Application of F NMR relaxometry to the determination of porosity and pore size distribution in hydrated cements and other porous materials

The potential to determine the porosity of cements and other porous materials by employing ¹⁹F NMR relaxometry was explored for samples of hydrated Portland cement, white cement and calcium aluminate, filled with Freon 11. The dependence of ¹⁹F signal amplitudes on the content of Freon in samples with completely filled pores was linear with a small (< 6%) intercept thus allowing a direct determination of total porosity. Additionally it was found that magnetic susceptibility (hence the content of paramagnetic compounds) of the solid can be evaluated from ¹⁹F NMR spectra of samples containing exterior liquid. Information concerning pore size distribution can be obtained from the analysis of multiexponential relaxation of ¹⁹F nuclei in samples with completely filled pores. Mathematical models employing sets of distributions of relaxation rates have been developed for this purpose. Distributions were assumed to be of a (fixed) square/triangular type allowing for the calculation of moments of any order from the estimated initial value, width of distribution and asymmetry factor. Longitudinal and transverse relaxation were characterised by either single or several continuous distributions of relaxation rates. One of the relaxating phases (assigned to fluid occupying throats connecting the pores) was found to selectively disappear when Freon was evaporated from samples. Another approach is to analyse the dependencies of means and variances of relaxation rates on Freon content. This has been done by employing the two-site relaxation model with a permanent adsorption layer and pore-dependent relaxation enhancement. Simultaneous fitting of dependencies of means and variances of relaxation rates on Freon content yields parameters of pore size distribution and of the dependence of relaxation enhancement on the pore radius. Analysis of experimental data shows agreement between these two approaches.     

EFFECT OF ISO- AND TERE-PHTHALOYL CONTENT ON THE PORE SIZE AND THE PORE SIZE DISTRIBUTION OF AROMATIC POLYAMIDE RO MEMBRANES

Poly-m-phenylene-iso(x)-co-tere(100-xc)-phthalamide copolymers with different x values were synthesized and viscoelastic properties of the polymer solution were studied in the presence of lithium chloride nonsolvent additive in order to determine the size of the polymer aggregate in the solution. Reverse osmosis membranes were also prepared using the polymers synthesized above. The membrane performance was then tested for the separation of sodium chloride solute and reference organic solutes, in order to obtain the average pore size and the pore size distribution of the membrane. The data for the pore size were further used to calculate the length of the polymer segment in the noncrystalline region. An interesting correlation was found between the above length and the isophythaloyl content, x, of the copolymer.     

几种椰壳活性炭材料的孔结构分析

为了筛选在异丙苯法生产苯酚工艺中吸附分离α-甲基苯乙烯的吸附剂，测定了4种椰壳活性炭材料的氮吸附等温线，并用BET模型、t图法、BJH理论等方法对孔结构进行分析与表征。结果表明：2号活性炭为微孔型，具有大量2．3nm以下的孔隙。1号、3号、4号活性炭除了微孔外还含有一定量的中孔。4号活性炭中孔率超过50％，拥有最小的平均中孔孔径，对α-甲基苯乙烯有较强的吸附能力，较适合作为异丙苯法生产苯酚工艺中α-甲基苯乙烯的吸附剂。     

Effect of additives concentration on the surface properties and performance of PVDF ultrafiltration membranes for refinery produced wastewater treatment

The aim of this study was to investigate the effect of pore-forming hydrophilic additives on the porous asymmetric polyvinylideneflouride (PVDF) ultrafiltration (UF) membrane morphology and transport properties for refinery produced wastewater treatment. PVDF ultrafiltration membranes were prepared via a phase inversion method by dispersing lithium chloride monohydrate (LiCl·H₂O) and titanium dioxide (TiO₂) nanoparticles in the spinning dope. The morphological and performance tests were conducted on PVDF ultrafiltration membranes prepared from a different additive content. The top surface and cross-sectional area of the membranes were observed using a field emission scanning electron microscope (FESEM) and energy dispersive X-ray (EDX) analysis. The surface wettability of porous membranes was determined by the measurement of a contact angle. The mean pore size and surface porosity were calculated based on the permeate flux. The results indicated that the PVDF/LiCl/TiO₂ membranes with lower TiO₂ nanoparticles loading possessed smaller mean pore size, more apertures inside the membrane with enhanced membrane hydrophilicity. LiCl·H₂O has been employed particularly to reduce the thermodynamic miscibility of dope which resulted in increasing the rate of liquid–liquid demixing process. The maximum flux and rejection of refinery wastewater using PVDF ultrafiltration membrane achieved were 82.50 L/m² h and 98.83% respectively at 1.95 wt.% TiO₂ concentration.     

EFFECT OF POLYVINYLPYRROLIDONE ADDITIVE ON THE PORE SIZE AND THE PORE SIZE DISTRIBUTION OF POLYETHERSULFONE (VICTREX) MEMBRANES

Investigations were made on the effect of the presence of polyvinylpyrrolidone (PVP) additive in the casting solution on the performance of polyethersulfone (PES) ultrafiltration membranes in the range of PVP/PEs weight ratio below 0.5. It was further attempted to investigate the effect of PVP additive on the size of PES polymer in the casting solution and the pore size and the pore size distribution of PES membranes. It was found that the presence of PVP additive decreases both the polymer size and the membrane pore size. Discussions were made on the mechanism of the formation of the pore based on the correlation between the polymer size and the pore size.     

Determination of the effects of the pore size distribution and pore connectivity distribution on the pore tortuosity and diffusive transport in model porous networks

An efficient computation method to study flow and transport process of small molecules in porous media using a dual site-bond lattice model, DBSM, is described. The microscopic properties of the porous network take into account the influence of local heterogeneities during the simulations. The numerical experiments demonstrated the combined effect of pore size distribution and connectivity distribution on the mass transport properties and the structural tortuosity. The results indicate that the pore size distribution and percolation phenomena related with pore shielding effects, influence significantly the tortuosity and the effective diffusivity of the porous network. Also, the simulations raise the important role of the connectivity distribution among the various pores in the gas diffusive properties of the poorly connected networks.     

RO and NF membrane fouling and cleaning and pore size distribution variations

The variations of porosity parameters of some reverse osmosis (RO) and nanofiltration (NF) polyamide thin-film composite membranes were determined in order to explain the changes of membranes' performances caused by membrane fouling and chemical cleaning of the fouled membranes. The pore size distribution curves and the effective number of pores in the membrane surface indicated plugging of the tight network pores in the membrane surface and even their disappearance during fouling. The enlargement of the wider aggregate pores was responsible for the noticed reduction in salt rejection. The initial pore structure of the fouled RO membrane was restored by immediate chemical cleaning. A delay of chemical cleaning of the fouled membranes led to irreversible changes in the porous structure of both the RO and NF membranes, which were caused by a microbial activity.     

多孔Al2O3陶瓷底膜的表征方法

研究了Al2O3陶瓷底膜的表征方法。首先,根据毛细管作用原理、润湿现象及Laplace方程,采用异丙醇作为润湿剂,求出底膜的孔径、干膜流量和湿膜流量,用这些数据,导出孔径分布函数,并作出孔径分布函数与孔径关系图,其次,采用GB1966-80的方法,测定了Al2O3陶瓷底膜的孔隙率。     

Characterization of heterogeneity in concrete and cement by mechanical spectroscopy

We propose a method to obtain the degree of heterogeneity of samples of cement or concrete due to the presence of cracks, bubbles or simply the native ingredients of the material. A number of samples are prepared in a cylindrical shape, and their transverse vibration resonant frequencies are measured. A given mode of oscillation will correspond to slightly different frequencies in different samples due to the random nature of the system. For example, for a given mixing formula, the ratio of sand to cement may be known, but the precise position of sand grain cannot be determined. We studied the statistical distributions of frequencies for each mode and found that there exists a relationship between the width of the probability functions and the degree of heterogeneity.     

How the coarse fraction influences the microstructure and the effective thermal conductivity of alumina castables – An experimental and numerical study

This study investigates the particle size distribution's effect on the microstructure and effective thermal conductivity (ETC) of alumina castables. The ETC was measured by the transient plane source method and predicted numerically based on a two-scale model describing the structure on a fine and coarse scale. The prediction considered particle and pore size distributions, porosity (around 20%) and grain morphology. The microstructure was investigated by scanning electron microscopy. For a constant fines content, increasing the coarse grain fraction while decreasing the medium fraction enhanced sintering of the matrix. Small pores (≤250 nm) increased the sintering activity. The densest castable contained the most small pores. The particles’ and pores’ contributions to the sintering activity led to intensified microcracking and a decreased ETC. The numerical model did not consider constituents ≤500 nm like the small pores and microcracks and the calculated ETC values consequently deviated from the measured values.     

Study on some factors affecting the results in the use of MIP method in concrete research

Effects of rate of pressure application and forms and type of sample on porosity and pore size distribution of concrete estimated through mercury intrusion porosimetry (MIP) are presented in this experimental work. Two different forms of concrete sample, namely, crushed chunks of concrete and small core drilled out from the concrete beam specimens, were used for this study. The results exhibit that the rate of pressure application in mercury porosimetry has little effect on porosity and pore size distribution of concrete. It is also demonstrated that small cores drilled out from large concrete specimens are preferable as samples for performing porosimetry test on concrete.     

The pore solution phase of carbonated cement pastes

Samples of hydrated cement pastes were exposed to atmospheres with various carbon dioxide concentrations at relative humidities controlled by different saturated salt solutions. When carbonated throughout their thickness, as indicated by the phenolphthalein test, they were resaturated with water and subjected to pore solution expression and analysis. The effects of the various carbonating environments on the pore solution composition and on aspects of the pore structure and mineralogy of the carbonated products are reported. Implications regarding the likely effects of different accelerated carbonation regimes on the corrosion behaviour of steel in concrete are discussed. In particular, it is shown that the use of saturated sodium nitrite solution to control the relative humidity of atmospheres with high concentrations of carbon dioxide may cause an evolution of gaseous oxides of nitrogen, which can result in the contamination of the pore solution with nitrite and nitrate salts.     

Porosity, pore size distribution and in situ strength of concrete

In this study, in situ strength of concrete was determined through compression test of cores drilled out from laboratory cast beams. The apparent porosity and pore size distribution of the same concrete were determined through mercury intrusion porosimetry, performed on small-drilled cores. The normal-strength concrete mixes used in the experimental investigation were designed to exhibit a wide variation in their strengths. To ensure further variation in porosity, pore size distribution and strength, two modes of compaction, two varieties of coarse aggregates, different levels of age, curing period and exposure condition of concrete were also introduced in experimental scheme. With the data so generated, an appraisal of the most frequently referred relationships involving strength, porosity and pore size of cement-based materials was carried out. Finally, a new empirical model relating the in situ strength of concrete with porosity, pore size characteristics, cement content, aggregate type, exposure conditions, etc., is presented.