利用粉煤灰常压干燥合成SiO2气凝胶的研究

为了实现粉煤灰回收利用的新途径,研究了一种利用粉煤灰为硅源常压干燥合成SiO2气凝胶的工艺.通过正交实验研究粉煤灰与氢氧化钠反应生成水玻璃的最佳工艺条件;所得水玻璃溶液通过硫酸催化或树脂交换碱催化法获得水凝胶,利用三甲基氯硅烷/乙醇/正己烷对水凝胶进行改性处理,在常压干燥下制备了SiO2气凝胶.利用BET、SEM和FTIR对气凝胶的微观结构及性质进行了研究,结果表明,所得气凝胶的比表面积为362.2～907.9m2/g、孔体积为0.738～4.875cm3/g、平均孔径为7.69～24.09nm,其中树脂交换碱催化法所得气凝胶的比表面积可达907.9m2/g,孔体积达4.875cm3/g.     

疏水SiO_2气凝胶的常压制备及应用进展

阐述了常压制备疏水SiO2气凝胶的关键技术,综述了表面后处理法、原位法等常压制备疏水SiO2气凝胶的工艺措施,介绍了疏水SiO2气凝胶在电磁学、光学、原子核物理等领域的应用现状,指出疏水处理可明显提高SiO2气凝胶的性能(低介电常数、高激光损伤阈值和耐环境能力等),从而拓宽了SiO2气凝胶的应用范围,最后展望了疏水SiO2气凝胶的发展和应用前景。     

二氧化硅气凝胶的制备和应用研究

三维纳米网络结构的SiO2气凝胶具有许多独特的性能,在光学、热学、声学、微电子学、高功率激光等领域具有广阔的应用前景,是目前材料科学家研究的热门材料之一.简要评述了SiO2气凝胶的制备技术和应用前景.     

纳米孔隔热材料——SiO_2气凝胶的表面改性及其表征

采用溶胶-凝胶法制备SiO2气凝胶,以三甲基氯硅烷/环己烷、二甲基二氯硅烷/环己烷体系为化学表面修饰剂,通过衍生法制备了疏水性SiO2气凝胶.利用气质联用对表面改性过程中发生的反应进行了剖析;并利用扫描电镜、红外光谱、比表面测定等测试方法对2种改性方法所获得的SiO2气凝胶的结构、形貌及性能组成进行了比较.研究表明,2种改性方法均可获得连续网络结构、多孔纳米材料.所得SiO2气凝胶的比袁面积分别为652m2/g和656m2/g,主要孔径尺寸为2～10nm.样品表面连有疏水基团,呈现明显的疏水性.     

SiO2气凝胶/壳聚糖复合药物载体材料的制备和表征

采用常压干燥法制备了SiO2气凝胶,所得气凝胶为介孔结构,比表面积618.8m2/g,孔径分布5～20nm.以SiO2气凝胶为原料,通过静电吸附法制备了SiO2气凝胶/壳聚糖复合药物载体材料,采用扫描电镜(SEM)、红外光谱(FTIR)等对复合材料的结构形貌进行了分析,研究了复合材料对硫酸庆大霉素药物的担载和释放性能.结果表明,所得SiO2气凝胶/壳聚糖复合材料为多孔网络结构,其中,由450℃处理的SiO2气凝胶制得的气凝胶/壳聚糖复合材料对硫酸庆大霉素具有较好的药物担载和缓释性能.     

SiO2气凝胶增强增韧方法研究进展

SiO2气凝胶的轻质纳米多孔网络结构使其在热学、声学、光学及电学等领域具有广阔的应用前景,但强度低和韧性差的缺点很大程度上限制了其应用发展.分析了SiO2气凝胶的主要力学性能指标和提高其强度的两种途径(控制制备工艺参数与材料复合法),讨论了这两种增强途径的最新研究进展.通过控制制备工艺参数获得结构均匀的SiO2气凝胶,并采用材料复合技术进一步提高SiO2气凝胶的强韧性能,可望获得结构稳定并具有良好强韧性的SiO2气凝胶材料.     

超级绝热材料SiO2气凝胶的制备及应用

介绍了超级绝热材料SiO2气凝胶的特殊纳米结构和超级绝热性能,对SiO2气凝胶新的制备方法和应用前景进行了综述,并指出SiO2气凝胶的未来研究重点和方向。     

SiO_2气凝胶力学性能的影响因素及改善方法

SiO2气凝胶的纳米多孔网络结构使其在热学、声学、光学、电学等领域具有广阔的应用前景,但其强度低和韧性差限制了发展和应用。对SiO2气凝胶力学性能的表征方法进行了概述,综述了制备工艺参数(如溶液酸碱度、组分配比、反应温度及时间、后处理工艺、干燥工艺等)对SiO2气凝胶网络结构及力学性能影响的研究进展,并介绍了材料复合法用于改善SiO2气凝胶力学性能的研究进展。     

超低密度SiO2气凝胶快速制备的新方法

以正硅酸乙酯(TEOS)为硅源,采用酸碱二步催化溶胶-凝胶法,结合超临界干燥技术制备了超低密度SiO2气凝胶.通过采用CH3CN为稀释剂,有效地降低了胶凝温度(室温),缩短了胶凝时间,从而达到了快速制备SiO2气凝胶的目的.利用SEM、BET等方法对干燥不同程度的低密度SiO2气凝胶微结构进行了研究.     

由多聚硅氧烷制备二氧化硅气凝胶

SiO2 气凝胶是一种新型轻质纳米多孔材料。本工作以多聚硅氧烷 (E -4 0 )为硅源 ,用溶胶 -凝胶法制备出了SiO2气凝胶。研究了催化剂、温度、水等因素对其溶胶 -凝胶过程的影响 ;讨论了F离子以及水对E -4 0水解 -缩聚反应的作用机理 ;并用孔径分布测定仪、TEM等方法对其微结构进行了研究。结果表明 :由E -4 0为硅源制备的SiO2 气凝胶骨架颗粒为十几nm ;孔径分布较广 ,其峰值在 3 0nm附近 ,此外 ,还存在几nm的微孔。     

氧化硅气凝胶隔热复合材料研究进展

氧化硅气凝胶由于纤细的纳米结构,具有极低的热导率,是一种新型轻质保温隔热的理想材料。但其力学性能和高温遮挡红外辐射能力差限制了气凝胶在该领域的应用。通过添加增强体和遮光剂研制气凝胶隔热复合材料是主要的解决方法。本文综述了近年来气凝胶隔热复合材料的制备方法,重点分析了气凝胶复合材料在力学性能和隔热性能方面的研究进展,并指出了存在的问题,对今后的研究提出了展望。     

纳米SiO2与粉煤灰协同改性水泥基材料性能研究

通过调整纳米SiO_2与粉煤灰的比例,研究了两者协同作用对水泥基材料性能的影响。结果表明,纳米SiO_2(NS)和粉煤灰协同作用效果优于NS单一掺加,3%(质量分数,下同)纳米SiO_2和不大于30%的粉煤灰同时掺加可以补偿粉煤灰引起的早期强度降低,且砂浆28d抗压强度不降低。随着NS掺量增加水泥基材料的干燥收缩增大,粉煤灰可以改善纳米SiO_2对干燥收缩的不利影响。随着NS掺量的增加,试件的抗冻性和抗氯离子渗透性能均得到提升,掺加3%NS与30%粉煤灰使水泥基材料达到最佳耐久性能。NS可以缩短水泥水化诱导期,加速水泥水化进程,且使胶凝体系总放热量增加。在水泥粉煤灰体系中掺入NS后,非蒸发水含量在早期明显增多,但在后期增长缓慢。     

图像处理方法在粉体粒度分布测量中的应用

为考察图像处理方法在粉体粒度分布测量中的应用,用数码相机和显微镜获取二氧化硅粉和粉煤灰样品的颗粒形貌图像,分别运用图像处理方法中的手动方式和自动方式测算了所选粉体样品的粒度分布。结果表明,二氧化硅粉的微分分布呈双峰特征,其累积分布曲线呈近似"S"型;粉煤灰的微分分布呈指数下降,其累积分布曲线为不严格"S"型;图像处理方法是适用于测量粉体粒度分布的方法之一。     

Influence of a fine glass powder on the durability characteristics of concrete and its comparison to fly ash

A detailed investigation carried out to ascertain the durability characteristics of fine glass powder modified concretes is reported in this paper. Tests were designed to facilitate comparisons between concretes modified with either glass powder or fly ash at the same cement replacement level. The optimal replacement level of cement by glass powder is determined from strength and hydration tests as 10%. The later age compressive strengths of glass powder and fly ash modified concretes are seen to differ by only 5%. The durability characteristics are ascertained using tests for rapid chloride permeability, alkali–silica reactivity, and moisture transport parameters. The chloride penetrability values indicate some amount of pore refinement. The potential of glass powder to reduce the expansion due to alkali–silica reaction is established from tests conducted in accordance with ASTM C 1260, but fly ash is found to perform better at similar replacement levels. Glass powder–fly ash blends that make up a 20% cement replacement level are found to be as efficient as 20% fly ash in reducing expansion. The control concrete is seen to exhibit the lowest overall moisture intake after 14 days of curing, and fly ash concrete the highest, with the glass powder concrete in between. The trend is reversed at later ages, demonstrating that both the replacement materials contribute to improved durability characteristics. The sorptivity and moisture diffusion coefficient values calculated from the moisture intake-time data also demonstrate a similar trend. These studies show that fine glass powder has the potential to improve the durability of concretes.     

Evaluation of blends of high and low calcium fly ashes for use as supplementary cementing materials

The present paper outlines the results of a research attempt aimed at developing and evaluating the performance of ternary blended cements, incorporating mixtures of two different types of fly ash (of high and low calcium content). The main target of this study was to investigate whether and by what means, the introduction of a certain type of fly ash into a fly ash–cement (FC) matrix containing a different type of ash, can improve the performance of the initial binary system. For achieving this, new pozzolans were prepared by mixing, in selected proportions, a high lime fly ash with an ash of lower calcium content. The efficiency of the new materials was examined in terms of active silica content, pozzolanic activity potential, strength development, k-values and progress of the pozzolanic action by means of fixed lime capabilities. The results obtained demonstrated that the mixtures containing equal amounts of each fly ash were the most effective for moderate cement substitution, whilst for higher replacements the intermixture possessing the highest active silica content shows supremacy at almost all hydration ages. The superior performance of the ternary fly ash blends was mainly attributed to synergistic effects detected for all the ashes utilized. These were quantified in each case and almost linear correlations were obtained with the k-values of the most efficient ternary mixes.     

Development of recycled strain-hardening cement-based composite (SHCC) for sustainable infrastructures

This paper presents the experimental results of an attempt to develop sustainable strain-hardening cement-based composite (SHCC) using recycled materials. SHCC exhibits desirable mechanical properties, including strain hardening and ductility. However, SHCC is composed of silica sand and a high volume of cement, which makes it more energy intensive than conventional concrete. The aim of this study is to promote SHCC sustainability in infrastructure design through the use of recycled materials. Alternative recycled materials – sand, fly ash, and polyethylene terephthalate (PET) fibers – are used to partially replace silica sand, cement, and polyvinyl alcohol (PVA) fibers, respectively, in SHCC specimens. The effects of the recycled materials on the mechanical behavior of the SHCC specimens are examined by conducting compressive tests, four-point bending (flexural) tests, and uniaxial tensile tests. Fundamental information is then used in the constitutive model to analyze and design infrastructures using SHCC with recycled materials. Test results indicate that fly ash improves both the bending and uniaxial tensile performance of SHCC due to an increase in chemical bond strength at the interface between the PVA fibers and cement matrices. However, SHCC that contains PET fibers does not perform well in the bending and uniaxial tensile tests due to the inferior material properties of the PET fibers, although its compressive behavior is similar to that of the PVA2.0 specimen. Also, it is noted that recycled sand increases the elastic modulus value of SHCC due to its larger grain size compared to that of silica sand. Based on the desire to maintain well-performing SHCC, a replacement ratio below 20% for fly ash or below 50% for recycled sand is deemed appropriate for creating sustainable SHCC, as concluded from this study.     

Investigating the role of reactive silica in the hydration mechanisms of high-calcium fly ash/cement systems

High-calcium fly ashes (ASTM Class C) are being widely used as a replacement of cement in normal and high strength concrete. In Greece such fly ashes represent the majority of the industrial by-products that possess pozzolanic properties. Even thought the contribution of factors, such as fineness and water/binder ratio, on the performance of fly ash/cement (FC) systems has been a common research topic, little work has been done on examining whether and to what extent reactive silica of fly ashes affects the mechanisms occurring during their hydration.The work presented herein describes a laboratory scale study on the influence of active silica of two high-lime fly ashes on their behavior during hydration. Volumes up to 30% of Greek high-calcium fly ashes, diversified both on their reactive silica content and silicon/calcium oxides ratio, were used to prepare mixes with Portland cement. The new blends were examined in terms of compressive strength, remaining calcium hydroxide, generation of hydration products and microstructural development. It was found that soluble silica of fly ashes holds a predominant role especially after the first month of the hardening process. At this stage, silica is increasingly dissolved in the matrix forming additional cementitious compounds with binding properties, principally a second generation C–S–H. The rate however, that fly ashes react in FC systems seems to be independent of their active silica content, indicating that additional factors such as glass content and fineness should be taken into account for predicting the contribution of fly ashes in the final performance of pozzolanic cementitious systems.     

Preparation and characterization of silica aerogels from oil shale ash

The silica aerogels were successfully synthesized using oil shale ash which is a by-product of oil shale processing via ambient pressure drying. The physical and textural properties of the silica aerogels have been investigated and discussed. The results showed that the organic modification of hydrogels was a crucial step during the processing which preserved mesopores in ambient pressure drying. The unmodified hydrogel underwent tremendous shrinkage during the drying and yielded microporous silica aerogel. Using this novel route, it could produce silica aerogel with low tapping density of 0.074 g/cm³, high specific surface (909 m²/g) and cumulative pore volume of 2.54 cm³/g. From the industrial point of view, the present process is quite suitable for a large scale production of powdered silica aerogel. Furthermore, it provides a new way to solve the problem of oil shale ash pollution.     

Influences of fly ash on magnesium oxychloride mortar

The application of magnesia-based construction materials draws much research interests nowadays due to the ever increasing awareness of environmental protection. By incorporation of fly ash into magnesium oxychloride (MOC) cement, an energy efficient and environmentally friendly repair material can be formed for successful industrial applications. In the current research, an appropriate formulation of the MOC matrix with a suitable combination of the molar ratios MgO/MgCl₂ and H₂O/MgCl₂ has been characterized by using phase diagram, X-ray diffractograms and scanning electronic microscope. Subsequently the influences of fly ash on the properties of both MOC cement and mortar are investigated. It is found that the incorporation of fly ash can enhance the workability or fluidity, retard the setting time, and improve the water resistance of the MOC mortars. With the enhanced performance and a slightly expansive nature, the MOC mortars incorporated with fly ash has a good potential to be used as a repairing material.