Viscoelastic behaviour of macro-defect-free cement

The viscoelastic behaviour of macro-defect-free (MDF) cement was studied by dynamic mechanical analysis. MDF specimens with various moisture contents in the range 1.70%–3.20% moisture, were measured at 1 Hz as a function of temperature from 34–96 °C and as a function of frequency from 0.05–5 Hz at 34°C. The viscoelasticity of MDF was found to be affected by moisture content through its plasticizing effect on the PVA binder. Time-moisture and temperature-moisture superposition of the shear moduli were found to be possible for MDF, and linear relationships between log time and linear moisture and between temperature and moisture were found. How the microstructure of MDF affects the viscoelastic response is also discussed through mechanical models. A comparison of the models with known experimental data suggests that the viscoelastic response arises from both direct connections between the inorganic particles and from connections through the polymer binder. Inorganic links are estimated to connect 45% of the inorganic phase.     

Effects of an organotitanate cross-linking additive on the processing and properties of macro-defect-free cement

We have investigated the effects of an organotitanate cross-linking additive on the processing and moisture sensitivity of macro-defect-free (MDF) cements. The modified samples were formed by a two-part process: first, the as-received calcium aluminate cement powder was coated with the organotitanate compound, followed by the standard MDF cement fabrication procedure. It was estimated that 6 wt% (by weight of the cement powder) of the additive was adsorbed. Rheological measurements showed that cross-linking reactions between the organotitanate additive and the polymer (polyvinyl alcohol-acetate) used to form MDF cement occurred during the high shear mixing process. This led to a 65% reduction in the processing window for the modified pastes as compared to the standard MDF cement pastes. However, the organotitanate-modified MDF cement samples exhibited improved moisture resistance as compared to the standard samples when exposed to 100% relative humidity at 22°C. The most significant result was that the modified samples retained nearly 100% of their initial strength after 200 days of exposure to these conditions, whereas the strength of the standard MDF samples was decreased by approximately 45%.     

Unreacted cement content in macro-defect-free composites: impact on processing-structure-property relations

The effect of unreacted cement content on the processing, structure, and properties of macro-defect-free (MDF) composites fabricated from calcium aluminate cement (CAC), -alumina (Al₂O₃), and polyvinyl alcohol-acetate (PVAA) has been investigated. Three systems were formed with initial CAC: Al₂O₃ ratios of 5050, 3565, and 2575 by volume in their respective formulations. The amount of unreacted cement was reduced from 68.1 vol% which is present in standard (100% CAC) MDF cement, to 14.9 vol % for composites with an initial CAC: Al₂O₃ ratio of 2575, while the hydration product content was reduced from 18.1 vol % to 11.4 vol % for these respective systems. A hard core/soft shell continuum percolation model was used to determine that alumina substitution did not significantly affect the percolative nature of the interphase and bulk polymer regions. However, experiments showed that the reduction in unreacted cement content through Al₂O₃ substitution affected both the processing and microstructural features of these composites. The moisture absorption kinetics and flexural strength of composites exposed to 100% relative humidity were also evaluated, and it was found that their moisture sensitivity improved with decreasing unreacted cement content. A hypothesis is presented to explain the role of unreacted cement on the moisture sensitivity of these materials.     

硅烷偶联剂改性MDF水泥的水敏性研究

为了改善MDF水泥的水敏性，以N-β-(氨乙基)-γ-氨丙基三甲氧基硅烷偶联剂对高铝水泥-聚乙烯醇基MDF水泥进行化学改性，测试了硅烷偶联剂改性MDF水泥的力学性能及耐水性，分析了改性MDF水泥的孔结构，并与未改性同种MDF水泥进行比较．结果表明：MDF水泥经硅烷偶联剂改性后，不仅抗弯、抗压强度、断裂韧性和微区硬度提高，孔隙减少，结构致密，而且表现出良好的耐水性，水浸泡90d的抗弯强度保持率由改性前的38%提高到改性后的80%，同时吸湿量和水的扩散系数下降。     

Mechano-chemical modification of cement with high volumes of blast furnace slag

The application of chemical admixtures significantly improves the performance of cement-based materials. Some admixtures can also be used to modify the cement grinding process and induce changes in the structure of cement minerals due to mechano-chemical activation. A reactive silica-based complex admixture was developed for the modification of cement grinding. This paper examines the effect of grinding on the strength of a modified cement containing granulated blast furnace slag in high volumes. According to the test results, mortars based on the modified cement possess a compressive strength of up to 91.7 MPa, a 62% increase over the reference.     

Evaluations of residual axial stresses and interfacial friction in Nicalon fibre-reinforced macro-defect-free cement composites

The residual axial stress and the interfacial frictional stress of Nicalon fibre-reinforced macro-defect-free cement composites have been evaluated using the indentation technique. In these composites, both the residual axial stress in the fibre and the interfacial bonding are relatively high compared to other ceramic composites. Owing to the high residual axial stress, Poisson's effect can be ignored in analysing the stress-displacement relation of the fibre, and the interfacial frictional stress along the sliding length can be assumed to be a constant during indentation loading and unloading. Because of the strong interfacial bonding, consideration of the upper bound of the sliding zone length during unloading is essential. The sliding zone length during unloading is limited by the debond length at peak loading. The theoretical predictions of stress-displacement relations are in excellent agreement with the experimental results.     

State of the art of macro-defect-free composites

Macro-defect-free (MDF) composites, developed and patented by scientists from Imperial Chemical Industries in the early 1980s, are very high strength cement–polymer composites. The preparation of MDF composites is different from the production of conventional cement paste in that high shearing with a roller mill as well as moderate pressure (about 5 MPa) and moderate temperature (about 80–100 °C) are applied during the production. Very low water/cement ratio (w/c) levels are achieved (as low as 0.10) in this composite, much lower than in other cement-based materials. Of the many unique properties exhibited by MDF composites, surely the most remarkable is their high flexural strength. This is generally attributed to their low porosity and to cross-linking reactions between cement and polymer. MDF composites may reach a flexural strength of 200–300 MPa levels, whereas ordinary cement pastes have generally around 5–10 MPa. However, serious durability problems are observed in MDF composites, particularly their significant reductions in strength when immersed in water. Comprehensive information about MDF composite research will help in understanding the reasons behind the high strength, microstructure and water sensitivity of MDF composites. This review summarizes the materials, production methods, properties, microstructure, hydration reactions, durability and potential application areas of MDF composites as published since 1981.     

Nature and behaviour of cement bonded particleboard: structure, physical property and movement

The paper records work carried out to analyse the structure and determine the behaviour of cement bonded particleboard (CBPB). The structure was quantified with respect to the structural parameters (distribution, size, shape and occupied area) of and interaction between individual components (pore, wood particle and cement paste), and the movement was analysed with respect to the structure of CBPB and the nature of the cement paste and wood particles. The results showed that: (1) The volume of CBPB occupied by the detectable pores is less than 1%, the areas occupied by wood particles are about 42% and 38% in vertical and horizontal surfaces respectively, and those by cement paste about 58% and 62%. (2) Orientation, size and shape of wood particles are very different between across the thickness and along the transverse directions of CBPB. (3) Equilibrium moisture content (EMC) and density have been complicated by the penetration of cement paste into wood particles and interfacial region between wood and cement paste. (4) CBPB was unstable under both constant and changing environmental conditions: exposure to a constant environmental condition resulted in an increase in mass and decrease in dimensions. Under a single change in relative humidity, the changes both in mass and dimensions on both adsorption and desorption consisted of two distinct stages: a significant change in the early stage of exposure and a gradual change in the later stage. Cycling under changing environmental regimes resulted in corresponding changes in mass and dimensions, and both reversible and irreversible behaviour occurred, giving rise to a series of displaced hysteresis loops which are very dissimilar to those for other materials. (5) The movement of CBPB has been attributed to the combined effects of moisture reaction, carbonation and degradation of CBPB, and all of these parameters gave rise to the development of incompatible stresses, which aggravated the above effects. Both mass and dimensional changes were essentially Fickian and non-Fickian. Models have been developed and able to predict both mass and dimensional changes effectively and efficiently. (6) The change of CBPB also reflected the change of wood particles and cement paste, and the strain and sorption of the CBPB have been successfully quantified in terms of moduli and volume/mass concentration of the wood particles and cement paste (the rule of mixtures).     

Cohesion aspects in rheological behaviour of fresh cement mortars

The cohesion of fresh cement mortars was measured with a device currently used for shear test on clays, the Laboratory Vane. The effect of the water/cement ratio and the specific surface of the cement on cohesion was examined. The influence of the addition of a superfluidizer based on melaminic resins on the cohesion, flow and compressive strength of the mortars was investigated.

---

Résumé Dans cet article on considère une composante du seuil de cisaillement des mortiers de ciment à l'état frais, la cohésion, mesurée avec un appareil normalement employé pour essayer le comportement au cisaillement des argiles, le ≪Laboratory Vane≫ On examine l'influence de la teneur en eau du mortier et de la surface specifique du ciment sur la cohésion, en déterminant l'existence d'un intervalle de rapports eau/ciment critiques (0,46–0,48), indépendant de la surface spécifique du ciment, où la cohésion du mortier est maximale et sa valeur est une fonction de la surface spécifique du ciment. Le rapport eau/ciment critique représente la limite inférieure du champ du comportement plastique du mortier et peut être défini comme la limite de plasticité. L'addition aux mortiers de ciment d'un adjuvant superplastifiant à base de résines de mélamine provoque le glissement de la limite de plasticité vers des rapports eau/ciment plus bas, au-dessous de 0,40 pour une addition de 5% en poids de ciment. On examine l'influence de l'addition d'adjuvant en différents pourcentages sur l'écoulement, la cohésion et la résistance mécanique dans un mortier plastique.

---

     

挤压316L金属蜂窝的烧结及其组织性能研究

以金属粉末、粘结剂为原料,经炼料制成膏状挤压料,通过挤压模成型为蜂窝状,再经高温烧结制备成316L不锈钢蜂窝.研究了烧结气氛、烧结温度对蜂窝烧结组织结构的影响,并对烧结后的蜂窝进行力学性能测试.结果表明,在氢气中烧结的316L蜂窝组织,金属颗粒间形成烧结颈,呈网状连接在一起,并随温度升高颗粒合并长大成晶粒,基体组织为Fe-Cr-Ni-C(γ-Fe)固溶体,第二相球形颗粒为富含硅的低熔点化合物;在真空中烧结,金属颗粒表面形成氧化物Fe2Cr4O4、Cr2O3,以及SiO2,大量的表面氧化物阻碍了金属粉末颗粒的结合,直接影响烧结蜂窝的强度,致使烧结蜂窝强度远低于氢气中烧结的蜂窝.在氢气中烧结的316L金属蜂窝,其径向抗压强度可达40～50 MPa,远高于目前广泛应用的陶瓷蜂窝载体,是作为载体材料的一种理想选择.     

对位芳纶/尼龙66的加工工艺对其力学性能的影响研究

由不同的加工工艺来研究对位芳纶/尼龙66(PPTA/PA66)复合材料的力学性能,结果表明:①随着PPTA纤维含量的增多,使用黏度较大的PA66基体进行复合时,PPTA/PA66复合材料的拉伸强度、弯曲强度都随之变大;对于冲击强度来说,PPTA纤维的加入,增加了其脆性,造成PPTA/PA66复合材料冲击强度不如原基体的冲击强度;②PPTA纤维含量在5%的情况下,当挤出机转速达到300r/min时材料的拉伸强度、冲击强度和弯曲强度出现极大值,此时的混合效果最理想;③较高的挤出温度有助于提高复合材料的拉伸强度和弯曲强度,对于冲击强度来说,PPTA/PA66复合材料冲击强度同样不如原基体的冲击强度;④PPTA纤维的长径比集中在65左右,芳纶纤维起到了增强效果。     

磷酸钙骨水泥化学活性及机械性能研究

在研究磷酸四钙制备方法的基础上,讨论其化学活性,发现该物质在高温时热稳定性不好,易于向能量更低的羟基磷灰石转化;在室温下又容易吸附空气中的水分子,发生缓慢水解.因此最好真空保存.     

MWNTs/丝素/聚酰胺共混静电纺纳米纤维毡的结构和性能

静电纺再生丝素纤雏具有在水中易溶胀、结构稳定性和力学性能差等缺陷.在前期研究聚酰胺6/66对静电纺丝素纤维的结构和性能改进效果的基础上,以多壁碳纳米管(MWNTs)增强静电纺丝素/聚酰胺6/66复合纳米纤维.研究发现,随着MWNTs含量的增加复合纤维毡的颜色由白色变成黑色,纤维直径逐渐减小,并且都在90nm以下.MWNTs的加入,还有效地提高了纤维的结晶度、热稳定性以及纤维毡的拉伸力学性能.     

Hierarchy of spatial interactions in the processing of contrast-defined contours

Both psychophysical and neurophysiological evidence suggest that there are two visual cortical processing streams, a linear stream that processes first-order stimuli and a nonlinear stream that also processes second-order stimuli. This evidence also suggests that before the extraction of the second-order signal, the nonlinear pathway broadly but not completely pools signals across initial linear filters that encode the orientation of the carrier of the second-order signal. The evidence suggests that such pooling does not occur across carrier spatial frequencies. We show that similar results are obtained with repulsion tilt illusions but not with attraction effects. Attraction effects exhibit complete orientation crossover (while retaining spatial frequency selectivity), perhaps indicating higher-level processing; an experiment on interocular transfer of the effects supported this conclusion.     

PVDF/PMMA共混挤出流延膜结构与性能研究

探讨了聚偏氟乙烯(PVDF)/聚甲基丙烯酸甲酯(PMMA)共混挤出流延膜的结构和性能.研究结果表明:PMMA的加入能够大大改善其微观结构,不但使结晶度降低,而且红外分析(IR)和广角x射线衍射(WXRD)证实,其中部分α晶型能明显地转变成β晶型;TGA研究表明,共混体系的稳定性比纯粹的PMMA稳定性提高,但PVDF的热稳定性只有很少降低;流变性能研究显示,PMMA含量在很宽的范围内体系扭矩变化不大,为选择加工条件提供了依据;力学性能测试显示出共混膜具有很好的力学性能.PVDF/PMMA共混挤出膜被证明是一种很有前途的薄膜材料.     

Effects of cement size distribution on capillary pore structure of the simulated cement paste

The evolution of tricalcium silicate (C₃S) microstructure during hydration is tri-dimensionally simulated based on an “Integrated Particle Kinetics Model”. The hydration degree, the contact surfaces between the hydrated particles, the hydraulic radius and the capillary pore size distribution of the simulated cement paste at various degrees of hydration are calculated. Three examples of the C₃S microstructure development with different size distributions are presented. The effects of the cement size distribution on pore structure of cement paste are demonstrated and the results are discussed. In these examples, the cement size distribution varies between 3–40, 5–40 and 10–40 μm, respectively.     

Multiscale mechanics of hierarchical structure/property relationships in calcified tissues and tissue/material interfaces

This paper presents a review plus new data that describes the role hierarchical nanostructural properties play in developing an understanding of the effect of scale on the material properties (chemical, elastic and electrical) of calcified tissues as well as the interfaces that form between such tissues and biomaterials. Both nanostructural and microstructural properties will be considered starting with the size and shape of the apatitic mineralites in both young and mature bovine bone. Microstructural properties for human dentin and cortical and trabecular bone will be considered. These separate sets of data will be combined mathematically to advance the effects of scale on the modeling of these tissues and the tissue/biomaterial interfaces as hierarchical material/structural composites. Interfacial structure and properties to be considered in greatest detail will be that of the dentin/adhesive (d/a) interface, which presents a clear example of examining all three material properties, (chemical, elastic and electrical). In this case, finite element modeling (FEA) was based on the actual measured values of the structure and elastic properties of the materials comprising the d/a interface; this combination provides insight into factors and mechanisms that contribute to premature failure of dental composite fillings. At present, there are more elastic property data obtained by microstructural measurements, especially high frequency ultrasonic wave propagation (UWP) and scanning acoustic microscopy (SAM) techniques. However, atomic force microscopy (AFM) and nanoindentation (NI) of cortical and trabecular bone and the dentin–enamel junction (DEJ) among others have become available allowing correlation of the nanostructural level measurements with those made on the microstructural level.