C-S-H凝胶的制备与性能研究

利用水热合成法制备了C-S-H凝胶,重点研究了不同的制备工艺、原材料与钙硅比对C-S-H凝胶结构的影响。经过详尽的X衍射(XRD)与核磁共振(NMR)分析表明,制备的样品都具备C-S-H凝胶的峰型特征;活性反应法相对于沉淀反应法制备的C-S-H凝胶层间距小、聚合度低;证实了C-S-H凝胶的层状结构,且层间距随钙硅比的增大而减小;Q~2/Q~2值与直链平均长度随钙硅比的增大而减小。     

氯离子环境下钙矾石和水化硅酸钙体系铝配位分布

通过化学合成钙矾石和水化硅酸钙,利用XRD、27 Al核磁共振测试手段,研究了氯离子环境下钙矾石(AFt)和水化硅酸钙(C-S-H)凝胶体系中铝配位分布规律。结果表明,在C-S-H凝胶和氯离子共存环境中,第三类水化铝酸盐(TAH)凝胶中六配位铝(Al[6])首先转化为四配位铝(Al[4])进入C-S-H结构,且Al[4]总量不受C-S-H钙硅比限制;同时氯离子不影响Al[6]-TAH向Al[4]转化,当达到C-S-H凝胶容纳Al[4]的极限时,剩余的Al[6]-TAH才与氯离子反应形成Friedel盐(F盐)。TAH比AFt更容易与氯离子反应形成F盐;低钙硅比C-S-H环境不利于TAH与氯离子结合,但利于钙矾石分解并与氯离子反应生成F盐。     

碳纳米管纤维力-电耦合效应的实验研究

CNT纤维是由沿轴向排列的高度取向的数十万根碳纳米管加捻组装而成。以实验方法研究CNT纤维在拉伸载荷与电流共同作用下的力学性质。实验结果表明,电流的引入会使纤维的模量和断裂强度显著降低。纤维在有电流通过时会产生轴向电致收缩力,电流强度越大,电致收缩力越大,在5mA时的收缩力约为2.5mN。将纤维轴向拉伸到2%应变,经过应力松弛使载荷趋于平稳后,首次通入电流或加大电流的强度,发现纤维的张力明显下降,主要原因是纤维模量变化引起的应力下降大于电致收缩力。在一定强度的电流下将纤维轴向拉伸到2%应变,经过应力松弛使载荷趋于平稳,然后改为通入相同强度的交流电流时,发现电致收缩力的响应很敏捷,当交流电流变化400个周期后,电致收缩力依然展现出较好的变化规律,这可使得CNT纤维作为新型电致驱动材料。     

C—S—H凝胶的制备与性能研究

利用水热合成法制备了C-S-H凝胶，重点研究了不同的制备工艺、原材料与钙硅比对C-S-H凝胶结构的影响。经过详尽的X衍射（XRD）与核磁共振（NMR）分析表明，制备的样品都具备C-S-H凝胶的峰型特征；活性反应法相对于沉淀反应法制备的C-S-H凝胶层间距小、聚合度低；证实了C-S-H凝胶的层状结构，且层间距随钙硅比的增...     

考虑纤维随机分布的复合材料热残余应力分析及其对横向力学性能的影响

建立了考虑纤维随机分布并包含界面的复合材料微观力学数值模型,模拟玻璃纤维/环氧复合材料固化过程中的热残余应力。通过与纤维周期性分布模型的计算结果进行对比,发现纤维分布形式会对复合材料的热残余应力产生重要影响,纤维随机分布情况下的最大热残余应力明显大于纤维周期性分布的情况下。研究了含热残余应力的复合材料在横向拉伸与压缩载荷下的损伤和破坏过程,结果表明:热残余应力的存在显著影响了复合材料的损伤起始位置和扩展路径,削弱了复合材料的横向拉伸和压缩强度。在横向拉伸载荷下,考虑热残余应力后,复合材料的强度有所下降,断裂应变显著降低;在横向压缩载荷下,考虑热残余应力后,复合材料的强度略有下降,但失效应变基本保持不变。由于热残余应力的影响,复合材料的横向拉伸和压缩强度分别下降了10.5%和5.2%。      

PAN基碳纤维微晶结构对拉伸强度的影响

碳纤维由微晶和非晶碳所构成。利用X射线衍射(XRD)和拉曼光谱(Raman)研究聚丙烯腈(PAN)基碳纤维微晶结构对拉伸强度的影响机理。结晶度、石墨化程度、微晶无序化程度、微晶尺寸都对拉伸强度有显著影响。结晶度、石墨化程度越大,微晶无序化程度越小拉伸强度越大。微晶尺寸越大拉伸强度越小。比较T300和T700,结晶度、石墨化程度的增加,微晶无序化程度的减小所导致的拉伸强度增量大于微晶尺寸增加所导致的拉伸强度减小量,从而使得T700的拉伸强度大于T300的拉伸强度,同理可知T800的拉伸强度大于T700的拉伸强度。比较M35J和M40J,结晶度、石墨化程度的增加,微晶无序化程度的减小所导致的拉伸强度增量小于微晶尺寸增加所导致的拉伸强度减小量,从而使得M40J的拉伸强度小于M35J的拉伸强度,同理可知M46J的拉伸强度小于M40J的拉伸强度。M35J,M40J和M46J内的较大的微晶对拉伸强度的影响起决定性作用。     

取样方向对含单边裂纹牛卡纸拉伸断裂行为的影响

目的 探究取样方向对含单边裂纹牛卡纸拉伸断裂行为的影响,以期提高牛卡纸为原料制作的包装盒等成品的强度。方法 首先在不同取样方向上制备含单边裂纹的牛卡纸试样,然后基于数字图像相关(Digital Image Correlation, DIC)方法,结合显微张力测试台测得不同取样方向与拉伸断裂载荷的关系图,分析不同取样方向下裂纹尖端微观应变场,最后结合电镜扫描测试进行对比分析。结果 结果表明,通常情况下,含单边裂纹的牛卡纸依然符合纸张纵向拉伸载荷大于横向拉伸载荷的规律,取样方向越靠近横向,裂尖区域应变值越大,发生断裂时的拉伸载荷越小。结论 牛卡纸取样方向和纤维与纤维的结合强度是影响牛卡纸拉伸断裂行为的重要因素。     

含孔天然纤维织物复合材料力学性能

研究了含孔天然苎麻纤维织物/异氰酸酯复合板在双轴向拉伸载荷下的力学行为。对0.5、1.0、2.0、4.0mm 4种孔径板进行了单向和双轴向载荷拉伸试验, 同时采用数字散斑相关方法对全场位移及孔径大小对应变的影响进行了表征。结果表明: 当载荷线性变化时孔周围的位移场分布较为均匀; 随着载荷接近破坏值, 位移场呈非线性分布并出现高应变值点, 破坏以极快的速度沿孔边在这些点首先发生。随着孔径的增大, 在1000~2000N双轴向载荷下孔周围相同面积内x、y方向正应变的平均值减小, 应变值波动小但范围增大。材料在单向和双轴向拉伸时表现出不同的力学特征: 双轴向载荷下失效强度要比单向拉伸时低, 降低比例为14%～27%, 且随着孔径的增加而增加。      

基于碟片法的溶蚀作用下水泥净浆微观结构演变研究

溶蚀作用下水泥基材料的微结构变化导致混凝土材料服役性能退化。为了克服传统试验因试样厚度影响而产生的脱钙不均匀现象,以NH4NO3溶液对约1mm的碟片状水泥净浆试件进行加速脱钙,通过控制质量损失来控制溶蚀程度。然后把质量损失作为衡量溶蚀程度的参数,结合压汞法(MIP)、环境扫描电子显微镜(ESEM)及能谱分析(EDS),研究不同溶蚀程度下水泥净浆的微观形貌、化学组成和孔隙结构变化规律。试验结果表明:试件钙硅比(Ca/Si)与质量损失大致呈两段线性关系。在第一阶段,水泥浆体试件Ca/Si逐渐降低,氢氧化钙(CH)分解,质量损失10%试件中CH已被严重溶蚀;水化硅酸钙(C-S-H)凝胶的Ca/Si随着溶蚀程度增加而降低,证明C-S-H凝胶第一阶段已开始分解。在第二阶段,主要表现为C-S-H凝胶继续分解。试验还发现,溶蚀后水泥净浆试件内部孔径小于50nm的孔略微增加,50~200nm的孔含量有部分减少;孔径大于200nm的多害孔大量增加。     

含孔CFRP正交层合板的双轴拉伸力学行为

采用加载臂开槽的中心开孔等厚度十字形试样,实验研究了正交对称铺层碳纤维增强聚合物基复合材料（CFRP）层合板在双轴拉伸载荷作用下的力学行为,分析了3种双轴加载比对其拉伸强度和破坏行为的影响。研究表明:纤维被切断的铺层部分在拉伸作用下容易与其相邻铺层脱粘,导致层合板承载力下降;等双轴加载时,在孔边的被切断纤维与连续纤维间基体在横向拉伸和纵向剪切组合作用下首先开裂;非等双轴加载时,在垂直于快速拉伸方向的铺层中沿孔边应力集中处先出现基体裂纹;随着加载比的增大,快速拉伸方向的细观结构损伤随载荷的增大发展更快,刚度下降更快,破坏时主裂纹的扩展方向更趋于垂直于快速拉伸方向;强度包络线的分析表明快速拉伸方向的拉伸强度随加载比的增大呈缓慢增大的趋势。      

The hydration of tricalcium silicate in the presence of colloidal silica

Reactions of colloidal silica fumes with calcium hydroxide or hydrating tricalcium silicate (C₃S) have been studied using calorimetry, chemical analyses, and scanning electron microscopy. Silica fume reacts immediately with calcium hydroxide forming a colloidal calcium silicate hydrate (C-S-H) similar to that formed by the hydration of C₃S. When excess silica is present it reacts with C-S-H already formed to produce a new, highly polymerized C-S-H, having a very low C/S ratio (1.0). Silica fume accelerates the hydration of C₃S, reduces the amount of calcium hydroxide formed by reacting with it, and slightly lowers the C-S-H ratio of the C-S-H formed by hydration. When large amounts of silica fume are present the formation of calcium hydroxide may be entirely suppressed and a highly polymerized C-S-H is formed. Silica fume is considered a good model for reactive pozzolans used in concrete.     

Molecular dynamics simulation of the nonlinear behavior of the CNT-reinforced calcium silicate hydrate (C–S–H) composite

Calcium–Silicate–Hydrate (C–S–H), which is the major constituent of the cement at the nanoscale, is responsible for the strength and fracture properties of concrete. This research is dedicated to the numerical study of enhanced mechanical properties of C–S–H reinforced by embedding carbon nanotube (CNT) in its molecular structure. Series of molecular dynamics (MD) simulations indicate that the tensile strength of CNT-reinforced C–S–H is substantially enhanced along the direction of CNT as compared to the pure C–S–H. The results of tensile loading reveal that CNT can efficiently bridge the two sides of cracked C–S–H. In addition, CNTs can severely intensify the “transversely isotropic” response of the CNT-reinforced C–S–H. Furthermore, the pull-out behavior of CNT reveals that the force-displacement response can be estimated by a bilinear model, which can later be used for simulation of cohesive crack propagation and multiscale simulation of crack bridging at macro scale specimen of CNT-reinforced cement.     

Molecular dynamics study of the interfacial strength between carbon fiber and phenolic resin

The interfacial strength between carbon fiber and phenolic resin is studied using molecular dynamics simulations to demonstrate that carbon fiber-reinforced carbon matrix composites (C/C composites) have improved tensile strength. Simulations are performed using two carbon fiber models, one of which has only carbon atoms and the other has carbon atoms and some fluorinated carbon groups. The carbon fiber models are regarded as two-layer graphite, and the phenolic resin model is treated as cross-linked structures. All force field parameters are based on the Dreiding force field. The tensile stress and interfacial fracture energy are calculated for the estimation of the interfacial strength. The results show that the model including the fluorinated carbon groups has lower interfacial strength than the model having only carbon atoms, up to a certain coating ratio of fluorinated carbon groups. Similarly, within the limits of the coating ratio, the interfacial fracture energy of the fluorinated carbon fiber model is lower than that of carbon fiber model having only carbon atoms.     

Aging damage model of concrete behavior during the leaching process

This paper deals with the effect of the leaching process of cement-based materials on their mechanical properties. This process mainly induces a total leaching of Ca(OH)₂ and a progressive decalcification of the C-S-H leading in turn to a gradient of C/S ratio in the leaching zone. Modeling of the deterioration of cement paste exposed to leaching consists of a decrease in the local elastic modulus with both a damage function d and an aging function L:E=E₀ (1-d)(1-L). The main parameter fo the aging function is the residual calcium content in the material. This calcium content depends directly on the thickness of the degraded zone and on the different types of hydrates in the cement paste. The non-linearity of the mechanical behavior of the cement-based material is described by a damage function whose main state parameter is the equivalent strain. The characteristic parameters of the material are identified by a compressive loading test. The model of aging damage behavior proposed in this paper corresponds perfectly with the experimental results obtained in the case of a uniaxial compressive load.     

水分子对无定形C-S-H凝胶微结构和力学性能的影响

基于随机系统,利用Ca离子、H₂O分子、OH基团和Si₃O₁₀集团作为基本单元构建无定形C-S-H凝胶模型,并结合分子动力学方法进行结构优化,得到不同H₂O/Si含量的无定形C-S-H凝胶。通过分析不同原子对的径向分布函数和近邻原子数,研究了水分子含量对C-S-H凝胶体系的微观结构特性和力学性能的影响。研究结果表明:无定形C-S-H凝胶体系符合非晶态结构特征即短程有序长程无序。由于Ca离子附近的O原子从水中的O原子变成了硅氧四面体中的O原子,导致水分子含量减少时Ca-O原子对的近邻原子数变化不大。随着水分子含量的增高,硅链骨架层间距增大,层间水分子增多,粘聚力减小,材料容易变形,导致材料体积模量、杨氏模量和剪切模量均减小。无定形CS-H凝胶模型的力学参量与实验值吻合较好。     

Damage in cement pastes exposed to MgCl<Subscript>2</Subscript> solutions

Magnesium chloride (MgCl₂) reacts with cement pastes resulting in calcium leaching and the formation of calcium oxychloride, which can cause damage. This paper examines the damage in different cement pastes exposed to MgCl₂ solutions. Volume change measurement and low temperature differential scanning calorimetry are used to characterize the formation of calcium oxychloride. Thermogravimetric analysis and X-ray fluorescence are used to quantify calcium leaching from Ca(OH)₂ and C-S-H. The ball-on-three-balls test is used to quantify the flexural strength reduction. Calcium oxychloride can form in cement pastes exposed to MgCl₂ solutions with a (Ca(OH)₂/MgCl₂) molar ratio larger than 1. As the MgCl₂ concentration increases, two-stages of flexural strength reduction are observed in the plain cement pastes, with the initial reduction primarily due to calcium leaching from Ca(OH)₂ and the additional reduction due to the calcium leaching from C-S-H (at MgCl₂ concentrations above 17.5 wt%). For the cement pastes containing fly ash, there is a smaller reduction in flexural strength as less Ca(OH)₂ is leached, while no additional reduction is observed at high MgCl₂ concentrations due to the greater stability of C-S-H with a lower Ca/Si ratio. The addition of fly ash can mitigate damage in the presence of MgCl₂ solutions.     

Experimental and numerical evaluation of bending and tensile behaviour of carbon-fibre reinforced SiC

The tensile and bending strength of the Liquid Silicon Infiltrated (LSI) ceramic–matrix composite (CMC), C/C–SiC, were investigated in varying orientations relative to the 0°/90° woven carbon fibres. The ratio of bending to tensile strength was about 1.7–2 depending on the loading direction. The non-linear stress–strain behaviour under tensile load and the linear elastic behaviour under compression load were included in the finite element analysis (FEA) of bending behaviour. The bending failure of the CMC-material was modelled by Cohesive Zone Elements (CZE) accounting for the directional tensile strength and Work of Fracture (WOF). The WOF was determined by Single Edge Notched Bending (SENB) tests. Comparable results from FEA and bending test were achieved. It was demonstrated that the failure of C/C–SiC at room temperature may be described by a macroscopic fracture mechanical FE-approach. The presented approach could also be adapted for the design of CMC-components and structures.     

Tensile strength degradation of ceramic fibres due to cyclic loading

Interfacial strength of Ti β 21S/SCS 6 composite was increased via ageing. Composite specimens with a_o/W ratios of 0.24 and 0.07 were prefatigued in three point bending at ambient and at 500 °C both in the air and vacuum. Bridging fibres were extracted from the pre-fatigued specimens and tested in tensile loading to monitor the strength degradation due repeated opening and closing of the fatigue crack. A tri-modal Weibull distribution was employed to express strength distribution of the SCS 6 fibres. Results have shown that cyclic loading of the composites lower the mean tensile strength of the fibres by 20%, compared to the unfatigued composite. A marked effect of initial unbridged defect size was observed, when the tensile strength data were divided into two sub-population using the fatigue crack length values. Compared to unfatigued composite, tensile strength reductions of 41 and 22% were measured from the prefatigued composites with a_o/W of 0.24 and 0.07, respectively.     

An investigation into the influence of grain boundary misorientation on the tensile strength of SiC bicrystals

In this work, molecular dynamics (MD) and Car-Parrinello molecular dynamics (CPMD) simulation-based analyses are performed to understand the influence of grain boundary (GB) misorientation on the tensile strength of SiC bicrystals. The tensile strength is governed by the changes in electron density and bond strength of atoms in GBs. An investigation of dislocation activity during mechanical deformation shows that the extent of the propagation of dislocations across the bicrystal grains is directly proportional to the extent of GB misorientation. An analytical relation that predicts the tensile strength as a function of GB misorientation is developed.     

Hydration of tricalcium silicate (C3S) at high temperature and high pressure

Hydration products of tricalcium silicate (C₃S) are the calcium silicate hydrates (C-S-H) and Portlandite. Silica fume, added to anhydrous cement in industrial formulations reacts with Portlandite and leads to C-S-H different from the previous one. C₃S hydration with and without silica fume has been studied under high pressure (1000 bar) and high temperature (160°C) by numerous techniques (²⁹Si and ¹H NMR, XRD, Thermal analysis, SEM) for different hydration times. In these conditions, high temperature more stable crystalline phases are formed and their kinetics of formation is dependent on pressure. Besides, electrical conductivity measurement on hydrating cement under pressure have been carried out in order to evidence the great dependence of hydration kinetics with pressure. This study proposes a practical phase diagram which allows on a thermodynamical base to understand the change of equilibrium temperature with pressure. The kinetics of reaction has been studied and mechanisms of reaction proposed to explain the results.