Hydromagnesite Mg5(CO3)4(OH)2·4H2O is the most widespread form of hydrated Mg-carbonate minerals. To better understand the factors controlling the precipitation of hydrated Mg-carbonates, we measured ...hydromagnesite solubility product at 25 and 50°C and its growth rate between 25 and 75°C, using natural hydromagnesite from a cave as seed material.
The solubility products values derived in this study, Ksp–Hmgs=−37.08±0.50 and −38.90±0.54 at 25 and 50°C, respectively, are in the upper range of published values. Hydromagnesite growth rate normalized to the BET surface area at 8⩽pH⩽10 is consistent with the direct and reversible attachment of the reactants at the solid surface being rate-limiting. It may be described by:RHmgs=A0·e-Ea/RT(ΩHmgs1/5-1)where A0, the pre-exponential factor, and Ea, the activation energy, are equal to 5.12×10−7mol/cm2/s and 45.5±9kJ/mol, respectively, and ΩHmgs stands for the saturation state of the solution with respect to hydromagnesite. Comparison of hydromagnesite growth rates with recently published magnesite growth rates (Saldi et al., 2009, 2012) show that: (1) hydromagnesite apparent growth activation energy is lower by more than 100kJ/mol compared to the activation energy for magnesite obtuse step advancement, and (2) hydromagnesite growth rate constant extrapolated to 90°C is 2.5 orders of magnitude higher than corresponding magnesite growth rate constant. Thus, our results confirm the long-standing hypothesis that the slow dehydration kinetics of the Mg2+ cation is responsible for the sluggish magnesite formation at low temperature, and that the kinetic barrier for hydromagnesite growth is much lower. Nevertheless, simulation of hydromagnesite and magnesite growth rates as a function of solution composition at 50 and 90°C, and pH 7 and 9 reveal that, because of its much higher solubility, hydromagnesite would grow more quickly than magnesite in natural or industrial environments only at 50°C and pH 9. This is consistent with the formation of hydromagnesite during the surface alteration of ultramafic rocks.
The design of curved structural building envelopes is challenging as it requires to account for a wide variety of constraints. In particular, the shape must be mechanically efficient, fabricable, and ...fit the site geometry. This article shows how a family of surfaces, called isotropic Linear Weingarten surfaces (nicknamed i-liwien), may fulfil all these constraints together, and be used as an intuitive design tool. We start by showing that these shapes are funicular for a uniform vertical load, and that principal stress lines form a conjugate net. This allows in particular for the design of gridshells with planar faces and low bending moments or for the design of self-stressed cable nets cladded by planar glass panels. We then propose a discrete model based on recent advances in discrete differential geometry. We use this model to propose a generation method from boundary curves with two additional control parameters. We demonstrate the shape potential on several examples. The application to shells, membranes, cable-nets and gridshells are discussed.
Pressure on the coastline is escalating due to the impacts of climate change, this is leading to a rise in sea-levels and intensifying storminess. Consequently, many regions of the coast are at ...increased risk of erosion and flooding. Therefore coastal protection schemes will increase in cost and scale. In response there is a growing use of nature-based coastal protection which aim to be sustainable, effective and adaptable. An example of a nature-based solution is a dynamic cobble berm revetment: a berm constructed from cobble and other gravel sediments at the high tide wave runup limit. These structures limit wave excursion protecting the hinterland from inundation, stabilise the upper beach and adapt to changes in water level. Recent experiments and field applications have shown the suitability of these structures for coastal protection, however many of the processes and design considerations are poorly understood. This study directly compares two prototype scale laboratory experiments which tested dynamic cobble berm revetments constructed with approximately the same geometry but differing gravel characteristics; well-sorted rounded gravel (DynaRev1) and poorly-sorted angular gravel (DynaRev2). In both cases the structures were tested using identical wave forcing including incrementally increasing water level and erosive wave conditions. The results presented in this paper demonstrate that both designs responded to changing water level and wave conditions by approaching a dynamically stable state, where individual gravel is mobilised under wave action but the geometry remains approximately constant. Further, both structures acted to reduce swash excursions compared to a pure sand beach. However, their morphological behaviour is response to wave action varied considerably. Once overtopping of the designed crest occurred, the poorly-sorted revetment developed a peaked crest which grew in elevation as the water level or wave height increased, further limited overtopping. By comparison, the well-sorted revetment was characterised by a larger volume of submerged gravel and a lower elevation flat crest which responded less well to changes in conditions. This occurred due to two processes: (1) for the poorly-sorted case, gravel sorting processes moved small to medium gravel material (D) to the crest and (2) the angular nature of the poorly-sorted gravel material promoted increased interlocking. Both of these processes led to a gravel matrix that is more resistant to wave action and gravitational effects. Both revetments experienced some sinking due to sand erosion beneath the front slope. The rate of sinking for the well-sorted case was larger and continued throughout due to the large pore spaces within the gravel matrix. For the poorly sorted revetment in DynaRev2, sand erosion ceased after approximately 28 h due to the development of a filter layer of small gravel at the sand-gravel interface reducing porosity at this location, hence a larger volume of sand was preserved beneath the structure. Both designs present a low-cost and effective solution for protecting sandy coastlines but from an engineering viewpoint it appears better to avoid well-sorted gravel material and greater gravel angularity has been seen to increase crest stability.
Experiments in which heated nitrogen/steam mixtures are passed through model cracks in concrete with thermocouples embedded along the inner surfaces of the cracks provide insights about the ...thermodynamic processes occurring within them. The thermal behaviour that is observed results from several interlinked processes that influence one another and the resulting transfer of nitrogen, steam, water and heat. By correlating the temperatures measured at different times and positions within cracks with different separations, one can observe how the flow is modified by the formation of condensation. When pure nitrogen is passed through the cracks, only a small amount of water evaporates from the concrete and partial obstruction of the cracks by condensed water only occurs in the cracks with small separations (∼30 µm). As the proportion of steam in the mixture is increased, the thermal fluctuations caused by the formation of condensation become more important and evolve over time as the concrete is heated.
The present volume contains the Proceedings of the 14th International Conference on Hydroinformatics – HIC 2022, held from the 4th to the 8th of July 2022 in Bucharest, Romania. The conference was ...jointly organised by the Technical University of Civil Engineering Bucharest and the University “Politehnica” of Bucharest.
Abstract Structural failure of concrete buildings on fire and complete destruction of the monolithic refractory lining during their drying stage are dangerous examples of the effect of explosive ...spalling on partially saturated porous media. Several observations in both cases indicated the presence of moisture accumulation ahead of the drying front, which are in tune with the most common theories on the explosive spalling of concrete. Previous studies have shown evidence of the existence of this phenomenon, however, they were biased by artifacts and experimental limitations (such as the beam hardening effect and changes in the microstructure of the material due to the presence of pressure and temperature sensors). In the current work, rapid neutron tomography was used to investigate the in‐operando drying behavior of a high‐alumina refractory castable, proposing a novel experimental layout aimed at a truly one‐dimensional drying front. This setup provided more realistic boundary conditions, such as the behavior of a larger wall heated from one of its sides, while also preventing some nonphysical artifacts (notably beam hardening). By eliminating these aspects, a direct proof that moisture accumulates ahead of the drying front was obtained. This work also lays the basis for further studies focusing on the response sensitivity analysis to boundary conditions and other parameters (e.g., heating rates and properties of the sample related to the moisture clog formation), as well as useful data for the validation and characterization stages of numerical models of partially saturated porous media.
AbstractDue 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.
