The Pore Structure of Hydrated Cementitious Compounds of Different Chemical Composition

The pore structure ofβ-C₂S, C₃S, and portland cement pastes was investigated using mercury porosimetry and H₂O and N₂ adsorption. The β-C₂S had more total macro- and mesoporosities than C₃S and portland cement pastes of a similar degree of hydration. C₃S and portland cement pastes had similar total porosities but differed in the porosity size distribution. In the mesopore range, the various test methods gave different results. These differences are discussed on the basis of the various models proposed for cement paste. It is shown that shrinkage could be correlated with the volume of pores <0.03 μm, but not with total porosity.     

Rearrangement of Porous CaO Aggregates During Calcite Decomposition in Vacuum

High-resolution SEM photographs, N₂ adsorption isotherms, Hg porosimetry, and micrometer measurements were used to characterize CaO particle shapes and pore-size distributions that result when calcite crystals are decomposed in vacuum at 686°C. The surface area of the CaO produced from large calcite crystals is constant at 116.4 m²/g independent of the extent of reaction. The volume occupied by a CaO aggregate is 98±2% that of the original calcite crystal. The ∼54% total porosity is comprised of 42% pores of ∼5 nm cross section and 12% pores of ∼10 μ m cross section. The duplex pore structure is formed by a diffusionless repacking of CaO particles that initially form with a more uniform distribution of particles and pores.     

Acoustic and Elastic Properties of PZT Ceramics with Anisotropic Pores

It has been shown that anisotropic porosity in PZT reduces the d₃₁/d₃₃, ratio, raising the hydrostatic piezoelectric response of PZT significantly. In the present work, this behavior is related to Poisson's ratio by applying the Gibbs thermodynamic energy function, literature data for dense PZT ceramics and for materials with hexagonal crystal symmetry, and experimental results for PZT with anisotropic porosity. Acoustic as well as mechanical tests of PZT ceramics with anisotropic pores demonstrate the elastic anisotropies that cause the reduction in Poisson's ratio. The dependence of d₃₁/d₃₃ on Poisson's ratio is also detailed. In addition, a close relation between d₃₁/d₃₃ and the longitudinal acoustic velocity is demonstrated. Flexural strength tests indicate that anisotropic pores provide dh_n-enhanced PZT ceramics with significantly greater strength than isotropic pores.     

Effects of Porosity on the Threshold Strength of Laminar Ceramics

Ceramic laminates exhibiting a threshold strength have been fabricated by dip-coating thick tape-cast Al₂O₃ layers into slurries containing mixtures of Al₂O₃ and either unstabilized zirconia (MZ-ZrO₂) or mullite to produce thin compressive layers via both a molar volume change and a differential thermal contraction. Porosity was introduced into the thin compressive layers by adding rice starch to the dip-coating slurries, which decomposed during densification of the laminate. As the volume fraction of porosity is increased, the residual compressive stress (σ_C), as measured by piezospectroscopy, is reduced and approaches zero at approximately 0.65 volume fraction of porosity. The elastic modulus mismatch ( E ₁/ E ₂) between the thin and thick laminate layers accounted for approximately one-half of the threshold strength for volume fractions of porosity ≤ 0.30 ( E ₁/ E ₂<0.4). Above 0.40 volume fraction of porosity, the strength significantly increased as did the scatter in strength values, and it was observed that the highly porous layers completely arrested crack extension; these materials no longer exhibited a threshold strength. For these laminates, failure occurred by the independent, sequential failure of one layer after another, followed by catastrophic failure due to delamination.     

Densification of Large Pores: I, Expriments

Three different diameters (0.50, 1, and 2 μm) of monosized spherical pores were introduced, at different volume percents up to 15%, into two submicrometer commercial Zr(Y)O₂ powders which exhibited either sluggish (ZrO₂–3 mol% Y₂O₃) or rapid (ZrO₂–8 mol% Y₂O₃) grain growth kinetics. Densification and grain growth behavior was studied as a function of heat-treatment period at 1400°C and/or 1500°C. Pores in the finer-grain Zr(3Y)O₂ densified more rapidly. The grain growth kinetics of the Zr(3Y)O₂ material were unaffected by porosity, because the pores were always larger than the grains. Consistent with the Zener concept applied to mobile pores on grain junctions, the spherical pores influenced the grain growth kinetics of Zr(8Y)O₂ once the grain size was greater than the pore size. Measurements indicated that the number of pores per unit volume decreased dramatically during densification, whereas their average size remained approximately constant.     

Mercury Porosimetry of Pharmaceutical Powders and Granules

Effect of scanning speed and moisture content of the sample on the result of mercury porosimetry analysis of mannitol and microcrystalline cellulose (Emcocel® 50M) powders and granules produced by wet granulation were studied. In high-pressure porosimetry analysis, the smallest pores (diameter <20 nm) of the powders and granules could not be detected accurately when high scanning speeds were used. If the total pore volume is the only parameter of interest, fast scanning speed can be used, because the scanning speed does not affect this value. In high-pressure porosimetry analysis, the volume of the smallest pores (diameter <40 nm) of mannitol and microcrystalline cellulose granules increased with increasing water content. For powders, no effect of moisture on the volume of the smallest pores was observed. Thus, the increase in the volume of the smallest pores of granules with increasing moisture is related to the structure of the granules. Measurement of the water content of the samples together with proper drying of the samples before the porosity measurement is extremely important.     

Investigations on the structure of fully hydrated portland cement and tricalcium silicate pastes. II. Total porosity and pore size distribution

On a series of OPC pastes hydrated at 20 °C and 90 °C and Ca₃SiO₅ pastes hydrated at 20 °C the total porosity and pore size distribution were measured. Data obtained by mercury porosimetry and nitrogen adsorption did not agree well as the volumina of pores filled with Hg and N₂ differed. Regardless on the method employed the found pore size distribution depended on the starting water-solid ratio; as equal porosity, it was different in different hydrated materials.     

Nanostructure and Micromechanical Properties of Silica/Silicon Oxycarbide Porous Composites

The microhardness–nanostructure correlation of a series of silica/silicon oxycarbide porous composites has been investigated, as a function of pyrolysis temperature, T _p. The pyrolyzed products have been studied by means of scanning electron microscopy, mercury porosimetry, chemical analysis, solid-state ²⁹Si-NMR, X-ray diffraction, Raman spectroscopy, and microindentation hardness. Two distinct regimes are found for the microhardness behavior with T _p. In the low-temperature regime (1000°C ≤ T _p < 1300°C), the material response to indentation seems to be dominated by the large amount of pores present in the samples. In this T _p range, low microhardness values, H , are found (<110 MPa). Above T _p= 1300°C, a conspicuous H increase is observed. In this high-temperature regime ( T _p= 1300–1500°C), microhardness values are shown to notably increase with increasing pyrolysis temperature. The H behavior at T _p= 1300–1500°C is discussed in terms of (i) the volume fraction of pores and the average pore size, (ii) the bond density of the oxycarbide network, and (iii) the occurrence of a nanocrystalline SiC phase.     

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.     

Non-destructive investigations of pore morphology of micropore carbon materials

Pore morphology of experimental micropore carbon materials was explored using non-destructive methods. The influence of the heat treatment on porosity of two micropore carbon materials was investigated using helium porosimetry (HP) and X-ray micro-computed tomography (µCT). Methodology of µCT image processing and separation of the total and the open porosity is presented. In both materials open pores were found over the whole height of the sample. The local pore thickness and tortuosity of the materials were calculated. The increase of the porosity after heat treatment is observed for both materials.     

Properties of Highly Porous Hydroxyapatite Obtained by the Gelcasting of Foams

Open-cell hydroxyapatite (HA) foams, produced through the novel technique of gelcasting foams with relative porosities ranging from 0.72 to 0.90, were characterized for pore-size distribution, surface area, permeability, compressive strength, elastic modulus, and microstructural features. The porous structure, which is composed of an array of spherical cells interconnected through windows, had a mode pore diameter in the range 17–122 μm, as demonstrated by mercury porosimetry. The BET specific surface area increased from 1.5 to 3.8 m²/g as the sample porosity increased. The compressive strength and elastic modulus were in the range 1.6–5.8 MPa and 3.6–21.0 GPa, respectively. The permeability constants, k ₁ (Darcian) and k ₂ (non-Darcian), were strongly dependent on porosity fraction and varied widely, from 1.22 × 10⁻¹¹ to 4.31 × 10⁻¹⁰ m² and from 1.75 × 10⁻⁶ to 8.06 × 10⁻⁵ m, respectively. This combination of properties make the HA foams suitable for a variety of potential applications in the biomedical field, preferentially nonloading, including materials for bone repair, carriers for controlled drug-delivery systems, and matrixes for tissue engineering.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Effect of Grain Growth on Pore Coalescence During the Liquid-Phase Sintering of MgO-CaMgSiO4 Systems

During the liquid-phase sintering of MgO-CaMgSiO₄ systems in N₂ atmosphere, the total porosity and the average pore size increase while the number of pores decreases. The negligible permeability of entrapped nitrogen through the liquid matrix and the observed linear relationship between the number of pores and that of the MgO grains suggest that the pores coalesce as a consequence of grain growth during sintering. An analysis of the balance between the pressure of the entrapped N₂ gas and the capillary pressure shows that pore coalescence in turn causes the observed porosity increase. When Fe₂O₃ or Cr₂O₃ is added to MgO-CaMgSiO₄, the pore size and the total porosity become larger or smaller, because the grain growth is accelerated or retarded, respectively.     

Processing of a Novel Multilayered Silicon Nitride

A new type of silicon nitride with a layered structure of alternating dense and porous layers was obtained by addition of β-Si₃N₄ whiskers to the porous layers. The materials consisted of dense layers 60 μm thick and porous layers 40 μm thick with a final porosity of about 30%. Highly anisotropic shrinkage behavior was observed during sintering. A large addition of whiskers to the porous layers resulted in layers with well-oriented and tightly tangled elongated grains, where porosity is represented by anisotropic shaped pores.     

Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina

The real part of the permittivity (epsilon') and the tan of sintered alumina (Al₂O₃) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan is found to depend very strongly on the pore volume, purity, and grain size. ɛ' is far less sensitive to impurities and grain size. The dependence of ɛ' on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and ɛ' of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near-theoretical density ranges between 9.1 × 10₋₅ and 2.4 × 10₋₅ depending on grain size.     

Study of the Bimodal Pore Structure of Ceramic Powder Compacts by Mercury Porosimetry

Mercury porosimetry was used to study the pore-size distribution of both green and partially sintered alumina compacts. Experiments showed that using the mercury-penetration volume data to study the pore structure changes during sintering for a compact with a bimodal pore-size distribution could lead to wrong conclusions because the mercury-penetration volume was not linearly proportional to the porosity. Instead, the porosity distribution should be used. When the porosity of large pores was <10%, some large pores within the compact were observed to become isolated or channeled to the compact surface; this caused errors in the porosity distribution measured by mercury porosity.     

Preparation and Sintering of Colloidal Silica-Potassium Silicate Gels

Porous silica gelled from colloidal silica and potassium silicate mixtures as developed by Shoup has been investigated. Various gels exhibit an average pore size from 50 to 250 nm and a porosity up to 85%. Dealkalization by solution leaching is found to depend on the average pore size and is easier for large pore sizes. A domain model has been proposed for the gel networks to account for the observation of both macropores and mesopores as measured by Hg porosimetry and N₂ desorption, respectively. The pore coarsening observed during sintering is explained by the material transport from areas around interdomain pores to those around intradomain pores in order to minimize the total surface energy.     

Fabrication of Mullite Ceramics With Ultrahigh Porosity by Gel Freeze Drying

Porous mullite (3Al₂O₃·2SiO₂) ceramics with an open porosity up to 92.9% were fabricated by a gel freeze-drying process. An alumina (Al₂O₃) gel mixed with ultrafine silica (SiO₂) was frozen and sublimation of ice crystals was carried out by drying the frozen body under a low pressure. Porous mullite ceramics were prepared in air at 1400°–1600°C due to the mullitization between Al₂O₃ and SiO₂. A complex and porous microstructure was formed, where large dentritic pores with a pore size of ∼100 μm contained small cellular pores of 1–10 μm on their internal walls. Owing to the complete mullitization, a relatively high-compressive strength of 1.52 MPa was obtained at an open porosity of 88.6%.     

Microstructure Development in Al2O3-Platelet-Reinforced Ce-ZrO2/Al2O3 Composites

Composites containing Ce-ZrO₂, Al₂O₃, and aligned Al₂O₃ platelets were produced by centrifugal consolidation and pressureless sintering, followed by heat treatments at 1600°C for varied duration. Constituents in the consolidated microstructures were either uniformly distributed throughout or segregated into gradient layers, depending critically on platelet content. Quantitative image analysis was used to examine microstructure development with heat treatment. Changes in the volume fraction, dimensional anisotropy, and gradient of pores and platelets, as well as changes in the phase gradient, were quantified. Microstructure development was strongly dependent on the initial microstructure design attained from suspension processing.     

Porosity in Spinel Compacts Using Small-Angle Neutron Scattering

Unfired spinel (MgAl₂O₄) compacts and sintered materials with small hard agglomerates (<5 μm) were studied using small-angle neutron scattering (SANS) techniques. The SANS results were compared with those from mercury porosimetry and gas adsorption. The results from green-state samples are consistent with interconnected "ink-bottle"-type porosity. In the latter stages of densification the average void size is significantly larger than that found in the unfired compact. The presence of the hard agglomerates affects the observed SANS scattering much more in the partially densified samples than in the unfired compacts. It was demonstrated that the use of multiple SANS techniques to study large voids (<0.1 μm) and large pore fractions (45%) is a useful, sensitive, nondestructive diagnostic probe for the evaluation of porosity during sintering.