The1HT 1 study of the influence of clay addition on Portland cement hydration

Effects of addition of three standard clay minerals, Na-montmorillonite, illite and kaolinite, on Portland cement hydration properties were studied. The¹H spin-lattice relaxation of exchangeable water was monitored during hydration time and the data were processed by spin-grouping analysis. The values and evolution dynamics of both resolvedT ₁ components and corresponding magnetization fractions show that each day mineral lowers the fluidity of Portland cement paste and accelerates its hydration in dormant. In advanced stages of hydration, the Na-montmorillonite provides the accelerating influence, while the kaolinite exhibits the retarding effect. The final values of gel pores to capillaries percentage fractions ratio indicate a slightly lower porosity of samples with Namontmorillonite and a higher porosity of pastes with the same percentage of illite or kaolinite, regarding to the pure hardened Portland cement.     

Two-dimensional correlation relaxation studies of cement pastes

Two-dimensional nuclear magnetic resonance relaxation correlation studies of cement pastes have been performed on a unilateral magnet, the Surface GARField. Through these measurements, the hydration process can be observed by monitoring the evolution of porosity. Characteristic relaxation time distributions have been observed in different cement pastes: fresh white cement, prehydrated white cement and ordinary Portland cement. The observed T(1)/T(2) ratio in these cements has been shown to agree with expectations based on high field values.     

A remark on nano-particle stability of cement C-S-H gel

Hydrated pastes of ordinary Portland cement prepared with different water-to-cement ratios were investigated by using the small-angle neutron scattering technique in the region of Q ∈ (0.0045, 0.11) Å^?1. Samples of cement pastes were subjected to non-standard hydration conditions using a mix with D₂O, low RH, and water-to-cement ratios spread over a very wide interval (0.4; 1.4). The investigation was focused on testing the structural stability of nano-metric particles in the cement C-S-H gel. Owing to the high structural stability of these nano-particles, their average diameter might be used as a microscopic parameter characterizing the nano-metric structure of C-S-H gels. The average diameter of the nano-particles of the studied ordinary Portland cement CEMI 42.5 R-SC was found to be close to the value of 4.2 nm and independent of the water-to-cement ratios.     

Increase of the reactivity of densified silica fume by sonication treatment

Five silica fumes from different manufacturers were subjected to ultrasonic treatment in order to decrease particle agglomeration and improve particle dispersion. The effectiveness of the sonication was observed as a reduction in particle size distribution of sonicated silica fume (SSF) compared to non-sonicated silica fume. SSF was added to Portland cement, and then the hydrated paste was analysed by thermogravimetric analyses (TGA/DTG) and scanning electron microscopy (SEM/EDX). The results were compared with those of control pastes made with untreated densified silica fume (DSF), as well as a reference cement paste of ordinary Portland cement (OPC). A maximum grade of de-agglomeration by the sonication was obtained, with a high volume of particles of diameter less than 1 μm. Images obtained by transmission electron microscopy (TEM) of the SSF showed sintered particles that could not be fragmented by the treatment. Micro-structural characterisation results showed an increase in the reactivity of the silica fume after the treatment.     

Shielding of gamma radiation by hydrated Portland cement–lead pastes

Portland cement mixed with different percentages of granulated lead is studied as shields for gamma radiation. The samples are cured at 100% relative humidity for various time intervals. It is found that cement containing 5% lead has the higher attenuation coefficient value. The results are interpreted by study of the bulk density and the compressive strength of the pastes. The formation of more packed structure of tobermorite by lead addition is studied using the XRD analysis and the SEM micrographs.     

Blended pastes of cement and lime: Pore structure and capillary porosity

Microstructure of blended pastes of lime and cement were studied in this paper. An increment of complexity of the microstructure was found when pastes increase their percentage in cement. Microstructural characteristics as porosity, morphology of the pores, pore size distribution and surface fractal dimension were evaluated in the different pastes studying the modification with the variation of composition. The capillary water absorption is also evaluated obtaining higher capillary coefficients values for the pastes with higher amounts of lime. The increase of complexity of the microstructure, due to the cement in the pastes, leads to slight deviations of the parallel tube model.     

Photo-directed organization of silver nanoparticles in mesostructured silica and titania films

Carbon nanotubes (CNTs) have the potential to enhance the strength, toughness, and multifunctional ability of composite materials. However, suitable dispersion and interfacial bonding remain as key challenges. Composites that are formed by reactions with water, like Portland cement concrete and mortar, pose a special challenge for dispersing the inherently hydrophobic nanotubes. The hydration of Portland cement also offers a specific chemical framework for interfacial bonding. In this study, nanoscale silica functional groups are covalently bonded to CNTs to improve their dispersion in water while providing interfacial bond sites for the proposed matrix material. The bond signatures of treated nanotubes are characterized using Fourier transform infrared spectroscopy. In situ dispersion is characterized using cryogenic transmission electron microscopy and point of zero charge (PZC) measurements. At the nanoscale, interparticle spacing was greatly increased. A slight increase in the PZC after treatment indicates the importance of steric effects in the dispersion mechanism. Overall, results indicate successful functionalization and dramatically improved dispersion stability in water.

     

Hot filament CVD of Fe-Cr catalyst for thermal CVD carbon nanotube growth from liquid petroleum gas

A hot filament chemical vapor deposition (HFCVD) method was used to prepare Fe-Cr thin film on Si substrate. The produced layers were used as catalysts for growing carbon nanotubes (CNTs) from liquid petroleum gas (LPG) at 825 °C by thermal CVD (TCVD) method. To characterize the obtained catalysts or CNTs, X-ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Raman spectroscopy were used. CNTs were grown on HFCVD derived Fe-Cr catalyst with the LPG as carbon source successfully. It was found that an annealing process on catalysts enhances the surface concentration of Cr atoms and reduces the sizes of catalyst particles. The grown CNTs on annealed sample were morphologically denser with smaller diameters compared to the as deposited one. In addition, the effect of filament temperature on CNTs was investigated. By increasing the filament temperature from 850 to 1050 °C the surface density and diameters of CNTs were improved.     

Variation of microstructure with carbonation in lime and blended pastes

Carbonation, as a reaction of the curing process of both, cement and lime binders, modifies the microstructure. Several microstructure properties, namely porosity, pore size distribution, surface fractal dimension, and specific surface area have been investigated in this study to describe the effect of carbonation on microstructure. Both carbonated and non-carbonated pastes of lime and blended pastes of lime and cement having varying water/binder (W/B) ratios are studied. Results show that carbonation decreases the porosity, but not with the same intensity in all pore size ranges. The highest modification is between 0.03 μm and 0.01 μm in lime pastes and between 0.2 μm and 0.02 μm in 50% lime pastes, while in 80% lime pastes the modification is very small. It is also observed that carbonation is a function of the binder composition but not of the W/B ratio. Moreover, surface fractal dimension decreases during the carbonation process, while the specific surface area varies depending of the binder composition.     

Electron paramagnetic resonance study on metakaolin-admixtured cement paste at different hydrated periods

The present work reports the effect of metakaolin (MK) on the properties of Portland cement through electron paramagnetic resonance study. Cement pastes containing 0%, 10%, 20% and 30% replacements of MK with cement and a water to cement ratio of 0.4 have been prepared. The g factors of Fe(III) and Mn(II) impurities at different hydration ages have been related to changes in the setting time of cement. The increase in g _Fe values and simultaneous decrease in g _Mn values with an increase in the replacement percentage of MK are explained due to pozzolanic reaction.     

The effect of the autoclaving process and addition of silica fume on Portland cement in shielding gamma radiation

An autoclaving treatment for disks made of neat Portland cement and cement with 5% silica fume hardened pastes was carried out to obtain suitable shield blocks for gamma radiation sources. The attenuation parameters such as the total linear attenuation coefficients, the mean-free-paths and the half-value thickness for different thicknesses at various periods of autoclaving were obtained. It was found that the attenuation by the neat cement disks autoclaved for 12 h and of 13.8 cm thickness and the disks of cement with 5% silica fume autoclaved for 6 h and of 12.9 cm thickness were the same and identical to that obtained by 3 cm thick sheets of lead. These results are interpreted by measuring the bulk density of the samples and are supported by measuring the compressive strength for the hardened samples.     

生长温度对碳纳米管阴极场发射性能的影响

碳纳米管(Carbon Nanotubes,CNTs)场发射平面显示器(Field Emission Display,FED)与其他显示器比较显示了其独特优点,被认为是未来理想的平面显示器之一。碳纳米管阴极作为器件的核心部分,其性能的好坏直接影响显示器的性能。针对30~60英寸(76.2~152.4cm)大屏幕显示器所用的厚膜工艺,即采用丝网印刷法制备了碳纳米管阴极阵列,研究了化学气相沉积法在不同温度下生长的CNTs的场发射电流-电压特性,找到了适合FED用碳纳米管的最佳生长温度。结果表明生长温度越高(750℃),CNTs场发射性能越好。并用荧光粉阳极测试这些CNTs的场发射发光显示效果,验证了上述结论。     

A novel continuous process for synthesis of carbon nanotubes using iron floating catalyst and MgO particles for CVD of methane in a fluidized bed reactor

A novel continuous process is used for production of carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CVD) of methane on iron floating catalyst in situ deposited on MgO in a fluidized bed reactor. In the hot zone of the reactor, sublimed ferrocene vapors were contacted with MgO powder fluidized by methane feed to produce Fe/MgO catalyst in situ. An annular tube was used to enhance the ferrocene and MgO contacting efficiency. Multi-wall as well as single-wall CNTs was grown on the Fe/MgO catalyst while falling down the reactor. The CNTs were continuously collected at the bottom of the reactor, only when MgO powder was used. The annular tube enhanced the contacting efficiency and improved both the quality and quantity of CNTs.The SEM and TEM micrographs of the products reveal that the CNTs are mostly entangled bundles with diameters of about 10-20 nm. Raman spectra show that the CNTs have low amount of amorphous/defected carbon with I_G/I_D ratios as high as 10.2 for synthesis at 900 °C. The RBM Raman peaks indicate formation of single-walled carbon nanotubes (SWNTs) of 1.0-1.2 nm diameter.     

Microstructure and properties of Pb(Mg1/3Nb2/3)O3–Portland cement composites

Lead magnesium niobate, Pb(Mg_1/3Nb_2/3)O₃ (PMN), ceramics particles was mixed with Portland Cement (PC) using a water to cement ratio of 0.50 and PMN content at 40% and 60% by weight to produce 0–3 Pb(Mg_1/3Nb_2/3)O₃–Portland cement (PMN–PC) composites. Microstructure and hydration of the composites were investigated. Calcium silicate hydrate gel can be seen surrounding the PMN particles. Dielectric constant of the composites was found to increase with PMN content. Successful poling of the composites was achieved. Interestingly, SEM micrographs of the PMN40 composite clearly showed calcium silicate hydrate gel (an essential hydration product of Portland cement) surrounding the PMN particles. In PMN60 composite, the gel can also still be seen but of less quantity. This is thought to be due to the increase in the volume ratio of the ceramics where the amount of calcium silicate hydrate gel was reduced in relation to the overall volume.     

NMR T1–T2 correlation analysis of molecular absorption inside a hardened cement paste containing silanised silica fume

ABSTRACT

The influence of silanised silica fume addition on the pore size distribution and wettability of white cement paste was investigated using T₁–T₂ correlation nuclear magnetic resonance (NMR) relaxometry. Surface silanisation of silica fume particles was achieved by the hydrolysis reaction of APTES (3-Aminopropyltriethoxysilane) and condensation of the silanol functional groups on the surface. The methods used for characterisation of the silanised silica fume particles were scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). By adding silanised silica fume to the cement paste, the accessibility of water molecules to the porous system becomes restricted, leading to a lower permeability in comparison with the unmodified cement paste. Differential scanning calorimetry (DSC) measurements on the cement pastes saturated with Octamethylcyclotetrasiloxane confirm also that the size of inter-C–S–H and capillary pores is not influenced by the addition of silica fume in a detectable manner.     

Characteristics of Co-filled carbon nanotubes

The Co-filled carbon nanotubes (CNTs) film was produced on silicon substrate by electron cyclotron resonance microwave plasma chemical vapor deposition (ECR-CVD). The effects of different plasma powers of 200, 300, 400 and 500 W, on the morphology, structure and electrical properties of the CNTs film, were studied. The results showed that the surface density of the vertical nanotubes decreased when the plasma power was higher than 200 W. When plasma power of 300 W was used, the ends of the metal-filled carbon nanotubes (MF-CNTs) became straighter and more uniform. The Co-filled CNTs grown at 300 and 400 W had a current discharge at the applied voltages of 30 and 40 V, respectively. In addition, the surface morphology and the structure of the CNTs film were examined using scanning electron microscopy (SEM) and high-resolution field emission gun transmission electron microscopy (TEM). Energy dispersive X-ray spectroscopy (EDXS) analyses were performed to identify the composition of the material inside the CNTs.     

Photoconductivity enhancement in alkali metal doped multiwall carbon nanotubes

Photoconductivity effects in pristine and alkali-metal (K, Li) doped multiwalled carbon nanotubes (CNTs) were studied under xenon (100 mW) and also halogen (10 mW) light continues sources. To perform the measurements, the pristine and alkali doped CNTs were deposited into pores of a silver foam plate with nano-metric porosity by electrophoresis technique. The foam acted as a conducting frame for sweeping the photo-induced electrons to prevent rapid local electron–hole recombination in the CNTs. The radiation spectrum of the xenon source was similar to the Sun light spectrum and under normal ambient condition the photocurrents in the alkali doped samples were enhanced noticeably in comparison with the pristine CNTs. These results present a functional photoconductive performance of a heap of as-prepared alkali-metal doped CNTs that would be applicable as a light sensor without the necessity of separation between metallic and semiconducting CNTs (m- and s-CNTs).     

Acoustical properties of aerated autoclaved concrete

This work analyses acoustic qualities of autoclaved aerated concrete (AAC). Three the most widely used types of AAC are chosen for the analysis: gas cement concrete, gas cement concrete with combined binder (Portland cement and lime), and foam cement concrete. The procedure and technique of the materials’ formation is presented in this work. The evaluation of acoustic qualities of AAC is based on the material’s air permeability and porosity (i.e., ratio of the volume of the interconnected pores to the total volume of pores). For this purpose the measurements obtained by an acoustic interferometer are used. The results of the experiment show that regression equations for the AAC types, which density ranges from 250 to 500 kg/m³, may be used to estimate the materials’ normal incidence absorption coefficient values, which depend on the air permeability and porosity. Results show that absorption coefficient of not specially treated AAC is rather low. According to the measurements obtained in a special reverberation room of 202 m³, a sound absorption coefficient may increase up to 0.6, provided that slits of Helmholtz resonator’s type are made in the slabs of AAC gas cement concrete with combined binder.     

沉积工艺参数对碳纳米管薄膜场发射性能的影响

利用微波等离子体化学气相沉积(MWPCVD)方法,在不锈钢衬底上直接沉积碳纳米管膜。通过SEM、拉曼光谱和XRD表征,讨论了制备温度和甲烷浓度对碳纳米管膜场发射的影响。结果表明:不同条件下制备的碳纳米管膜的场发射性能有很大差异,保持氢气的流量(100sccm)、生长时间(10min)、反应室压力不变,当甲烷流量为8sccm、温度为700~800℃时,场发射性能最好,开启场强仅为0.8V/μm,发射点分布密集、均匀。     

Study of the dehydration of Portland Cement by Mössbauer spectrometry

Egyptian Portland Cement in the form of one inch cube was hydrated at different times of hydration. Nine cubes of each period of hydration were heated for five minutes 200, 300, 400 up to 1000°C then were quenched in air. The compressive strength was measured for these samples and related to unheated ones. These cubes were ground and measured by Mössbauer spectrometry to correlate the effect of dehydration of cement pastes on the states of iron, with the decrease of compressive strength. It was observed that starting from 400°C the central doublet characteristic of the hydration process decreased as the dehydration temperature was increased. At 1000°C the dehydration process was complete, the central doublet disappeared and the compressive strength vanished. The hydration process was found to be reversible. The application of Mössbauer spectrometry to estimate the degree of fire in concrete building was demonstrated.