砂浆中邻近集料表面最近间距分布的数值模拟

采用具有粒子动态混合密实功能的SPACE 系统, 实现了高集料体积分数模型砂浆结构的生成。从而, 以3 种细集料粒径分布的模型砂浆(其中一种的粒径范围为0.125～1.34 mm , 另外两种的粒径范围为0.25～5.00 mm) 结构为例, 研究了集料细度和集料体积分数(φ= 40 %～70 %) 对邻近集料表面最近间距分布的影响。结果表明, 集料细度增加和集料体积分数增大都会使大尺度邻近集料表面最近间距出现的概率减小、小尺度邻近集料表面最近间距出现的概率增加, 且峰值概率的位置向小间距方向偏移。另外, 邻近集料表面最近间距的分区段累计概率结果分析表明, 3 种模型砂浆结构的邻近集料表面最近间距分布的57 %以上小于10μm。最后, 邻近集料表面最近间距的平均值的分析结果显示, 当集料体积分数在40 %～70 %之间变化时, 邻近集料表面最近间距的平均值在54.6～1.1μm 之间变化; 当砂浆中集料的体积分数在50 %～70 %之间变化时, 邻近集料表面最近间距的平均值与集料的体积分数基本上呈线性关系; 单位砂浆体积下集料的表面积的变化对邻近集料表面最近间距的平均值有影响, 但二者之间的比值并非常数。     

引气混凝土气泡尺寸分布的三维重构

基于体视学和几何概率理论给出了引气混凝土三维气泡尺寸重构方法,由二维平面上气泡截面圆的直径分布计算气孔的实际尺寸分布,并生成了一个多尺度分布的立方体模型结构验证了该三维重构方法的合理性.然后,使用邻近粒子表面最近间距的解析解研究了气泡细度和混凝土含气量对邻近气泡表面最近间距平均值的影响,并与用传统方法得到的气泡间距因子进行了比较.结果表明,在含气量相同的条件下,用传统方法得到的气泡间距因子是邻近气泡表面最近间距平均值的3-4倍.该方法的给出,为从二维截面上获得的引气混凝土中的气泡截面圆信息获取实际气泡在三维空间中的气泡间距信息提供了依据.     

水泥基复合材料界面过渡区体积分数的定量计算

借助邻近函数公式, 给出了水泥基复合材料集料与浆体间界面过渡区体积分数的定量计算公式, 并采用随机点采样方法验证采用该定量公式计算水泥基复合材料界面过渡区体积分数的可行性。在此基础上, 预测了由符合Fuller 分布的集料制备的混凝土中界面过渡区体积分数随集料体积分数和集料粒径范围的变化曲线。讨论了界面厚度变化、集料体积分数变化以及集料粒径分布变化对界面过渡区体积分数影响的差别, 并给出了衡量界面过渡区重叠程度的定性和定量方法。结果表明: 在常规集料体积分数下, 三者中对界面过渡区体积分数以及界面过渡区重叠程度影响的主次顺序均为: 界面过渡区厚度变化带来的影响> 集料体积分数变化带来的影响> 集料细度变化带来的影响。      

天然火山灰-水泥-粉煤灰复合浆体流变性能

采用C-LTD80 QC型旋转粘度计研究了天然火山灰-水泥-粉煤灰复合浆体的流变行为,分析了天然火山灰掺量对浆体屈服应力、塑性粘度以及触变性的影响,并采用Zeta电位仪探索其作用机理。结果表明：天然火山灰-水泥-粉煤灰复合浆体流变可分区段用Bingham模型及Herschel-Bulkley模型模拟;天然火山灰粒径分布跨度大,颗粒形状不规则,且密度小于水泥,等质量代替会增加浆体固相体积分数;天然火山灰使水泥浆体Zeta电位降低,浆体中粒子间静电作用力减弱,粒子更易相互吸附黏聚。而粉煤灰颗粒呈球状且表面光滑,使水泥浆体Zeta电位降低,粒子不易相互吸附黏聚。因此天然火山灰-水泥-粉煤灰复合浆体的屈服应力、塑性黏度和触变性随着天然火山灰掺量增加而增加,随粉煤灰的掺量增加而降低。     

连续粒径水泥2矿粉双组分体系在浆体中的堆积密度计算公式

首先推导出双组分连续粒径粉体在浆体中的堆积密度公式;通过测定多组粒径分布不同的复合水泥在流动度相同的情况下的需水量,得到不同粒径分布对应着的不同堆积密度。所采用的15 种试样的需水量相差4%～10%。用该公式计算上述不同粒径分布的复合水泥浆体的堆积密度及需水量与试验值基本吻合。对公式的适用性进行了验证,说明该公式可以用来模拟诸如水泥和磨细矿粉体系、水泥和粉煤灰体系等胶凝材料细颗粒的堆积密度。     

水泥浆体中连续粒径粉体的堆积密度研究

为了研究粒径分布对水泥石结构与性能的影响,通过测定多组粒径分布不同的矿粉在流动度相同情况下的需水量,得到矿粉不同粒径分布所对应的不同的堆积密度,推导了浆体中连续粒径粉体的堆积密度公式。用该公式计算上述不同粒径分布的矿粉浆体的堆积密度及需水量,计算结果与试验值基本吻合。说明该公式可以用来模拟诸如水泥、磨细矿渣、粉煤灰等胶凝材料细颗粒的堆积密度。     

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。     

外加剂和粉煤灰特性对三峡大坝混凝土亚微结构和耐久性能的影响

分别用中热水泥、低热水泥和花岗岩沙石、一级和二级粉煤灰、木钙、萘系高效减水剂和引气剂,配制成三峡大坝混凝土,测定了混凝土的强度、抗渗性、抗冻性等性能和孔隙率、水泥浆体和集料界面上晶体取向、气泡间隔系数等.提出:为确保三峡大坝混凝土耐久性,必须使用一级粉煤灰、高效减水剂和引气剂,水/胶比应低于0.50.     

水泥粒度分布对水泥性能影响的研究进展

从两个方面总结了水泥粒度分布对水泥性能影响的研究进展,一方面总结了反映水泥颗粒群整体情况的特征参数比表面积S、特征粒径x′和均匀性指数n对水泥性能的影响;另一方面总结了不同粒径区间水泥颗粒的性能。介绍了描述水泥粒度分布的RRB方程和Fuller曲线,综述了理论上粒度分布对水泥性能的影响情况,认为水泥的粒度分布是与其性能有明确定量关系的细度参数,是水泥粉磨细度控制的最终目标。     

废水泥浆对水泥胶砂性能的影响研究

研究了废水泥浆澄清液、浆体以及干燥后的粉体对水泥胶砂流动度以及抗压强度的影响,为废水泥浆在预拌混凝土生产中的的回收利用提供技术基础,解决废水泥浆的环境污染和资源化利用问题。     

Particle size effect on the strength of rice husk ash blended gap-graded Portland cement concrete

Rice husk ash (RHA) has been used as a highly reactive pozzolanic material to improve the microstructure of the interfacial transition zone (ITZ) between the cement paste and the aggregate in high-performance concrete. Mechanical experiments of RHA blended Portland cement concretes revealed that in addition to the pozzolanic reactivity of RHA (chemical aspect), the particle grading (physical aspect) of cement and RHA mixtures also exerted significant influences on the blending efficiency. The relative strength increase (relative to the concrete made with plain cement, expressed in %) is higher for coarser cement. The gap-grading phenomenon is expected to be the underlying mechanism. This issue is also approached by computer simulation. A stereological spacing parameter (i.e., mean free spacing between mixture particles) is associated with the global strength of the blended model cement concretes. This paper presents results of a combined mechanical and computer simulation study on the effects of particle size ranges involved in RHA-blended Portland cement on compressive strength of gap-graded concrete in the high strength/high performance range. The simulation results demonstrate that the favourable results for coarser cement (i.e., the gap-graded binder) reflect improved particle packing structure accompanied by a decrease in porosity and particularly in particle spacing.     

Effects of microstructure on restrained autogenous shrinkage behavior in high strength concretes at early ages

Fluorescence microscopic examinations were conducted to identify damages induced by restraining autogenous shrinkage. Characteristics of fluorescent areas and their correspondence to autogeneous shrinkage behavior of high strength concretes were discussed. Silica fume concrete exhibited a greater creep potential when loaded at very early ages. The microstructure in sealed concretes with an extremely low water/binder ratio was porous. The vicinity of aggregate grains was more porous and weaker than the bulk matrix in sealed concretes. In addition, sealed silica fume concretes contained many unhydrated cement particles which were profiled by thin gaps between the core cement particles and the surrounding cement paste matrix. These features of microstructure were not observed in water ponded concretes. The detected fluorescent areas may be defects caused by selfdesiccation and autogenous shrinkage. The flaws had little effects on the development of strength. However, the presence of thin gaps around remnant cement particles may increase creep deformation to relieve internal stresses.     

Design of green concrete made of plant-derived aggregates and a pumice-lime binder

The use of particles from agricultural lignocellulosic resources in concrete gives it desirable environmental and multiphysics qualities. In this study, parallels are drawn between particles derived from hemp and sunflower stems, in terms of their morphological and physical properties. A pumice-lime binder is proposed as an alternative to the traditional cement or lime based solutions for both environmentally friendly and mechanical qualities. Compaction is applied during casting and its effects on mechanical properties are analysed. A principal finding of this study is that the hemp and sunflower materials show large similarities in terms of morphology and mechanical performance of the resulting concrete. The pumice-lime binder provides desirable properties even with raw pumice sand, which represent 90% of the binder mass proportion. Compaction level during casting induces an orthotropy, even with low plant content, and increases the compressive strength. A simple analytical model using Powers’ equation is proposed to predict plant concrete compressive strength with low plant quantities.     

Computer simulation of interfacial packing in concrete

Computer simulation of particle packing against an aggregate surface was undertaken to show the effects of four variables on interfacial porosity profiles. The variables in order of significance and their assumed physical meaning are: sticking probability (tendency of cement particles to flocculate), amplitude of particle motion (energy of mixing), travel distance of particle to surface (thickness of water film surrounding aggregate), and original particle density (roughly related to water/cement ratio). In all cases, simulations demonstrated that interface porosity decreased from nearly 100% directly at the interface to that of the bulk paste at two to three particle diameters. Flocculation (sticking probability) was found to be the single most-significant variable. Highly flocculated systems produced very porous interfaces. When flocculation was reduced, packing became more efficient. It was also found that energy of mixing (amplitude of motion), was not an entirely independent variable. The simulation showed that, if the tendency to flocculate was high, gentle mixing (low amplitude of motion) was found to result in better packing and a less porous interfacial zone. If, on the other hand, flocculation was low, then vigorous mixing (high amplitude of motion) promoted better packing near the interface. The thickness of the water film surrounding the aggregate (travel distance) was found to have only a minor effect on the outcome of simulations, while original packing density (w/c) resulted in no significant differences at all.     

Étude des effets du cobroyage d'un sable et d'un clinker sur les propriétés d'un béton de sable

Mineral additions can be mixed with the clinker before the grinding process. Thus, one obtains a ‘coground’ binder which can allow some reduction in the cement ratio. Some dune sand is added, in variable proportions, to the clinker, such as to obtain a coground binder from which different sandcrete mixes are defined. To distinguish the effects of the binder fineness from those of the addition ratio, a two-stage experimental design process has been followed. In a first stage, the fineness of the coground binder has been quantified. Thereafter, the influence of several parameters (fineness, addition ratio, water/binder ratio) on the mechanical properties has been evaluated and modelled. The degree of activity of the mineral additions has been confirmed. It can lead to some gain in the cement content.     

Effects of high friction aggregate and PG Plus binder on rutting resistance of hot mix asphalt mixtures

The rutting resistance of hot mix asphalt (HMA) Superpave? mixes in surface course materials was investigated using asphalt material characterisation tests and a digital imaging processing (DIP) technique. The effects of the type of aggregate, the type of binder and the binder content on rutting resistance were quantified. Two types of aggregate were examined: Superpave? SP12.5 and high friction SP12.5 FC2. Both a modified (PG Plus) and an unmodified binders were considered at the optimum binder content and the optimum content plus an additional 0.5%. To accurately identify the effect of each variable, the shear upheave of these mixes was also quantified. The DIP technique involved estimating the number of aggregate contacts, the total contact length and internal structure index of two-dimensional images of the experimentally tested samples. The results showed that both the rutting resistance and stiffness of HMA surface mixes were sensitive to aggregate type, binder type and binder content. A high friction aggregate provided a better internal structure characteristic, as well as superior rutting resistance and stiffness for HMA mixes. The use of PG Plus and the addition of 0.5% to the optimum binder content negatively affected HMA stiffness and rutting resistance. However, the levels of rutting resistance for all mixes were acceptable (rut depth < 12.5 mm), even when the shear upheave was considered. Internal structure indices measured by DIP were effective for capturing changes in the internal HMA structure with respect to aggregate type and asphalt cement content.     

The Evolution of Microstructure in Three-Component Granulation and Its Effect on Dissolution

The relationship between microstructure and dissolution rate of three-component granules was investigated. Granules were prepared by fluid bed granulation from sucrose spheres as model excipient, sodium chloride as model active ingredient, and polyethylene glycol (PEG) as in situ melt binder. A novel method for controlling the distribution of active ingredient within the granule was developed, based on suspending its particles in the binder prior to granulation. Granule microstructure was varied by systematically changing the NaCl particle size and the active/excipient ratio in granules. The dissolution rate of granules in water was measured by conductometry. A minimum was found in the functional dependence of dissolution time on NaCl fraction in the granule, in line with earlier computer simulations. The primary particle size was found to influence dissolution time in a nonlinear way depending on the fraction of available particle surface immersed in the binder. The intrinsic binder dissolution can therefore be rate-controlling if primary particles of the active ingredient are totally coated by binder. This was confirmed by comparing the dissolution times of granules prepared with PEGs of different molecular weight.