Coupled effects of sulphate and temperature on the strength development of cemented tailings backfills: Portland cement-paste backfill

Cemented paste backfill (CPB), which is a mix of tailings, water and cement, is subjected to the combined actions of temperature and sulphate during its service life. There is a need to acquire solid knowledge on the coupled effects of temperature and sulphate on the strength of CPBs for a safe, durable and cost-effective design of CPB structures. Hence, the main objective of this paper is to use an experimental approach to study the combined effect of temperature and sulphate on the strength development and microstructure (mineralogical composition of the hardened cement paste) of CPBs. About 200 CPB specimens with various initial sulphate contents (0, 5000, 15,000, and 25,000 ppm) and cured at different temperatures (0 °C, 25 °C, 20 °C, 35 °C, and 50 °C) are tested at different curing times (28, 90, and 150 days). The results show that the coupled effect of temperature and sulphate has a significant impact on the strength and mineralogical composition of the CPB. Depending on the curing time, temperature and initial sulphate content, the sulphate can have a positive or negative impact, i.e., leads to an increase or decrease of CPB strength. The obtained results show a strong indication that the absorption of sulphate by calcium–silicate–hydrate (C–S–H) could lead to the formation of lower quality C–S–H, thereby decreasing the strength of the CPB. This study has demonstrated that the coupled effect of sulphate and temperature on CPBs is an important factor for consideration in the designing of cost-effective, safe and durable CPB structures.     

Experimental characterization of the stress–strain behaviour of cemented paste backfill in compression

It is of great interest for economical and security reasons to understand the compressive properties of underground cemented paste backfill. In this paper, the stress–strain behaviours of cemented paste backfill (CPB) subjected to uniaxial compression and conventional triaxial tests are presented and discussed. The effect of CPB basic components, strength, ageing and confining pressure on the deformation behaviour of CPB are evaluated and discussed. The results show that the stress–strain behaviour of CPB is strongly influenced by the confinement, the age and strength of CPB, and its components. The increase in confining pressure leads to a change in the mode of failure, in the stiffness, and an increase in the strength.     

Initial temperature-dependence of strength development and self-desiccation in cemented paste backfill that contains sodium silicate

Advanced knowledge of the effect of the initial temperatures of cemented paste backfill (CPB) on its strength development and self-desiccation ability is needed to provide a rational basis for mixture proportioning as well as the cost-effective design of CPB structures and speeding up of the mining cycle. An experimental testing and monitoring program has been undertaken to determine the influence of various initial temperatures (2 °C, 20 °C, 35 °C and 50 °C) on the strength development and evolution of the self-desiccation (measured by the volumetric water content and suction) of the CPB that contains sodium silicate as the admixture (S-CPB) at early ages (up 28 days). The evolution of the temperature, electrical conductivity (gives information about the cement hydration progress), volumetric water content and suction of S-CPB samples with the specified initial temperatures have been monitored for 28 days. Moreover, mechanical tests and microstructural analyses are performed on these S-CPB specimens after specific curing times (6 h, 1 day, 3 days, 7 days and 28 days). The results obtained show that the initial backfill temperature has a significant influence on the curing temperature of S-CPB, its strength development and self-desiccation. Furthermore, the results reveal that the temperature-time history, cement hydration, self-desiccation and strength development of S-CPB are strongly coupled. The initial S-CPB and these couplings should be taken into account for a safe and economical design of S-CPB structures and the speeding up of mining cycles.     

Variability in rheology of cemented paste backfill with hydration age,binder and superplasticizer dosages

The use of (CPB) material to ameliorate geotechnical stability of underground mine is in nascent stage in India. Rheological properties of CPB change with travelling time as it is transported to underground mine stope through pipeline reticulation. In this paper, rheological properties of CPB based on mill tailings of a carbonate rich mineral processing waste are evaluated for different dosages of polycarboxylate (PC) based (SP). Each CPB sample having 78?wt% solids is mixed separately with 4%, 6% or 8% of binder dosages (ratio of the weight of dry binder to the weight of dry tailings) and, 0%, 0.5%, or 1.0% of SP dosages as weight of dry binder. The paper presents a methodology for determining yield stress, plastic viscosity and thixotropic behaviour of CPB mixture as a function of hydration age, binder and SP dosages. Results from the experimental campaigns indicate that SP content has significant influence on rheological behaviour of CPB and can be suitably exploited to enhance the flow characteristics of the carbonate rich process tailings. The study also develops multivariate linear regression models of yield stress, plastic viscosity and thixotropy of CPB depending on the hydration age, binder and SP dosages.     

Modelling the leaching of calcium hydroxide from cement paste: effects on pore space percolation and diffusivity

As concrete is exposed to the elements, its underlying microstructure can be attacked by a variety of aggressive agents. For example, rainwater and groundwater can degrade the concrete by dissolving soluble constituents such as calcium hydroxide. Using computer simulation, this paper examines the effects of calcium hydroxide dissolution on two material properties: the percolation properties or connectivity of the capillary pore space, and the relative ionic diffusivity. A microstructural model for cement paste is used to produce a hydrated specimen which is subsequently subjected to the leaching process. Pore space percolation characteristics and relative ionic diffusivity are computed throughout the leaching process as a function of total capillary porosity. Material variables examined are water: solids ratio and silica fume content. Percolation theory is used to develop the concept of a critical volume fraction of calcium hydroxide plus capillary pore space. It is shown that this critical combined volume fraction determines the magnitude of the effect of leaching on relative ionic diffusivity.

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Resume Quand le béton est exposé aux éléments, sa microstructure sous-jacente peut être agressée par divers agents. Ainsi, la pluie et l'eau souterraine peuvent détériorer le béton en dissolvant les composants solubles tels que l'hydroxyde de calcium. A l'aide d'une simulation par ordinateur, on examine ici les effects de la dissolution d'hydroxyde de calcium sur deux propriétés du matériau: la percolation de l'espace interstitiel et la diffusivité ionique relative. On utilise un modèle microstructural de la pate de ciment pour produire une éprouvette hydratée qu'on soumet ensuite au lessivage. On calcule les caractéristiques de la percolation interstitielle et la diffusivité ionique relative tout au long du phénomène de lessivage en fonction de la porosité capillaire totale. Les variables du matériau considérées sont le rapport eau/solides et la teneur en micro-silice. On se sert de la théorie de la percolation pour développer le concept de fraction volumique critique d'hydroxyde de calcium en combinaison avec l'espace interstitiel capillaire. On montre que cette combinaison critique détermine l'importance de l'effet de lessivage sur la diffusivité ionique relative.

     

Likelihood ratio test-based interval estimate of the number of signals present in a measurement

A multi-step procedure is proposed to construct a confidence interval for the number of signals present and a comparative analysis of our procedure with other existing procedures is conducted. The proposed procedure uses likelihood ratio statistics and its simulated percentiles sequentially to determine the upper and lower limits for the confidence interval. A preference zone in the parameter space of the population eigenvalues is denned and it is used to separate the signals and the noise. The least favourable configuration (LFC) is derived asymptotically under the preference zone. Under the LFC, the procedure parameters necessary to attain the required confidence level are determined. A comparative analysis with other procedures is presented in tables.     

Dynamic tests of cemented paste backfill: effects of strain rate,curing time,and cement content on compressive strength

This article investigates the compressive strength of cemented paste backfill (CPB) under dynamic loading. To accommodate the low impedance CPB, a modified split Hopkinson pressure bar (SHPB) system is adopted. In contrast to traditional solid steel transmitted bar, a hollow aluminum transmitted bar is introduced to reduce the impedance. With this system, the dynamic stress equilibrium is achieved, which guarantees the valid dynamic material testing condition. The dynamic tests are conducted for CPB with different cement contents and curing time. It is observed that: (1) for CPB with the same curing time and cement content, the dynamic strength increases with the strain rate, (2) for CPB with the same cement content, the dynamic strength increases with the curing time, and (3) for CPB with the same curing time and tested under similar strain rate, the dynamic strength increases with the percentage of cement. This observation can be understood by considering the hydration process of cements.     

A probabilistic source assessment framework for leaching from secondary materials in highway applications

Recovered materials from the transportation sector or secondary or by-product materials from the industrial, municipal, or mining sector can be used as substitutes for natural materials in the construction of highway infrastructure. The environmental impact of traditional and newer secondary materials needs to be determined for the conditions of their expected use. The purpose of this paper is to introduce a probabilistic framework for evaluating the environmental acceptability of candidate secondary materials based on the risk of soil and groundwater contamination from leached metals and organics from the pavement. The proposed framework provides a structured guidance for selecting the appropriate model, incorporating uncertainty, variability, and expert opinion, and interpreting results for decision making. This new approach is illustrated by a probabilistic analysis of arsenic leaching from Portland cement concrete and asphalt concrete materials that were constructed using virgin and secondary products.     

Optimization of municipal solid waste leaching test procedure: assessment of the part of hydrosoluble organic compounds

Despite national recycling campaigns, the amount of municipal solid waste (MSW) to be treated remains very important in France with almost 39% of the waste produced going to landfills. Therefore with the increasing concern over sustainable development and energy valorization, it seems essential to optimize current treatment methods and develop new preparation techniques of the waste. Nevertheless an important first step to take into account is to evaluate the waste using a different method than biogas production. In this perspective, the leaching test (LT) could be used as a tool to evaluate the ability of a waste to mobilize organic and mineral compounds. This research aims at optimizing a leaching test protocol mainly adapted to organic waste in order to be used on MSW to assess the fractions of both fast and slow mobilized organic matter. Several leaching tests have thus been implemented, optimized and compared in terms of accessible organic matter in the waste. Results have shown that the test conditions have a great influence on the mobilization of pollutants. The duration of the test affects mainly the quantity and quality of organic molecules extracted. The renewal of the eluent does not properly simulate the conditions of a landfill. The results would be used to assess the performance and the efficiency of new ways of waste pretreatment.     

CdS-sensitized TiO2 in phenazopyridine photo-degradation: Catalyst efficiency,stability and feasibility assessment

Mineralization of phenazopyridine, 1, in water, under solar-simulator radiation was efficiently achieved using nanoparticle CdS-sensitized rutile TiO₂, TiO₂/CdS, 2, as photo-catalysts. Despite that, 2 showed two main drawbacks. Firstly, the system was difficult to recover by simple filtration, and demanded centrifugation. Secondly, the sensitizer CdS showed relatively high tendency to leach out hazardous Cd²⁺ ions under photo-degradation reaction conditions. In an attempt to solve out such difficulties, 2 was supported onto sand surface. The sand/TiO₂/CdS system, 3, was easier to recover but showed slightly lower catalytic activity compared to 2. On the other hand, the support failed to prevent leaching of Cd²⁺. This indicates limited future applicability of CdS-sensitized TiO₂ photo-catalyst systems, in solar-based water purification strategies, unless leaching out tendency is completely prevented.     

Flow of Molten Arsenic Selenide in a Cylindrical Channel

The flow rate of molten arsenic selenide in cylindrical channels is measured at channel diameters of 3.0 to 5.5 mm, channel lengths of 40 to 120 mm, temperatures of 285–320 and 375–470°C, and inert gas gage pressures of up to 1.5 × 10⁵ Pa. It is found that there is a motionless melt layer on the inner surface of the channel. For a channel 4.6 mm in diameter, its thickness is 0.7 mm at 290°C and 0.1 mm at 420°C. In the temperature range 280–315°C, there is a threshold gas pressure below which the melt does not flow. Partial crystallization may occur in the flowing melt. Its effect on the melt flow rate grows as the holding time at 270–320°C increases. The data obtained can be used to choose conditions for producing As₂Se₃ optical fibers by the crucible method.     

Mass transfer in the leaching of a solid from a porous sphere

We derive a sequence of approximate solutions of the problem of the kinetics of the extraction of a solid material uniformly filling the volume of a porous particle when it interacts with a liquid which selectively dissolves this material.     

Copper leaching from a sandy soil: mechanism and parameters affecting EDTA extraction

A series of 24h batch tests of copper extraction from a sandy soil were performed by washing the soil with aqueous solution of ethylenediaminetetraacetic acid, EDTA. EDTA versus copper molar ratio were in the range between 1 (equimolar tests) and 13.3. The tests were performed at three selected liquid/solid ratio, 5, 12.5 and 25. Results show that decreasing the pH of the washing solution an higher copper extraction yield was achieved: an almost complete copper extraction was achieved after 23 h of mixing at a L/S = 5, and after 5 h of mixing at an L/S = 12.5. The mechanism of copper extraction was found to involve two sequential processes: the former dissolution of metal salts, that lead to an initial high concentration of both copper and some competitive cations (essentially Ca(2+)), and the following EDTA exchange reaction between calcium and copper complexes, which corresponded an increase of pH in the washing solution. A negligible extraction of Fe(3+) was also observed at the investigated operative conditions.     

Superconductor stability, 1983: a review

Superconductor stability, as a characteristic of steady states, is reviewed. Reference is made to internally cooled superconductors, metastable magnets and superfluid helium.     

Noise assessment of a Fourier transform infrared spectroradiometer subject to the stability of a conventional laboratory blackbody source

A method is described for the measurement of the noise-equivalent spectral radiance (NESR) of Fourier transform infrared (FTIR) spectroradiometers at all wave numbers of a selected range. The method requires minimal detailed knowledge of the sensor and no support equipment beyond a blackbody source. The NESRs of the FTIR spectroradiometer are determined at every wave-number increment in the 700-1300 cm(-1) range, for six resolutions, with a conventional blackbody source and ensembles of differential spectra. The NESRs are well behaved and consistent with the expected dependence on resolution; however, they depend on source temperature at the highest (1 cm(-1)) and lowest (32 cm(-1)) resolutions, with little or no statistical dependence at intermediate resolutions. Residual source drift is shown to be the likely cause of the dependence at 1 cm(-1); the dependence on the source at 32 cm(-1) resolution is shown to be most probably due to photon noise. At intermediate resolutions the sensor noise is dominant.     

Flow of a Viscoplastic Arsenic Selenide Melt in Circular-Cylindrical Channels

The flow rate of molten arsenic selenide in circular-cylindrical channels was measured as a function of temperature and excess gas pressure over the melt with application to the fabrication of optical fibers by crucible methods. The channel diameter was 3.0, 4.2, and 4.5 mm, and the channel length was 120 mm. The temperature was varied from 285 to 320°C, and the excess pressure of the inert gas was up to 1.5 × 10⁵ Pa. The results demonstrate that there is a threshold pressure for melt flow, suggesting that the melt is a viscoplastic fluid. The experimental data are interpreted in terms of the Shvedov-Bingham rheological model.     

Strain stability in a material

A parameter is proposed for describing the state of a material, which is the derivative of the strains with respect to the stresses with unaltered loading conditions and changes only in the material's compliance. It is shown that this parameter is related to the rigidity of the loading system and also to the shape and size of the loaded element. The scope is considered for using this parameter to estimate the strain stability. It is found that the strain loses stability if is numerically equal to or less than the compliance of the material.Translated from Problemy Prochnosti, No. 3, pp. 74–81, March, 1994.     

Chromium behavior during cement-production processes: a clinkerization, hydration, and leaching study

The behavior of chromium during the production of cement clinker, during the hydration of cement and during the leaching of cement mortars was investigated. The microstructures of clinker and mortar properties were investigated using free lime, XRD, SEM/EDS, and TG/DTA techniques. Chromium was found to be incorporated in the clinker phase. The formation of new chromium compounds such as Ca(6)Al(4)Cr(2)O(15), Ca(5)Cr(3)O(12), Ca(5)Cr(2)SiO(12), and CaCr(2)O(7), with chromium oxidation states of +3, +4.6, +5, and +6, respectively, was detected. After the hydration process, additional chromium compounds were identified in the mortar matrix, including Ca(5)(CrO(4))(3)OH, CaCrO(4)·2H(2)O, and Al(2)(OH)(4)CrO(4), with chromium oxidation states of +4.6, +6, and +6, respectively. Additionally, some species of chromium, such as Cr(3+) from Ca(6)Al(4)Cr(2)O(15) and Cr(6+) from CaCr(2)O(7), CaCrO(4)·2H(2)O, and Al(2)(OH)(4)CrO(4), were leached during leaching tests, whereas other species remained in the mortar. The concentrations of chromium that leached from the mortar following U.S. EPA Method 1311 and EA NEN 7375:2004 leaching tests were higher than limits set by the U.S. EPA and the Environment Agency of England and Wales related to hazardous waste disposal in landfills. Thus, waste containing chromium should not be allowed to mix with raw materials in the cement manufacturing process.     

The absence of a mixed-alkali effect in the leaching of corrosion-resistant glasses

Alkali depth profiles in water-leached glasses with molar composition 16A₂O·8CaO· 2MgO·1Al₂O₃·73SiO₂ (A=Na, K and Na+K) were determined using secondary ion mass spectroscopy (SIMS). In the glass containing only sodium an apparent correlation exists between the alkali diffusion coefficient derived from the leaching kinetics, assuming this process is governed by ion exchange, and that determined via electrical conductivity measurements in the dry glass. In the other two glasses the diffusion coefficients derived on the basis of ion exchange are much higher than the ones found in the dry glasses. These data can be interpreted in terms of a model for the leaching of glass where the rate-determining step is the diffusion of molecular water.