Characterization of the granular packing and percolation threshold of reactive powder concrete

Reactive powder concrete (RPC) is a new cement-based material developed through microstructural engineering. RPC is composed of very fine powders: sand, crushed quartz and silica fume, all with particle sizes comprised between 300 and 0.02 μm, and a low water content, W/C<0.20. A very dense matrix is achieved by optimizing the granular packing of these powders. This compactness confers to RPC ultrahigh strength and durability.

The hydration kinetic of the cementitious matrix using electrical conductivity and isothermal calorimetry is presented. A linear relationship between the logarithm of conductivity ln(σ) and the degree of hydration , has been found:

ln(σ)=A_v−B_v

where A_v depends on the nature and the dosage of the different mineral and organic components, and B_v is an invariant related to the granular packing. From 19 different RPC compositions, an average value of 12.1 with a standard deviation of 1.7 were found for B_v. The capillary porosity percolation threshold was also deduced from this relationship. It has been found that when the degree of hydration is equal to 26%, the capillary pore space is discontinuous. This last result is in agreement with the prediction of the NIST microstructural model by Bentz and Garboczi [D., Bentz, E. Garboczi, Percolation of phases in a three-dimensional cement paste microstructural model, Cem Concr Res 21 (2) (1991) 325–344]. A continuous determination of the degree of hydration based on electrical conductivity is proposed.     

A study of the electrical properties of carbon nanotube-NiFe2O4 composites: Effect of the surface treatment of the carbon nanotubes

Novel carbon nanotube-NiFe₂O₄ composite materials have been prepared for the first time by in situ chemical precipitation of metal hydroxides in ethanol in the presence of carbon nanotubes (CNTs) and followed by hydrothermal processing. The obtained composite powders were characterized using XRD, TEM and EDS. The effect of surface oxidation treatment of CNTs on their properties was investigated by FTIR, zeta potential and hydrodynamic radius distribution characterization. Electrical conductivity measurements show that surface oxidation treatment of CNTs can improve the electrical conductivity of the composites more pronouncedly than pristine CNTs do. With 10 wt.% addition of surface treated CNTs, the electrical conductivity is increased by 5 orders of magnitude. The surface oxidized CNTs are crucial for this significant increase in electrical conductivity, which provides strong adhesion between the nanotubes and the matrix to give a homogeneous carbon nanotube-NiFe₂O₄ composite.     

Thermal behaviour and phases evolution during the sintering of porous inorganic membranes

Anorthite-based highly porous membranes were successfully produced using calcined oyster shell to enhance the pore network. The calcined oyster shells produce CaO responsible for the crystallisation of gehlenite and anorthite at relatively low temperature. While the crystallisation produced nano and meso size of intergranular pores, vitrification of feldspar is responsible for development of the capillary porosities. The increasing sintering temperature from 1200 °C to 1300 °C implies the increase in average pores radius from 1.2 μm to 14.3 μm due to the formation of spherical pores from vitrification. The combination of different class of porosities in the matrices results in the interconnection with improvement of the permeability of the porous network. Porosity, permeability and chemical stability were improved with 20 wt.% of calcined oyster shell addition allowing the possible development of high strength porous network which is promising for the membranes support and other applications including liquid separation as well as liquid filtration where high pressure is used.     

Evolution of elastic behavior of alite paste at early hydration stages

Elastic behavior is an important aspect of fresh cement-based materials. In this paper, the elasticity development of fresh pure alite paste within 5 hours was monitored in situ by small amplitude oscillation shear. Isothermal microcalorimetry, X-ray diffraction, scanning electronic microscopy, ¹H nuclear magnetic resonance, zeta potential measurements were used to investigate the underlying mechanisms. It was found that the evolution of storage modulus can be characterized by three different stages. The first stage stems from the compression of electrical double layer and the slight increase of solid volume fraction; the second stage is only controlled by the colloidal interaction; and the third stage is associated to the significant growth of solid volume fraction, the enhancement of short-range attraction and the rigidification of calcium silicate hydrate (CSH) network. Moreover, the relationship between the formation of CSH and the growth of elasticity was also quantified.     

Electrical conductivity and rheological properties of ordinary Portland cement-silica fume and calcium hydroxide-silica fume pastes

Two groups of solids mixtures were prepared: (i) the first group includes four mixes having different ordinary Portland cement/silica fume (OPC/SF) weight ratios and (ii) the second group consists of four blends having different Ca(OH)₂/SF molar ratios. Electrical conductivity measurements were carried out on the pastes of the first group mixes using two initial water-to-solid (W/S) ratios of 0.55 and 0.70 by weight; while the W/S ratios used for the second group mixes were 1.00 and 1.20 by weight. The measurements were done at 25 and 45 °C for each paste during setting and hardening processes after gauging with deionized water. Rheological properties were studied at room temperature for all mixes using various W/S ratios. The results obtained indicate clearly the effect of SF and W/S ratio on the rheological properties and electrical conductivity of all pastes under investigation. The relation between the electrical conductivity and rheological properties for different mixes were discussed based on the chemical nature and physical state of the hydration products formed at early ages of hydration.     

Capillary porosity depercolation in cement-based materials: Measurement techniques and factors which influence their interpretation

The connectivity of the capillary porosity in cement-based materials impacts fluid-and-ion transport and thus material durability, the interpretation of experimental measurements such as chemical shrinkage, and the timing and duration of curing operations. While several methods have been used to assess the connectivity of the capillary pores, the interpretation of some experimental procedures can be complicated by the addition of certain chemical admixtures. This paper assesses capillary porosity depercolation in cement pastes using measurements of chemical shrinkage, low temperature calorimetry (LTC), and electrical impedance spectroscopy. The experimental results are analyzed to identify the time of capillary porosity depercolation. In addition, the factors that influence the interpretation of each technique are discussed. Experimental evidence suggests that capillary porosity depercolation, as defined by Powers, occurs after hydration has reduced the capillary porosity to around 20% in cement paste systems. The influence of capillary porosity depercolation on the transport properties is demonstrated in terms of a reduction in the electrical conductivity of the cementitious material. Special attention is paid to understand and interpret the influence of shrinkage-reducing admixtures (SRAs) on the freezing behavior of cementitious systems, particularly in regard to the inapplicability of using LTC to detect porosity depercolation in cement pastes containing such organic admixtures.     

多孔材料中甲烷水合物生成的传热数值模拟研究

水合物储运(NGH)是近几年发展起来的天然气储运技术,已具备实现工业化的潜力。但水合物的生长是传质传热控制的反应,因此在放大实验中存在诸多不确定因素。针对该问题,对水合物反应器中多孔材料内甲烷水合物生成传热过程建立了基于化学反应动力学和多孔材料内传质传热的甲烷水合物生成传热数学模型,可用于计算反应器内水合物生成分布和热量分布,指导水合反应器的设计和优化。通过模拟与实验数据对比验证了该模型的可靠性,并对使用了不同热导率填料的水合反应过程进行数值模拟。结果显示,模拟值与实验值的绝对平均相对误差小于6%,生成传热模型准确性高;在水合反应过程中,热量传递是影响水合物生成速率的关键因素之一。导热不良时,易在水合物生成中心部分形成局部过热,对水合物生长造成热抑制。在进行水合物生成放大实验时,应特别注意反应器内部的热量控制。     

多孔材料中甲烷水合物生成的传热数值模拟研究

水合物储运(NGH)是近几年发展起来的天然气储运技术,已具备实现工业化的潜力。但水合物的生长是传质传热控制的反应,因此在放大实验中存在诸多不确定因素。针对该问题,对水合物反应器中多孔材料内甲烷水合物生成传热过程建立了基于化学反应动力学和多孔材料内传质传热的甲烷水合物生成传热数学模型,可用于计算反应器内水合物生成分布和热量分布,指导水合反应器的设计和优化。通过模拟与实验数据对比验证了该模型的可靠性,并对使用了不同热导率填料的水合反应过程进行数值模拟。结果显示,模拟值与实验值的绝对平均相对误差小于6%,生成传热模型准确性高;在水合反应过程中,热量传递是影响水合物生成速率的关键因素之一。导热不良时,易在水合物生成中心部分形成局部过热,对水合物生长造成热抑制。在进行水合物生成放大实验时,应特别注意反应器内部的热量控制。     

Simulated microstructure and transport properties of ultra-high performance cement-based materials

Ultra-high performance cement-based materials expected to be used in nuclear waste containers were submitted to a leaching test in order to evaluate their long-term durability. Reactive powder concretes (RPC) were attacked by de-ionized water. Previous studies revealed a superficial degradation after leaching with a sound zone underneath an altered porous zone in which anhydrous silicates C₃S and C₂S were dissolved. To predict the long-term durability of RPC, the hydration rate of cement minerals, pozzolanic reactivity of silica fume, pore structure, and mechanisms of chemical reactions were needed. So first, the microstructure of RPC matrix was simulated using the NIST microstructural model. Then the transfer of Ca ions through percolating water was estimated using DIFFU-Ca, a model based on the local chemical equilibrium. This double modeling validates the damage process related to an instantaneous dissolution of anhydrous cement silicates at the degradation front which results in a higher connected pore space, and is in good agreement with experimental results. The long-term behavior is expressed as the depth of the altered zone, the leaching kinetics and the evolution of Ca concentration in the material.     

ONE- AND TWO-PHASE FLOW IN NETWORK MODELS OF POROUS MEDIA

We discuss the low Reynolds number flow of one or two immiscible Newtonian fluids in network models of microscopically random porous media. For the case of a single fluid, we reduce the flow problem to an analog random electrical resistor problem and use an 'effective medium theory' to express the permeability of such networks in terms of the pore space geometry. For the flow of two fluids we use the Washburn approximation to incorporate capillary pressure differences, and show that this problem may also be formulated as a random electrical network. In this case, the capillary menisci correspond to moving batteries, and we follow the motion of the fluid-fluid interface (the ensemble of analog batteries) by a time-step procedure. We study the time evolution of the interface and the dynamics of blobs of one fluid contained in the other, as a function of the network geometry.’     

ONE- AND TWO-PHASE FLOW IN NETWORK MODELS OF POROUS MEDIA

We discuss the low Reynolds number flow of one or two immiscible Newtonian fluids in network models of microscopically random porous media. For the case of a single fluid, we reduce the flow problem to an analog random electrical resistor problem and use an 'effective medium theory' to express the permeability of such networks in terms of the pore space geometry. For the flow of two fluids we use the Washburn approximation to incorporate capillary pressure differences, and show that this problem may also be formulated as a random electrical network. In this case, the capillary menisci correspond to moving batteries, and we follow the motion of the fluid-fluid interface (the ensemble of analog batteries) by a time-step procedure. We study the time evolution of the interface and the dynamics of blobs of one fluid contained in the other, as a function of the network geometry.'     

Structural transformations in thermally modified porous glasses

The changes in the structural parameters of porous glasses (pore radius, pore volume, specific surface of pores, structural resistance coefficients) upon heating are investigated as a function of the composition of initial two-phase alkali borosilicate glasses with the use of a number of independent methods, such as the adsorption techniques (water vapor adsorption, mercury porosimetry, thermal desorption of nitrogen), transmission electron microscopy, small-angle X-ray scattering, and membrane conductivity measurements. It is demonstrated that the structural transformations in thermally modified porous glasses are associated with the processes of overcondensation of pores and viscous flow in the silica network.     

Some aspects of wetting/dewetting of a porous medium

Various features of wetting/dewetting of porous media are examined. The phenomenon of capillary hysteresis is illustrated by a vertical capillary tube which consists of an alternating sequence of convergent—divergent conical sections. A study of the kinetics of wetting of this tube by a liquid shows that when the velocity of the liquid/vapour meniscus is plotted against the height of penetration, it oscillates about the Washburn velocity—distance curve and performs Haines jumps. A general macroscopic equation is derived for the rate of wetting/dewetting of a porous medium having randomly distributed, finely divided particles or pores. Use is made of the Forchheimer equation, which is an extension of Darcy's equation to higher Reynolds numbers. Dissipative energy terms due to internal fluid calculaton and to irreversible movements of the meniscus strongly affect the initial rate of imbibition, but as the wetting progresses the Reynolds number decreases and Washburn's equation prevails.The application of percolation theory to wetting/dewetting phenomena in porous media is studied. The use of percolation theory by Kirkpatrick and Stinchcombe to find the electrical conductivity of inhomogeneous solid mixtures is adapted to determining the permeability of a porous medium to fluid flow. It is also shown how the relation between the “precolation probability” and the concentration of “unblocked” channels or pores can be applied in calculating the capillary pressure—desaturation curve in drainage. In particular, percolation theory predicts that a threshold pressure or break-through pressure is required before a non-wetting fluid can displace a wetting fluid in a porous medium. It is often convenient to use tree-like or branching lattice networks as models of a porous medium, because these are amenable to exact solutions in regard to percolation probability and permeability. The percolation properties of porous medium models which consist of lattice networks of cylindrical channels with a distribution of cross-sections and also of randomly packed rotund particles are examined and their relevance to wetting/dewetting phenomena discussed.     

NiO as an Efficient Counter Electrode Catalyst for Dye-Sensitized Solar Cells

NiO is an important heterogeneous catalyst employed in chemical processes. However, it is a new topic to explore NiO as a counter electrode catalyst for dye-sensitized solar cells (DSSCs). In this paper, NiO with poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) was demonstrated an efficient DSSC counter electrode with a maximum power conversion efficiency of 7.58 %. Furthermore, electrochemical impedance spectroscopy and cyclic voltammetry measurements revealed that the excellent photovoltaic performance is due to the combination between the high catalytic activity of NiO and the superior electrical conductivity of PEDOT:PSS. The optimum weight ratio of NiO to PEDOT:PSS is 48.     

Electrical properties of polybithiophene-polystyrene composites

Conducting composites consisting of polybithiophene and based on porous crosslinked polystyrene as host polymer have been synthetized in the vicinity and above the percolation threshold by oxidative polymerization with FeCl₃. Electrical conductivity and thermoelectric power measurements for different degrees of doping have been carried out in the temperature range 80–300 K. The electrical conductivity variations are weakly thermally activated while the thermoelectric power has metallic magnitude with positive sign and increases with temperature. Conduction mechanisms are interpreted on the basis of an hopping model involving bipolaronic clusters. © 1996 John Wiley & Sons, Inc.     

Dialkylimidazolium halide ionic liquids as dual function inhibitors for methane hydrate

Six dialkylimidazolium halide ionic liquids have been investigated for their potential application as novel gas hydrate inhibitors. Their effects on the equilibrium methane hydrate dissociation curve in a pressure range 105–205 bar and the induction time of methane hydrate formation at 114 bar and a high degree of supercooling, i.e., about 25 °C, are measured in a high-pressure micro-differential scanning calorimeter. Similar to dialkylimidazolium tetrafluoroborate investigated in our previous work, these ionic liquids are found to shift the equilibrium hydrate dissociation/stability curve to a lower temperature and, at the same time, retard the hydrate formation by slowing down the hydrate nucleation rate. To understand the performance of these ionic liquids in inhibiting the hydrate formation, the electrical conductivity and infrared spectra of ionic liquids are also obtained and analyzed.     

How to Investigate the Setting Kinetics of Refractory Castables——A Review of Methods and Their Relevancy for Technical Use

This article contributes to a meaningful interpretation of results gathered by in-situ measurements of sonic velocity,electrical conductivity and the change of temperature during setting and curing of LC refractory castables. All said monitoring techniques are well known in the refractory community and are well documented in the literature. However,the time dependent changes of the said properties are not well correlated to mineralogical and in consequence technological changes of the material during setting and curing. The basic interest of refractory users of course is to define the time at which the installation or the pre-shape construction element can be demoulded. This is in principle possible with the methods listed above. However,after water addition the time dependent changes of sonic velocity,electrical conductivity and temperature are diverse as there are possible combinations of cements,microfines and surface-active additives. In further the ambient conditions,temperature and relative humidity have a strong influence on these properties and this does not only mean a simple time-shift. Up to now the results are more confusing than helpful to determine the best time for demoulding pre-shapes and refractory linings. Recent research at Koblenz University of Applied Science contributes to a deeper understanding of the setting behaviour,because besides the physical evolution of the said parameters the time dependent formation of hydrate phases is also investigated by means of gravi-metric method. The proposed presentation will show detailed insights in the evolution of refractory castables during setting and curing.Key words: refractory; setting kinetics; hydrate bond water     

Influence of laser wavelength on the critical energy density for initiation of energetic materials

Critical densities of the energy of laser initiation of PETN containing nanoscale aluminum inclusions at radiation wavelengths of 1064 and 532 nm were measured experimentally. The critical initiation-energy density that corresponds to a 50%th probability of explosion was 1.15 J/cm² for the first harmonic of a neodymium laser and 0.7 J/cm² for the second. The dependence of the efficiency of radiation absorption by aluminum on the size of metal nanoparticles for the first and second harmonics of a neodymium laser is calculated. It is shown that the particle diameter corresponding to the absorption efficiency maximum and the amplitude of the maximum depend on the radiation wavelength. The absorption efficiency maximum for the first harmonic is observed in an inclusion 204 nm in diameter, and for the second, in an inclusion 96 nm in diameter. The amplitude of the maximum increases from 0.351 at a wavelength of 1064 nm to 0.490 at a wavelength of 532 nm. Dependences of the critical initiation energy density for energetic materials on the radius of metallic nanoparticles are calculated. Qualitative agreement between theoretical and experimental results is shown.     

Estimation and determination of the permeability of porous ceramics

The possibility of estimating the permeability of porous ceramic materials both with mercury and with gas liquid pore diagrams and based on model concepts concerning the porosity and average size of the constituent particles in the material was demonstrated. It was shown that the permeability of a porous material and the average pore radius can be directly determined with the linear segment of the reverse GLP curve and the capillary sinuosity can be calculated with it.     

Proton Spin–Spin Relaxation Study of the Effect of Temperature on White Cement Hydration

The chemical and microstructural changes within a white cement paste were characterized in situ using proton nuclear magnetic resonance spin–spin relaxation at 30 MHz, and X-ray diffraction. Paste samples with a water-to-cement ratio of 0.42 were cured at constant temperatures of 2°, 20°, 60°, and 100°C. Proton nuclear magnetic resonance spin–spin relaxation allows tracking the evolution of the mixing water into the solid fractions of calcium silicate hydrate, calcium hydroxide, and monosulfate, and the liquid phases: the calcium silicate hydrate interlayer water, gel pore water, and capillary pore water. It is shown that the hydration process is markedly accelerated with increasing hydration temperature, and that proton nuclear magnetic resonance relaxation measurements can quantitatively determine the proportions of water phases, their magnetic resonance characteristics, as well as the setting times of the cement during the hydration process.