A review of cement-based materials as electroceramics

The dielectric behavior of unpoled cured cement-based materials enables these materials to serve as electroceramics. The behavior entails the DC polarization (apparent DC electrical resistivity increase), permittivity (AC polarization, capacitance measurement) and DC electret (permanent electric dipole, voltage measurement). The dielectric behavior is not derived from functional admixtures such as the perovskite ceramics. The polarization involves charge-carrier polarization, with the carriers being primarily the ions in the pore solution. Dipolar polarization associated with the polar water molecules plays a minor role. Silica fume, if present, decreases the permittivity, partly due to the pore refinement. A polymer admixture, if present, increases the permittivity, with significant polarization resulting from the cement-polymer interface. Carbon fiber, if present, affects the electronic and ionic conduction, with the fiber’s ozone treatment promoting the ionic conduction and enhancing the permittivity. As the water/cement ratio increases, the permittivity increases, but the DC polarization decreases. The DC polarization occurs faster and more significantly than the subsequent depolarization. This reflects the electret, which discharges upon short circuiting (as in capacitor discharge) and subsequently charges back upon open circuiting. The temperature increases the permittivity or the electret’s electric field, whereas tension decreases the same, enabling capacitance-based/voltage-based self-sensing of temperature and stress/strain.     

Mixed electron and proton conductivity of polyaniline films in aqueous solutions of acids: beyond the 1000 S cm−1 limit

BACKGROUND: The application potential of conducting polymers depends on their conductivity. It is generally assumed that the conductivity determined in the dry state is a parameter that unambiguously characterizes them. RESULTS: The conductivity of polyaniline (PANI) films immersed in aqueous solutions of sulfuric acid may be more than 1000 times higher compared with that obtained by measurement of dry films in air, and is estimated to reach a value exceeding 3300 S cm^?1 in 1 mol L^?1 sulfuric acid. This is explained by the reduction of conductivity barriers between conducting PANI islands. CONCLUSION: The organized polymer chains in the conducting islands of a PANI film are separated by disordered regions of low conductivity in the dry state. The penetration of sulfuric acid solution into the disordered areas increases the overall conductivity of the PANI film by improving the electrical contact between the islands through ionic charge transport. The electronic conductivity of the PANI film in the dry state thus converts to mixed electron–proton conduction in acidic aqueous solutions, electron conductivity being dominant in ordered regions and ionic conductivity in disordered regions separating them. Weakly bound protons are the most important ionic charge carriers hopping along the PANI chains. Copyright © 2009 Society of Chemical Industry     

Supplementary cementitious materials for mitigating degradation of kraft pulp fiber-cement composites

Kraft pulp fiber reinforced cement-based materials are being increasingly used where performance after exposure to environmental conditions must be ensured. However, significant losses in mechanical performance due to wet/dry cycling have been observed in these composites, when portland cement is the only cementitious material used in the matrix. In this research program, the effects of partial portland cement replacement with various supplementary cementitious materials were investigated. Binary, ternary, and quaternary blends of silica fume, slag, Class C fly ash, Class F fly ash, metakaolin, and diatomaceous earth/volcanic ash blends were examined for their effect on the degradation of kraft pulp fiber-cement composite mechanical properties (i.e., strength and toughness) during wet/dry cycling. After 25 wet/dry cycles, it was shown that binary composites containing 90% slag, 30% metakaolin, or greater than 30% silica fume did not exhibit any signs of degradation, as measured through mechanical testing and microscopy. Ternary blends containing 70% slag/10% metakaolin or 70% slag/10% silica fume were also effective in preventing degradation. A reduction in calcium hydroxide content and the stability of the alkali content due to supplementary cementitious material addition were shown to be primary mechanisms for improved durability.     

Microstructural effect of the shrinkage of cement-based materials during hydration, as indicated by electrical resistivity measurement

The effect of shrinkage during hydration on the microstructural change in cement-based materials was studied by measuring electrical resistivity and shrinkage strain. The microstructural change caused by shrinkage, as indicated by the fractional change in resistivity at the same shrinkage strain, is diminished with silica fume and increased with sand. The resistivity abruptly and irreversibly increases at a shrinkage strain of 3×10⁻⁴. The increase is negligible when silica fume is present. The fractional change in resistivity per unit shrinkage strain is much larger than the fractional change in resistivity per unit compressive strain in the cured state.     

Dispersion of carbon fibers and conductivity of carbon fiber-reinforced cement-based composites

Dispersion of carbon fibers in the cement matrix remains a hot topic in the preparation of carbon fiber-reinforced cement-based composites (CFRC) because it affects greatly both the mechanical and electrical properties of the composites. In this work, a new dispersant hydroxyethyl cellulose was used with the aids of pre-dispersion by ultrasonic wave to realize the uniform distribution of chopped carbon fibers in the cement matrix. The fracture surface of the prepared CFRC was observed by scanning electron microscopy, the elemental distribution was investigated by energy dispersive spectroscopy, and the components was analyzed by X-ray diffraction. Influences of carbon fiber lengths and contents, water/cement weight ratio, molding process, curing time, and silica fume content over the conductivity of the CFRC composites were studied. The mechanism of conductivity was discussed. Results shown that the electrical resistivity intended to decrease with the increasing of carbon fiber contents. The mass fraction 0.6% of carbon fibers was a turning point. The concentration of hydroxyethyl cellulose between 1.66% and 1.86% was mostly beneficial for the dispersion of carbon fibers. The resistivity was increased first and decreased then with the increase of water/cement ratio. When the CFRC sample was prepared by the vibrating pressing method, the resistivity of the sample was reduced far greatly than that of the sample by the vibrating method. The incorporation of silica fume into the CFRC composites exerted not only a good effect on the dispersion of carbon fibers, but also increased the density of the composites to further influence the conductivity of the CFRC.     

Preparation of scrap tire rubber fiber–silica fume mixtures for modification of clayey soils

This experimental work was performed to investigate the influence of silica fume–scrap tire rubber fiber mixture inclusion on the geotechnical properties of clayey soils. The natural and modified clayey soil samples were subjected to the unconfined compression, the shear box, the odometer and the falling-head permeability tests after compaction at optimum moisture content. The results of experimental research indicated that silica fume, fiber and silica fume–fiber mixture modification enhanced both the unconfined compression strength and strength parameters. Although, the fiber modification increased in the hydraulic conductivity, it decreased in the swelling pressure. It was observed also that the silica fume and silica fume–fiber modification decreased both the hydraulic conductivity and swelling pressure. Consequently, it is concluded that the silica fume–fiber mixture materials can be successfully used for the modifications of clayey soils in the geotechnical applications.     

Partial replacement of carbon fiber by carbon black in multifunctional cement-matrix composites

Cement reinforced with discontinuous carbon fiber is known for its piezoresistivity-based strain sensing ability, its electrical conductivity and the consequent multifunctionality. The high cost of carbon fiber is disadvantageous. Both carbon fiber and carbon black (used with silica fume in the amount of 15% by mass of cement) increase the DC conductivity and the EMI shielding effectiveness of cement, but carbon fiber is more effective than carbon black. Partial (50%) replacement of carbon fiber by carbon black lowers the cost, in addition to increasing the workability, while the electrical conductivity and the electromagnetic interference shielding effectiveness are maintained. However, the partial replacement reduces the strain sensing effectiveness. Total replacement of carbon fiber by carbon black diminishes both the conductivity and the shielding effectiveness, further reduces the strain sensing effectiveness, decreases the compressive modulus and increases the compressive strain at failure, while the compressive strength is maintained. The increased workability due to the partial replacement enables a higher total conductive admixture content to be attained. The maximum total conductive admixture content is 3.5% by mass of cement. In contrast to fiber replacement, the addition of carbon fiber to cement with carbon black decreases the compressive strength, strain at failure and density.     

超细高活性矿物掺合料对UHPC水化和收缩性能的影响

本试验研究了超细高活性矿物掺合料(超细掺合料)与硅灰以单掺、复掺的方式制备超高性能混凝土(UHPC),分析了复掺不同掺量超细掺合料对UHPC的工作性、力学性能、水化热和收缩性能的影响。结果表明:UHPC流动性随超细掺合料掺量的增加而增加,跳桌流动度最高为275 mm;将超细掺合料与质量分数为10%的硅灰以复掺的方式制备UHPC时,随超细掺合料掺量的增加,UHPC抗折强度先增加后降低,抗压强度先增加后趋于平稳,最大抗折强度和抗压强度分别为25.9 MPa和150.0 MPa;超细掺合料与质量分数为10%的硅灰复掺制备的UHPC水化热随超细掺合料掺量增加,先增大后减小;复掺质量分数为10%的超细掺合料与质量分数为10%的硅灰制备的UHPC早期收缩量最小,比单掺质量分数为20%的硅灰制备的UHPC低50.92%。     

Electric polarization and depolarization in cement-based materials, studied by apparent electrical resistance measurement

Electric polarization in cement-based materials (without conductive admixture) under an applied DC electric field was found by apparent electrical resistance measurement to be faster than subsequent depolarization under a reverse field by a factor ranging from 5 to 8. The slow depolarization suggested a degree of ionic trapping. In contrast, depolarization was even faster than polarization in carbon fiber cement, due to the fast hole response. Sand addition slowed down polarization saturation, but enhanced the polarization. Silica fume addition did not slow down polarization saturation, but diminished the polarization slightly. An increase in temperature enhanced the polarization due to increase in ionic mobility.     

Use of fly ash as an admixture for electromagnetic interference shielding

The use of fly ash as an admixture results in enhancement of the electromagnetic interference (EMI) shielding effectiveness from 4 to 8 dB at 1 GHz, whereas the use of silica fume as an admixture results in negligible effect on the shielding effectiveness. The DC electrical resistivity is decreased slightly by silica fume, but is essentially not affected by fly ash. Both fly ash and silica fume cause slight increases in the reflectivity. The effectiveness of fly ash for shielding is attributed to the Fe₂O₃ component (15.4 wt.%) in the fly ash.     

低温(10℃)-干湿循环下水泥砂浆抗硫酸盐侵蚀试验研究

在低温(10 ℃)-干湿循环双重环境下,对不同水灰比不同胶凝材料方案的水泥砂浆试件的抗硫酸盐侵蚀性能进行了试验研究,其中水灰比采用0.5和0.36,胶凝材料分别为普通硅酸盐水泥、中抗硫酸盐硅酸盐水泥和在普通硅酸盐水泥中分别掺入15%矿粉+1%硅灰和15%矿粉+3%硅灰.结果表明:在低温(10 ℃)-干湿循环双重条件下,既存在化学侵蚀又存在物理侵蚀,但是以物理侵蚀为主;通过降低水灰比或者使用抗硫酸盐硅酸盐水泥能显著提高砂浆抗硫酸盐侵蚀性能;在不同的水灰比下,复掺矿粉和硅灰会得到不同的效果,在低水灰比时能提高抗硫酸盐侵蚀的性能,在高水灰比时反而会降低抗硫酸盐侵蚀的性能.     

竹笋壳纤维的力学性能

为了了解竹笋壳纤维的基本力学性能,为竹笋壳纤维的进一步开发利用提供理论依据,研究了竹笋壳纤维拉伸断裂性能、松弛性能和定伸长弹性性能等力学指标。结果表明,竹笋壳纤维断裂性能指标拉伸断裂强度、初始模量和断裂伸长率分别为3．21cN／dtex、214．32cN／dtex和2．01％,在湿态下竹笋壳纤维的拉伸断裂强度和初始模量下降较大,分别下降了38．6％和33．1％,断裂伸长率变化不大;比较干、湿态下竹笋壳纤维的抗应力松弛性能和定伸长抗伸回弹性,湿态下的抗应力松弛性能和弹性能力较优。     

Studies on chemical resistance of low water/cement ratio concretes

Solutions containing mineral acids, and certain organic acids and salts are highly corrosive to portland cement concrete. Since permeability is the key factor governing the rate of deterioration, it is customary to use a low water-cement ratio in making concretes or concrete overlays required to resist corrosive action of aggressive chemical solutions. Pozzolanic admixtures are often used to provide additional protection against acidic attack. Highly reactive pozzolanic admixtures, such as condensed silica fume, which rapidly react with calcium hydroxide and reduce both the alkalinity and permeability of concrete are now being used for improving durability. During the last two decades, latex admixtures have also found widespread application. The polymeric constituents of a latex seem to coat the alkaline hydration products of portland cement, thus protecting them from attack by aggressive solutions.

An experimental study was undertaken to evaluate the relative chemical resistance of low water-cement ratio concretes, containing either a styrene-butadiene latex or a silica fume admixture, to the following solutions; 1% HCl, 1% H₂SO₄, 1% lactic acid, 5% acetic acid, 5% ammonium sulfate, and 5% sodium sulfate. Time taken to register 25 percent weight loss by fully submerged concrete specimens was used as a criterion for failure. From the data it appears that, except for the ammonium sulfate solution, the concrete containing the silica fume generally showed better resistance to chemical attack than other concrete types.     

超高性能混凝土基体的组成与微结构关系研究

用交流阻抗、差热分析等技术研究了不同矿物掺合料组成和水胶比下,UHPC基体的组成与微观孔结构关系.结果表明,矿粉与硅灰的掺入可以极大地改善基体的孔隙结构.在粉煤灰和矿粉最密堆积下,UHPC基体的体积电阻随着硅灰掺量的增加而增加,当硅灰掺量大于20％,低水胶比时,基体的体积电阻达到相对最大值.当水胶比大于0.18时,UHPC基体的体积电阻随着水胶比的增大而减小.通过孔隙吸水率研究表明,在热养护2 d条件下,UHPC基体的孔隙吸水率总体上随着硅灰掺量的减少以及水胶比的增大而增加,在水胶比0.22以下孔隙结构可能开始不连通.差热分析的研究表明,硅灰的掺入降低了Ca(OH)2含量,增加了C-S-H凝胶含量,说明硅灰通过发生火山灰反应改善UHPC基体的孔隙结构,在掺量大于20％、水胶比相对较低的情况下连通孔隙最少,达到最优孔隙结构.     

天然橡胶湿法混炼的研究进展

根据补强填充体系种类,分别介绍炭黑、白炭黑和石墨烯等与天然橡胶的湿法混炼工艺技术,并对天然橡胶湿法混炼所面临的问题及发展趋势进行了分析和展望。湿法混炼研究包括不同补强填充体系水乳液的配制方式和配方优化及湿法混炼胶的后处理工艺。今后应对湿法混炼多组分复杂反应的特征和混炼胶结构与性能的关系开展深入研究,并加强自动化与连续化生产工艺的研发。     

Effects of silica fume, latex, methylcellulose, and carbon fibers on the thermal conductivity and specific heat of cement paste

Due to their poor conductivity, latex (20–30% by weight of cement), methylcellulose (0.4–0.8% by weight of cement), and silica fume (15% by weight of cement) decreased the thermal conductivity of cement paste by up to 46%. In addition, these admixtures increased the specific heat of cement paste by up to 10%. The thermal conductivity decreased and the specific heat increased with increasing latex or methylcellulose content. Short carbon fibers (0.5–1.0% by weight of cement) either did not change or decreased the thermal conductivity of cement paste, such that the thermal conductivity decreased with increasing fiber content due to the increase in air void content. The fibers increased the specific heat due to the contribution of the fiber-matrix interface to vibration.     

沸石粉取代部分硅灰的超高性能混凝土力学性能研究

相比于硅灰,沸石粉是一种可就地取材、价格低廉的矿物掺合料。采用沸石粉取代硅灰制备超高性能混凝土(UHPC),研究了沸石粉掺量、水胶比和钢纤维体积掺量对沸石粉UHPC力学性能的影响。结果表明:沸石粉取代部分硅灰降低了UHPC的3 d强度,而随着龄期的增加,15%(质量分数)取代率的沸石粉增加了其强度,30%(质量分数)沸石粉取代率影响不大;沸石粉有助于改善UHPC后期韧性;水胶比的增加降低了沸石粉UHPC强度,但水胶比为0.16和0.14的试件强度相差不大;适量钢纤维有助于提高沸石粉UHPC强度,其最佳体积掺量范围为2.5%~3.0%。     

硅灰对超轻水泥基复合保温材料性能的影响

超轻水泥基复合保温材料(UCIM)是以水泥为胶凝材料,膨胀聚苯乙烯泡沫塑料(EPS)颗粒、掺合料、泡沫剂、改性剂和水等为主要原料,采用物理发泡工艺制备而成。UCIM由EPS颗粒与泡沫混凝土基体互穿构成,不同品种的掺合料等效替代水泥后,能不同程度影响水泥浆体对EPS颗粒的包裹性,从而影响UCIM结构的均匀性与制品性能。通过设计不同掺量的掺合料,对比硅灰、偏高岭土及矿粉所制备的UCIM的均匀性及强度,结果表明,当采用硅灰时,UCIM未产生分层离析现象且制品强度试验结果较好;通过微孔拍摄及强度、热工性能测试,系统研究了硅灰掺量对UCIM的泡沫混凝土基体的孔结构、强度和导热系数的影响,结果表明,适宜掺量的硅灰能提高UCIM的力学性能,使UCIM的泡沫混凝土基体的平均孔径减小,进而有利于降低UCIM导热系数。     

Novel zein‐based electrospun fibers with the water stability and strength necessary for various applications

BACKGROUND: Some of the problems with electrospun zein fiber are that it has very low tenacities in the dry and wet states and that mats of the fiber become films when immersed in water. The fibers are therefore unusable for various applications despite their good biocompatibility and biodegradability. This research was conducted to overcome these problems by electrospinning novel fibers containing various concentrations of zein, citric acid (CA) and sodium hypophosphite monohydrate (SHP) and by crosslinking the zein with CA and with SHP serving as a catalyst. RESULTS: The CA‐crosslinked electrospun zein fiber has as much as 10‐fold greater wet tenacity and 15‐fold greater dry tenacity than regular electrospun zein fiber. The average diameter of these fibers is 451 nm, which is the smallest diameter ever reported for zein‐based electrospun fiber. A mat of this fiber retains its fibrous structure when immersed in water, and the fiber retains about 70% of its tenacity after 16 days at 50 °C and 90% relative humidity. CONCLUSION: The high dry and wet tenacities, good water stability and small diameter of the novel CA‐crosslinked electrospun zein fiber make it attractive for biomedical and other applications that expose zein to water or that require high surface area. Copyright © 2008 Society of Chemical Industry     

活性掺合料对环氧树脂修补砂浆综合性能的影响

为了探究活性掺合料对环氧树脂修补砂浆的改性效果,为修补工程应用提供依据.研究硅灰和粉煤灰对环氧树脂修补砂浆力学性能、粘结强度、尺寸稳定性和抗冻性的影响,并采用扫描电子显微镜(SEM)和压汞法探究和分析影响机理.结果表明:环氧树脂使砂浆抗压强度降低,掺入硅灰可补偿强度损失,掺粉煤灰砂浆的强度随龄期增加而增加,但其中28 ...