175 research outputs found
Shrinkage characterization of alkali-activated slag/fly-ash blends
No full textAlkali-activated materials offer the potential for more durable, sustainable and low-CO2 construction and building materials with reduced environmental footprints when compared to Portland cement concrete. However, this new concrete technology suffers substantially from early-age autogenous shrinkage and micro cracking. The aim of this work is to illuminate the intrinsic reasons that are responsible for larger autogenous shrinkage in alkali-activated slag/fly-ash (AASF) blends by understanding the essential link between solidification process (reaction mechanism, kinetics, phase formations and binder structures) and early-age autogenous shrinkage deformations.
In this study, six different compositions of AASF are studied by varying the type and the concentration of three different activators: sodium metasilicate, combinations of sodium metasilicate with sodium carbonate and sodium sulfate . The results show that the solidification process strongly depends on both the alkalinity and anion type of the alkaline solution. A higher alkalinity (high amount of Na2O) accelerates the reaction process, as the presence of OH- ions enhances the dissolution of slag and also increases the solubility of silica and alumina. Therefore, the intense autogenous shrinkage of alkali-activated slag at early-ages can be attributed to the high amount of chemical shrinkage. Autogenous shrinkage of alkali-activated slag is not only caused by well-known self-desiccation process in hardened state, but related to the condensation shrinkage. The larger autogenous shrinkage in AAS may also be attributed to refined pore structure and silica polymerization, which is controlled by the nature of anion presence in the solution.status: Publishe
The kinetic effect of carbonic anhydrase on CaCO3 phase transformations during carbonation of Ca(OH)2 solutions
No full textstatus: Publishe
Effects of Alkali Activation and CO<inf>2</inf> Curing on the Hydraulic Reactivity and Carbon Storage Capacity of BOF Slag in View of Its Use in Concrete
No full text© 2019, Springer Nature B.V. This work investigates the sequential application of alkali activation and CO2 curing to BOF steel slag as a technique for improving its hydraulic reactivity for use in concrete, while also exploiting its potential as a carbon sink. Activation with either a sodium hydroxide/sodium-silicate or a sodium hydroxide/sodium carbonate solution was first evaluated in a preliminary calorimetric study for selecting the solution compositions leading to the formation of early stage hydration products. The pastes produced with the selected solutions were then cured either in a humidity chamber or in a carbonation chamber (at 20 or 50 °C) for up to 28 days, in order to assess long term reaction products. Mineralogical and thermal analysis showed the formation of a C–S–H like phase, specifically in the samples activated by the sodium hydroxide/sodium-silicate solution, whereas significant occurrence of gaylussite was noticed in the samples activated with the sodium hydroxide/sodium carbonate solution. A maximum CO2 uptake of 6% by wt, due to calcium carbonate formation, was observed in the latter samples, whereas a 5% value was achieved in the former ones. The compressive strength of the mortars prepared with sodium hydroxide/sodium silicate and cured in the carbonation chamber at 50 °C was above 2 MPa, while it was lower for the other samples, particularly those activated with sodium hydroxide/sodium carbonate. Alkali activation employing sodium hydroxide/sodium silicate solutions followed by CO2 curing at relatively high temperature (i.e. 50 °C) resulted a promising treatment for BOF slag valorization in the manufacturing of concrete for non structural applications.status: Publishe
Characterization of protein-flavor interactions using inverse gas chromatography
No full textIn this research, we investigated the retention/release mechanism of selected flavor compounds on or from protein matrices by establishing quantitative design principles for these interactions. Thermodynamic parameters (partition coefficient Kp, free energy of adsorption ∆Gs and the enthalpy of adsorption ∆Hs) of the interaction between selected flavor compounds (hexane, hexanal, hexanol and d-limonene) and protein systems (soy protein isolate and zein) were determined by using inverse gas chromatography under different temperatures and relatively humid conditions. The inverse gas chromatography system was fitted with an additional humidification system that could maintain the relative humidity of the carrier gas, thus enabling the evaluation of the effect of relative humidity on the measured quantities. Increasing temperature and relative humidity led to less favorable interaction between selected flavors and proteins. Flavor retention at high relative humidity was less than at low relative humidity or at dry conditions. This suggests that flavor compounds and water molecules might be competing to bind to the available sides of the protein. Quantitative characterization of the mechanism and thermodynamics of flavor binding and release in protein matrices will benefit the food industry to efficiently develop flavored foods.M.S.Includes bibliographical referencesby Ozlem Dol
Risk-averse control of undiscounted transient Markov models
No full textThe classical optimal control problems for discrete-time, transient Markov processes are infinite horizon, undiscounted expected total cost or reward models. Some examples of these models are optimal stopping problems and stochastic shortest or longest path problems, which may have applications in health-care, finance, and maintenance. However, such expected value models implicitly assume the decision maker is risk-neutral, so they may not be appropriate for several real-life problems. In this study, we use Markov risk measures to formulate a risk-averse version of the optimal control problem for transient Markov processes with general state and compact control spaces. We derive risk-averse dynamic programming equations and show that they have a unique solution which is also the optimal value of the Markov control problem. Furthermore, it is shown that a randomized policy may be strictly better than deterministic policies, when risk measures are employed. We suggest two algorithms, value iteration and policy iteration methods, for solving the dynamic programming equations and show their convergence. In general, each policy evaluation step of the policy iteration algorithm requires solving a system of nonsmooth equations. We use a version of nonsmooth Newton method to solve these equations and show its global convergence. We further consider a risk-averse finite horizon Markov control problem under randomized policies and derive a value iteration method for its solution. Finally, we work on asset selling, organ transplant, and credit card examples to illustrate the theory for infinite horizon problem, and present numerical results.Ph. D.Includes bibliographical referencesIncludes vitaby Ozlem Cavu
Modeling traveler behavior via day-to-day learning dynamics
No full textTravel behavior lies at the core of analysis and evaluation of transportation related measures aiming to improve urban mobility, environmental quality and a wide variety of social objectives. A better understanding of travel behavior will improve travel demand forecasting and the assessment of emerging transport policies, and will improve our means to increase road safety. The day-to-day models reflect the travelers’ learning and forecasting mechanisms. These models predict travelers’ choices for any given day based on their experienced choices in the previous days. Day-to-day approaches allow the use of wide range of behavioral rules, and levels of aggregation, and capture the heterogeneity in users’ learning and adaptation processes, and behavioral characteristics. This thesis aims to develop a novel framework to model the interdependence between travelers’ choice decisions, learning and adaptation behavior and the day-to-day update mechanism of traffic flows. The novelty of this thesis is that the proposed approach combines traveler heterogeneity and rationality in a single framework to predict travelers’ day-to-day departure time and route decisions, and develops a novel day-to-day dynamic traffic assignment approach. The empirical results obtained from real transportation network, New Jersey Turnpike, confirm that the proposed day-to-day learning and dynamic traffic assignment framework model can successfully capture the significant learning dynamics, demonstrating the possibility of developing a psychological framework (i.e., learning models) as a viable approach to represent travel behavior. The other contributions of this thesis include a novel route choice set generation approach based on stochastic integer programming approach. The proposed methodology takes into account travel time variability and reliability in the transportation network. The path relevance criteria are directly incorporated into the optimization model by minimizing mean travel time, travel time variability and path overlap. Unlike previous approaches in the literature, proposed methodology eliminates the filtering step from the choice set generation and generates paths sets at desired dissimilarity level while minimizing the travel time and variability of these paths. Several case studies show the applicability of the proposed methodology on real transportation networks.Ph.D.Includes bibliographical referencesIncludes vitaby Ozlem Yanmaz-Tuze
Blended cement-lime mortars for conservation purposes: Microstructure and strength development
No full textBlended lime-cement mortars are commonly used in conservation practices even though they may show lack of adequate strength and durability for certain cement-lime compositions. This paper focuses on understanding the hardening reactions and their influence on the strength development, microstructure and porosity for the cement-lime mortars in various compositions. Mortars composed of 30%, 50% and 70% ce-ment replacement with lime hydrate and lime putty by mass were studied. Cement hydration has been found to contribute to the early stage strength development while carbonation is mostly favoured after 3 days and con-tributes to the strength development until 180 days. The degree of carbonation is much more pronounced with increasing lime content and porosity of the mortars and the reaction is still in progress at 90 days. All the blended mortars revealed lower compressive and flexural strength than that of the reference cement mortar due to their lower cement content and higher porosity. Long-term compressive strength development has been achieved after 180 days while flexural strength for certain mortar compositions does not increase beyond 28 days. Unlike cement mortar, the mortars blended with lime hydrate and lime putty exhibit an elastic-plastic deformation before failure occurs, which is preferred for repair mortars to adapt to differential settlements and to allow more deformation under critical stresses in the masonry.sponsorship: This study is part of an ongoing research project (OT/04/27 3E030765) conducted at the Department of Civil Engineering at the K.U. Leuven. The research grant offered to Ozlem Cizer by the university is gratefully acknowledged. Department of Metallurgy and Materials Engineering (MTM) at the K.U.Leuven is gratefully acknowledged for the use of the Scanning Electron Microscope. (Department of Civil Engineering at the K.U. Leuven|OT/04/27 3E030765)status: Publishe
Carbonation and hydration of mortars with calcium hydroxide and calcium silicate binders
No full textHardening of calcium hydroxide and calcium silicate binders composed of cement, rice husk ash (RHA) and lime in different compositions were studied with mortars using thermal analysis, mechanical strength and SEM. When cement is partially replaced with RHA and lime, hardening occurs as a result of combined hydration, pozzolanic reaction and carbonation reaction. While hydration of cement contributes to the early strength development of the mortars, carbonation is much more pronounced at later stage with the decrease in the cement content and increase in the porosity of the mortars. RHA-cement mortars indicated a long-term strength development, which is lower than that of the reference cement mortar. This was attributed to their higher porosity due to the high water demand of the porous RHA grains. Strength reduction was recorded at the very early stage for RHA-cement-lime mortars containing 10%-wt cement as well as RHA-lime mortars. This has been explained with the insufficient cement content, destructive effect of the calcium carbonate phases on the initially hydrated cement phases and partial carbonation of the initially hydrated phases. This decrease can also be related to the presence of excess RHA in the matrix as excess silica can react with the initially formed C-S-H phase, which can lead to the formation of polymerized C-S-H phase.sponsorship: This study is the part of an ongoing Research Project (OT/04 3E030765) conducted at the Department of Civil Engineering and a Master thesis research "Performance Evaluation of Pozzolanic Rice Husk Ash Binder in Tanzania" by Jan Campforts at the K.U.Leuven. The research grant offered to Ozlem Cizer by the OT/04 project funded by the K.U.Leuven is gratefully acknowledged. (Jan Campforts at the K.U.Leuven|OT/04 3E030765, K.U.Leuven |OT/04 )status: Publishe
Competition between Carbonation and Hydration on the Hardening of Calcium Hydroxide and Calcium Silicate Binders
No full textLime-based mortars composed of calcium silicate binders harden as a resu lt of a combined reaction of hydration and carbonation. Understanding th is combined reaction mechanism and the consequences on the hardened mort ar properties are essential while assessing how they affect the overall durability of historic masonry. In this research, this combined reaction is studied in mortars with lime-pozzolana, hydraulic lime and cement-li me binders, and the consequences on the physical properties, mechanical properties and microstructure of the mortars are discussed. In-depth res earch has been performed on the hydration and carbonation reactions to p rovide fundamental insight on the reaction mechanisms. Carbonation reaction of calcium hydroxide proceeds as initial carbonatio n at the early stage and increased carbonation at the later stage. Durin g initial carbonation, CO2 molecules are rapidly absorbed by the alkalin e water, resulting in the precipitation of calcite crystals instantly. T his phase involves a dormant period in which the reaction controlling fa ctor is the dissolution rate of calcium hydroxide and CO2 in the pore wa ter. Increased carbonation starts once open pore space is created with t he evaporation of the water, allowing diffusion paths for CO2 through th e pore structure. Therefore, at later stage diffusion of CO2 is the rate controlling factor in the carbonation process. CO2 concentration, relat ive humidity conditions and physical properties of the lime particles ar e found to have an important impact on the habit, size and morphology of the calcite by altering calcium and carbonate ions concentration in the pore water. The consequence of this on the mechanical properties of the mortar is negligible. Hydration reactions are the first reaction order in hydraulic lime and l ime-pozzolana mortars, which proceed more gradually when compared with f ast cement hydration. Carbonation is the complementary reaction in the s trength gain of the mortars and starts once pore structure is formed to allow CO2 diffusion. Competitive reaction order between hydration and ca rbonation can occur in lime-pozzolana mortars depending on the pozzolani c reactivity of the pozzolana. As a consequence of the consumption of th e calcium hydroxide by the carbonation reaction, the degree of the pozzo lanic reactions is lowered. The consequence of this on the mechanical pr operties is remarkable while the same impact is not observed on the poro sity. The degree and the order of hydration and carbonation reactions ar e strongly influenced by the moisture content. Hydration reactions are p romoted under moist conditions while carbonation reaction is retarded. H ydraulic lime and lime-pozzolana mortars require at least 28-day moist c uring to improve the hydration reactions and to ensure sufficient streng th development. In cement-lime mortars, an interaction has been found between the unhydr ated or hydrated calcium aluminates and calcium carbonate originating fr om the addition of lime. Complementary effect of the carbonation reactio n of free lime on the strength development of these mortars is more pron ounced than the lime mortars. Competitive reaction order between cement hydration and carbonation of lime does not occur under atmospheric condi tions because the fast cement hydration is always the first-reaction ord er building up a sound hydrated microstructure. It is the CO2-rich condi tions under which carbonation prevails without negatively affecting the mechanical properties of cement-lime mortars despite decalcification of the hydrated phases.status: Publishe
In situ monitoring of mineral waste carbonation under high CO₂ pressure
No full textThe treatment of fine grained steel slags with CO2 at elevated pressure and temperature conditions is a process that allows to produce high quality building materials. In this process the mineral waste is stabilized and CO2 is permanently sequestered in the form of carbonates. However, in order to enhance the sequestration potential and further improve the quality of the building materials, it is essential to understand the carbonate formation and the impact of this process on the pore network in the building material. A better understanding of the process will translate in improved building materials and enhanced CO2 sequestration. In this study, the process of carbonate formation stainless steel slags is investigated. The carbonate formation in the pore network of the fine grained steel slags is monitored during CO2 exposure in a specially designed mini reactor using high resolution X-ray tomography (HRXCT). The use of this non-destructive technique allows to visualize and quantify the amount carbonate that is precipitated inside the building material. The technique also provides a 3D representation of the pore network from which the porosity and permeability reduction through time is deducted. This new approach using HRXCT provides new insights in the carbonation process and provides better input parameters for modelling of carbonation models
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