Hardened portland cement pastes of low porosity IV. Surface area and pore structure

This paper deals with the total surface areas, total pore volumes, and the distribution of pore surface and pore volume in pores of different sizes of 25 low-porosity pastes. A Type II clinker was ground with two grinding aids: diethyl carbonate and Reax 70. The water-cement ratios were 0.2 and 0.3. The hydration temperatures were 5°, 25°, and 50°C. Nitrogen adsorption isotherms at -196°C and water vapor adsorption isotherms at 25°C were used for the surface area and pore structure determinations. The methods used in the analyses of micropores and wider pores were those developed by Brunauer and his coworkers.     

Surface area and pore structure of ceric oxide

Adsorption-desorption isotherms of nitrogen at ?196°, and of methanol and cyclohexane vapours at 35°, were determined on a sample of microcrystalline hydrated ceria and its products of dehydration formed in air at temperatures up to 1000°. The nitrogen adsorption isotherms were analysed by the de Boer t-method. In one particular sample, the V_a-t plot gave a straight line passing through the origin, which continued until near the saturation pressure. This sample is known to be porous, and its pore size distribution curve gave a most probable hydraulic radius of 35 ǎ. In analysing the pore structure, the corrected modelless method was used, and there is an indication that the pores are most likely to be cylindrical in shape. Sintering seems to take place through material transport from wider pores to fill narrower ones, where the total number of pores is reduced. Adsorption of larger molecules than nitrogen, namely methanol and cyclohexane, gave surface area values which are slightly lower than those calculated from nitrogen adsorption. It seems that slight persorption effects can take place in minor fractions of micropores present. However, the remainder of pores present in all samples investigated are accessible to the three adsorbate molecules.     

Autoclaved slag-clinker-sand pastes surface area,pore structure and compressive strength

Surface areas, total pore volumes and pore size distributions were measured from the adsorption of both nitrogen and water vapour on hydrothermally hardened slag-clinker-sand pastes. The compressive strength of the hardened specimens could be related to the pore structure and pore size distribution curves, and the interesting result obtained is that mesopores affect the compressive strength of the autoclaved pastes more than micropores, thus providing another factor to be considered in addition to the total porosity of the paste.     

Hardened portland blast-furnace slag cement pastes: I. Effects of additives on surface area and on pore structure

Portland blast-furnace slag cement pastes were prepared with various water/cement ratios. Specific surface areas and pore structures of the hardened pastes were investigated by nitrogen adsorption. The “accessibility” of the nitrogen molecules to the pore structure is discussed in terms of degree of hydration and total porosities of the pastes. Effect of presence of CaCl₂, a typical steel reinforcement corrosive agent, was also studied, and results indicated that it alters the area and pore structure extensively, to a more “open structure,” thus facilitating its own accessibility. Lime and gypsum addition was also studied in presence and in absence of CaCl₂, and the effect of the Blaine surface area of the unhydrated cement is particularly emphasized in this investigation.     

Studies on water and nitrogen adsorption on hardened cement pastes. II-Thermodynamics of adsorption

Heats as well as entropies of water and nitrogen adsorption on hardened cement pastes of various pore structures were computed. Water vapor adsorption is accompanied by a decrease in integral entropy relative to the liquid state, while nitrogen adsorption is accompanied by an increase in integral entropy. The results could be related to the pore structures of the various pastes. For water adsorption, the molecules seem to be oriented in a definite array with more order than the liquid structure, and depending on the pore size, the normal liquid structure is resumed at some distance far from the surface. Nitrogen results could be described in terms of the inaccessibility and exclusion of the molecules from parts of the pore system.     

Hardened slag–cement pastes of various porosities. II. Water and nitrogen adsorption

Specific surface areas and pore structure studies were carried out on hardened slag-cement pastes. Water and nitrogen adsorption isotherms were measured, and the derived parameters were critically compared. Nitrogen areas and total pore volumes were found to be much smaller than water values. For water vapour adsorption, the specific surface areas derived from the adsorption branches are found to be lower than those derived from the resorption isotherms. Some evidence is presented in this paper that the surface area values derived along the adsorption branches are underestimated values, and that the resorption values might be considered to be nearer to the actual areas of the various pastes. Calculations of the hydraulic radii refute the hypothesis that the areas inaccessible to nitrogen are interlayer areas.     

Microstructure of hardened gypsum pastes

Specific surface areas and pore size distribution analysis were taken as essential parameters to explore the microstructure of hardened gypsum pastes. The hydration of gypsum plaster is essentially affected by the water/plaster ratio of the mix, and by the presence of additives which might act as retarders or accelerators to the hydration process. Affecting the microstructure would mutually lead to variations of the compressive strength of the hardened pastes, and these variations are also discussed. Results are mainly expressed in terms of the solid (measured by the extent of its specific area) to the space (pore) ratios. Dual distribution of pore sizes was observed and the two groups of pores are designed as S (small) and L (large) pores. Similarities to the microstructure of hardened Portland cement pastes are indicated. Accelerators appear to function differently from retarders, and the effects of water/plaster ratios were investigated in both cases. The results obtained may carry many significant practical implications to the future use of hardened gypsum pastes.     

Influence of Heat Treatments on the Pore and Adsorption Characteristics of Sodium Doped Alumina Pillared Bentonite

Various amounts of Na+ ions were exchanged into alumina pillared bentonite (Al-PILB) sample, by controlling the pH of the dispersion of Al-PILB and sodium chloride solution. The Na+ doped pillared clays were calcined at elevated temperatures and adsorption of nitrogen at –196°C, cyclohexane and water at ambient temperature (21 ± 1°C) by the calcined samples were conducted. The results revealed a wide size distribution of the micropores in the pillared clay. Introduction of sodium ions converted the surface of the pore walls from hydrophobic to hydrophilic and blocks some micropores, enhancing water adsorption but reducing nitrogen and cyclohexane adsorption. Existence of Na+ ions in the pores did not improve the thermal stability of the pillared clay. Calcination at high temperatures resulted in a decrease in adsorption capacity. After calcination at 700°C, cyclohexane was inaccessible to the remaining micropores in the Na+ doped pillared clays. The adsorption behavior was clearly related to the cation content as well as the calcination temperature. These results may be useful in developing desiccants and adsorbents from pillared clays for dehumidification and adsorptive cooling applications.     

Effect of first drying upon the pore structure of hydrated alite paste

Water losses and gains, nitrogen adsorption, and imbibition of organic fluids were measured in a study of the pore structure of two, well-hydrated alite pastes that had original water/alite ratios of 0.4 and 0.6 by weight. Drying from saturation to a relative humidity in the range 0.3 to 0.8 was a particularly slow process probably because some pores emptied via smaller pores. The nitrogen adsorption measurements, on pastes in which the water has been removed by organic fluid replacement, suggested that micropores were closing as a result of capillary tensions developed at relative humidities between 0.7 and 0.4. The loss of surface area was recoverable with resaturation of the pastes with water. Imbibition of water and organic fluids indicated that there was an irrecoverable pore volume loss after drying and rewetting that was dependent upon the relative humidity of drying.     

Modeling of diffusive mass transport in micropores in cement based materials

In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity.     

Studies on expansive cement II. Hydration kinetics,surface properties and microstructure

Hydration kinetics were followed by measuring non-evaporable water and free sulphate contents. Surface areas, total pore volumes and the microstructure of the hardened expansive cement pastes are discussed. Nitrogen and water vapour were used as adsorbates in the measurement of the surface areas and pore volumes and the results obtained are compared with each other. Scanning electron microscopy was employed to study the microstructures of the hardened pastes.     

Ettringite formation in compressed expansive cement pastes

Type I expansive clinker pastes of w/c=0.25 were compressed at pressures of 50, 200 and 500 Kg/cm². Several parameters of the hardened pastes, measured at various curing ages, were compared with those of the non-compressed (control) pastes. Relative values ( values) were developed to express the changes due to compression in total porosity, non-evaporable water, compressive strength, as well as in specific surface area. Surface area changes are due to changes in the size and subsequent growth of the ettringite crystallites. This work is affected by the available pore space. SEM gave direct evidence in favour of a through-solution mechanism at least beyond one day of hydration.     

Surface properties of cement hydration products. I. Pore structure of calcium silicate hydrates prepared in a suspension form

Adsorption–desorption isotherms of water vapour at 30° were determined on preparations of calcium silicate hydrates of different molar ratios of lime/silica before and after free lime and free silica extractions. From these isotherms specific surface areas were measured and total pore structure analysis was evaluated. A surface layer thickness (t) curve for the adsorption of water vapour on calcium silicate hydrates was constructed, based on experimental curves previously published for the number of layers adsorbed on a variety of non-porous solids with the same heat of adsorption as the calcium silicate hydrates. The existence of both wide pores and micropores was detected in the calcium silicate hydrates. When the samples were free from uncombined lime and silica the micropores were estimated to have an average hydraulic radius of 4–5 Å, whereas the wide pores had an average size of 10 Å. Wide pores up to 200 Å also exist. The pores were predominantly parallel plates.     

Hardened alite pastes of various porosities II. Morphology and microstructure

The hardened alite pastes were prepared by using initial water:alite ratios of 0.20, 0.30, 0.45 and 0.60. The microstructures of the low and high porosity pastes were investigated by scanning electron microscopy. The results obtained indicated that the cementing system is really a composite material having a structure controlled by the total pore system of the hardened paste. In low porosity pastes, the hydration products are semi-crystalline; whereas in high porosity pastes, the hydration products possess a massive tabular structure.     

Some comments on the microstructure of hardened cement pastes

The effect of water/cement ratio of hardened cement pastes in the range 0.23 to 0.71 on the micropore structure as revealed by mercury intrusion and nitrogen adsorption porosimetry is discussed. The relationship between these micropores and the solid minerals as revealed by electron microscopy is also discussed.     

Effect of Curing Conditions on the Capillary Porosity of Hardened Portland Cement Pastes

The capillary pore structure of hardened portland cement pastes cured by high-pressure steam, chemical acceleration, high-pressure steam with reactive SiO₂, water immersion, water immersion and high-pressure steam, and hot-pressing was measured using mercury porosimetry to 50,000 psi. Differences of > 2 orders of magnitude exist in the average capillary pore diameters of the cement pastes studied. The largest pores (∼1 to 3 μm in diameter) are associated with high-pressure steam-cured pastes. The smallest average capillaries observed were 0.02 μm for pastes steam-cured with reactive SiO₂. Hot-pressed pastes had essentially no porosity accessible to mercury. The application of pore size control to problems of polymer-impregnated concrete is discussed.     

Pore structure of air-entrained hardened cement paste

The effect of air entrainment on the pore structure of hardened cement paste was investigated. Air-entrained and air-free samples of various water-cement ratios and ages were prepared by a well-defined procedure. The first and second-intrusion pore-size distribution curves of the samples were determined by mercury intrusion porosimetry. It was observed that sample preparation technique affects the pore-size distributions of hardened cement pastes. The second-intrusion curves indicated a decrease in the total volume and a reduction in the size of pores that are uniform in cross section with decreasing age and water: cement ratio. The second-intrusion curves of air-entrained and air-free pastes of equal water: cement ratio and age matched with each other. It was concluded that air entrainment introduces only large air voids observable by a naked eye and does not alter the characteristic fine pore structure of hardened cement paste appreciably.     

Influence of pozzolans and slag on the microstructure of partially carbonated cement paste by means of water vapour and nitrogen sorption experiments and BET calculations

The influence of water-to-binder ratio (0.33 to 0.50) and additions (fly ash, slag, silica fume) on the microstructure of partially carbonated cement pastes was studied by nitrogen sorption and static and dynamic water vapour sorption. The selected technique affects macropore condensation and accessibility of pores, while predrying influences removal of CSH interlayer water. BJH calculations showed the increased amount of capillary pores with higher water-to-cement ratio, and the decrease of micropores (< 2 nm), in pastes with 50% or more fly ash or slag. Paste with 10% SF showed a high amount of gel pores, related to the higher amount of CSH gel, calculated from adsorption at 23% RH. A linear relation was observed between BET specific surface and water-cement ratio. Thermogravimetric analysis illustrated the influence of water-cement ratio and pozzolanic materials on the portlandite content. Introduction of silica fume, increased the specific surface accessible to water, but not to nitrogen molecules.     

The significance of physical factors on the adsorption of polyaromatic compounds by activated carbons

The adsorption of three synthetic organic compounds (SOCs) (i.e., phenanthrene, biphenyl, and 2-chlorobiphenyl), with similar physicochemical properties but different molecular conformations (i.e., planar and nonplanar), by an activated carbon and an activated carbon fiber was investigated. The physical characteristics of the carbons were obtained from low temperature nitrogen adsorption isotherms using BET, DR, and DFT models. Their surface chemistry was characterized by water vapor adsorption, pH of the point of zero charge, acid/base uptakes, and elemental analysis. The results indicated that adsorbent pore structure characteristics and adsorbate molecular conformation are important in the adsorption of SOCs by porous carbonaceous adsorbents. To predict the adsorption of SOCs by activated carbons, accurate characterization of pore shape and size distribution in primary micropores is important. The results indicated that adsorbate molecules can access and fill more efficiently the slit-shape pores than ellipsoidal pores, whereas the ellipsoidal pores create higher adsorption potential than slit-shape pores. Both molecular conformation and dimensions of adsorbate affect the adsorption. Planar molecules appear to access and pack in slit-shape pores more efficiently as compared to nonplanar molecules. Nonplanar molecular conformation weakens the interactions between adsorbate molecules and carbon surfaces.     

碳化过程中大水灰比水泥浆体孔结构的变化(英文)

使用特殊的增黏剂与聚羧酸减水剂,制备了掺加石灰石粉、高炉矿渣、硅灰等混合材的普通波特兰水泥浆体和和低热硅酸盐水泥浆体(水粉比为1.0)。这些水泥浆体在20℃的水中养护4年后基本完全水化。这些硬化水泥浆体在5%(质量分数)CO2、相对湿度66%和温度20℃条件下进行碳化,对比研究碳化前后水泥浆体孔结构的变化。结果显示:碳化浆体内孔直径大于10nm的孔体积明显减少;碳化浆体的孔径分布向大孔径范围偏移;掺加混合材的硬化水泥浆体结构明显趋于松散;与不掺加任何混合材的水泥浆体相比,掺加混合材的水泥浆体的孔径更大。