Long-term coupled behaviour of cemented paste backfill in load cell experiments

A pressure cell apparatus has been developed in this research work to study the long-term hydro-mechanical behaviour of cemented paste backfill (CPB) cured under applied stress. The samples are cured for 7, 28, 90 and 150 days and the evolution of their mechanical, hydraulic, physical and microstructural properties is studied. Also, the suction, temperature and electrical conductivity are monitored for a period of 150 days of curing. The testing and monitoring programmes are conducted in undrained conditions, with and without pressure application. The obtained results show that the curing stress affects the hydro-mechanical behaviour of CPB for up to 28 days. Within this curing period, the CPB exhibits enhanced hydro-mechanical performance. However, application of sustained excessive curing stress onto the CPB samples induces the propagation of microcracks in the backfill structure, thus causing lower mechanical strength and higher fluid permeability at the more advanced ages. Furthermore, the mineralogical and chemical compositions of the tailings (e.g., sulfidic tailings) can significantly alter the mechanical strength properties (uniaxial compressive strength and elastic modulus) and the permeability of the CPB. The evolution of coupled factors and characteristics of the CPB at an early age control and influence its long-term behaviour and performance.     

Role of curing temperature in progress of lime-soil reactions

An expansive black cotton soil was used to assess if ambient temperature influences the progress of lime-soil reactions. Lime-soil mixes containing lime additions above the Lime Modification Optimum (or Initial Consumption of Lime value) of the expansive black cotton soil was cured for periods ranging from 1 hour to 400 days at 25°C. The curing temperature of 25°C is representative of mean temperatures occurring in semi-arid regions of Karnataka, India. The in situ progress of lime treatment was monitored by the use of electrical conductivity and pH measurements. The trend of decrease in conductivity and pH of the 4 and 7 lime-soil mixes suggested that pozzolanic activity commenced after 1 day of curing at 25°C. Comparatively, an earlier study by other researchers had reported that pozzolanic activity does not commence before 7 days of curing at 11.5°C for lime additions above the Lime Modification Optimum value of swelling clay. Combining the results of studies at 25°C and 11.5°C, it appears that higher ambient temperatures do accelerate the progress of lime-soil reactions.     

Unsaturated hydraulic properties of cemented tailings backfill that contains sodium silicate

Recycling the mine waste (tailings) into cemented tailings backfill has economical and environmental advantages for the mining industry. One of the most recent types of cemented tailings backfill is gelfill (GF), a backfill that contains sodium silicate as chemical additive. GF is typically made of tailings, water, binder and chemical additives (sodium silicate gel). It is a promising mine tailings backfill technology. From a design point of view, the environmental performance or durability of GF structures is considered as a key factor. Due to the fact that GF structures are cementitious tailings, their durability and environmental performance depend on their ability to resist the flow of aggressive elements (water and oxygen). Thus, understanding the unsaturated hydraulic properties of GF is essential for a cost-effective, environmentally friendly and durable design of GF structures. However, there is a lack of information with regards to unsaturated hydraulic properties of GF, the factors that affect them and their evolution with time. Hence, the unsaturated hydraulic properties (water retention curve (WRC) or water characteristic curve, air entry value (AEV), residual water content, unsaturated hydraulic conductivity) of GF are investigated in this paper. GF samples of various compositions and cured in room temperature for different times (3, 7, 28, and 90 days) are considered. Saturated hydraulic conductivity and microstructural tests have been conducted; WRCs are measured by using a WP4-T dewpoint potentiameter and the saline solution method. Unsaturated hydraulic conductivity is predicted using the van Genuchten (1980) equation. The water retention curve (WRC) is determined as the relationship between volumetric water content and suction for each GF mix and curing time. The van Genuchten (1980) equation is used to simulate the WRC to best-fit the experimental data. AEV and residual water content are also computed for each mix and curing time. Furthermore, functions are developed to predict the evolution of AEVs, residual water content and fitting parameters of the van Genuchten model with degree of hydration. Important outcomes have been achieved with regards to unsaturated hydraulic properties. The unsaturated hydraulic conductivity of GF was calculated to decrease when the suction, binder content, and degree of hydration increase. The effects of binder content and degree of hydration are more obvious at low suction ranges. The obtained results would contribute to a better design and assessment of the durability and environmental performance of GF structures.     

Coupled Modeling of Temperature Distribution and Evolution in Cemented Tailings Backfill Structures that Contain Mineral Admixtures

Cemented paste backfill (CPB, a mixture of tailings, water and binder) is widely utilized to fill underground mine voids. To achieve a good, economical performance, one approach is to proportionally use mineral admixtures such as fly ash and slag as partial substitutes for Portland cement. Binder hydration is one of the most significant factors that can generate heat within hydrating CPB structures, which in turn, influences the mechanical and hydraulic properties of CPB, as well as the pore structure within CPB. However, the temperature evolution due to the hydration of Portland cement that contains fly ash or slag is different from that of hydration with solely Portland cement. Hence, in consideration of the heat generated by both binder hydration and transferred between CPB and its surrounding media, a numerical model is developed to predict and determine the temperature development within CPB that contains mineral admixtures. After that, data from field and laboratory studies are employed to validate the developed model. The validation results demonstrate a good consistency between the model and the field and laboratory studies. Consequently, the proposed model is applied to simulate and determine the temperature evolution with time via mineral admixtures, binder content, initial rock and CPB temperatures, stope geometry, backfilling rate, curing time and backfilling strategy. The obtained results will contribute to better designs and preparation of CPB mixtures, as well as predict the temperature distribution within CPB structures.     

Compression Behavior of Mine Tailings Amended with Cementitious Binders

The objective of this study was to evaluate the effect of fly ash amendment on the compression behavior of mine tailings. Natural and synthetic (i.e., laboratory prepared) mine tailings were used to assess the effects of tailings composition and tailings solids content on compressibility. Three types of off-specification fly ashes and Type I–II Portland cement were used as cementitious binders. Tailings-fly ash mixtures were prepared at solids content of 60–75% (water content = 33–67%), water-to-binder ratios of 2.5 and 5, and were cured for 0.1 days (2 h), 7, and 28 days. Bi-linear compression curves on semi-log plots were observed in most of the binder-amended tailings specimens. The break in slope on the compression curve was identified as the breaking stress, whereupon cementitious bonds were broken. The breaking stress increased with an increase in fly ash content, which was attributed to a lower water-to-binder ratio and void volume-to-binder volume ratio that produced more effective particle bonding. Breaking stress also increased with an increase in CaO content and CaO-to-SiO₂ ratio of fly ash, which resulted in more effective bonding between particles. The effect of curing time on the breaking stress of fly ash amended specimens was characterized by (1) an increase in breaking stress via increase in curing time and cementitious bond formation or (2) a constant breaking stress with curing time due to competing mechanisms during loading. Specimens cured under a vertical stress showed an increase in breakings stress with applied load water removal prior to cementitious bond formation that reduced the water-to-binder ratio and led to more effective cementation.     

Experimental study of thermal effects on the mechanical behaviour of a clay

The paper presents the results of an experimental study of thermal effects on the mechanical behaviour of a saturated clay. The study was performed on CM clay (Kaolin) using a temperature-controlled triaxial apparatus. Applied temperatures were between 22 and 90°C. A comprehensive experimental program was carried out, including: (i) triaxial shear tests at ambient and high temperatures for different initial overconsolidation ratios; (ii) consolidation tests at ambient and high temperatures; and (iii) drained thermal heating for different initial overconsolidation ratios. The obtained results provide observations concerning a wide scope of the thermo-mechanical behaviour of clays. Test results obtained at 90°C were compared with tests performed at ambient temperature. Based on these comparisons, thermal effects on a variety of features of behaviour are presented and discussed. Focus is made on: (i) induced thermal volume change during drained heating; (ii) experimental evidence of temperature influence on preconsolidation pressure and on compressibility index; (iii) thermal effects on shear strength and critical state; and (iv) thermal effects on elastic modulus. Thermal yielding is discussed and yield limit evolution with temperature is presented. The directions of the induced plastic strains are also discussed. Several remarks on the difference in the mechanical behaviour at ambient and high temperatures conclude the paper. Copyright © 2004 John Wiley & Sons, Ltd.     

Modeling the heat development in hydrating CPB structures

Cemented paste backfill (CPB) is a mixture of dewatered tailings, hydraulic binders and water. In addition to contributing to the stability of mine workplaces, CPB greatly benefits the environment by minimizing surface tailings disposal. Hence, it has become one of the most commonly used ways in mine backfilling around the world. Temperature can significantly affect the mechanical properties of cemented backfill. A source of heat in CPB is produced by binder hydration. Hence, a FLAC based numerical model is developed to predict and analyse the heat developed by hydrating CPB structures. To validate the model, results of the developed model are compared with three case studies (mathematical, laboratory, and field investigations). The validation results show a good agreement between the developed model and these cases. The effects of stope geometry, thermal properties of both rock and CPB, filling rate, binder content and initial boundary conditions are also investigated.     

不同温度应力路径下饱和黏土剪切特性

在精准温控动三轴试验系统上开展了不同温度及不同升温路径饱和黏土剪切试验研究,探讨了不同温度对饱和软黏土不排水剪切特性的影响,分析不同升温固结方式对饱和软黏土孔压发展、体变、强度以及模量的影响规律。试验结果显示：在4～76 ℃试验研究范围内,环境温度升高导致饱和软黏土的不排水剪切强度有所减少,但温度升高对土体模量增加影响明显,温度T和模量ET关系可用ET = 2.69T 0.3表达;升温变化时正常固结黏土产生超孔隙水压力并随着温度增大而增大,升温热固结后土的剪切强度将明显提高,且排水状态下升温固结对土剪切强度增长小于升温完成后再固结情况;土体从26 ℃分别升高20、40 ℃时,升温引起的超孔压比分别为0.41、0.61,剪切峰值强度分别增加8.23%、22.37%。研究表明：升温幅值增大会使土体热固结程度越大,升温分级越多,热固结也越充分,其对应的体变、强度增长率则越大;同时最终温度及热固结路径对其剪切相转换特征存在影响,升温越高、热固结路径越多其剪胀性越明显,但温度变化范围、固结分级、热固结路径总体上对孔隙水压力的发展基本不产生影响。     

Thermal expansion of alkali halides at high pressure: NaCl as an example

Thermal defects in ionic materials can have important effects on their thermal expansion at high temperatures. Earlier treatments of thermal expansion generally have neglected or not considered such effects. Here an analytical expression for the thermal defect contribution and its dependence on pressure is derived. We show that such contributions, which are significant at high temperature and atmospheric pressure, become negligible at pressures above approximately 0.25 to 0.35 of the bulk modulus at standard conditions. At very high pressure, based on Birch's (1968) relationship between high and low pressure thermal expansion, and assuming αK _Tis independent of pressure, NaCl thermal expansion can be calculated within the constraints of a semi-empirical quasi-harmonic perfect crystal model. The calculations are compared with available theoretical and experimental values over an extended temperature/pressure regime. The method should have broad applicability for other ionic crystals.     

Hydraulic Conductivity of Fly Ash-Amended Mine Tailings

The objective of this study was to evaluate the effect of fly ash addition on hydraulic conductivity (k) of mine tailings. Mine tailings used in this study were categorized as synthetic tailings and natural tailings; two synthetic tailings were developed via blending commercially-available soils and natural tailings were collected from a garnet mine located in the U.S. Two fly ashes were used that had sufficient calcium oxide (CaO) content (17 and 18.9 %) to generate pozzolanic activity. Hydraulic conductivity was measured on pure tailings and fly ash-amended tailings in flexible-wall permeameters. Fly ash was added to mine tailings to constitute 10 % dry mass of the mixture, and specimens were cured for 7 and 28 days. The influence of fly ash-amendment on k of mine tailings was attributed to (1) molding water content and (2) plasticity of the mine tailings. Tailings that classified as low-plasticity silts with clay contents less than 15 % exhibited a decrease in k when amended with fly ash and prepared wet of optimum water content (w _opt ). Tailings that classified as low-plasticity clay exhibited a one-order magnitude increase in k with addition of fly ash for materials prepared dry or near w _opt . The decrease in k for silty tailings was attributed to formation of cementitious bonds that obstructed flow paths, whereas the increase in k for clayey tailings was attributed to agglomeration of clay particles and an overall increase in average pore size. The results also indicated that the effect of curing time on k is more pronounced during the early stages of curing (≤7 days), as there was negligible difference between k for 7 and 28-days cured specimens.     

Microstructural and mechanical properties of marine clay cemented with industrial waste residue-based binder (IWRB)

Improving the engineering properties of low-strength soft clay in an environmentally friendly way becomes a challenge in coastal areas. Conventional ground treatment techniques for marine clay using cement can cause significant environmental pollution. In this study, the potential use of industrial waste residue-based binder (IWRB), a silicate-based chemical modified by a powdery polymer, as a substitute for Portland cement (PC) is investigated. Collected marine clay was treated with various IWRB-to-PC ratios (0:8, 4:4, 2:6 and 8:0 wt. %) to measure the mechanical properties, through unconfined compression (UC) test and one-dimensional consolidation (ODC) test, and the microstructural and mineralogical characters, through scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The strength and the deformation of specimens treated with mixed IWRB and PC in a ratio of 1:1 were similar to those improved with PC alone, but the toughness was significantly improved. The microstructural results demonstrated that the cementitious compounds (C–S–H and C–A–S–H) increased significantly in the early curing stage of marine clay treated with IWRB, contributing to the improvement of mechanical properties. It is suggested that IWRB can be an effective substitute for PC to reduce the cost and environmental pollution.

     

养护温度对水泥固化淤泥强度影响试验研究

采用水泥固化海泥（CSMC）作为围海垦地填料正在被广泛关注。由于水化放热,当环境温度较高时大体积填筑的CSMC内部实际养护温度可能明显高于室内标准养护温度（≈ 20 ℃）,这必然会对CSMC的强度发展产生影响,但目前工程实践中并未考虑这一温度差异的影响。因此,基于室内试验,测试了不同配合比CSMC试样在不同养护温度下的强度发展过程,并重点探讨了养护温度对CSMC强度发展的影响规律及其内在机制。结果表明：养护温度对CSMC强度发展影响显著,高的养护温度不仅会显著提高CSMC的早期强度,而且会比较明显地提高CSMC的晚期强度,这就意味着在CSMC垦地工程实践中考虑养护温度的影响是十分必要的。鉴于此,文中最后给出了如何在CSMC垦地工程实践中考虑养护温度对强度发展影响的建议和设计流程。     

Flow Characteristics of Cemented Paste Backfill

Cemented paste backfill (CPB) is primarily used for backfilling underground voids at George Fisher Mine (Mount Isa, Australia). The objective of this paper is to summarise the geotechnical characterisation of the tailings and the rheological properties of the CPB as determined from a laboratory testing program undertaken at James Cook University. Two binders were examined [a General Purpose cement and a slag blend cement] over a range of dosages from 0 to 6% and CPB mix solids content in the range of 72–78%. The slump tests were carried out using the standard cone (ASTM C 143) used for concrete and a cylinder with 110 mm (diameter) × 110 mm (height), whereas the yield stress was measured using a shear vane (Brookfield vane spindle V-73). The index characteristics of the tailings including the grain size distribution, liquid limit, plastic limit, specific gravity were determined as per ASTM standards. This paper will then discuss the interrelationships among the solid content, slump, saturated density and the yield stress of the CPB. It is shown that there is strong correlation between the two different slump test devices used in this study. The smaller cylindrical device appears to have good potential for slurries like mine tailings or dredged mud that have high water content for slump test. There is also strong inter-relationship among solid content, slump, yield stress, and bulk density. Increasing the solid content increases the bulk density and yield stress, but reduces the slump. While there is hardly any difference between the two binder types used in this study in terms of flow parameters, namely the yield stress and slump, the binder dosage has an effect. At any specific solid content, higher binder dosages lead to a drop in the slump and increase in the yield stress. The difference is more pronounced in dense slurries. It is also strongly believed that the trends and relationships developed in this study may be valuable for the other mining operations using CPB.     

Temperature dependence of the relationship of thermal diffusivity versus thermal conductivity for crystalline rocks

Thermal diffusivity governs the transient heat transport equation. Thus, a realistic characterisation of this parameter and its temperature dependence is crucial for geothermal modelling. Due to sparse information from boreholes, lack of samples, and elaborate measurement procedures, there is often insufficient data on thermal diffusivity at elevated temperatures. We make use of existing data on crystalline (metamorphic and magmatic) rock samples from the Kola Peninsula and the Eastern Alps and develop a general relationship for the temperature dependence of thermal diffusivity up to 300°C. The temperature dependence of thermal conductivity is parameterised itself, using an empirical relationship which is set up for both data sets as well. Hence, only thermal conductivity at ambient temperatures is required for determining the temperature dependence of thermal diffusivity. We obtain different coefficients for both data sets which can be explained by different geological settings, and therefore different mineral compositions and internal structures. Comparisons with other expressions for these rock physical parameters show a good agreement at ambient conditions. General relations for thermal diffusivity at elevated temperatures are rare. A comparison of our results with data from two crystalline samples from the KTB and data from the Southern Indian Granulite Province highlights the need for further data, which will help to quantify uncertainties.     

Tools for assessing the impact of climate change on freshwater fish populations

We identify and discuss ways to use existing information on the thermal ecology of freshwater fishes to assess the potential impact of climate change on wild populations of these organisms. Two primary questions are identified: (i) how do aquatic habitats change in response to atmospheric climate change? (ii) how do fish respond to habitat change at both the individual and population levels? In lakes, climate warming will lead to higher surface water temperatures, longer ice-free periods, and longer periods of thermal stratification. In rivers, climate warming will lead to higher groundwater temperatures with corresponding increases in both summer and winter temperatures, from headwaters to mouth. We describe several methods for predicting the biological effects of these changes in habitat. We examine the use of bioenergetic models to predict the impact of climate change on the growth of individual fish. We examine the use of thermal habitat models to assess the impact of climate change on population abundance. We examine the use of life cycle models to assess the impact of climate change on the zoogeographic distribution of species. Finally, we identify new research required to further develop these methods.     

Thermal expansion of kyanite at ambient pressure: An X-ray powder diffraction study up to 1000 ℃

The thermal expansion coefficients of kyanite at ambient pressure have been investigated by an X-ray powder diffraction technique with temperatures up to 1000 ℃. No phase transition was observed in the experimental temperature range. Data for the unit-cell parameters and temperatures were fitted empirically resulting in the following thermal expansion coefficients: aa = 5.8(3) × 10-5, ab = 5.8 (1) × 10-5, ac% = 5.2(1) × 10-5, and av = 7.4(1) × 10-3 ℃-1, in good agreement with a recent neutron powder diffraction study. On the other hand, the variation of the unit-cell angles a, β and γ of kyanite with increase in temperature is very complicated, and the agreement among all studies is poor. The thermal expansion data at ambient pressure reported here and the compression data at ambient temperature from the literature suggest that, for the kyanite lattice, the most and least thermally expandable directions correspond to the most and least compressible directions, respectively.     

南海北部海底地热测量的数据处理方法

为了解南海北部地热分布特征,使用EWING型海底地热探针在3个区域进行海底地热测量,利用TK04型热导率仪测量采自3个区域的沉积物样品的热导率,并对相关数据进行一系列校正处理,获得3个代表性站位HD343、HS82、HX129的热流密度。数据分析表明:对于EWING型探针需要对原始温度记录进行偏移量校正和平衡温度计算;不同自容式温度传感器(MTL)间的相对温度偏移量是基本固定的,取决于记录器本身的性质,与测站位置无关;插入海底后,EWING型探针记录的温度随时间的变化取决于摩擦生热的大小,如果摩擦引起的温度变化小于环境温度,温度将随时间缓慢上升,否则随时间缓慢降低;室内测量的热导率经过温度和压力校正后,一般降低4％～5％;3个站位的地温梯度分别是81 ℃/km、109 ℃/km、94 ℃/km,热导率分别为0.979 W/(m·K)、0.785 W/(m·K)、0.886 W/(m·K),热流密度分别为79 mW/m²、86 mW/m²、83 mW/m²。     

Development of a high modulus paste fill using fine gold mill tailings

Summary In the mining industry today there is some caution about using fine tailings as a backfill material. Traditionally, hydraulic backfill has only used the coarse fraction of tailings, excluding the fines by a classification process. With the development of paste fill, the percentage of fine tailings being sent underground has increased, but still remains low due to the high percentages of sand and gravel which usually make up these fills. Sand and gravel have been added to paste fills to aid pumpability and to increase fill strength and stiffness. This leaves the remainder of fine tailings destined for surface disposal. The main focus of this paper is to evaluate what effect the addition of fine gold mill tailings in the form of agglomerated tailings pellets has on the strength and stiffness characteristics of a total tailings paste fill. The purpose is to create a high modulus fill which is made up entirely of fine tailings. A constant slump design of 20 cm (8 in) was used for each mix. Various binder dosages, curing periods and combinations of pellet to tailings ratio were studied. Raw fill slump and density, and cured fill compressive strength and modulus of elasticity were also examined. Results from the above study indicate that agglomerated tailings paste fill (ATPF) has superior strength and stiffness characteristics. Compressive strengths were enhanced while the modulus of elasticity values was tripled when compared to total tailings paste fills of the same binder content and consistency. ATPF minimizes the surface disposal of tailings and maximizes the utilization of fine tailings underground as a useful backfill material.     

Novel sensible thermal storage material from natural minerals

Novel sensible thermal storage materials (TSM) were first synthesized via thermally treating the green compact obtained using clay, kaolin tailings, and hematite as major raw materials. The samples were characterized using differential scanning calorimetry and thermogravimetric, X-ray diffraction, thermal conductivities, petrography analysis, Fourier transformation infrared spectroscopy, and scanning electron microscopy. The thermal conductivity of the green compact reached 1.11–1.64 W m^?1 K^?1 after thermally treated at 200–1,000 °C. The clay component was proven to have a predominant effect on the thermal conductivity of the green compact. Kaolin tailings could act as a “modulator” for adjusting the thermal conductivity from 1.42 to 1.92 W m^?1 K^?1. Affecting mechanism of microstructural change of main components during sintering on thermal conductivity of TSM was prominently investigated. TSM could provide a potential candidate for thermal energy storage systems of concentrated solar power.     

Effects of thermal discharges on the microstructural growth ofMercenaria mercenaria

The effects of thermal discharges from the Oyster Creek Nuclear Generating Station at Barnegat Bay, New Jersey, are recorded in the microstructural growth ofMercenaria mercenaria, a common coastal marine pelecypod. The analysis of the shell microstructure shows that this bivalve acts as an effective monitor of the environmental conditions existing in the marine waters adjacent to the power station. Many physiological and environmental events such as spawning, winter (freeze) shocks, summer (heat) shocks, thermal shocks, tidal cycles, and major storms are clearly recorded in the shell microstructure. The exact time of occurrence of these events can be determined by counting daily growth increments backwards from the outer shell margins of freshly killed individuals. Microstructural growth patterns reflected in Barnegat Bay specimens indicate that these pelecypods were affected mainly by temperature extremes, temperature variations, tides, type of substratum, and age. Growth patterns in specimens from areas surrounding Oyster Creek (affected by thermal effluent) are significantly different from those from other bay localities (unaffected by thermal effluent).Mercenaria mercenaria within approximately a 1.6km radius of Oyster Creek show a lower summer growth rate (10 percent to 25 percent lower) and a greater number of growth breaks (2 to 6 more per clam) than those away from the creek. The lower summer growth rates occur in bivalves subjected to the effluent because the added heat during the summer months causes water temperatures to exceed a critical threshold for optimum growth in the species. The greater number of growth breaks takes place, in turn, because many of the breaks (thermal shock breaks) are generated by rapidly fluctuating temperatures associated with abrupt shutdowns, massive load reductions and rapid renewal of operations following shutdowns or load reduction periods at the nuclear power station. In addition, the effluent may be upsetting natural spawning events in the clams when abrupt changes in power station operations overlap with spawning periods. In this respect, spawning may be precluded by sharp temperature changes which result in physiological shocks to the animal.