Solidified Specimens Research Articles

Formulation and properties of a new cleaner double liquid alkali-activated grouting material

Herein, a new cleaner double liquid alkali-activated grouting material (DLAG) was successfully developed, which could be applied similarly to cement by adding water. Desulphurisation gypsum (DG) was selected to activate ground granulated blast-furnace slag (GGBFS) and fly ash (FA) to completely replace cement. The optimum proportion of the basic materials and admixtures were determined by a series of orthogonal and single-factor controlled variable experiments. The mechanism for the strength enhancement of the materials was interpreted by microscopic experiments. The optimum proportion of the basic materials was determined that slurry A comprised of only GGBFS, whereas DG, FA, and metakaolin (MK), were present in a mass ratio of 6:3:1 in slurry B. The single slurry maintained good workability over a long period before mixing, and the fluidity of mixed slurry could be dynamically adjustable from 138 mm to 215mm. The strength of solidified specimens reached 45.7 MPa at 28 days curing time, which was 220% higher than that of Portland cement. Microscopic analyses showed that Al2O3 and SiO2 in the precursor materials were continuously dissolved and reorganised to produce hybridised phase gels (C, N)-A-S-H (calcium, sodium aluminium silicate hydrate), and also hydrated with Ca2+, SO42− provided by DG to produce ettringite and calcium silicate hydrate (C-S-H) in the alkaline environment. The size and content of hydration products gradually increased with increasing curing time and acted together to enhance the strength of solidified body. The proposed material can promote the development of alkali-activated technology in the field of special grouting materials.

Excellent thermal stability of nanostructured Al2O3–Y3Al5O12–ZrO2 eutectic ceramic composites by high-speed directional solidification

The contradiction between the nanostructure and thermal stability is the major obstacle on the way of the practical applications of nanomaterials. Here we demonstrated a simple solution for reconciling this contradiction, i.e., the high-speed directional solidification of a ternary eutectic ceramic composite. Its highly textured lamellar and rod-like nanostructures and low-energy interfaces contributed to an outstanding thermal stability that the nanostructure was maintained after being heated at 1573 K for 100 h. It was uncovered that the high solidification rate of 1080 mm/h had an evident restriction on the growth orientation of the faceted crystals during the solidification process, forming a highly textured nanostructure. Combining with the interface energy minimization mechanism, the low-energy interfaces were simultaneously obtained. The evolution of microstructure and interface characteristics during the coarsening process at the higher temperature was further studied. The lamellar structure presented a better thermal stability than that of the rod-like structure since the former had a constraint on the diffusion dimensions. Moreover, a novel structural evolution pattern in the coarsening process was revealed, which was completely different from that of traditional sintered ceramics. The solidified specimens could further keep stable in density, homogeneity of structure and size of internal pores during heat treatment, contributing to a better stability in hardness than that of traditional sintered specimens.

Hydrothermal solidification of underground construction wastes into building materials: Waste slurry recycling, industrial application and evaluation

A novel hydrothermal technology was developed to recycle underground construction wastes into tobermorite-based building materials and immobilize polyvalent Cr. The hardening mechanism involved in tobermorite synthesis and immobilization of Cr were studied by various techniques, including XRD, SEM, STA, XPS and ICP-OES. A simulated waste slurry was converted hydrothermally to well-crystalline tobermorite for both promoting strength and structurally accommodating Cr. A higher curing temperature and longer time facilitated the tobermorite formation, with a high proportion of tobermorite (∼93.67 wt%) being obtained at 200 °C for 24 h at a Ca/Si molar ratio of 0.6. Along with tobermorite formation, the in-situ immobilization of Cr in the tobermorite structure was also examined. Cr(Ⅲ) was found to replace the Ca2+ in the Ca–O octahedron to form Cr–O bonds or connect with H2O in the interlayer as Cr–OH bonds; whereas Cr(Ⅵ) groups linked with Ca2+ in the interlayer as stable CaCrO4, which adversely affected tobermorite formation. Both Cr(Ⅲ) and Cr(Ⅵ) had very low leaching rates from the solidified specimens, although the lower leaching rate of Cr(Ⅲ) (∼4.05% of 5000 ppm) compared to Cr(Ⅵ) (∼7.23% of 5000 ppm) confirmed that tobermorite had a stronger immobilization capacity for Cr (III). An actual waste slurry was used as the dominant raw material (∼70 wt%) for the hydrothermal synthesis of a tough tobermorite-based building material with a flexural strength (∼25.3 MPa), which was even higher than that of common concrete materials. The hydrothermal technology developed here has been successfully applied to the industrial production of standard bricks containing 50–70 wt% waste slurry that were fully compliant with Chinese standards. Life cycle assessments revealed that the hydrothermal slurry-recycled bricks effectively reduced environmental impacts by ∼13%–152% compared to similar building products, thereby demonstrating economic benefits and sustainability.

Evaluating the role of feldspar on the resistivity of alkali-activated slag towards seawater attack.

In this study, the effect of K-feldspar on the behavior of alkali-activated slag pastes (AAS) before and after exposure to seawater was evaluated by measuring the compressive strength and the microstructure. To achieve this objective, the mixtures were prepared upon partial replacement of ground granulated-blast furnace slag (GGBFS) with calcined and un-calcined feldspar fine particles (termed as FS and CFS) at levels ranging from 5 to 20%, by mass. Under processing conditions, the solidified specimens were immersed for 1, 7, 14, and 28 days in 100% RH at 40 ± 2 °C. After 28 days, a group of samples having the same composition were soaked in seawater solution for 1, 3, and 6 months. The new phases were evaluated using X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG), and scanning electron microscopy (SEM). The inclusion of FS and/or CFS in AAS enhances the performance of the pastes in normal and harsh conditions in relation to their reference counterparts. Among all the mixtures that were tested, alkali-activated composite comprising 15% CFS indicated more durable properties in terms of increased compressive strength and dense morphological structure after exposure to an aggressive environment.

Laser additive manufacturing of melt-grown Al2O3/GdAlO3 eutectic ceramic composite: Powder designs and crack analysis with thermo-mechanical simulation

Directed energy deposition method is an efficient one-step laser additive manufacturing technology to achieve eutectic ceramic composite with high property and ultra-fine microstructures. In this paper, melt grown dense Al2O3/GdAlO3(GAP) eutectic ceramic composites have been directly fabricated from the spherical powder reconstructed by an optimized spray granulation method. Effects of the powder size distribution, feeding rate, and heat treatment on the morphology and microstructure of the as-solidified eutectic ceramic composites have been investigated. Results show that the powder fluidity plays an important role in the heat conduction of the laser process. Finer powder (imperfect spherical powder with diameter less than 10 µm) gives rise to the disturbance of molten pool. Moreover, this powder greatly aggravates the phenomenon of powder’ sticking on surface of the specimen, which subsequently induces the leading growth of coarse dendrite-like GAP primary phases (higher interface temperature of dendrite tip for GAP phase) and sintered eutectic phases at the specimen edge. The finite element modeling (FEM) method is used to analysis the coupled thermal dynamic during the process after verification with infrared thermal image. It shows that longitudinal maximum principal stress exhibits a steep gradient at the edges of the as-solidified ceramic, making the specimen susceptible to cracking along the deposited direction at the first few layers. By optimizing the feedstock powder characteristics and the directed energy deposition process, completely solidified cylindrical and thin-walled Al2O3/GAP eutectic ceramic composites with the maximum dimensions of ϕ 4 × 95 mm3 and 10 × 4 × 44 mm3 have been successfully fabricated. The solidified specimens present smooth glossy surface and fine microstructures with the average eutectic spacing of 0.31 µm. The average micro-hardness and fracture toughness of 15.16 ± 0.29 GPa and 4.3 ± 0.09 MPa·m1/2 have been obtained, respectively.

Effect of Micro-Structural Dispersity of SiMo Ductile Iron on High Temperature Performance during Static Oxidation

High silicon and molybdenum (SiMo) ductile iron is commonly used for car exhaust systems, and its micro-structural dispersity depends on intrinsic parameters, which include alloy composition and inoculation efficiency, as well as extrinsic factors, such as casting wall thickness and molding material, which define cooling rate during solidification. Micro-structural dispersity is referred to as the degree of heterogeneity of sizes of structural constituencies within the microstructure. A variation in the micro-structural dispersity could impact the high temperature performance of SiMo ductile iron during static oxidation and transient thermo-mechanical loading conditions. In this study, static high temperature tests were performed on SiMo ductile iron solidified in a casting with varying wall thicknesses from 5 mm to 100 mm. The faster solidified specimens (taken from near chilled casting surfaces) had extremely high micro-structural dispersity as compared to the thicker section samples. After thermal exposure, each of the samples were characterized using 2D sections and 3D µCT images, and the results indicated an order of magnitude difference in graphite phase dispersity. The surface degradation was quantified after static oxidation experiments were implemented at temperature intervals between 650 °C and 800 °C. Non-destructive µCT 3D analysis and SEM/EDS were performed on cross sections and used to quantify the scale topology and structure. Carbon analysis was used to decouple the scale formation and decarburization phenomena that occurred within the samples. These methods enabled the quantification of the oxidation of the SiMo cast iron with different micro-structural dispersity levels after being exposed to high temperature static oxidation. Additionally, the complex material behavior during oxidation-assisted transient thermo-mechanical loading will be presented in a separate article.

Effect of Heat Treatment on the Corrosion Behaviour of Rapidly Solidified Al-8Ni-2Co Alloy in NaCl Solution.(Dept.M)

The corrosion behaviour of as-rapidly solidified and heat treated AI 8Ni-2Co alloy ribbons produced using melt spinning technique was investigated. The decomposition behaviour of the rapidly solidified ribbons was followed up using x-ray diffraction, differentialscanningcalometry. microhardness, and metallography. The stable phases which are generally formed in conventional solidified alloy in the as cast condition, were not detected in the as-rapidly solidified specimens. Instead, super-saturated solid solution α-AI was present in addition to fine dispersoids of A19(CoNi)2. The corrosion behaviour was investigated through potentiostatic technique in 3.5% NaCL aqueons solution. The corrosion rate was the lowest in the as-rapidly solidified condition and increased with increasing the annealing temperature as a result of the formation and coarsening of the intermetallic Al3Ni.

An environment-friendly pretreatment process of municipal solid waste incineration fly ash to enhance the immobilization efficiency by alkali-activated slag cement

Abstract Municipal solid waste incineration (MSWI) fly ash can cause serious early volume expansion when stabilized and solidified by cementitious materials. Many pretreatment processes have been reported, such as alkali solution, milling and chelating etc. However, these processes may lead to second pollution or higher costs. This study proposed an original pretreatment process of MSWI fly ash, and investigated its effect on the immobilization efficiency by alkali-activated slag (AAS) cement. The possible reasons for volume instability were assumed and analyzed by gas chromatography, scanning electron microscopy, X-ray diffraction, hydrostatic testing and leaching tests. It was found that the 0.377 wt% of metallic aluminum is the main expansive source of volume expansion in early stage (within 24 h). Due to the amphoteric characteristic of Al, it is possible to eliminate it using either acid or alkaline. Considering the fact that alkali-activated system and the fly ash were both alkaline, this study chose water immersion to pre-treat the fly ash, aiming to avoid the possible neutralization process of acid treatment and the entrained extra alkali component of alkali treatment, which may greatly affect many characteristics of alkali-activated slag. After pretreatment, the solidified specimens showed lower water demand (with a water to cement ratio of merely 0.27) and higher compressive strength due to the refined and smoothed particles. The volume expansive rate was greatly reduced, giving the better volumetric stability and encapsulation rate. Up to 90% MSWI fly ash-added solidified bodies feature a volume expansive rate of less than 0.09%. The alkali-activated slag pastes can effectively immobilize up to 70% treated fly ash with the leaching rate of solidified particles lower than the limits recommended by Chinese standard, Identification for extraction toxicity (CSEPA GB 5085.1, 2007). Therefore, the pretreatment process is easy to operate and is a promising way to increase the immobilization efficiency of alkali-activated slag cement.

Hydrothermal solidification of sepiolite into a cemented sepiolite aggregate for humidity regulation and formaldehyde removal

AbstractSepiolite powder was hydrothermally solidified into a cemented, designed to function both in humidity regulation and volatile organic compound (VOC) removal. The solidification process mimicked the cementation of sedimentary rocks. The formation of the calcium aluminium silicate hydrate (C-A-S-H) or Al-tobermorite enhanced the strength (maximum flexural strength >17 MPa) and improved the porosity of the solidified materials. Due to the low temperature of hydrothermal solidification (≤473.15 K), most sepiolite remained in the matrix of the solidified specimens. The cemented sepiolite aggregate shows outstanding humidity-regulating performance (moisture adsorption of 430 g m–2), and the synergistic effects of the residual sepiolite and neoformed Al-tobermorite exerted a positive influence on the humidity regulation performance of the material. Similarly to the behaviour of sepiolite, the solidified material also displayed good formaldehyde-removal capacity (60–68%). The pore dimensions controlled the humidity regulation and formaldehyde removal. The humidity regulation depends on the mesopores, which originate mainly from both the original sepiolite and the neoformed C-A-S-H phases and Al-tobermorite, while the formaldehyde removal depends on the micropores from the original sepiolite in the matrix. As such, the cemented sepiolite aggregate might be hydrothermally synthesized and might be used to improve the comfort and safety of indoor environments for human beings.

High-Strength Solidification of Fly Ash/Carbide Slag and Its Fixing Ability for Heavy Metals

Hydrothermal hot-pressing will occur between silica in fly ash and calcium in carbide slag and give rise to high-strength hydrates. High-strength solidified specimens were prepared with fly ash and carbide slag via hydrothermal hot-pressing at 200°C and 7 MPa. The effect of concentration of activator and content of carbide slag on compressive strength of the specimens was analyzed. It was found that when the mass ratio of fly ash to carbide slag was 1:1 and 20% 5 mol/L NaOH solution was added, the solidified specimens contained a large amount of cross-linked fibrous tobermorite with compressive strength of 70.93 MPa. The solidified specimens had a good fixation effect on such heavy metals as Cu, Zn, Cd and Pb. Hydrothermal hot-pressing, featured by simple procedures, easy automation and low cost, is an ideal potential way of utilizing fly ash and carbide slag and fixation of heavy metals.

STRENGTH IMPROVEMENT CHARACTERISTIC OF CEMENT-SOLIDIFIED DREDGED MARINE CLAY WITH RELATION TO WATER-CEMENT RATIO

Dredged marine sediments were retrieved from a maintenance dredge site for examination on the solidification efficacy. The high plasticity clay (CH) sample had low shear strength and high compressibility, making it unacceptable as a geomaterial for construction purposes. Due to its poor engineering properties, the material was destined for disposal offshore: incurring cost, time as well as contamination risks along the transportation route. With solidification, the soil could be improved for potential reuse in reclamation works, for instance. The laboratory investigation involved admixing ordinary Portland cement with the soil at different water-cement (WC) ratios, ranging from 1.5 to 3.5. Left to mature in a confined, damp environment for periods up to 56 days, the solidified specimens were subjected to the unconfined compression test. The measurements were conducted at predetermined intervals of 3, 7, 14, 28 and 56 days. Duplicate specimens were tested using the unconfined compression apparatus in accordance with BS1377 (1990). The unconfined compressive strength (qu) and Young's modulus (EP) were derived from the stress-strain plots. Both the strength and stiffness were found to increase with lower WC ratio and prolonged maturing, where the solidified soil transformed from a soft, weak material to that of a hard, strong one. The strength improvement was as high as 2.5 times that of the 3-day old specimens, while the stiffness increased by 4 times for the large strain range, as derived from the compression tests. Expediency of solidifying the soil with cement was further examined by correlating the strength, stiffness and deformation with WC as well as rest period.

A novel hydrothermal method to convert incineration ash into pollucite for the immobilization of a simulant radioactive cesium

The Fukushima nuclear accident in Japan on March 11, 2011 produced huge amounts of Cs-polluted incineration ashes; conventional solidification methods seem unsuitable for the treatment of large amounts of Cs-polluted ashes. A novel hydrothermal method was developed to directly convert Cs-polluted incineration ash (rice husk ash) into pollucite to immobilize Cs in its crystal structure in situ. Results revealed that pollucite could be synthesized readily over a wide range of added Cs (Cs/Si=0.2–0.6); the addition of more Cs (Cs/Si≥0.5) caused the formation of a small amount of cesium aluminosilicate (CsAlSiO4), which exhibits poor immobilization behavior for Cs. Pollucite could be formed even for a short curing time (1h) or at a low curing temperature (150°C). However, a high curing temperature or a long curing time favored the formation of a pure pollucite. With the added calcium hydroxide, a tough specimen with a flexural strength of approximately 22MPa could be obtained, which suggested that this technology may be applied directly to the solidification of Cs-polluted incineration ashes. Hydrogarnet and tobermorite formations enhanced the strength of the solidified specimens, and meanwhile the formed pollucite was present in a matrix steadily. Leaching test demonstrated that the amount of Cs that leached from the synthesized specimens was very low (0.49×10−5–2.31×10−5) and even lower than that from the reference hollandite-rich synroc (2.0×10−2), although a higher content of Cs was found in the synthesized pollucite specimens (6.0–31.7%) than in the reference (3.7%). Therefore, the hydrothermal conversion of Cs-polluted incineration ash into pollucite can be applied to immobilize Cs directly.

Influence of melt overheating treatment on solidification behavior of BiTe-based alloys at different cooling rates

Persistent efforts have been made to improve the performance of thermoelectric (TE) materials by different preparation methods. However, scarce attention has been paid to the effects of liquid state on solidified structures and properties of TE materials. In this work, the abnormal resistivity-temperature (ρ–T) behaviors of Bi2Te3 and Bi0.5Sb1.5Te3 melts during a definite temperature range in the first heating process were observed, which implied a temperature-induced liquid–liquid structural transition (TI-LLST) may exist in the liquid alloys. Based on this information, by melting specimens at different temperatures before and after TI-LLST and then solidifying at various cooling rates, it is found that the microstructures of solidified specimens are refined and the hardness elevate with cooling rate increasing for a specific composition. More significantly, the effects of crystal size decreasing and hardness increasing are further optimized by melt overheating treatment. In addition, this technique can also alter the crystallographic preferred orientation in some degree, which can also influence the transport of electrons and phonons. Since the TE and mechanical properties are highly susceptible to microstructures, the present work may provide an alternative approach to adjust the performance of TE materials and to improve the reliability and sustainability of TE devices in service.

Development of tool for physical simulation of skin formation during investment casting of nickel-based superalloys

Abstract Development of investment casting process has been always a challenge for manufacturers of complex shape parts with thin elements. Particularly, misruns often occur in the as-cast complex shape parts due to the formation of solid skin by freezing of melt in contact with colder ceramic mould. This work presents a new tool for physical simulation of skin formation during investment casting. Special ceramic tubes are designed and fabricated from the material used for the manufacturing of ceramic moulds for investment casting. Melting/solidification experiments are carried out in the thermo-mechanical simulator, where the melt is contained in the ceramic tube, which is heated to the temperature of ceramic mould in investment casting. Detailed microstructural characterization of the solidified specimens is performed; the obtained results predict the thickness of skin and its microstructure. This concept is applied to investment casting of complex shape nozzle guide vanes from the Mar-M247 Ni-based superalloy. Experimental casting trials are performed, and the outcomes of physical simulation tool are validated against experimental results.

Physical Simulation of Investment Casting of Complex Shape Parts

Development of investment casting process has been a challenge for manufacturers of complex shape parts. Numerous experimental casting trials are typically carried out to determine the optimum casting parameters for fabrication of high-quality products. In this work, it is demonstrated that physical simulation of investment casting can successfully predict microstructure and hardness in as-cast complex shape parts. The physical simulation tool consists of a thermal model and melting/solidification experiments in thermo-mechanical simulator. The thermal model is employed to predict local cooling rate during solidification at each point of a casting. Melting/solidification experiments are carried out under controlled cooling rates estimated by the thermal model. Microstructural and mechanical characterization of the solidified specimens is performed; the obtained results predict the local microstructure and mechanical properties of the casting. This concept is applied to investment casting of complex shape nozzle guide vanes from Mar-M247 Ni-based superalloy. Experimental casting trials are performed and the outcomes of physical simulation tool are validated against experimental results. It is shown that phase composition, secondary dendrite arm spacing, grain size, γ/γ′ eutectic size and volume fraction, size and shape of carbide particles, and local microhardness can be predicted at each point of the casting via physical simulation.

Hydrothermal Hot-pressing Solidification of Coal Fly Ash and Its Ability of Fixing Heavy Metal

In this paper, hydrothermal hot-pressing solidification of coal fly ash has been done by mixing with different contents of calcium hydroxide and distilled water for 45 min at the condition of 7 MPa and 200°C. The strength of solidified specimens can reach the highest value of more than 63 MPa with 20% calcium hydroxide and 20% distilled water. During strength development, tobermorite may have a larger effect than CSH, and the immobilization of the solidified specimens for heavy metals can be also attributed to the new phases formed in the solidified bodies with different heavy metals. The results showed that Zn and Cd were to be fixed more readily than Cu and Pb.

Effect of drying-wetting cycles on leaching behavior of cement solidified lead-contaminated soil

Lead contaminated soil was treated by different concentration of ordinary Portland cement (OPC). Solidified cylindrical samples were dried at 40°C in oven for 48h subsequent to 24h of immersing in different solution for one drying-wetting. 10 cycles were conducted on specimens. The changes in mass loss of specimens, as well as leaching concentration and pH of filtered leachates were studied after each cycle. Results indicated that drying-wetting cycles could accelerate the leaching and deterioration of solidified specimens. The cumulative leached lead with acetic acid (pH=2.88) in this study was 109, 83 and 71mg respectively for solidified specimens of cement-to-dry soil (C/Sd) ratios 0.2, 0.3 and 0.4, compared to 37, 30, and 25mg for a semi-dynamic leaching test. With the increase of cycle times, the cumulative mass loss of specimens increased linearly, but pH of filtered leachates decreased. The leachability and deterioration of solidified specimens increased with acidity of solution. Increases of C/Sd clearly reduced the leachability and deterioration behavior.

Solidified morphology and mechanism of bulk nanocrystalline Fe80P13C7 alloy at isothermal undercooling

The molten Fe80P13C7 alloys can achieve a large undercooling up to 320 K by fluxing technique. With the help of fluxing technique, the molten Fe80P13C7 alloys can be solidified at different undercooling (ΔT) through isothermal undercooling experiment. It is indicated that the microstructure of the solidified Fe80P13C7 alloy specimens is refined significantly with the increasing undercooling and the grain size is about 20 μm, 10 μm, 200 nm and 70 nm for ΔT=50 K, 150 K, 250 K and 320 K, respectively. The solidification morphologies of the solidified Fe80P13C7 alloy specimens under different undercooling are quite different. When ΔT=50 K, it presents a traditional solidification microstructure under a undercooling condition, composed of the primary dendrite and anomalous eutectic within the dendrites. When ΔT=150 K, a cell-like solidification morphology can be found, which can be proposed to be formed based on the nucleation and growth of spinodal decomposition mechanism. When ΔT=250 K, there is a strong direction of the solidification under an optical micrograph, two zones can be divided, and the microstructure of each zone presents a network which results from a liquid spinodal decomposition. When ΔT=320 K, the microstructure presents a random network completely. Microhardness test shows that the hardness of the solidified specimens increases with the undercooling.

소석회-광물찌꺼기 고형화의 초기 수화에 미치는 황산염의 영향

폐광산의 광물찌꺼기의 고형화처리 시 황산염이 초기 수화에 미치는 영향을 확인하기 위하여 본 연구를 수행하였다. 광물찌꺼기에 소석회를 결합재로 사용하였고, <TEX>$Na_2SO_4$</TEX>를 혼합수로 이용하여 9가지 조건의 고화체를 제작하였다. 제작된 고화체를 7일, 14일, 28일 양생 후 압축강도 측정, 중금속 용출시험, XRD 분석을 실시하였다. 증류수를 이용하여 양생을 실시한 고화체는 양생기간이 경과함에 따라 압축강도가 증가하였으며, <TEX>$Na_2SO_4$</TEX>를 혼합한 고화체는 양생기간 경과에 따라 압축강도가 감소하였다. <TEX>$Na_2SO_4$</TEX>의 농도가 증가할수록 고화체 외부 균열이 확실하게 나타났고, 시간이 경과함에 따라 균열이 내부까지 진행되었다. 고화체의 Cu, Cd, Zn, As 농도는 양생기간 경과에 따라 감소하였고, Pb의 경우 소석회 함량이 10 wt% 이상 혼합될 경우 농도가 증가함을 확인하였다. 균열이 발생된 고화체의 수화 생성물에서 석고와 <TEX>$MgSO_4$</TEX> 피크가 XRD 분석으로 확인되었고, SEM-EDS를 통해 주상의 결정이 Ca, S, O로 구성된 석고임을 확인하였다. 소석회를 이용하여 광물찌꺼기를 고형화시킬 경우 황산염 생성으로 균열이 심하게 발생되므로, 황 함량이 높은 광물찌꺼기의 처리 시에는 이를 보완할 수 있는 대책이 강구되어야 할 것으로 판단된다. Influence of sulfate on the early hydration in the solidification treatment of abandoned mine tailings was characterized. Solidified specimens using hydrated lime as a binder were prepared with various amounts of added <TEX>$Na_2SO_4$</TEX> and different curing days. Unconfined compressive strength measurement, heavy metal leaching test, XRD analysis were performed after 7-, 14- and 28-days curing. According to curing days strength of solidified specimens using only distilled water increased but those with addition of <TEX>$Na_2SO_4$</TEX> decreased. External cracks of specimens developed definitely with increasing <TEX>$Na_2SO_4$</TEX> concentration and curing days. Concentrations of Cu, Cd, Zn, and As in the leached solutions from solidified specimens decreased significantly but Pb was leached readily in cases of hydrated lime dosage more than 10 wt%. Gypsum and <TEX>$MgSO_4$</TEX> were identified in the cracked solidified specimens by XRD analysis, and pillar-shaped crystals of SEM image were identified as gypsum in reference with EDS analysis. Crystallization of sulfate in the process of lime-tailing solidification caused cracking, which should be supplemented for solidification treatment of highly sulfur-contained tailing.

Hydrothermal solidification behavior of municipal solid waste incineration bottom ash without any additives

Municipal solid waste incineration (MSWI) bottom ash could be solidified with and without slaked lime (calcium hydroxide) addition by a hydrothermal method under steam pressure of 1.56MPa at 200°C for up to 72h. Experimental results showed that CSH gel or tobermorite exerted a main influence on strength development, and without any additives CSH gel was easy to form, while slaked lime addition favored to form tobermorite. Tobermorite seemed to exert a larger effect on the strength development than CSH gel. Leaching results showed that the concentrations of heavy metals dissolved from the solidified specimens were effectively reduced after hydrothermal processing. The immobilization was mainly due to the tobermorite or CSH gel formation, and Pb2+ and Zn2+ seemed to be fixed more readily than Cr6+, which might be the reason that the structural Ca2+ within tobermorite or CSH gel was exchanged by Pb2+ and Zn2+ more easily than Cr6+. In addition, there existed a close relationship between leaching concentration and strength enhancement, and a higher strength seemed to exert a larger effect on immobilization of heavy metals.