Concrete Leaching Research Articles

TRASH OR TREASURE?: Putting Coal Combustion Waste to Work

Even as public debate rages over the question of whether coal should continue to provide the majority of U.S. electric power needs, the U.S. Energy Information Administration predicts in International Energy Outlook 2009 that, absent new policies to the contrary, the United States—along with China and India—is expected to account for 88% of the projected net increase in coal consumption between 2006 and 2030. Meanwhile, coal combustion waste (CCW)—the noncombustible remains from coal burning—continues to pile up at the rate of about 131 million tons per year in the United States alone, and electric utilities are looking to recycle a larger proportion of this material.

PIXE/RBS as a tool to study cementitious materials: Application to the dynamic leaching of concrete

The suitability of the application of Ion Beam Analysis (IBA) techniques such as Particle Induced X-ray Emission spectrometry (PIXE) and Rutherford Backscattering Spectrometry (RBS) to elemental depth profiling in concrete is analysed, studying hardened samples of concrete prior to and after a dynamic leaching test. A calibration of the data has been done by comparing the results obtained by IBA techniques with those obtained by Chemical and Thermogravimetric Analysis (TG/DTA). From PIXE and RBS data relevant information about the migration of minor elements, within the concrete matrix after leaching is obtained.

Stability of phyllosilicates in Ca(OH)2 solution: Influence of layer nature, octahedral occupation, presence of tetrahedral Al and degree of crystallinity

This paper presents the results of a comprehensive investigation of the interaction of layered silicates with Ca(OH)2 in hydrothermal conditions. The study is intended to evaluate the stability of the clay buffer in radioactive waste repositories, at the intermediate stages of concrete leaching, when the pH is controlled by the dissolution of portlandite. The influence of layer nature, octahedral occupation, presence of tetrahedral Al and degree of crystallinity will be assessed by analysing the behaviour of a set of well-selected phyllosilicates and using the combined capabilities of 29Si and 27Al MAS-NMR spectroscopy, powder X-ray diffraction and SEM/EDX. The results show that the main factor affecting the stability of the clay is the octahedral occupation, so that trioctahedral phyllosilicates are much more stable than dioctahedral ones. The nature and expandability of the layer does not seem to much influence the stability of the clay, so that a 2:1 expandable phyllosilicate shows the same stability as a chemically analogous 1:1 non-expandable phyllosilicate. However other factors like the poor crystallinity of the starting material or the presence of Al in the tetrahedral sheet of trioctahedral phyllosilicates weaken the clay structure in alkaline conditions and favour the transformation towards other phases.

Modelling and simulation of concrete leaching under outdoor exposure conditions

Recently, a demand regarding the assessment of release of dangerous substances from construction products was raised by European Commission which has issued the Mandate M/366 addressed to CEN. This action is in relation with the Essential Requirement No. 3 “Hygiene, Health and Environment” of the Construction Products Directive (89/106/EC). The potential hazard for environment and health may arise in different life cycle stages of a construction product. During the service life stage, the release of substances due to contact with the rain water is the main potential hazard source, as a consequence of the leaching phenomenon. The objective of this paper is to present the development of a coupled chemical-transport model for the case of a concrete based construction product, i.e. concrete paving slabs, exposed to rain water under outdoor exposure conditions. The development of the model is based on an iterative process of comparing the experimental results with the simulated results up to an acceptable fit. The experiments were conducted at laboratory scale (equilibrium and dynamic leaching tests) and field scale. The product was exposed for one year in two types of leaching scenarios under outdoor conditions, “runoff” and “stagnation”, and the element release was monitored. The model was calibrated using the experimental data obtained at laboratory scale and validated against measured field data, by taking into account the specific rain water balance and the atmospheric CO 2 uptake as input parameters. The numerical tool used in order to model and simulate the leaching behaviour was PHREEQC, coupled with the Lawrence Livermore National Laboratory (LLNL) thermodynamic data base. The simulation results are satisfying and the paper demonstrates the feasibility of the modelling approach for the leaching behaviour assessment of concrete type construction materials.

Physico-mechanical characteristics and mechanism of concrete leaching in cracked zone of reinforced-concrete penstock linings at the Sayano-Shushenskaya HPP

Results are presented for investigations of strength and deformation characteristics of the monolithic concrete and concrete in the cracked zone of the linings, the temperature_moisture-content regime of the linings, and the condition of the reinforcement. The causes and mechanism of leaching of cement stone from the concrete in the cracked zones are outlined; and, recommendations are given for diminishing the negative effect of the leaching.

Coupling between leaching and creep of concrete

In a radioactive waste disposal, concrete containment structures must be studied over extended periods during which it is necessary to account for a possible degradation by calcium leaching due to on-site water. An experimental investigation is described where the effects of an accelerated calcium leaching process of concrete on creep of concrete are highlighted. The comparison with a creep test where the sample is immersed in water shows that leaching generates tertiary creep and rupture of the specimen. A Dirichlet series coupled to a mechanical damage are used to model the coupled tertiary creep. With this method we can evaluate the lifetime of concrete structures subjected to chemical and mechanical loading.

Leaching of rock-concrete interfaces

It has been shown that contacts between host rock and engineered barriers may be critical in the design of deep radioactive waste repositories. Water is expected to reach the interface zone after the resaturation of the geological massive and its presence may lead to concrete leaching. Such a phenomenon could increase the interface transmissivity and compromise the confinement of radioactive waste. This paper intends to investigate the influence of concrete leaching on the hydromechanical behaviour of host rock-concrete interfaces. Some concrete specimens have been subjected to an accelerated leaching process using ammonium nitrate. The hydromechanical response of degraded concrete-rock interfaces has been studied under shearing and compared to that of sound interfaces. Consistent with the results available in the literature on bulk concrete, a loss of mechanical strength has been observed for the degraded interface. Unlike the sound specimens, the degraded interfaces do not dilate when sheared and they tend to be closed, thereby preventing water from flowing.

Concrete mechanical behaviour and calcium leaching weak coupling

Concrete mechanical behaviour and calcium leaching weak coupling

Modelling of radionuclide release from a concrete container

This study presents numerical simulations of radionuclides (RN) release from a concrete container performed with a chemo-transport model, which describes simultaneously radionuclides (RN) migration in concrete and concrete leaching. The mathematical description of leaching is based on the generalized transport equation of calcium in concrete. The equation is solved numerically using the CAST3M code. 2D simulations of concrete leaching with the possibility to account for the influence of cracking have been performed. A second transport equation has also been implemented in the CAST3M code to determine the extent of radionuclide diffusion in (partially) leached concrete. The radioactive decay and the retention of the RN in concrete are taken into account. Results show that leaching is particularly important for RN having long decay periods. The crack depth appears to be the most important geometric parameter (compared to the crack opening and the crack width) with respect to RN diffusion.

Concrete mechanical behaviour and calcium leaching weak coupling

ABSTRACT A rheological model with orthotropic short-time and long-time damage is developed in order to model leached concrete behaviour. Coupling with calcium leaching is obtained by affine attenuation of model parameters according to a microhardness profile measured on leached cement pastes. Calibration shows that most model parameters managing mechanical behaviour have the microhardness law as their attenuation law.

Chemomechanics of concrete at finer scales

Concrete, like many other materials (whether man-made, geological or biological), is a highly heterogeneous material with heterogeneities that manifest themselves at multiple scales. As new experimental techniques such as nanoindentation have provided unprecedented access to micro-mechanical properties of materials, it becomes possible to identify the mechanical effects of chemical reactions at the micro-scale, where the reactions occur, and trace these micro-chemo-mechanical effects through upscaling techniques to the macro-scale. The focus of this paper is to review recent developments of a microchemomechanics theory which ultimately shall make it possible to capture chemomechanical deterioration processes at the scale where physical chemistry meets mechanics. This is illustrated through application of the theory to early-age concrete and calcium leaching, and an outlook to biologically mediated deterioration processes in solid materials is given.

Modification of the Tournemire Argilite in Contact with Simplified Concrete Fluid

ABSTRACTArgillaceous rocks are considered as potential host rocks for radioactive waste repositories. The design of an underground disposal often includes large amounts of concrete which are proposed as waste matrix, backfill material and structural components. The leaching of concrete by groundwater produces alkaline solutions that will react with the host rock inducing modifications of the geological barrier performances. In order to examine the effects of an alkaline plume on the properties of claystone, an experimental program is being developed in our institute involving batch, diffusion and percolation experiments. This paper presents the results obtained with first batch studies performed at 25°C and 70°C over 3 months. The effect of high pH solutions (corresponding to the early period of leaching) on the composition of a claystone (originating from Tournemire - France) was examined. The changes in solutions composition were monitored as a function of temperature and V/m ratio. In spite of the short contact time between solution and solid, some modifications in these two phases were noticed. The interactions between solids and solutions were more pronounced at 70°C than at 25°C, resulting in pH and potassium concentration decrease, while the sodium concentration remained constant. Moreover, the dissolution of mineral phases such as dolomite and kaolinite was proposed.

Reuse of municipal solid wastes incineration fly ashes in concrete mixtures

This study is aimed at assessing the feasibility of concrete production using stabilized m.s.w. (municipal solid waste) incineration fly ashes in addition to natural aggregates. The tested fly ashes were washed and milled, then stabilized by a cement-lime process and finally were reused as a “recycled aggregate” for cement mixture production, in substitution of a natural aggregate (with dosage of 200–400 kg m −3). These mixtures, after curing, were characterized with conventional physical-mechanical tests (compression, traction, flexure, modulus of elasticity, shrinkage). In samples containing 200 kg(waste) m −3(concrete), a good compressive strength was achieved after 28 days of curing. Furthermore, concrete leaching behavior was evaluated by means of different leaching tests, both on milled and on monolithic samples. Experimental results showed a remarkable reduction of metal leaching in comparison with raw waste. In some cases, similar behavior was observed in “natural” concrete (produced with natural aggregates) and in “waste containing” concrete.

Residual design strength of cement-based materials for nuclear waste storage systems

Calcium leaching of concrete may be critical for the mechanical integrity of hazardous waste storage systems, in which cement-based materials are employed as construction material or grouting material. Results from a recent triaxial strength test campaign on calcium depleted cementitious materials highlight that these materials exhibit decohesion and frictional softening and become increasingly pressure sensitive. But the overall emerging picture is one of a material with a residual, finite accountable strength, which is well above either the particulate or the matrix strength. This allows us to propose a ‘safe’ lower bound for calcium depleted cement-based composites, which may serve for the durability performance design of concrete structures subjected to calcium leaching-one critical design scenario of e.g. nuclear waste storage systems.

Water leaching of high and ultra high performance concrete: a nuclear magnetic resonance study

The consequences in terms of microstructure and texture of a prolonged contact between concrete and a continuous flow of mineral water have been investigated here by Nuclear Magnetic Resonance (NMR) because of its non-invasiveness and sensitivity to local environment. In particular, we evidence the dissolution of residual anhydrous cement, which leads to the further precipitation of hydrates occurring over 12 months of leaching tests in High Performance Concrete (HPC) and Ultra High Performance Concrete (UHPC). The study of the longitudinal relaxation of proton magnetization shows that the difference of pore size distribution between these two types of concrete remains mostly in the number of capillary pores. Its evolution with the time of water leaching up to the end of our experiment is not significant. L’étude des conséquences d’un point de vue microstructural et textural d’un contact prolongé entre un béton et de l’eau minérale circulant en système ouvert fait l’objet de ce travail. En raison de la nature hétérogène et multiphasique de ces matériaux, la résonance magnétique nucléaire (RMN) s’impose comme une technique non invasive de choix. Plusieurs formulations de bétons à hautes performances (BHP) et de bétons à ultra hautes performances (BUHP) ont été considérées ici, afin d’identifier l’effet de la quantité et de la réactivité chimique de leurs différents constituants. La consommation partielle du ciment anhydre et la précipitation d’hydrates secondaires ont, entre autres, pu être observées par RNM du silicium 29 Si et de l’aluminium 27 Al. Il ressort de l’étude de la relaxation magnétique nucléaire des protons que les différences de porosité entre les BHP et les BUHP résident essentiellement dans la proportion de pores capillaires et que leur évolution jusqu’à 12 mois d’essai de lixiviation est faible.

Instantaneous and noninstantaneous dissolution: approximation by the finite volume method

This paper is devoted to the study of finite volume schemes for nonlinear dissolution-diffusion equations. The approximate solution is shown to converge to the continuous one, which is also shown to converge in the limit of instantaneous dissolution. The scheme is applied to the interpretation of an experiment of leaching of cement concrete.

Diffusion of radionuclides in concrete and concrete-bentonite systems

Various construction materials are under consideration for nuclear waste repositories. Two important materials are concrete and bentonite clay, which will act as mechanical barriers and prevent convective water flow. These barriers will also retard transport (diffusion controlled) of dissolved radionuclides by a combination of mechanical constraints and chemical interactions with the solid. An important issue is the possible change of the initial sodium bentonite into the calcium form due to interaction with calcium from the concrete. The initial leaching of concrete was studied using radioactive spiked concrete in contact with compacted bentonite. Measurement were made of the diffusion of Cs, Am and Pu into 5 different types of concrete in contact with pore water. The diffusivity measured for Cs agrees reasonably well with data found in the literature. No movement could be measured for Am and Pu (< 0.2 mm), even though the contact times were extremely long (2.5 and 5 yr, respectively). The diffusion of Na, Ca and Cs from concrete into bentonite was also measured.