Due to their great performance and ease of installation, refractory castables are common ground materials to enable high‐temperature processes. However, their fully operational condition is slowed ...down by the gradual drying stage required. Therefore, better understanding of the moisture transport is essential to improve their efficiency and reduce the likelihood of explosive spalling events due to vapor pressurization. Neutron tomography provides a relevant inner view of the moisture distribution across a sample and its evolution over time. In this work, the effect of the heating rate on moisture clog was investigated and compared with available laboratory and industrial observations. It was found out that higher heating rates resulted in a faster and longer lasting water accumulation ahead of the drying front, in agreement with other macroscopic studies and explaining the common reasoning behind using slower heating rates and safer industrial operations. This study highlights the potential of neutron imaging for the ongoing effort to maximize the efficiency of the refractory castables drying process by controlling the moisture accumulation without exclusively relying on slower heating rates.
•Unprecedented numerical convergence study of widely used finite element model for concrete and refractory castable at high temperature.•It was shown that the strategy of the numerical implementation ...of the sorption isotherm does not change dramatically the convergence order (both in time and space).•Sensitivity analysis of the input parameters that helps the end users to decide which properties must be carefully characterized or not.•Such analysis showed that the permeability is the most important quantity, followed by the thermal conductivity.•A new mixed-element formulation is proposed and validated, providing a framework for extreme cases with high heating rates and strong discontinuities on the mesoscale domains properties.
Structural materials are broadly used in applications such as nuclear vessels, high-temperature processes, and civil construction. Usually, during their placing and lifespan, they may present free or chemically bonded liquid phases in their structure, demanding careful attention when exposed to high heating rates. Their behavior in such conditions is a challenging problem as it comprises numerous highly nonlinear properties (not easily measured via experimental tests), strongly coupled equations and unreliable experimental benchmarks. Nonetheless, such simulations are of great interest. This work aims to provide a numerical study, checking whether its solution indeed converges and yields reliable results. Additionally, as the model needs several input parameters, this work conducts a sensitivity analysis and also assesses its applicability to more complex scenarios, as such issues remain open in the literature. In order to do that, a simple model that can be easily adapted for mixed formulations and complex geometries was proposed. It was found out that when considering unidimensional models the choices regarding the interpolation of the sorption isotherms are not essential to the numerical stability of the system. Besides that, the permeability and thermal conductivity of the material are the most important parameters that affect the simulation results of pressure, temperature and evaporable water content profiles. Finally, the 2D mesoscale simulation of concrete with polymeric fibers (based on the mixed formulation of the problem) yielded results that agreed with experimental observations. Thus, the model proposed herein can provide a solid base for future works and also important insights towards simpler methodologies.
Taking into account the increasing interest in calcium aluminate cement (CAC) applications as a biomaterial in the endodontic area, the mechanical strength and phase transformations of a commercial ...cement (Secar 71) containing 15 or 20 wt% CaCO3 were evaluated. The compressive strength, apparent porosity, dimensional linear changes, XRD and TGA tests were carried out to evaluate samples behaviour after immersion in water and maintained at 37 C for 1 to 30 days of curing. According to the results, CAH10 and C2AH8 formation were inhibited in the CaCO3-containing compositions and the presence of the C3A.CaCO3.11H phase led to a significant increase in the cement mechanical strength. Nevertheless, the partial decomposition of this monocarbonate hydrate was detected at 37 C after 1-7 days and the continuous hydration of CA and CA2 also affected the compressive strength behaviour of the samples.
Due to their high corrosion resistance to basic slags, either pre-formed or
in situ spinel (MgAl
2O
4) containing refractory castables are nowadays widely used as steel ladle linings. Nevertheless, ...whereas the pre-formed spinel castables present high volumetric stability and a well-known processing technology, the
in situ spinel castables still require further understanding due to the challenges related to magnesia hydration and their expansive behaviour at high temperatures. Therefore, the objective of this paper is to review the knowledge already available for high-alumina spinel-containing castables (preformed and
in situ) in order to provide a support for novel technological developments in the area. The main variables considered are the spinel content and grain size, the effect of calcium aluminate cement and hydratable alumina on the general castables’ properties, the influence of different alumina and magnesia sources and the silica fume content. Nowadays research subjects, including the use of mineralising compounds, the addition of nano-scaled particles and the evaluation of the effect of expansion under constraint will also be addressed, pointing out alternatives for the design of high-performance alumina-magnesia refractory castables.
•DIC highlights damage growth by means of increased crack density and crack openings for networks on monitored surfaces, whereas traditional techniques only monitor Young's modulus ...changes.•Tomography scans showed that cracks initiate on the surface, and then propagate toward the center of the specimen.•DIC results indicate that after the crack density reaches its maximum value, the crack opening displacements continue to increase as in-depth propagation is still active.
MgO particles are added to high-alumina castables to provide in situ spinel formation at high temperatures. However, the MgO hydration upon curing may damage the material because of localized volumetric expansion. Usually, damage is evaluated by the ex situ overall measurement of Young's modulus changes during processing via Impulse Excitation Techniques. In this paper, an experimental setup was designed to use Digital Image Correlation (DIC) as an in situ alternative to evaluate damage. Tomographic scans highlighted that all cracks initiated on the sample surfaces, and propagated in the bulk in an intergranular mode. Crack initiation and growth were assessed, for different temperatures, via Surface Crack Density (SCD) measurements, and Mean Crack Opening Displacement (MCOD) fields. They provided important insights into heterogeneous expansion phenomena and crack network quantification; for example, the SCD flattened while the overall damage was still increasing. The results attested the usefulness of DIC for in-situ quantification of ceramics cracking during processing.
Magnesia hydration is a key concern in refractory castable processing. The volumetric expansion that follows this reaction can result in cracks or even explosion during the first heating-up. Citric ...acid (CA) and other chelants can significantly reduce MgO hydration rate in aqueous suspensions by forming an insoluble magnesium citrate protective coating on the magnesia particles’ surface. In the present work, the performance of CA as an anti-hydration additive in refractory castables was evaluated by hydration tests, mechanical strength and apparent volumetric expansion (AVE) measurements and thermogravimetry. The results attained have shown that CA effectiveness depends strongly on the amount added and by the interaction with other raw materials in the composition, in particular calcium aluminate cement.
Calcium aluminate cement (CAC) contents higher than 3wt% in refractory castables can have some drawbacks in the various processing steps (mainly drying) and also in their refractoriness when in ...contact with SiO2. The use of colloidal silica as an alternative binder has been studied by many researchers in recent years and recently reports have also explored the use of colloidal alumina for the same purpose. This article reviews the recent developments in nano-bonded refractory castables focusing on the use of colloidal silica or alumina. In the first part of the paper, a comparison of different binding systems for refractory castables is shown. The benefits of replacing CAC or hydratable alumina by colloidal binders are discussed. In the second part, the advantages of colloidal silica/alumina as a refractory binder are highlighted. Meanwhile, the characterization techniques and functional mechanisms of these binders are presented in order to understand the behavior of these systems. Finally, in the last section, the challenges for suitable use of colloidal binders are discussed and the future direction of nano-structured refractory castables is outlined.
MgO based refractory castables draw wide technological interest because they have the versatility and installation advantages of monolithic refractories with intrinsic MgO properties, such as high ...refractoriness and resistance to basic slag corrosion. Nevertheless, MgO easily reacts with water to produce Mg(OH)
2, which is followed by a large volumetric expansion, limiting its application in refractory castables. In order to develop solutions to minimize this effect, a better understanding of the main variables involved in this reaction is required. In this work, the influence of temperature, as well as the impact of the chemical equilibrium shifting (known as the common-ion effect), on MgO hydration was evaluated. Ionic conductivity measurements at different temperatures showed that the MgO hydration reaction is accelerated with increasing temperature. Additionally, different compounds were added to evaluate their influence on the reaction rate. Among them, CaCl
2 delayed the reaction, whereas KOH showed an opposite behavior. MgCl
2 and MgSO
4 presented similar results and two other distinct effects, reaction delay and acceleration, which depended on their concentration in the suspensions. The results were evaluated by considering the kinetics and the thermodynamics of the reaction, and the mechanical damages in the samples that was caused by the hydration reaction.
Calcium aluminate cement reactions with water result in anhydrous phases dissolution, followed by nucleation and crystal growth of hydrate compounds. Due to the dynamic characteristics of this ...process and in order to evaluate the phase transformation kinetics of such materials, suitable methods to halt hydration are required. In this work, the use of acetone and microwave drying, aiming to withdraw free water and inhibit further reactions, was evaluated. X ray diffraction was used to quantify the phases generated in the cement samples kept at 37
°C for 1–15 days. The advantages and drawbacks of these procedures are presented and discussed. Using microwaves to halt the hydration process in shaped samples seems to be effective to withdraw the cement free water, if a previous grinding of the material is carried out. The procedure suggested can be used in researches in the refractory castable area, endodontic cements and others.
Abstract Polymeric fibers are often used as a drying additive for refractory castables because they can increase their permeability, reducing the risk of pressurization that is believed to trigger ...explosive spalling. Despite the potential of synthetic polymers to be engineered and obtain desired properties, the required parameters for inducing permeability enhancement remain unclear. This inhibits the development of novel designed drying additives and improvement of the numerical models. This work investigates the effect of polypropylene (PP), polyethylene (PE) and cellulose fibers on the water transport in refractory castables through rapid neutron tomography, enabling the in situ visualization of the water distribution, the drying front advance and the size, intensity and duration of moisture accumulation. PE and cellulose fibers accelerate drying fronts earlier than PP, in which PE exhibits larger moisture accumulation, residual moisture behind its drying front and a slower drying rate at higher temperatures despite the early water removal initiation. In contrast, cellulose emerged as a better candidate, due to a swelling–shrinkage based mechanism. The neutron tomography observations unveil the dynamic and intricate effect of fibers in the permeability, emphasizing that safer industrial processes require a deeper understanding of the underlying mechanisms to develop better fibers and accurate numerical models.
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