Solidification Mechanism Research Articles

Mechanisms of enhancing MgO for stabilization/solidification of Pb-contaminated red clay through CO2 sequestration

Pb-contaminated soil poses significant environmental and health risks as well as soil stability issues. Research on sandy soils highlights CO2-enhanced reactive MgO as a promising solution for improving the solidification of Pb-contaminated soils. However, carbonation effects can differ markedly between soil types owing to varying soil properties. In this study, we evaluated the effects of CO2-enhanced reactive MgO on the engineering and environmental characteristics of Pb-contaminated red clay and explored its mechanism of carbonation solidification. The results showed that CO2-enhanced reactive MgO increased the strength of Pb-contaminated red clay to over 3 MPa within 1 h, which was approximately 25 times the strength of untreated soil (0.2 MPa) and significantly higher than that of reactive MgO-treated, uncarbonated soil (0.8 MPa). The pH of the carbonated soil (9–10) facilitated Pb2+ immobilization, and the increase over the initial parameter elevated the electrical conductivity value. Moreover, CO2-enhanced reactive MgO reduced the Pb2+ leaching concentration to below 0.1 mg/L, even at high Pb concentrations (10,000 mg/kg). Pb2+ transformed into lead carbonates during the carbonation process, with the hydrated magnesium carbonates forming a dense internal structure. This solidification mechanism included chemical precipitation, physical adsorption, and encapsulation. Notably, the carbonation time should be controlled within 1 h to prevent soil expansion. Together, these findings support the potential of CO2-enhanced reactive MgO for efficient and low-carbon application in the solidification of Pb-contaminated red clay.

Recycling of arsenic residue to basalt fiber via vitrification

The management of arsenic residue (AR) poses a significant environmental challenge globally. Conventional methods such as landfill, adsorption, and purification often fail to prevent arsenic migration, leading to uncertainties about the final disposition of AR and risks of secondary pollution. Vitrification offers a more stable approach for solidifying AR. This study explores the vitrification mechanism of AR using basalt and evaluates the performance of the resulting fibers. Our findings indicate that increasing the arsenic concentration decreases the high-temperature viscosity of basalt, making fiber drawing more energy-efficient. Incorporating a small amount of arsenic (approximately 0.4 mol%) significantly enhances the tensile strength of basalt fibers (BFs) by 10 %, attributed to reduced viscosity and improved fiber formation with fewer internal defects. Additionally, the flexural strength and stiffness modulus of asphalt mixtures are notably improved with the addition of AR-BF composites. We also examined the solidification and leaching mechanisms of arsenic in the basalt glass system. BFs containing up to 0.4 mol% As2O3 exhibited arsenic leaching levels below the standard threshold (<67.7 μg/dm2) under strong acid/alkali immersion conditions. These results highlight the potential for in-situ curing and recycling of hazardous AR, particularly in the development of mechanically reinforced materials.

Effect of Niobium Modification on Solidification and Crystallization Mechanism of Medium-Carbon Cast Steel

Effect of Niobium Modification on Solidification and Crystallization Mechanism of Medium-Carbon Cast Steel

The cementitious properties of alkali-activated municipal solid waste incineration fly ash-phosphorus slag-secondary aluminum dross matrix composites and the mechanism of solidification of heavy metals

Based on the purpose of safe and resourceful disposal of hazardous waste, this paper proposes a new method for the combined treatment of municipal solid waste incineration fly ash (MSWIFA), phosphorus slag (PS), and secondary aluminum dross (SAD). A novel alkali-activated binder was synthesized by orthogonal experimental design, and the strength formation mechanism of the binder and the solidification and stabilization (S/S) mechanism of heavy metals were revealed. The results show that the maximum strength of the sample at 60 d is 22.4 MPa. The addition of SAD will affect the development of the strength of the solidified body and the types of hydration products of the gel. The hydration products of the solidified body are mainly calcium silicate hydrate (C-S-H), calcium aluminosilicate hydrate (C-A-S-H), and calcium alumina (AFt). The solidified body of alkali-activated cementitious material has a good S/S effect on heavy metals, and its solidification efficiency is Zn > Cd > Pb > Cu > Ni > As > Cr. The maximum solidification rates of Zn, Pb, and Cd were 99.97 %, 97.90 %, and 99.68 %, respectively. At the same time, the leaching concentration of various heavy metals meets the threshold requirements stipulated by relevant international norms and has good environmental safety.

Optimization of silty soil solidification agent ratio and mechanism of strength change based on design-expert

ABSTRACT In order to achieve the resource utilization of silty soil, based on the solidification mechanism of existing inorganic solidification agents, coal gangue and fly ash were selected as substrates, and quicklime and sodium silicate were used as activators to study a new type of muddy soil solidification agent. This project adopts mixture experimental design and unconfined compressive strength testing methods, and uses design expert software to analyze each component to obtain independent variable relationship strength prediction formulas. By using scanning electron microscope, Particles (Pores) and Cracks Analysis System, the changes in micro pore structure characteristics of Reinforced soil were obtained. The research results indicate that the relationship between the influence of various variable factors on strength gain is as follows: sodium silicate>calcium oxide>fly ash>coal gangue. As the age increases, the porosity shows a trend of first increasing and then decreasing. It has been revealed that the strength growth of Reinforced soil with curing agents is mainly due to the compaction and filling effects of hydrated calcium silicate, hydrated calcium aluminate, and a small amount of ettringite. This study is expected to provide reference for the solidification of high moisture content muddy soil.

Permeability and Disintegration Characteristics of Loess Solidified by Guar Gum and Basalt Fiber.

Loess has the characteristics of loose, large pore ratio, and strong water sensitivity. Once it encounters water, its structure is damaged easily and its strength is degraded, causing a degree of subgrade settlement. The water sensitivity of loess can be evaluated by permeability and disintegration tests. This study analyzes the effects of guar gum content, basalt fiber content, and basalt fiber length on the permeability and disintegration characteristics of solidified loess. The microstructure of loess was studied through scanning electron microscopy (SEM) testing, revealing the synergistic solidification mechanism of guar gum and basalt fibers. A permeability model was established through regression analysis with guar gum content, confining pressure, basalt fiber content, and length. The research results indicate that the addition of guar gum reduces the permeability of solidified loess, the addition of fiber improves the overall strength, and the addition of guar gum and basalt fiber improves the disintegration resistance. When the guar gum content is 1.00%, the permeability coefficient and disintegration rate of solidified soil are reduced by 50.50% and 94.10%, respectively. When the guar gum content is 1.00%, the basalt fiber length is 12 mm, and the fiber content is 1.00%, the permeability of the solidified soil decreases by 31.9%, and the disintegration rate is 4.80%. The permeability model has a good fitting effect and is suitable for predicting the permeability of loess reinforced with guar gum and basalt fiber composite. This research is of vital theoretical worth and great scientific significance for guidelines on practicing loess solidification engineering.

Study on the Shear Strength of Loess Solidified by Guar Gum and Basalt Fiber.

Loess is widely distributed in the northwest and other regions, and its unique structural forms such as large pores and strong water sensitivity lead to its collapsibility and collapse, which can easily induce slope instability. Guar gum and basalt fiber are natural green materials. For these reasons, this study investigated the solidification of loess by combining guar gum and basalt fiber and analyzed the impact of the guar gum content, fiber length, and fiber content on the soil shearing strength. Using scanning electron microscopy (SEM), the microstructure of loess was examined, revealing the synergistic solidification mechanism of guar gum and basalt fibers. On this basis, a shear strength model was established through regression analysis with fiber length, guar gum content, and fiber content. The results indicate that adding guar gum and basalt fiber increases soil cohesion, as do fiber length, guar gum content, and fiber content. When the fiber length was 12 mm, the fiber content was 1.00%, and the guar gum content was equal to 0.50%, 0.75%, or 1.00%, the peak strength of the solidified loess increased by 82.80%, 85.90%, and 90.40%, respectively. According to the shear strength model, the predicted and test data of the shear strength of solidified loess are evenly distributed on both sides of parallel lines, indicating a good fit. These findings are theoretically significant and provide practical guidance for loess solidification engineering.

Performance evaluation of combined passive/active thermal charge/discharge latent heat tank with stabilized-solid-liquid storage material for intermediate thermal storage system

This study proposes a new model operation for an intermediate thermal storage (ITS) tank for solar water heater system. It combines passive-charge active-discharge (PCAD) in a single storage tank, potentially reducing the number of components of the system and improving the net energy balance. The charge process is done by heating the storage material using an electric heater, which is also suitable for direct photovoltaic heating. The temperature profile of stabilized storage material in this stage shows a close plateau line in the solid-liquid transition, indicating a steady phase transition takes place. It minimizes the temperature deviation between the solid/liquid region, which is advantageous for determining the operation protocol. For the discharge process, the working fluid circulates from the mantle side and continues to the inner side of the tank, resulting in two-consecutive heating. It allows the fluid to harness the stored heat up to 72.8%. It is also achieved by the stabilized storage material, which has a better solidification mechanism. Thus, the proposed PCAD tank is able to meet the criteria of an ITS tank, which requires energy exchange during the charge/discharge stage. The finding demonstrates that a simplification of the ITS tank is achievable. The implication of the finding can be used as a fundamental basis for developing new solar water heating systems, including for direct charge operation using photovoltaic-heating systems where the heat is taken as alternative energy storage instead of electricity.

Study on performance and application of O-QGS soil curing agent for waste clay with low liquid limit

To reuse industrial solid wastes and waste clay with low liquid limit, a kind of soil solidification material by using cement, quicklime and industrial solid wastes such as ground granulated blast-furnace slag (GGBS), silica fume (SF) was developed in this study. Response surface methodology (RSM) based on central composite design (CCD) was used to design the experiment and optimize the mix ratio of GGBS, quicklime and SF under certain cement content conditions (i.e., the content ratio of cement, GGBS, quicklime, and SF was 5: 9.14: 1.7: 2.13). A soil solidification agent named O-QGS was developed to solidify waste clay with low liquid limit. To clarify the solidification mechanism of solidified soil, a series of laboratory experiments such as UCS test, water stability test, and scanning electron microscopy (SEM) test were carried out to capture the mechanical properties, water stability, and microstructure of O-QGS solidified soil and cement solidified soil. For practical purpose of O-QGS, a method for forming prefabricated pile by using O-QGS solidified soil was developed, and a method for strengthening soft foundations with prefabricated O-QGS solidified soil pile was proposed. Based on the results of load tests, the bearing capacity of prefabricated O-QGS solidified soil pile and cement high-pressure rotary jet grouting pile, as well as the composite foundations bearing capacity of prefabricated O-QGS solidified soil pile and cement high-pressure rotary jet grouting pile used for strengthening soft foundations, were analyzed. The feasibility of prefabricated O-QGS solidified soil pile used for strengthening soft foundations was verified in practice. The present study shows that the UCS of O-QGS solidified soil is 7.25 MPa at 28 days, and the water stability coefficient of O-QGS solidified soil is larger than 0.8. Compared with the method of cement high-pressure rotary jet grouting pile to reinforce soft foundation, the bearing capacity of prefabricated O-QGS solidified soil pile to reinforce soft foundation is higher, and the cost can be saved by 22.4 %.

Solidification Mechanism of Bayer Red Mud under the Action of Calcium Hydroxide

Because of the strong alkalinity of red mud, it is difficult to recycle, and the long-term accumulation of red mud causes environmental pollution. The study shows that the solidification characteristics of bayer red mud (RM) under the action of Ca(OH)2 (CH) are obvious. The mechanical properties of Bayer RM paste with different amounts of CH at different ages were tested. The strength of RMCH gradually increases with the increase in CH content and age, reaching a turning point in strength at 26.4% content of CH, with the highest strength at 28 days, reaching 2.73 MPa. The solidification products were characterized by XRD, FTIR, TG-DTG, and SEM-EDS. The results show that under the action of CH, the main solidification products of RM are C-(A)-S-H, hemicarboaluminate, and monocarboaluminate. In the solidification process, hydroxysodalite and faujasite-Na react with CH to generate C-S-H, Al(OH)4−, and Na+, then react to generate hemicarboaluminate, monocarboaluminate and C-(A)-S-H, among which hemicarboaluminate is transformed into monocarboaluminate in the presence of calcite, and further monocarboaluminate decomposes to generate calcite. It provides a basis for the study of the interaction mechanism between a single substance and RM and provides a research basis for the sustainable utilization of red mud.

Adsorption behavior and solidification mechanism of Pb(II) on synthetic C-A-S-H gels with different Ca/Si and Al/Si ratios in high alkaline conditions

Calcium silicate aluminate hydrate (C-A-S-H) is the main hydration product and its porous adsorption property can effectively inhibit Pb(II) leaching from alkaline pore solution of alkali-activated municipal solid waste incineration (MSWI) fly ash compacts. The study examines how different ratios of Ca/Si and Al/Si in C-(A)-S-H affect Pb(II) adsorption behavior and mechanism in high alkaline conditions. The results showed that Al-doped C-A-S-H gels have increased specific surface area and cumulative pore volume, and slightly decreased average pore size. Pb(OH)3- adsorption by C-A-S-H was mainly chemisorption with spontaneity. Temperature, pH value, Al/Si and Ca/Si ratios of C-A-S-H affected Pb(II) adsorption. Equilibrium adsorption capacity (Qe) initially increased then decreased with higher Al/Si for C-A-S-H with Ca/Si 0.60 but decreased with higher Al/Si for Ca/Si 1.20. Active Si-OH (Q1 or Q2b) and Al-OH (Q2(1Al)) in aluminosilicate tetrahedra chains dehydrate and condense with Pb(OH)3- to form Si-O-Pb or Al-O-Pb bonds. Higher interlayer water content in low Ca/Si and Al/Si C-A-S-H facilitated ion exchange during Pb(II) adsorption. After adsorption, aluminosilicate tetrahedra chain length increased while interlayer water content decreased. Ca(II) and Pb(II) in C-A-S-H underwent partial ion exchange during the adsorption process. Simulation results indicate Pb(OH)3- was more likely to polycondensate with silica tetrahedral bridging oxygen in Si2AlO2(OH)8 to form Si-O-Pb than Al-O-Pb bonds. These findings provide theoretical support for designing dosages of alkali activators and doped aluminosilicate minerals to improve the solidification efficiency of Pb(II) in alkali-activated MSWI fly ash compacts.

Solidification Mechanism of Microstructure of Al-Si-Cu-Ni Alloy Manufactured by Laser Powder Bed Fusion and Mechanical Properties Effect

Based on previous work, where Al-Si-Cu-Ni alloy was successfully manufactured by laser powder bed fusion (PBF-LB/M) technology, in this study, we further observe the microstructure of the alloy, analyze the formation mechanism of the microstructure during solidification, and discuss their implications for the mechanical properties. The results indicate that the microstructure comprises multi-level cellular heterogeneous structures, with an α-Al matrix in the interior of the cellular structure and Cu- and Ni-rich phases clustered at the boundaries, intertwined with the silicon network. During solidification, α-Al solidifies first and occupies the core of the cells, while Si phases and Cu- and Ni-rich phases deposit along the cellular boundaries under the influence of surface tension. During the solidification process of cellular boundaries, influenced by spinodal decomposition and lattice spacing, Si phases and Cu- and Ni-rich phases interconnect and distribute crosswise, collectively forming multi-level cellular structures. The refined cellular microstructure of the PBF-LB/M Al-Si-Cu-Ni alloy enhances the mechanical properties of the alloy. The alloy exhibits a bending strength of 766 ± 30 MPa, a tensile strength and yield strength of 437 ± 6 MPa and 344 ± 4 MPa, respectively, with a relatively low fracture elongation of approximately 1.51 ± 0.07%. Subsequent improvement can be achieved through appropriate heat treatment processes.

Microscopic Mechanism and Reagent Activation of Waste Glass Powder for Solidifying Soil

Glass waste products represent a significant environmental concern, with an estimated 1.4 billion tons being landfilled globally and 200 million tons annually. This results in a significant use of land resources. Therefore, it would be highly advantageous to develop a new method for disposing of waste glass. Waste glass can be recycled and ground into waste glass powder (WGP) for use in solidified soil applications as a sustainable resource. This study is based on solidified soil research, wherein NaOH, Ca(OH)2, and Na2SO4 were incorporated as activators to enhance the reactivity of WGP. The optimal solidified soil group was determined based on unconfined compressive strength tests, which involved varying the activator concentrations and WGP content in combination with cement. X-ray diffraction (XRD) was used to study the composition of solidified soil samples. Microscopic pore characteristics were investigated using scanning electron microscopy (SEM), and the Image J software was employed to quantify the number and size of pores. Fourier-transform infrared spectroscopy (FTIR) was employed to examine the activation effect of waste glass powder. This study investigated the solidification mechanism and porosity changes. The results demonstrate that the addition of activated WGP to solidified soil enhances its strength, with a notable 12% increase in strength achieved using a 6% Ca(OH)2 solution. The use of 2% concentration of Na2SO4 and NaOH also shows an increase in strength of 7.6% and 8.6%, respectively, compared to the sample without WGP. The XRD and SEM analyses indicate that activated WGP enhances the content of hydrates, reduces porosity, and fosters the formation of a more densely packed solidified soil structure.

Investigation of Engineering Properties and Solidification Mechanism of Loess by Sodium Silicate Alkali-Activated Coal Gangue Powder

The aim of this study is to investigate the engineering properties and solidification mechanism of loess through the use of alkali-activated coal gangue powder with sodium silicate. Experimental methods and comprehensive analysis were employed to examine the effects of different proportions of alkali-activated coal gangue powder with sodium silicate on the engineering properties of loess, including mass shrinkage, compressibility, and shear strength. Additionally, scanning electron microscopy was utilized to gain in-depth insights into the interaction and solidification mechanism between loess and alkali-activated coal gangue powder. The results show that the sodium silicate alkali-activated gangue powder curing loess has significantly improved the compressive strength and shear strength of the loess. With a ratio of 7 : 2 : 1, the 28 days compressive strength of solidified loess is 1.7 MPa, and the shear strength is 67.92 kPa, which is 1.91 and 2.13 times the 28 days compressive strength and shear strength of unmixed gangue powder and sodium silicate specimens respectively. The hydration–hydrolysis reaction, ion-exchange reaction, and volcanic ash reaction of the gangue powder under an alkaline environment generated hydrides that filled the pores between soil particles, enhanced the interparticle cohesion, and made the internal structure of the specimens denser, improving the engineering performance of loess solidification. The proposed sodium silicate alkali-activated gangue powder curing loess mechanism can provide a theoretical reference for the engineering application of gangue powder and the curing modification of loess.

Mechanism of geopolymeric solidification in shield-tunnelling slurry from diverse sources: The role of bentonite adsorption in influencing the reaction process

Geopolymeric solidification is an effective process for treating engineering waste slurry, yet its efficacy varies with source of slurry. This study examined four shield tunneling spoil types for geopolymeric solidification. The unconfined compressive strength of the best sample was 17–126% higher than the least effective one. A significant negative correlation between strength and bentonite indices was found. Subsequently, the addition of bentonite to the shield tunneling spoil for experiments, followed by various analyses, comfirmed bentonite's negative impact on strength. Bentonite forms agglomerates during geopolymerization, adsorbing hydration gel, weakening the direct linkage of the hydration gel; they also adsorb elements such as Al, Ca, and Na, hindering the formation of geopolymer products. However, the final adsorption ratio was not within the optimal range for geopolymer reactions, impeding the improvement in curing strength. This study offers the theoretical guidance for improving engineering waste slurry treatment processes.

Experimental Study on Enhancing the Mechanical Properties of Sandy Soil by Combining Microbial Mineralization Technology with Silty Soil.

Currently, coastal sandy soils face issues such as insufficient foundation strength, which has become one of the crucial factors constraining urban development. Geotechnical engineering, as a traditional discipline, breaks down disciplinary barriers, promotes interdisciplinary integration, and realizes the green ecological and low-carbon development of geotechnical engineering, which is highly important. Based on the "dual carbon" concept advocating a green and environmentally friendly lifestyle, Bacillus spores were utilized to induce calcium carbonate precipitation technology (MICP) to solidify coastal sandy soils, leveraging the rough-surface and low-permeability characteristics of silty soil. The mechanical-strength variations in the samples were explored through experiments, such as calcium carbonate generation rate tests, non-consolidated undrained triaxial shear tests, and scanning electron microscopy (SEM) experiments, to investigate the MICP solidification mechanism. The results indicate that by incorporating silty soil into sandy soil for MICP solidification, the calcium carbonate generation rates of the samples were significantly increased. With the increase in the silty-soil content, the enhancement range was 0.58-3.62%, with the maximum calcium carbonate generation rate occurring at a 5% content level. As the silty-soil content gradually increased from 1% to 5%, the peak deviator stress increased by 4.2-43.2%, enhancing the sample shear strength. Furthermore, the relationship between the internal-friction angle, cohesion, and shear strength further validates the enhancement of the shear strength. Silty soil plays roles in adsorption and physical filling during the MICP solidification process, reducing the inter-particle pores in sandy soil, increasing the compactness, providing adsorption sites, and enhancing the calcium carbonate generation rate, thereby improving the shear strength. The research findings can provide guidance for reinforcing poor coastal sandy-soil foundations in various regions.

The Design of a Novel Alkali-Activated Binder for Solidifying Silty Soft Clay and the Study of Its Solidification Mechanism.

In order to overcome the problems of the high economic and environmental costs of a traditional ordinary portland cement-based binder, this study used self-combusted coal gangue (SCCG), granulated blast furnace slag (GBFS) and phosphorous slag (PS) to prepare a novel SCCG-GBFS-PS (SGP) ternary alkali-activated binder for solidifying silty soft clay (SC). Firstly, the parameters of the SGP ternary binder were optimized using orthogonal experiments. Then the effects of the SGP ternary binder content (mass ratio of the SGP ternary binder and the SGP-solidified soil), initial water content of SC (mass ratio of SC' water and SC) and types of additives on the unconfined compressive strength (UCS) of the SGP-solidified soil were analyzed. Finally, the hydration products and microstructure of the SGP-solidified soil were analyzed to investigate the solidification mechanism of the SGP ternary binder. The results showed that the optimal mass ratio of GBFS and PS is 2:1, and the optimal alkali activator content (mass ratio of Na2O and the SGP ternary binder) and modulus of alkali activator (molar ratio of SiO2 and Na2O of alkali activator) were 13% and 1.3, respectively. When the SGP ternary binder content was 16% and the initial water content of SC was 35%, the SGP-solidified soil met the requirement of UCS for tertiary cured soil. The incorporation of triethanolamine and polyvinyl alcohol improved the UCS, while the incorporation of Na2SO4 significantly deteriorated the UCS of the SGP-solidified soil. The C-S-H gels and C(N)-A-S-H gels generated by hydration of the SGP-solidified soil were interspersed, interwoven and adhered to each other to form a network-like space structure that played the roles of skeleton, bonding soil particles and filling pores, which improved the macroscopic properties of the SGP-solidified soil. The results of this study provide a reference for the design and development of a solid waste-based binder for solidifying SC.

Microstructure, solidification defects and mechanical properties of high-modulus and high-strength SiC/AlSi10Mg composites fabricated by selective laser melting

Since the aluminum matrix composites prepared by additive manufacturing are prone to defects such as pores and cracks, the properties of the formed composites cannot be further improved. Therefore, the preparation of SiC/AlSi10Mg composites with high modulus and high strength by selective laser melting technology is of great significance for the wide application of aluminum matrix composites. This work will further investigate the defect formation, microstructure evolution, and solidification mechanism of the composites. The matrix microstructure of the composites can be efficiently refined by adding SiC. In addition, the 5 wt.% SiC/AlSi10Mg composites showed improvements in the tensile yield strength, microhardness, bending strength, elastic modulus, and ultimate tensile strength as compared to the AlSi10Mg alloy. These increases were 21.1 %, 18 %, 8.7 %, 23.2 % and 9.2 % respectively. The strengthening mechanisms at room temperature include fine grain strengthening, thermal mismatch strengthening, solid solution strengthening and precipitation strengthening. It is also mentioned that the primary causes of the decrease in toughness are stress concentration and the existence of the brittle precipitation phase (Al4C3). The increase in high-temperature strength is attributed to the pinning role of SiC and precipitate phases relative to grain boundary and dislocation climbing.

Recycling multisource industrial waste residues as green binder for cemented ultrafine tailings backfill: Hydration kinetics, mechanical properties, and solidification mechanism

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

Chromium immobilization in basic magnesium sulfate cement: Experimental and density functional theory studies

This study investigated the effects of different concentrations of Cr(III) on the properties of basic magnesium sulfate cement (BMSC). In addition, the hydration process of BMSC with different concentrations of Cr(III) was studied using isothermal calorimetry and in-situ pH monitoring. A combined experimental approach and density functional theory simulation was further employed to investigate the solidification mechanism of Cr(III) in BMSC. The results indicated that Cr(III) affected the hydration heat, pH value, compressive strength and setting time of BMSC, and the influence became more obvious with the increase of Cr(III) concentration. In addition, the solidification efficiency of Cr(III) in BMSC was up to 99.9%; the efficient solidification of Cr(III) in BMSC primarily operated through ion exchange interactions, as revealed by density functional theory simulation and phase analysis results. Furthermore, the BMSC matrix after being immersed in simulated acid rain solutions for 28 days had a compressive strength of over 45 MPa, showing remarkable resistance to acids.