1,720,984 research outputs found
Transport in concrete with new CO<sub>2</sub> reduced cements - Reactive Transport Model for Durability Estimations
Full text linkConcrete is the most used building material in the world due to its cheap price and desired properties such as strength, workability and durability. Concrete is a composite material obtained by mixing cement, water, aggregates and small amounts of chemical additives. Following the mixing, a (fast) chemical process called hydration is initiated in which the different cement clinkers react with water and form different hydration products, such as calciumsilicate-hydrate (C-S-H), ettringite and monosulfate. The hydration process can continue slowly over the lifetime of concrete structures and is influenced by the so-called service environment, which is the environment surrounding the concrete. In aggressive service environments, for instance, in the case of marine exposure, the altering of hydrated cement can lead to the end of its service life. Most of the altering processes are related to moisture and gas transport within the pores of the concrete. This thesis was part of a project aiming to provide the basis for a green conversion of cement and concrete production in Denmark. The thesis contributes to a better understanding of long-term consequences of implementing new CO2-reduced cement in concrete through experimental investigations and reactive mass transport modeling. CO2-reduced cement refer to bindersystems where amounts of the Portland cement is replaced with supplementary cementitious materials such as calcined clay and limestone filler, in order to decrease the CO2 emission from the production of concrete. Three major areas of focus related to moisture and gas transport in cement-based systems were covered.First focus area was the explanation of vapor transport through a partially saturated capillary pore. In unsaturated porous materials, the presence of trapped fluid in a capillary pore increase the overall rate of vapor transport rather than blocking it. This is simply due to the fact that the trapped liquid can be considered as a bridge shortening of the apparent distance the vapor needs to travel through the pore system. A simple approach was proposed describing the process with a single equation using the gradient of the chemical potential as the driving force for diffusion, in this case, equal to the gradient of the relative humidity. The model was established without the use of fitting parameters. The proposed method was compared with, experimental results for isothermal vapor transport through a partially saturated cylindrically symmetric capillary tube of variable cross-sectional area, with excellent agreement. The study concluded that the enhancement of vapor transport in the presence of trapped pore liquid can be modeled using the geometry and the relative humidity at the gas-liquid interface. It is nearly impossible to directly apply this model to cement-based materials since it is not possible to accurately describe the geometry and boundary conditions at each gas-liquid interface, due to the complex microstructure of the pores of hydrated cement. However, the study provided insight into one important pore-scale vapor transport mechanism in unsaturated systems. Furthermore, the study also demonstrated the conceptual advantage of using the chemical potential in modeling vapor transport at the pore-scale. Secondly, liquid water and water vapor transport properties relevant to concrete durability modeling were investigated. Moisture properties of ten different CO2-reduced cement-based binder-systems containing supplementary cementitious material in the form of fly ash, calcined clay, burnt shale and gray micro-filler, where investigated using four different experiments being, (i) sorption tests (moisture fixation), (ii) cup tests in two different relative humidity intervals, (iii) drying tests, and, (iv) capillary suction tests. An inverse approach was developed for determining a separate description of the effective diffusion in the liquid phase and in the vapor phase as a function of the saturation degree, relevant for the proposed continuum moisture transport model adopted. The inverse approach in all essential parts uses the measured mass change over time, as obtained from all the experimental investigations, to inversely obtain the effective diffusion parameters. Due to the use of multiple diffusion experiments both in absorption and desorption, covering different relative humidity intervals, the moisture properties obtained with the proposed inverse analyses method provide a good description valid for a wide range of cases of the moisture transport for the ten different binder-systems. The proposed method does not explicitly account for the effect of pore structural change over time, but it provides a good description of moisture properties at the conditions the samples were tested at. Accurate models for moisture transport is crucial for more involved durability models also including ionic and gas transport features.Thirdly, a multi-phase reactive mass transport framework for durability estimation of cement-based materials was developed. The goal was to include the gas phase alongside the implemented liquid phase and solid phase developed in earlier works. The addition of the gas phase into an earlier developed framework includes the description of gaseous transport in the air-filled space and chemical gas-liquid interaction. The governing system of equations includes a modied version of the Poisson-Nernst-Planck system of equations including gaseous transport in the air-filled space, ionic transport in the liquid phase, electro-migration of ionic species, two-phase moisture transport consisting of water vapor and liquid water, and sorption. A stringent model for mass transport also including for gaseous constitutes contributes to a better understanding of the true mechanisms responsible for the altering of hydrated cement-based materials in unsaturated conditions as well as in cyclic drying-wetting conditions. Furthermore, the inclusion of gaseous constituents into the framework enhances the model's accuracy and enables the investigation of combined effects of different degradation processes acting simultaneously, for example, carbonation and chloride ingress. Different numerical examples are included demonstrating applications of the above mentioned added futures
A coupled phase-field and reactive-transport framework for fracture propagation in poroelastic media
No full textWe present a novel approach to model hydro-chemo-mechanical responses in rock formations subject to fracture propagation within chemically active rock formations
Kinec_v3: A computer database for the calculation of the dissolution rates of the major rock forming minerals
No full textThe Kinec_v3 database allows the calculation of the dissolution rates of 133 major rock forming minerals in aqueous solutions at temperatures from 0 to 300 °C, pH from 0 to 14 and mineral saturation states from far-from-equilibrium to equilibrium. Calculated rates are generated using equations and parameters reported by Hermanska et al. (2022,2023) and Oelkers and Addassi (2025). The database has been created to be used in conjunction with the PHREEQC, but can be adapted for use in other computer algorithms that take into account fluid-mineral equilibria, mineral reactions and chemical transport. The Kinec_v3 database also allows for calculation of mineral precipitation rates consistent with dissolution rates using transition state theory based rate equations, yet such calculations are largely unverified at present, due to the dearth of available experimental measurements. The Kinec_v3 database is available to download fromhttps://github.com/Mou1a/Kinect_V3
Reactive mass transport in concrete including for gaseous constituents using a two-phase moisture transport approach
No full textThis article presents the further development of a tool for multi-phase reactive mass transport modeling for durability estimation of cement-based materials, by the addition of the gas phase, adopting a truly separate two-phase moisture transport approach. The governing system of equations are based on physically sound hybrid mixture based version of the Poisson-Nernst-Planck system of equations including gaseous transport in the air-filled space, ionic transport in the liquid phase, electro-migration of ionic species, a two-phase moisture transport model, and sorption. The addition of the gas phase and the two-phase moisture description enables the user of the model to investigate individual and combined effects of different degradation processes in unsaturated systems. The altering of hydrated cement under three different environments were studied representing an accelerated carbonation environment, a submerged marine environment and a cyclic drying-wetting zone in a marine environment, to illustrate some of the model’s capabilities
A comprehensive and consistent mineral dissolution rate database: Part III: Non-silicate minerals including carbonate, sulfate, phosphate, halide, and oxy-hydroxide minerals
No full textThis paper describes the creation of a consistent database of the far-from-equilibrium dissolution rates in aqueous fluids of the major non-silicate minerals including carbonates, sulfates, phosphates, oxides and hydroxides. This work, based on the regression of available dissolution rate data normalized to their BET surface area as a function of pH and temperature, was performed using equations described in Heřmanská et al. (2022, 2023). For carbonate minerals, an additional term was added to account for the slowing of rates in response to increasing aqueous bicarbonate and carbonate concentration. All rate equations have been incorporated into a computer accessible database allowing for the direct application of these rates to assess the temporal evolution of chemical mass transfer in laboratory and field-based systems. It is anticipated that such applications will lead to further advances in our understanding the reaction rates of individual minerals and of natural water-rock interaction in the future.We are grateful to Martin Voigt, Chiari Mareni, Julien Declercq and Matylda Heřmanská for their enthusiasm and aid on completing the first two parts of this series, and to Hussein Hoteit for hosting EHO at KAUST during part of the completion of this project. We are indebted to David Parkhurst and Tony Appelo for creating and making available to all free of charge the PHREEQC computer code. It is only through the use of codes like these that our community can apply mineral-water reaction rates to understanding complex geochemical processes. We would also thank Sigurdur R. Gislason for constant encouragement and support during the creation of this series of manuscripts. This manuscript has been created in honor of Jacques Schott. Jacques, is not only a pioneer in the field of mineral-fluid reaction rates, but also has spent a lifetime introducing younger scientists, including EHO into rigorous experimental methods and the interpretation of these results. EHO owes a debt of gratitude to Professor Schott and his patience and understanding
Pore-Scale Assessment of CO2 Solubility and Capillarity Trapping
No full textCarbon Capture and Storage (CCS) has emerged as a pivotal technology in mitigating the escalating concern over anthropogenic CO2 emissions. This study undertakes a pore-scale investigation into the efficacy of CCS, emphasizing CO2 capillary and solubility trapping mechanisms within geological formations. Utilizing dynamic pore-scale imaging via x-ray micro-computed tomography (micro-CT), we examine the behavior of gaseous CO2 injected into water-saturated Idaho Gray sandstone under various flow rates. Our methodology involves a CT scan imaging through a flow cell, enabling precise control and observation of CO2 behavior during injection and imbibition processes. The research delineates the influence of injection rates on capillary trapping, revealing an optimal medium flow rate that maximizes CO2 retention within the pore network. Additionally, the study delves into the solubility dynamics of CO2 in response to pressure fluctuations, simulating real-world conditions of subsurface CO2 storage. Key findings suggest that injection rates play a critical role in trapping efficiency, with a 33.96% trapping efficiency observed at an intermediate rate of 0.05 ml/min. Moreover, the study captures the temporal evolution of CO2 solubility and exsolution, providing valuable insights into the long-term stability of sequestered CO2. By enhancing the understanding of CO2 phase behavior under varying reservoir conditions, this research offers significant contributions to the optimization of CCS strategies, with implications for both environmental sustainability and energy resource management.<br
The impact of secondary silicate mineral precipitation kinetics on CO2 mineral storage
No full textInternational audienceThe rates of subsurface mineral carbonation are commonly considered to be limited by the dissolution rates of the silicate minerals originally present in the target reservoir rocks. Nevertheless, the rates of secondary silicate precipitation can influence this rate by changing the fluid phase composition during reactive rock-CO2-water interaction. The degree to which secondary silicate mineral precipitation rates influence the extent and efficiency of mineral carbonation in the subsurface is explored via a suite of geochemical modeling calculations. Calculations were performed using the PHREEQC computer code either by assuming local equilibrium for secondary silicate minerals or calculating their precipitation rates using Transition State Theory-based equations. Calculated carbonation rates of fresh basaltic glass are found to be slower initially when accounting for the sluggish precipitation rates of secondary silicates, including clay minerals and zeolites. These slower rates are due to higher calculated aqueous aluminum concentration, which slows the dissolution rates of the primary basaltic glass. The slower precipitation rates of secondary aluminosilicate minerals, however, may result in less flow path clogging, leading to an overall larger total mineral storage capacity over time. In contrast, the sluggish precipitation rates of secondary silicate minerals accelerate significantly the carbonation rate of altered basalts as a larger percentage of the liberated divalent metals are available for carbonate mineral precipitation. Taken together these results illustrate the importance of considering the rates of secondary silicate precipitation rates to accurately predict the rate and extent of subsurface mineral carbonation efforts
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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