The effect of porosity on the strength of foamed concrete

A study has been undertaken to investigate the effects of replacing large volumes of cement on the properties of foamed concrete (up to 75% by weight) with both classified and unclassified fly ash. This is the third paper in a series; it investigates the relationship between porosity and compressive strength and presents mathematical models that have been developed to describe this relationship. The compressive strength of the foamed concrete was shown to be a function of porosity and age, and a multiplicative model (such as the equation derived by Balshin) was found to best fit the results at all ages up to 1 year. In addition, it was concluded that the equation derived by Hoff could effectively be used to predict the compressive strength of foamed concrete mixtures containing high percentages of ash.     

On the fracture statistics of open-porous alumina foam structures

In the present study, industrial fabricated alumina foams of three different cell sizes were under study in terms of compressive and bending strength. Questioned were the impact of the cell dimensions and, in terms of bending strength, the loading rate and the specimen aspect ratio. The foam structures were characterised by the aid of micro focus X-ray computer tomography and analysed by optical microscopy. By normalising and averaging the load-displacement curves, the foam elasticity was analysed. The resulting sets of strength data were compared by the aid of four different probability distributions, namely the Normal, Log-Normal, Weibull, and Gamma distribution. For most of the test sets of the open-porous alumina foams, either the Normal or the Log-Normal distribution proved the best representation of the strength data.     

Low strain rate compressive failure mechanism of coarse grain alumina

The present work reports the dynamic compressive strength (σ_c) of a dense (~95%) coarse grain (~20 μm) alumina measured at ~30 °C as a function of strain rates $\dot{\left(\mathit{\epsilon }\right)}$ ranging from 10⁻⁵ to 5×10⁻¹ s⁻¹. The results showed a unique 40% enhancement of (σ_c) with the increase in $\dot{\left(\mathit{\epsilon }\right)}$. Extensive post mortem examination of fracture fragments obtained from the compressive failure tests by FESEM, TEM and HRTEM confirmed the formation of micro-cracks, shear bands, nanoscale cleavages and dislocations whose recurrence had increased with strain rate. Both shear induced microplasticity and nanoscale cleavages as well as dislocations had occurred concurrently yet independently during compressive fracture of coarse grain alumina even at very low strain rates. Based on these evidences a new compressive failure mechanism of alumina was proposed.     

Modification of alumina bubbles with ammonium aluminum sulphate

Ammonium aluminum sulphate was introduced to modify alumina bubbles by the wet chemical method. Distribution of ammonium aluminum sulphate, in situ decomposition characteristics, microstructure and mechanical properties of the modified alumina bubbles were investigated by SEM, XRD, BET and by a purposely designed experimental device for the measurement of compressive resistance. Experimental results showed that an ammonium aluminum sulphate thin film was formed on the surface of the alumina bubbles, at the same time, ammonium aluminum sulphate also infiltrated into the flaws and cavities of the alumina bubbles. After heat-treated at 900 °C, the ammonium aluminum sulphate on the alumina bubbles could be in situ decomposed to γ-Al₂O₃ with high activity. The compressive resistance of the modified alumina bubbles was enhanced from 15.6N to 38.7N after heat-treatment at 1700 °C for 2 h.     

板状SiC增韧Al_2O_3的R－曲线行为

本工作用刻痕弯曲强度技术研究了板状ＳｉＣ增韧的热压Ａｌ２Ｏ３材料的Ｒ－曲线行为。随着裂纹的扩展，其断裂韧性显著地增加，最后趋向于平稳值约８．５ＭＰａ·ｍ１／２。这个结果与已报告过的纯Ａｌ２Ｏ３和ＳｉＣ晶须增强Ａｌ２Ｏ３的断裂阻力进行了比较。     

Strength distribution of particles under compression

From time immemorial people dealt with size reduction processes (mill, mineral liberation, etc.). As time has passed industrial units for comminution processes have become larger and more sophisticated, but still they perform with low efficiencies [1], [2] and [3]. The strength of a particle is one of its most crucial characteristics due to the mechanical stresses experienced by each particle within an industrial unit. This is because the final size of particles is mostly dependant on the strength distribution of the raw material [4]. In this present study, the ability of a number of statistical formulations to accurately describe the strength distribution of particles was examined. Additionally, selected equations were analyzed and a general expression including the effect of the material and particle size was developed. A number of approaches to define particle strength were considered, and strength in terms of crushing force was chosen. Particle strength in terms of force and in terms of energy was also compared and found to be size independent. Finally, particle strength in terms of stress was examined and compared to the particle strength in terms of force.The ability to describe the compression strength distribution will significantly improve the accuracy of the comminution processes simulation, design and optimization.     

焦炭气孔率对焦炭热性能的影响

通过研究40kg试验焦炉单种煤焦炭、配合煤焦炭及工业焦炉焦炭的气孔结构及焦炭热性能,得出气孔率对焦炭热性能的影响。焦炭气孔率对焦炭热性能有较大影响,随着气孔率的增加,CRI增加,CSR降低;工业焦炉焦炭气孔率与焦炭热强度之间关系密切,气孔率每增加1％,CRI增加0．48％,CSR降低1．46％。用气孔率预测焦炭热性能,对指导焦炭生产、控制焦炭热性能具有指导意义。     

A study on the relationship between porosity of the cement paste with mineral additives and compressive strength of mortar based on this paste

Mercury intrusion porosimetry study was carried out on ordinary Portland cement (OPC) pastes with 10% to 40% mineral additives, such as steel-making slag, granulated blast furnace slag and fly ash. For all samples, the porosity of paste and compressive strength of mortar based on this paste were determined at 3, 7, 28, 90 and 180 days. Relationship between the porosity and strength was investigated and some equations for the strength-porosity relationship were presented according to Balshin multiplicative model. Results show that mineral additives delayed process that micropore structure of OPC paste developed and strength development of sample with mineral additives was faster than that of OPC sample. Balshin equation fits the results of strength and porosity of all samples and there is a strongly quantitative relationship between strength and porosity. After being mixed with mineral additives, the intrinsic strength σ₀ and power n both increased and the sequence of σ₀ and n for different mineral additives was fly ash>steel-making slag>blast furnace slag.     

Semiconductor nanostructures in an alumina template matrix: micro- versus macro-scale photoelectrochemical behavior

We show herein that the photoelectrochemical behavior of a given semiconductor nanodot (p-CuSCN or n-TiO₂) in an alumina template matrix, is remarkably different than that of its macro-sized counterpart. Three separate examples of this distinct difference in behavior are presented. It is shown how the photoresponse (e.g. photocurrent) may be amplified (from a low level typical of the signal emanating from a ∼10⁻¹¹ cm² region corresponding to a semiconductor nanodot) by using a large number of electrically inter-connected Au nanowires to support the overlying semiconductor nanodots. The anomalous photoresponse of p-CuSCN nanodots in the template matrix was also numerically simulated by a simple parallel equivalent circuit consisting of a semiconductor and a photocapacitor. Possible practical application scenarios are finally presented for these nanostructures.     

Porosity features and gas permeability analysis of bi-modal porous alumina and mullite for filtration applications

Bimodal porous structures were prepared by combining conventional sacrificial template and partial sintering methods. These porous structures were analysed by comparing pore characteristics and gas permeation properties of alumina/mullite specimens sintered at different temperatures. The pore characteristics were investigated by SEM, mercury porosimetry, and capillary flow porosimetry. A bimodal pore structure was observed. One type of pore was induced by starch, which acted as a sacrificial template. The other pore type was due to partial sintering. The pores produced by starch were between 2 and 10 µm whereas those produced by partial sintering exhibited pore size of 0.1–0.5 µm. The effects of sintering temperature on porosity, gas permeability, and mullite phase formation were studied. The formation of the mullite phase was confirmed by XRD. Compressive strengths of 37.9 MPa and 12.4 MPa with porosities of 65.3% and 70% were achieved in alumina and mullite specimens sintered at 1600 °C.     

叔丁醇基凝胶注模成型制备氧化铝多孔陶瓷

以微米级Al2O3粉料为原料,叔丁醇为溶剂,采用凝胶注模成型工艺制备了氧化铝多孔陶瓷,并研究了Al2O3浆料的固相体积分数(分别为8%、10%、13%和15%)对1 500℃保温2 h烧后氧化铝多孔陶瓷的气孔率、气孔孔径分布、耐压强度、热导率和显微结构的影响.结果表明:当Al2O3浆料的固相体积分数从8%增加到15%时,氧化铝多孔陶瓷烧结体的总气孔率从71.2%逐渐降低至61.2%,气孔平均孔径从1.0 μm逐渐减小至0.78 μm,耐压强度从16.0 MPa逐渐增大至45.6 MPa,而热导率从1.03 W·(m·K)-1逐渐增大至1.83W·(m·K)-1.     

Inkjet printing and nanoindentation of porous alumina multilayers

The objectives of this study were to analyse the effect of inkjet 3-D printing parameters, particularly the splat overlap distance, for the fabrication of defect-free porous Al₂O₃ ceramic multilayers, and to correlate the resulting porosities with the mechanical properties measured using nanoindentation. An aqua-based alumina ink was used in this study to fabricate the multilayers on dense alumina substrates by inkjet printing. The as-printed specimens were dried and sintered at 1200–1500 °C. The resulting microstructural features of each specimen and their corresponding porosities were studied using FIB-SEM. Elastic modulus and hardness were determined using the spherical nanoindentation technique. Results showed that defect-free porous alumina multilayers with excellent layer to layer and layer to substrate integrity were successfully fabricated. The porosity-dependence of the elastic modulus and hardness was shown to be consistent with values predicted using empirical expressions, despite the presence of abnormal grain growth at higher temperatures.     

Preparation of alumina foams by the thermo-foaming of powder dispersions in molten sucrose

The alumina powder disperses in molten sucrose due to the hydrophilic interaction between the particle surface and sucrose hydroxyls. The thermo-foaming of the dispersions is due to the bubbles created by the water vapour produced by the OH condensation at 150 °C which are stabilized by the alumina particles adsorbed on the gas–liquid interface as well as the increase in viscosity. The foaming time, the foam setting time and the foam volume depend on the alumina powder to sucrose weight ratio. The alumina foams have interconnected cellular microstructure and the cells are having a near spherical morphology. The porosity (97.84–93.29 vol.%.) decreased and the average cell size (0.54–1.2 mm) increased with the increase in alumina powder to sucrose weight ratio (0.4–1.4). The alumina foams with density in the range of 0.239–0.267 g/cc showed compressive strength in the range of 1.02–1.47 MPa.     

Influence of preparation method and alumina content on crystallization and morphology of porous yttria stabilized zirconia

Influence of addition of alumina and preparation methods (sol-gel synthesis and mechanochemical preparation) on crystallization and morphology of yttria stabilised zirconia was examined. Presence of alumina was found to delay crystallization of zirconia, the effect being more pronounced at higher alumina content. The two oxides form easily distinguished separate phases. Milling lowers the crystallization temperatures of the sol-gel derived powders since nuclei are formed during the milling and smaller particle size allows easier removal of residual organic components. The milling results in crystallization of some monoclinic zirconia, both in sol-gel derived powders and in case of mechanochemical processing. There are no significant differences between the preparation methods in pore size and relative density of sintered tablets: powders obtained by mechanochemical processing and milled sol-gel derived powders both give tablets with homogeneous morphology. The advantage of sol-gel process is preparation of pure tetragonal zirconia phase without traces of monoclinic phase.     

Dispersion and gelation behavior of alumina suspensions with Isobam

As a copolymer of isobutylene and maleic anhydride, Isobam, a newly-developed gelling agent, plays a significant role in ceramic forming process. However, dispersion and gelation behavior of Isobam has not been discussed in detail yet. To investigate the influence of other dispersants, pH and electrolytes on dispersion and gelation behavior of Isobam, zeta potential, viscosity and modulus curves during gelation were measured from 50?vol% alumina suspensions with 0.1–0.7?wt% Isobam. A two-step bridging mechanism is proposed to explain the gelation behavior of Isobam, providing guidance on usage of Isobam in combination of other additives to achieve suspensions with high solids loading and adjustable gelation characteristic.     

Preparation of AlN micro-honeycombs with high permeability via freeze-casting

Highly permeable AlN micro-honeycomb (AlN-H) ceramics with unidirectional macropore channels and porous struts were fabricated via tertiary butyl alcohol (TBA)-based freeze-casting. The effect of the AlN solid loading of the freezing slurries on the microstructure, open porosity, N₂ gas permeability, and compressive strength of the as-prepared AlN-Hs were systematically investigated. The results showed that the honeycomb structure and open porosity of the AlN-Hs can be adjusted by altering solid loading. With the increase in solid loading from 10 vol% to 30 vol%, the approximate pore channel size of the AlN-Hs decreased from 50.1 μm to 15.6 μm, strut thickness increased from 8.2 μm to 16.6 μm approximately, and the corresponding open porosity decreased from 87.6–56.6%. The as-fabricated AlN-H with an open porosity of 64.9% possessed high N₂ gas permeability and essential compressive strength and can be used as a catalyst support or filter in industries.     

Correlations among processing parameters and porosity of a lightweight alumina

In this study, lightweight alumina was fabricated using α-Al₂O₃ micropowder as the raw material and corn starch as a pore-forming agent. Orthogonal experiments were designed to investigate the effect of particle size, pore-forming agent addition, and sintering temperature on the density and porosity of the lightweight alumina. The experimental results were analysed using a one-way analysis of variance and non-linear fitting, and the correlation between each processing parameter and property was discussed. The results indicate that the bulk density and total porosity of lightweight alumina are mainly affected by the pore-forming agent addition, while the sintering temperature is the main contributor to the apparent and closed porosity of the samples. Based on the Brook theory, dynamics analysis was performed on various samples. The difference in physical properties of various samples arose from differences in the relationship between grain boundaries and pore migration velocity. By adjusting the processing parameters, lightweight alumina with low bulk density, low apparent porosity, and high closed porosity could be obtained.     

Experimental study of the mechanical behavior of plastic concrete in triaxial compression

This paper is going to present the results of an extensive experimental parametric study of the mechanical responses of various types of plastic concrete in unconfined and triaxial compression tests. Plastic concrete consists of aggregates, cement, water and bentonite, mixed at a high water cement ratio, to produce a ductile material. It is used for creating an impermeable barrier (cut-off wall) for containment of contaminated sites or seepage control in highly permeable dam foundations. A plastic concrete cut-off wall acts essentially as a barrier to stop or reduce the groundwater flow. In this study the effect of specimen age, cement factor, bentonite content and confining pressure on shear strength and permeability of plastic concrete were investigated. The observed behavior is more and more ductile for increasing confining pressure. It is shown, also, that any increase in confining pressure increases the compressive strength as well as the elastic modulus and the deformability of the specimen. It is shown that an increase in cement factor increases the shear strength as well as the elastic modulus. It is obtained that increase of bentonite content, decreases the compressive strength as well as the elastic modulus. Increasing the age of the specimens causes an increase of the compressive strength as well as the elastic modulus and also the shear strength parameters are affected. Also, it is obtained that increase in confining pressure and cement factor reduces the permeability.     

Effect of additives on porosity of alumina materials obtained by freeze casting

The influence of two different additives, glycerol and polyvinyl alcohol (PVA), on the rheological behaviour and freeze casting performance of 35?vol.-% aqueous alumina suspensions is studied. Suspensions with PVA were prepared by either mixing all the components together or adding the PVA in a second step on the dispersed slurry. Although the slight increase in viscosity suggests that competitive adsorption occurs, the microstructure seems not to change depending on the order of addition of additives. Considering its lower molecular weight, glycerol provides lower viscosity, and as a consequence of its cryoprotective character, sintering leads to smaller pore size, being the porosity 35% for an added content of 10?wt-% with respect to solids. In the case of PVA, contents of 2?wt-% are enough to obtain solid firm bodies with a porosity of 48%. The porosity and the size of the pores decrease with increasing concentration of PVA.     

Low-temperature sintering of coarse alumina powder compact with sufficient mechanical strength

Coarse alumina powder compacts doped with various amounts of titania and copper oxide were pressurelessly sintered from 900 °C to 1600 °C. Their phase assemblages and microstructural evolution, as well as their properties, were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry/thermogravimetric (DSC/TG) analysis, and three-point bending and wetting test. The role of TiO₂ and CuO during the sintering is discussed in detail. The experimental results show that the liquid phase from the copper oxide appeared at approximately 1200 °C, so the solid-state reaction between alumina and titania took place at a lower temperature. Such solid state-reaction sintering had a strong impact on the grain growth and greatly promoted the densification of the alumina compact. In addition, the liquid phase inhibited the abnormal grain growth and microcracking. As a result, the coarse alumina powder compacts doped with 5 wt% TiO₂–CuO were fully densified and exhibited sufficient flexural strength (342±21 MPa) when sintered at a temperature of 1450 °C for 2 h.