deposited in underground mine stopes is suggested for laboratories who currently use a same mixing procedure, irrespective of the variable field specific energy. [ABSTRACT FROM AUTHOR]

Limestone calcined clay cement (LC3) is a sustainable alternative to ordinary Portland cement, capable of reducing the binder's carbon footprint by 40% while satisfying all key performance metrics. The inherent compositional heterogeneity in select components of LC3, combined with their convoluted chemical interactions, poses challenges to conventional analytical models when predicting mechanical properties. Although some studies have employed machine learning (ML) to predict the mechanical properties of LC3, many have overlooked the pivotal role of feature selection. Proper feature selection not only refines and simplifies the structure of ML models but also enhances these models' prediction performance and interpretability. This research harnesses the power of the random forest (RF) model to predict the compressive strength of LC3. Three feature reduction methods—Pearson correlation, SHapley Additive exPlanations, and variable importance—are employed to analyze the influence of LC3 components and mixture design on compressive strength. Practical guidelines for utilizing these methods on cementitious materials are elucidated. Through the rigorous screening of insignificant variables from the database, the RF model conserves computational resources while also producing high-fidelity predictions. Additionally, a feature enhancement method is utilized, consolidating numerous input variables into a singular feature while feeding the RF model with richer information, resulting in a substantial improvement in prediction accuracy. Overall, this study provides a novel pathway to apply ML to LC3, emphasizing the need to tailor ML models to cement chemistry rather than employing them generically. [ABSTRACT FROM AUTHOR]

Tensile strength is a crucial parameter involved in the design and analysis of cemented paste backfill (CPB). The ability of CPB to withstand tensile forces is essential for the stability of the backfilled stopes, particularly in areas with high stress or deformation. The tensile strength is a critical design parameter used in sill mats to perform underhand cut-and-fill operations. This study presents a novel technique that utilizes rectangular dog-bone specimens and compression to tensile load converters to perform the direct determination of tensile strength. This study indicates that the prevailing assumption regarding the ratio of unconfined compressive strength (UCS) to tensile strength (i.e., 10:1 or 12:1) underestimates the strength. The results suggest a ratio closer to 3:1 or 4:1. The findings indicate that the ratio varies with the curing interval. Specifically, the tensile-to-compressive strength ratios were higher in early-age specimens, as tensile strength values do not increase at the same rate as those of compressive strength. This disparity has notable implications, as underestimating tensile strength via traditional UCS-to-tensile strength ratios could potentially inflate binder consumption. Our study underscores the importance of using direct tensile strength measurements to optimize mining operations. [ABSTRACT FROM AUTHOR]

In world practice, the need for high-strength concrete with an intensive gain of early strength is due to an increase in requirements for characteristics of concrete and the desire to shorten the construction period. Alkali-activated cement, based on soluble sodium silicates (SSS), can demonstrate high strength and rapid gain due to the nano-modifying effect of amorphous silica present in SSS. However, the problem with the effective use of such cement compositions is unsatisfactory short setting times. This work investigates the effect of modifying admixtures on the structure formation of alkali-activated slag cement (AASC), its physical and mechanical properties depending on characteristics of SSS and the basicity of the aluminosilicate component (precursor), which was changed by the ratio of the ordinary Portland cement (OPC) clinker and granulated blast furnace slag (GBFS). A positive synergistic effect was noticed from glycerol and trisodium phosphate, as the components of a complex admixture, to control the setting of AASC. This resulted in extending the initial setting time from 1 to 5 min to the values of 21–72 min. The compressive strength of 21–26.3 MPa by 3 h, 36.5–43.4 MPa by 1 day, and 84.7–117.1 MPa by 28 days was obtained. Proper shrinkage deformations were equal to 0.47–0.6 mm/m. It was shown that with an increase in the basicity of the aluminosilicate component, the properties of AASC increased both in the early and late stages of hardening. [ABSTRACT FROM AUTHOR]

This study focused on mullite-based and forsterite-based ceramic bricks fired at 1000 °C from mixtures of fly ash (40 mass%) and kaolins or vermiculites (60 mass%). The structural, physical, and mechanical properties were characterized by X-ray powder diffraction, nitrogen physisorption, mercury porosimetry, thermogravimetry, and compressive strength. In the development of green-material-derived photocatalysts, we evaluated fly ash ceramic bricks based on kaolins and vermiculites, which deserve deeper research. Alkali potassium in the mixtures positively influenced the reduction of the firing temperature, shrinkage, small porosity, and high compressive strength of ceramic bricks. The crystallization of mullite in fly ash was observed on exotherm maxima from 813 to 1025 °C. Muscovite/illite admixture in kaolins precursor of mullite-based ceramics reduced the crystallization temperature of mullite by up to 70 °C. Vermiculite–hydrobiotite–phlogopite in mixed layers of a raw vermiculite precursor of forsterite-based ceramics controlled the formation of enstatite and forsterite in the temperature range from 736 ± 6 °C to 827 ± 6 °C. Mullite- and forsterite-based ceramic bricks were also investigated for photocatalytic hydrogen production. The photocatalytic generation of hydrogen in the presence of mullite-based ceramic bricks was positively correlated with the percentages of Fe2O3 in the lattice of mullites and in the presence of forsterite-based ceramics with the presence of diopside. Mullite-based ceramic produced the highest yield of hydrogen (320 µmol/gcat after 4 h of irradiation) in the presence of mullite with the highest 10.4% substitution of Fe2O3 in the lattice. The forsterite-based ceramic produced the highest hydrogen yields (354 µmol/gcat after 4 h of irradiation) over more active diopside than forsterite. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for determining the uniaxial and triaxial compressive strength of heterogeneous material composed of dacite (D) and altered dacite (AD). A zone of gradual transition from altered dacite to dacite was observed in the rock mass. The mechanical properties of the rock material in that zone were determined by laboratory tests of composite samples that consisted of rock material discs. However, the functional dependence on the strength parameter alteration of the rock material (UCS, intact UCS of the rock material, and mi) with an increase in the participation of "weaker" rock material was determined based on the test results of uniaxial and triaxial compressive strength. The participation of altered dacite directly affects the mode and mechanism of failure during testing. Uniaxial compressive strength (σciUCS) and intact uniaxial compressive strength (σciTX) decrease exponentially with increased AD volumetric participation. The critical ratio at which the uniaxial compressive strength of the composite sample equals the strength of the uniform AD sample was at a percentage of 30% AD. Comparison of the obtained exponential equation with practical suggestions shows a good correspondence. The suggested methodology for determining heterogeneous rock mass strength parameters allows us to determine the influence of rock material heterogeneity on the values σciUCS, σciTX, and constant mi. Obtained σciTX and constant mi dependences define more reliable rock material strength parameter values, which can be used, along with rock mass classification systems, as a basis for assessing rock mass parameters. Therefore, it is possible to predict the strength parameters of the heterogeneous rock mass at the transition of hard (D) and weak rock (AD) based on all calculated strength parameters for different participation of AD. [ABSTRACT FROM AUTHOR]

This paper presents the results of an extensive experimental study on the effect of the addition of two types of fly ash produced during fluidized bed combustion of bituminous coal and lignite, which differ substantially in their chemical and mineral compositions, on the compressive strength of concrete. Concretes with water/binder ratios of 0.65, 0.55 and 0.45 made with CEM I 42.5 R Portland cement and gravel aggregate were tested. The analyzed amounts of fly ash added to the binder were 0, 15% and 30% by weight. Based on the results of compressive strength testing after 28 and 90 days of curing, the relationships with the water/binder ratio and fly ash content in the binder were determined. The fly ashes used were highly active and capable of pozzolanic reaction. The relationships established allow the compressive strength of concretes based on composite cement-fly ash binder to be predicted with sufficient accuracy. The results presented in this study are an important contribution to the knowledge of concretes with combined binders. They have the exploratory value of establishing the dependence of compressive strength at 28 and 90 days on binder composition and water-binder ratio. In addition, they could be used almost directly in practical applications. [ABSTRACT FROM AUTHOR]

The Lower Globigerina Limestone Member, the oldest member of the Globigerina Limestone Formation, outcrops over most of the Maltese archipelago, notably Malta. It has provided the islands' main building material since the Neolithic period. This paper makes available a corpus of findings relating to the geochemistry, mineralogy and textural properties of this limestone—mostly unpublished and undertaken nearly three decades ago—which provide a useful source to understand its behavior. Bulk chemistry and mineralogy showed that non-carbonate and clay content is higher in limestone of inferior quality. Textural analyses gave insight into the fabric of the matrix, including inter- and intra-particle porosity. These analyses were supplemented by an array of petrophysical tests, including color (a parameter which has a correlation with density and Fe2O3 content), ultrasonic pulse velocity and compressive strength. The findings not only give insight into the composition of the limestone, using insoluble residue content of ≥5% as the threshold of inferior quality lithotype, but provided an insight into the physico-mechanical bonding present, a characteristic which has a bearing on the deterioration of this limestone. [ABSTRACT FROM AUTHOR]

Copper slag, a by-product of the pyrometallurgical process used for obtaining copper from copper ore in Bor, Serbia, contains mainly silicon, iron, calcium, and aluminium oxides. Due to such properties, it is disposed of in landfills. Despite the favourable technical properties copper slag aggregates possess, such as low-water absorption (WA24 0.6%), low resistance to fragmentation (LA 10%), and low resistance to wear (MDE 4%), its use in the construction industry is still limited. The results of testing the technical properties of copper slag aggregates (CSAs) as a potential replacement for natural river aggregate (RA) are presented in this paper. The experiments included tests on three concrete mixtures with partial replacement of coarse natural aggregate with copper slag. The replacement of RA particle sizes of 8/16 mm and 16/31.5 mm with CSA in the amount of 20% + 50% and 50% + 50% resulted in an increase in the compressive strength of 12.4% and 10.5%, respectively. The increase of CSA content led to a decrease in water penetration resistance and salt-frost resistance of concrete, whereas the resistance to chloride ion penetration did not change significantly. [ABSTRACT FROM AUTHOR]

The geopolymer is an environmentally friendly and high-performance material. Nowadays, how to improve the degree of the geopolymer's reaction and enhance its mechanical properties has become a hot topic. This study used orthogonal tests to design the precursor mixing ratio, considering GGBS content (A), water/binder ratio (B), and alkaline activator modulus (C). The fly ash (FA) ground granulated blast furnace slag (GGBS)-based geopolymers were cured under two standard curing conditions: 40 °C under water and 40 °C in the oven. Then, the influence of these factors on the mechanical properties of geopolymers under different curing conditions was summarized. The contribution of each factor was ranked, which was used to find out the most sensitive factors affecting the mechanical properties. Taking the 7 days and 28 days of compressive strength and flexural strength of the geopolymer specimens as the evaluation criteria, the optimum ratio method for preparing geopolymers was obtained. Then, the prediction model of compressive strength under different curing conditions was established. SEM and XRD were used to analyze the microstructure and hydration products of the samples. The test results showed that the optimum ratio of FA-based geopolymers varied under different curing conditions. The GGBS content was the key factor in determining the mechanical properties. The heat curing condition was the best curing condition, the 28-day compressive strength could reach 76.3 MPa, and the 28-day flexural strength could reach 7.4 MPa. The prediction models established for compressive strength under different curing conditions had high accuracy. The specimens under the best curing conditions exhibited a dense internal microstructure and the presence of C-S-H gels, C-A-S-H gels, and N-A-S-H gels. [ABSTRACT FROM AUTHOR]

The New Zealand sea sand ore is a kind of vanadia–titania magnetite formed by erosion in the coastal zone. Because of its coarse particle size, smooth spherical particles, complex chemical composition, it has been added to sinter as an auxiliary material. Based on the principle of optimizing ore blending to strengthen advantages and weaken disadvantages, this paper used New Zealand sea sand raw ore that has not undergone any pretreatment as the main material and prepared it into oxidized pellets using a disc pelletizer and explored the influence of high-proportion unground sea sand ore on the preparation process and reduction performance of oxidized pellets. The influence of unground sea sand ore on the falling strength, compressive strength, reduction swelling index, and reduction degree of pellets was analyzed by the ICPAES, XRF, XRD, SEM-EDS, and other detection methods, and the change laws and influencing factors of oxidized pellets were analyzed. With the increase of the amount of unground sea sand ore used, the falling strength and compressive strength of the green pellets first decreased and then gradually increased, while the compressive strength of the oxidized pellets first increased and then decreased. At the same time, as the amount of sea sand ore used increased, the reduction process of pellets was restricted. The reduction swelling index and the reduction degree index generally show a downward trend. However, the compressive strength of the pellets gradually increased after reduction. Through the research on the pellet-forming performance and reduction properties of unground sea sand ore, it is shown that when the amount of unground sea sand ore used was 40%, it can still be used as raw material for blast furnace ironmaking. Thus, this research provided specific data support for iron and steel enterprises to improve the ratio of unground sea sand ore and reduce production cost. [ABSTRACT FROM AUTHOR]

As part of an extensive research program exploring the potential benefits of using air-entraining admixtures in mine backfill, the experimental study presented in this paper investigates the effect of cement and entrained air dosages on mine backfill unconfined compressive strength (UCS), fresh density, and dry density. Backfill specimens were prepared using silica sand, normal Portland cement, water, and an entrained air admixture. An experimental design with response surface methodology was adopted to develop predictive mathematical models and analyze the results. The results demonstrated that an entrained air dosage of 3.5% could improve the UCS of the mine backfill owing to better dispersion of cement particles. However, a further increase in the dosage reduced the UCS as well as the fresh and dry densities by approximately 200 and 120 kg/m3, respectively. Study results imply that using air-entraining admixtures can potentially enhance mine backfill flowability and reduce the density, thus providing safer and more sustainable working conditions in an underground mining environment. [ABSTRACT FROM AUTHOR]

It is of great significance to study the effect of solid contents (SC), binder-to-tailings (b/t) ratio, types and dosage of fly ash (FA) on the viscosity (V) and uniaxial compressive strength (UCS) of backfill. It can improve filling efficiency and reduce filling costs to understand the relationship between SC, b/t ratio, FA dosage and viscosity, and UCS of backfill. Consequently, this paper carried out uniaxial compression tests and rheological tests on five different types of backfill specimens. Experimental results indicate that, with the increase of SC, the viscosity and UCS of all backfill samples increases as a power function. With the decrease of b/t ratio, the viscosity and UCS of all backfill samples decreases as an exponential function. The coupling effect of SC and b/t ratio has a great influence on the viscosity and UCS of backfill samples. The relationship between SC, b/t ratio and viscosity, and UCS is a quadratic polynomial function. The order of the viscosity of the backfill slurry is: pure tailings < backfill slurry mixed with Ordinary Portland Cement (OPC) < backfill slurry mixed with FA1 < backfill slurry mixed with FA2. The higher the FA dosage, the greater the viscosity. The order of the UCS of backfill is: backfill with OPC > backfill with FA1 > backfill with FA2. The higher the FA dosage, the smaller the UCS. The UCS of all backfill samples increased with the increase of curing time (CT). The relations between the viscosity and UCS of backfill present the positively linear functions. It is feasible to use viscosity to predict the UCS of backfill, and the error between the UCS predicted value and the test value is mostly controlled within 10%. Ultimately, the findings of the experimental work will provide a scientific reference for the mine to design the strength of the backfill. [ABSTRACT FROM AUTHOR]

The replacement of traditional cement with high clinker content should be achieved quickly to lower the carbon footprint of mortar and concrete. Cement is responsible for about 70% of the carbon footprint of cementitious materials. Current research suggests that the use of limestone and metakaolin or calcined clay could replace the current four gigatons of clinker produced. Here, binary systems composed of limestone/cement and metakaolin/cement are first studied to determine the individual impact of fine limestone and diverse fine metakaolins on the flow and compressive strength of the material. The flow properties are correlated with the surface areas of clinker and metakaolin and are almost independent of the limestone content. A model based on a linear relationship between compressive strength and porosity is used to estimate the reactivity of cement, limestone and metakaolin. An excellent correlation is obtained with the two binary systems and confirmed with the ternary systems using the same reactivity factors. The presented model allows the determination of the impact of each of the three components on compressive strength development. Limestone and metakaolin accelerate the hydration of clinker, leading to higher early strength, proportionally to their surface area. The reactivity of metakaolin is also found to be directly related to its mean size or surface area. [ABSTRACT FROM AUTHOR]

The thinning of tailings gradation during ore processing leads to a sizeable fall in the strength of cementitious paste backfill (CPB), increases operational risks, and encourages researchers to use alternative economic products. This study aims to increase the strength performance by improving CPB's gradation while cutting costs and reducing the sum of the binder employed per unit volume. An evolution of the slump/strength/structural properties of sand-substituted CPBs was explored experimentally. Samples were made with a fixed cement content (7 wt.%), diverse tailings/sand fractions (e.g., 100/0, 90/10, 80/20, 70/30, and 50/50), and diverse solid contents (e.g., 72 and 76 wt.%). After curing for 3–56 days, several experiments, such as slump, uniaxial compressive strength (UCS), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM), were undertaken for the filling samples. The results demonstrate that adding sand to the backfill greatly increases CPB's strength (up to 99%), but the replacement rate of sand was limited to 30% due to its segregation effect. Microstructural tests reveal that CPB's void volume decreases as the added amount of sand increases. To sum up, it was concluded that calcareous sand made a major contribution to the filling strength, incorporating the effects of enhancing the fill gradation's readjustment and reducing the sum of cement being used in the unit volume for CPB manufacturing. [ABSTRACT FROM AUTHOR]

The performance of five composite Portland cements (CPCs) with limestone filler (LF = 10%–25% by mass) and calcined illite clay (CIC = 10%–25% by mass) elaborated by intergrinding was analyzed in paste, mortar, and concrete. Hydration was studied by isothermal calorimetry, bound water, and XRD. Flow and compressive strength (2 to 90 days) were determined in standard mortar. Concretes (w/b = 0.45; binder content = 350 kg/m3; slump = 15 ± 3 cm) were elaborated to determine compressive and flexural strength, water penetration, and chloride migration. Intergrinding CPCs have a large specific surface area when LF + CIC increases, with a similar size range of clinker particles. Supplementary cementing material replacements decreased the heat rate, prolonged the dormant period, and decreased the acceleration rate at early ages. According to the Fratini test, all CPCs had positive pozzolanicity after 28 days, but XRD analysis showed Ca(OH)2 associated with monocarboaluminate phases. Mortar flow was slightly reduced when the proportion of CIC was increased. Mortar strength decreased when the sum of LF + CIC increased. CPC strength class was limited by compressive strength after 28 days. Concretes were workable, and the compressive strength after 28 days depended on the LF + CIC, and CIC contributed after 90 days. After 28 days, the water penetration depended mainly on the LF + CIC content. The chloride migration coefficient was also reduced when CPC contained more CIC and less LF. [ABSTRACT FROM AUTHOR]

Based on the acoustic emission (AE) system and the digital scattered-spot deformation monitoring system, uniaxial compression tests were conducted on composite samples consisting of sandstone and cemented coal gangue–fly ash backfill (CGFB) to investigate their deformation and failure characteristics. The results showed that the average uniaxial compressive strength of the composite samples was 83.09% higher than that of the pure CGFB samples and 92.28% lower than that of the pure sandstone samples. In the composite samples, damage occurred in the CGFB part, and they showed obvious plastic damage characteristics. On both sides of the intersection, the sandstone and the CGFB deformed synergistically in the absence of a macroscopic failure. After a macroscopic failure, the interface effect promoted sandstone deformation and restrained CGFB deformation, transforming the sandstone and the CGFB on both sides of the intersection into a nonsynergistically deformed state. The interface effect had the most obvious influence on the horizontal deformation of the sandstone and CGFB monitoring points near the intersection. The failure of the CGFB samples induced sandstone springback deformation with a springback capacity of 0.0089 mm in the vertical direction and 0.0055 mm in the horizontal direction, which led to the further rupture and failure of the CGFB. [ABSTRACT FROM AUTHOR]

Ca/Mg carbonate minerals, such as calcite and dolomite, play an increasingly important role in the development of alkali-activated binders or geopolymers, which are regarded as promising sustainable cement materials. In contrast to studies on calcite and dolomite, the effect on geopolymer properties of the addition of natural magnesite (magnesium carbonate) to aluminosilicate raw materials has not been investigated. The aim of this study is to investigate the influence of mechanical activation (MA) and natural magnesite addition to fly ash (FA) on the compressive strength of geopolymers based on the natural magnesite–FA blend. Magnesite substitutes FA in amounts of up to 20 wt.%. Geopolymers were prepared using NaOH solution as an alkaline agent. XRD, FT-IR spectroscopy, thermogravimetry, SEM, and a dissolution test are used to investigate the geopolymerization process. The major reaction product was sodium-containing aluminosilicate hydrogel. Magnesite is found to transform, to a minor degree, to hydrotalcite. MA of the blend significantly improves geopolymer strength. For geopolymers based on (FA + magnesite) blends mechanically activated for 180 s, the strength is on average 8.0 ± 1.5, 3.0 ± 0.9, 1.5 ± 0.2, and 1.7 ± 0.5 times higher than that for the geopolymers based on the blends mechanically activated for 30 s at the age of 7, 28, 180, and 360 d, respectively. Although blending FA with magnesite does not increase geopolymer strength, for the mixtures containing 1%–10% magnesite, in general, the strength is either not reduced or it is reduced to a small degree compared to the geopolymers based on 100% FA. Using previously obtained data, for the first time, the effect of the addition of three Ca/Mg carbonate minerals to FA and MA on geopolymer performance is compared. Under similar conditions, geopolymer strength decreases in the order calcite > dolomite > magnesite. The main factors affecting the strength of geopolymers based on the mechanically activated blends of FA with magnesite including filler, dilution, and chemical effects are discussed. [ABSTRACT FROM AUTHOR]

This paper explores the alkali activation potential of brick wastes and metallurgical slags. Inorganic polymers (IPs) were produced using an alkaline medium consisting of sodium hydroxide and sodium silicate solutions and the optimum synthesis conditions were determined. In this context, the variable parameters, such as solid to liquid (S/L) ratio, curing temperature (60, 80 and 90 °C) and ageing time (7 and 28 days) on the compressive strength and the morphology of the produced IPs were investigated. Specimens produced under the optimum synthesis conditions were subjected to high temperature firing and immersed in distilled water and acidic solutions for various periods of time, in order to assess their durability and structural integrity. The results showed that the IPs produced using a mix ratio of 50 wt % metallurgical slag and 50 wt % brick wastes, cured at 90 °C and aged for 7 days obtained the highest compressive strength (48.9 MPa). X-ray fluorescence analysis (XRF), particle size analysis, Fourier transform infrared spectroscopy (FTIR), mineralogical analysis (XRD), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and thermogravimetric (TG) analysis also confirmed the optimum microstructural characteristics and the chemical reactions that took place during synthesis. The overall results of this study indicate that the co-valorization of different waste streams, which are produced in large quantities and cause environmental problems if not properly managed, is a viable alternative for the production of binders or secondary construction materials with higher added value. [ABSTRACT FROM AUTHOR]

Recycling phosphogypsum (PG) for cemented paste backfill (CPB) has been widely used at phosphate mines in China. However, the impurities in PG prolong the setting time and reduce the uniaxial compressive strength (UCS), limiting the engineering application of PG. This paper aims to investigate the feasibility of treated PG (TPG) washed repeatedly using deionised water (DW) for CPB. A water-washing pre-experiment was first conducted to find the proportion with the least DW demand and the effects of water-washing on ordinary PG (OPG). Then, based on the PG:DW ratio obtained from the pre-experiment, the properties of the OPG-based CPB (OCPB) and TPG-based CPB (TCPB) were tested using slump tests, UCS tests, and microstructural analysis. The results show that (1) after 11 water-washings at the PG:DW ratio of 1:1.75, the pH of the supernatant (pH = 6.328) meets the requirements of Chinese standard GB 8978-1996. (2) Water-washing improves the particle gradation quality of PG and removes the soluble impurities adsorbed at the surface of PG crystals. (3) The initial slump values of TCPB are 0.19–1.15 cm higher than that of OCPB, furthermore, the diffusivity values of TCPB are better than the performance of OCPB, with 0.61–1.68 cm of superiority. (4) The UCS values of TCPB are up to 0.838 MPa, 1.953 MPa, and 2.531 MPa, after curing for 7, 14, and 28 days. These are 0.283 MPa, 0.823 MPa, and 0.881 MPa higher than that of OCPB, respectively. It can be concluded that water-washing pre-treatment greatly improves the workability and mechanical property of PG-based CPB. These results are of great value for creating a reliable and environmentally superior alternative for the recycling of PG and for safer mining production. [ABSTRACT FROM AUTHOR]

In this study, the dynamic compressive strength and dissipated energy of rocks were investigated under hydrostatic pressure after different freeze–thaw cycles (FTCs). A split Hopkinson pressure bar (SHPB) with a lateral confining pressure chamber was used for the dynamic testing of sandstone specimens. The results indicated that under a certain loading rate and hydrostatic pressure, both the dissipative energy and dynamic compression strength of rocks decreased with the increase in the number of FTCs. The hydrostatic pressure significantly increased the dissipated energy and dynamic compression strength of rocks, and the enhancement became more pronounced as the number of FTCs increased. By analyzing the energy and damage characteristics of the specimens, after 40 FTCs, the internal damage of the specimens became critical and desensitized the dissipated energy to the loading rate. Based on the testing results, an empirical function was proposed to describe how the dissipated energy was related to the number of FTCs, hydrostatic pressure, and loading rate. It was demonstrated that the dissipated energy had a negative linear relation with FTC, which was enhanced by confinement. The dissipated energy of the specimen corresponded to the dynamic compression strength according to a quadratic function. [ABSTRACT FROM AUTHOR]

Microwave radiation is a relatively new energy source that is being considered for several applications in mineral processing and extractive metallurgy. In the present research, various gibbsite-type bauxite ores were subjected to microwave radiation. The main objective was to assess the effect of microwave dewatering on the compressive strength indices of the ores and to compare the results obtained to those for conventional heating. Firstly, the fundamental interactions of the microwaves with the ores were evaluated by determining both the real and the imaginary permittivities as a function of temperature, and these were related to the water content. Secondly, the microwave heating behavior was modeled using a 24 factorial statistical analysis. Thirdly, the effect of dewatering by microwave heating on the compressive strength indices of roughly spherical bauxite ore pisoids was studied, and these results were compared to those obtained using conventional heating. Fourthly, the effect of particle size on the compressive strengths of irregular-shaped single particles of bauxite ore was investigated using both heating techniques. Finally, the energy requirements for dewatering of the ores, and hence reducing their compressive strengths, were compared for both processes. On the laboratory scale, the results showed that in comparison to conventional dewatering, microwave dewatering resulted in lower strength indices at both lower moisture removals and energy inputs. [ABSTRACT FROM AUTHOR]

A large amount of waste coming out from industries has posed a great challenge in its disposal and effect on the environment. Particularly fly ash, coming out from thermal power plants, which contains aluminosilicate minerals and creates a lot of environmental problems. In recent years, it has been found that geopolymer may give solutions to waste problems and environmental issues. Geopolymer is an inorganic polymer first introduced by Davidovits. Geopolymer concrete can be considered as an innovative and alternative material to traditional Portland cement concrete. Use of fly ash as a raw material minimizes the waste production of thermal power plants and protects the environment. Geopolymer concretes have high early strength and resistant to an aggressive atmosphere. Methods of preparation and characterization of fly ash-based geopolymers have been presented in this paper. The properties of geopolymer cement/mortar/concrete under different conditions have been highlighted. Fire resistance properties and 3D printing technology have also been discussed. [ABSTRACT FROM AUTHOR]

The stability of composite material that is composed of roof rock, cemented paste backfill (CPB), and floor rock has an important impact on safe mining within metal mines. In order to explore the mechanical properties, acoustic emission (AE), energy dissipation, and damage evolution of roof–CPB–floor (RCF) layered composite materials, uniaxial compression (loading rate 0.02 mm/min) AE tests on RCF materials with different CPB height ratios were performed. The test results show that: (1) the uniaxial compressive strength (UCS) and elastic modulus (ER) of the RCF material were lower than those of the roof or floor rock and higher than that of the CPB. With the increase in the CPB's height ratio from 0.2 to 0.7, the UCS and the ER decreased from 18.42 MPa to 10.08 MPa and 3.15 GPa to 1.79 GPa, respectively, and the peak strain first decreased from 0.695 to 0.510 and then increased from 0.510 to 0.595. The UCS increased as a polynomial function with the increase in the ER. (2) The AE ring count first increased slowly, then increased rapidly, and finally maintained a high-speed increase. The AE cumulative ring count at the peak point decreased with the increase in the CPB height ratio. The energy dissipation showed that the elastic energy UE accumulated slowly at first, then the dissipated energy UD increased, and finally the UE decreased and the UD increased almost linearly. The UT, UE, UD, UE–UT ratio and UD–UT ratio showed a decreasing trend, and the UE–UD ratio showed an increasing trend at the peak point with the increase in the CPB height ratio. (3) Two damage constitutive models were established based on the AE ring count and energy principle. The damage evolution process of RCF materials can be divided into three stages: the slow damage accumulation stage, stable damage growth stage, and rapid damage accumulation stage. [ABSTRACT FROM AUTHOR]

Cemented paste backfill (CPB) technology is widely used for environmental protection and underground goaf treatment. The influences of solid concentration, coarse aggregates dosage, and cement dosage on the rheological properties and compressive strength of CPB blended with coarse aggregates (CA-CPB) are investigated through three-factor and four-level orthogonal experiments. The dynamic shear stress and plastic viscosity are selected to characterize the rheological properties of CA-CPB. The uniaxial compressive strength (UCS) is used to describe the compressive strength. The effect of each factor on rheological properties is different from that on UCS. The most significant influences on rheological properties and UCS are solid concentration and cement dosage, respectively. The optimal levels of each factor for rheological properties and UCS are different, resulting in different optimal combinations obtained through range analysis. Therefore, the overall desirability function approach is employed to perform multiple response optimization. The optimal parameters for high fluidity and strength obtained provide valuable information for the CA-CPB process in the Chifeng Baiyinnuoer Lead and Zinc Mine. [ABSTRACT FROM AUTHOR]

The main aim of this investigation is to develop backfill concrete including coal gangue and metakaolin to reduce solid waste. For this purpose, a total of 30 concrete mixtures were designed by the inclusion of 0%, 25%, 50%, 75% and 100% coal gangue as coarse aggregates and 0%, 10% and 20% metakaolin as binder at 0.55 and 0.45 water to cement ratios. The compressive strength was tested after 3, 7 and 28 days for a total of 90 samples. Meanwhile, the influences of coal gangue and metakaolin on the elastic modulus, ultrasonic pulse velocity, rebound number and open porosity were explored. Then, the relationship between physical and mechanical properties was revealed by design code expressions and empirical models. Furthermore, an extreme learning machine was developed to predict compressive strength by concrete mixtures. The results show that the inclusion of coal gangue results in a poor performance in physical and mechanical properties of concrete. However, the drawbacks of concrete containing coal gangue can be compensated by metakaolin. The predicted results of design code expressions and empirical models are closed to the experiment results, with a 10% error. In addition, the findings reveal that the extreme learning machine offers significant potential to predict the compressive strength of concrete with high precision. [ABSTRACT FROM AUTHOR]

The properties of a binder developed by the alkali-activation of a single waste material can improve when it is blended with different industrial by-products. This research aimed to investigate the influence of blast furnace slag (BFS) and fly ash (FA) (0-50 wt %) on the microstructure and compressive strength of alkali-activated ceramic sanitaryware (CSW). 4 wt % Ca(OH)2 was added to the CSW/FA blended samples and, given the high calcium content of BFS, the influence of BFS was analyzed with and without adding Ca(OH)2. Mortars were used to assess the compressive strength of the blended cements, and their microstructure was investigated in pastes by X-ray diffraction, thermogravimetry, and field emission scanning electron microscopy. All the samples were cured at 20 °C for 28 and 90 days and at 65 °C for 7 days. The results show that the partial replacement of CSW with BFS or FA allowed CSW to be activated at 20 °C. The CSW/BFS systems exhibited better mechanical properties than the CSW/FA blended mortars, so that maximum strength values of 54.3 MPa and 29.4 MPa were obtained in the samples prepared with 50 wt % BFS and FA, respectively, cured at 20 °C for 90 days. [ABSTRACT FROM AUTHOR]

In this study, the effects of the initial sulfate content on the properties of cemented paste backfill (CPB) made from coarse tailings has been investigated via mercury intrusion porosimetry. The combined effects of the sulfate content and curing time on the total porosity, pore size distribution, and unconfined compressive strength of the produced material were discussed. It was found that the specimens with an initial sulfate content of 5000 and 35,000 ppm exhibited higher unconfined compressive strength, while the resulting fine porous structures characterized by pore radii of 10-400 and 1-10 μm significantly improved the mechanical properties of the CPB. In addition, an increase in the curing time decreased the overall pore volume in the radius range of 1-400 μm but increased the pore volume at pore radii less than 1 μm. [ABSTRACT FROM AUTHOR]

As lots of underground mines have been exploited in the past decades, many stope instability and surface subsidence problems are appeared in the underground mines, while the cemented paste backfill (CPB) technology has been applied for more than 40 years, and it can solve these problems. As it is shown that the effect of backfilling is mainly affected by the mechanical properties of the CPB, and there are lots of factors which can influence the strength of the CPB, but the coupled effects of curing conditions has not been reported. In this research, the coupled effects of curing conditions are importantly considered, and the uniaxial compressive strength (UCS) is adopted as the important evaluation index of CPB, then the evolution law of the UCS for CPB are analyzed, also the mathematical strength model of CPB is established. The findings suggest that the relationship between the UCS of CPB and curing stress develops the function of quadratic polynomial with one variable, while the UCS of the CPB shows the power function as the curing temperature increases. Moreover, the established mathematical strength model is verified on the basis of laboratory experiments, the error between the measured UCS and the prediction UCS is less than 15%. It shows that the established strength model of the CPB by considering the curing conditions can predict the UCS very well, it has great significance for the safety design of CPB. [ABSTRACT FROM AUTHOR]

Cement companies are significant contributors of the planet's anthropogenic CO2 emissions. With increased awareness of the substantial volume of CO2 emissions from cement production, a variety of mitigation strategies are being considered and pursued globally. Hybrid cements are deemed to be technologically viable materials for contemporary construction. They require less clinker than that for ordinary Portland cement, leading to a decrease in CO2 emissions per tonne of hybrid cement manufactured. The hybrids produced in this study consist of 70% siliceous coal fly ash and 30% Portland cement, and combines chemical (sodium sulphate) and mechanical (milling) activation. The aim of this work was to develop a better understanding of the hydration products formed and the resulting effect of activation on these hydration products, of hybrid coal fly ash cement pastes over an extended curing period of up to one year. The results indicated that chemical activation increases the formation of stable, well crystallised ettringite. Chemical activation as well as mechanical activation increased the rate of the pozzolanic reaction between portlandite contained in cement and coal fly ash. The application of combined chemical and mechanical activation definitely resulted in the fastest rate of portlandite consumption, hence an increased rate of the pozzolanic reaction. [ABSTRACT FROM AUTHOR]

The number of parallel joints has an impact on the size effect of the uniaxial compressive strength and characteristic strength of a rock; however, the relationships between them are yet to be derived. We studied the influence of the number of joints and rock size on the uniaxial compressive strength of the rock. This study established ten numerical simulation programs using numerical simulations and the RFPA software. Stress–strain curves of different numbers of parallel joints and sizes of rocks were analyzed. Relationships between the uniaxial compressive strength and number of parallel joints and rock size were proposed, and their special functions were obtained. Mathematical models between rock characteristic size, rock characteristic strength and the number of parallel joints were established. Simulations of the verification program confirmed that these relationships are still applicable after the angle of parallel joints changes. [ABSTRACT FROM AUTHOR]

Geopolymers are created by mixing a source of aluminosilicates, which can be natural or by-products from other industries, with an alkaline solution. These materials based on by-products from other industries have proven to be a less polluting alternative for concrete production than ordinary Portland cement (OPC). Geopolymers offer many advantages over OPC, such as excellent mechanical strength, increased durability, thermal resistance, and excellent stability in acidic and alkaline environments. Within these properties, mechanical strength, more specifically compressive strength, is the most important property for analyzing geopolymers as a construction material. For this reason, this study compiled information on the different variables that affect the compressive strength of geopolymers, such as Si/Al ratio, curing temperature and time, type and concentration of alkaline activator, water content, and the effect of impurities. From the information collected, it can be mentioned that geopolymers with Si/Al ratios between 1.5 and 2.0 obtained the highest compressive strengths for the different cases. On the other hand, high moderate temperatures (between 80 and 90 °C) induced higher compressive strengths in geopolymers, because the temperature favors the geopolymerization process. Moreover, longer curing times helped to obtain higher compressive strengths for all the cases analyzed. Furthermore, it was found that the most common practice is the use of sodium hydroxide combined with sodium silicate to obtain geopolymers with good mechanical strength, where the optimum SS/NaOH ratio depends on the source of aluminosilicates to be used. Generally speaking, it was observed that higher water contents lead to a decrease in compressive strength. The presence of calcium was found to be favorable in controlled proportions as it increases the compressive strength of geopolymers, on the other hand, impurities such as heavy metals have a negative effect on the compressive strength of geopolymers. [ABSTRACT FROM AUTHOR]

The pozzolans of the Moroccan Middle Atlas are derived from a low explosive volcanism, mostly strombolian. They are mainly composed of olivine and pyroxene, presenting a less homogeneous structure (irregular vesicles). The main target of this project is to study the use of natural pozzolans (NP) and metakaolin as precursors for the production of geopolymeric binders. The characterization of raw materials and elaborated geopolymers was carried out to study their mineralogical, chemical, microstructural, and mechanical properties. The studied pozzolans and kaolin were crushed, grinded, and sifted to get a fine grain size diameter of less than 100 µm. Then, they were calcined at 750 °C for 2 h to achieve an amorphous structure, increasing of their reactivity. Geopolymer production consists of mixing pozzolans and metakaolin with different amounts with an alkaline solution of sodium hydroxide and sodium silicates. The mass proportion of metakaolin (MK) used in this study was 10%, 20%, and 30%. In the present work, the amount of metakaolin was added as a source of alumina. The elaborated geopolymers were characterized using XRD, FTIR, TGA, and SEM analyses. The compressive strength was measured at 7, 14, and 28 days. The results showed interesting mechanical proprieties at about 18 MPa at 28 days with the mixture containing 20% MK. The addition of MK showed a significant increase in mechanical properties of the elaborated geopolymer. Meanwhile, the other results confirmed the training of new phases in addition to N-A-S-H gel. All these results indicate that the use of pozzolans in the production of geopolymers could be a great solution for the sustainable management of this mineral resource. [ABSTRACT FROM AUTHOR]