水泥水化过程的细观力学模型与性能演化

基于水泥水化过程的实验测量数据, 建立了一个研究水泥水化过程的细观结构和有效性能演化的细观力学模型。该模型由未水化的水泥颗粒、水泥凝胶和孔洞三相介质组成, 并假设细观结构呈周期性均匀分布。随着水化过程的进行, 模型中的组份是连续变化的, 并与实验测量的组份含量完全一致。利用本文中模型和均匀化方法(直接平均法和二尺度展开法) 计算了水泥浆体在各瞬时的杨氏模量和泊松比。研究表明, 在水泥的水化过程中, 水泥浆体的弹性性能随水化度而变化, 其变化规律和精度与现有的实验数据符合很好。该方法可容易地推广到三维情况和其他水化介质。      

混凝土微观力学基础研究进展及应用展望

混凝土是现代土木工程建设的基础和关键结构材料,现代工程建设的发展对混凝土性能提出新的挑战。水化硅酸钙(C-S-H)是水泥水化最重要的产物(约占水化产物的70%),也是混凝土中最重要的胶结性物质,起到胶结砂石骨料、发挥强度的关键作用。在微观尺度上,C-S-H是一种多孔介质材料,组成结构十分复杂。因此,对C-S-H微观力学性能进行解析和设计,是认识和提高水泥基材料宏观力学性能的关键,同时也是混凝土研究领域的基础科学问题。该文旨在介绍混凝土微观力学性能表征方法、混凝土微观力学计算理论以及其在混凝土应用过程的新发现,同时展望混凝土微观力学在工程建设中的应用。     

基于多项材料细观力学的混凝土干缩量预估

混凝土的干缩特性一直是学术与工程界关注的问题之一。该文用细观力学方法对混凝土相对于水泥浆体的干缩率进行研究。假定混凝土为由骨料和水泥浆体组成的二相材料,根据混凝土收缩量与组成混凝土的各相介质收缩量之和相等的原则,分别用基于Eshebly等效夹杂理论的Mori-Tanaka方法和基于载荷叠加思想的细观力学方法推导了混凝土干缩量。结果表明：Mori-Tanaka法、自洽方法和经验公式得到的相对干缩量基本相同,能很好地反映混凝土的干缩特性。混凝土相对干缩随骨料体积分数的增加而下降,随水泥基质泊松比的增大而增加。相对干缩随骨料与水泥基质体积模量比的增大而减小,但是当其值大于5后的影响比较小。用该文的方法估算混凝土的干缩,物理意义明确且计算简单,可以减少实验的工作量,同时也为混凝土结构的设计提供一定的参考。     

基于分子动力学的C-S-H凝胶性能研究进展

分子动力学是水泥基材料原子级尺度研究的基本方法之一,在混凝土的精细化设计中具有重要应用.C-S-H凝胶是水泥的主要水化产物,决定硬化水泥基材料的宏观性能.本文详细阐述了C-S-H凝胶分子结构模型和分子动力学力场的常见类别(这是决定模拟准确性的关键因素),探讨了C-S-H凝胶分子动力学模型与宏观性能的关系,总结应用分子动力学研究C-S-H凝胶的相关研究进展.本文对在工程应用中提升水泥基材料物理化学性能具有指导意义.     

水化硅酸钙凝胶的弹塑性和徐变特性

采用纳米压痕技术研究了不同龄期水泥硬化浆体中水化硅酸钙(C-S-H)凝胶的微观弹塑性和徐变特性。结果表明,随着龄期的增长,C-S-H凝胶在相同荷载作用下的弹性能逐渐减小,并趋于稳定值,且低密度C-S-H凝胶的弹性能均高于同龄期的高密度C-S-H凝胶。对于相同的荷载和持荷时间,高密度C-S-H凝胶的徐变显著低于同龄期低密度C-S-H凝胶。应用纳米压痕技术从微观层面测定了两种密度C-S-H凝胶的变形性能的差异,揭示了C-S-H凝胶的弹性能和徐变与凝胶的折合弹性模量呈反比关系。     

聚丙烯/聚苯乙烯共混体系模量的细观力学研究

应用复合材料细观力学理论和相应分析模型,预测了聚丙烯/聚苯乙烯(PP/PS)共混体系的弹性模量并进行相应的试验研究。系统研究了非相容PP/PS共混物的体积含量和共混时间对共混物形态和弹性模量的影响,分析了细观形态与宏观力学性能间的关系。对PP/PS共混物的拉伸实验结果表明:微观力学模型在模量预报上是可行的,其中Mori-Tanaka法更有效,因为与半经验模型(如Halpin-Tsai法和改进的混合模型)相比,它没有试验参数,在材料设计时具有更大的优越性。     

γ-TiAl 基合金有效弹性性能的微结构尺度效应

基于微极理论细观力学方法, 详细分析了近片层γ- TiAl 基合金材料有效弹性性能的微结构尺度效应。采用空间角度平均方法处理近片层γ- TiAl 基合金中横观各向同性PST( Polysynthetically twinned crystal) 颗粒夹杂的空间任意取向分布,并将Mori-Tanaka 法推广到微极介质, 建立了近片层γ-TiAl 基合金材料的有效弹性模量及其尺度效应的分析模型。结果表明: 当PST 夹杂颗粒直径尺度a 与微极基体材料(等轴γ颗粒) 的特征长度lm相当时, 合金材料的有效弹性模量的大小将受到夹杂PST 颗粒大小的影响, 夹杂颗粒尺度减小, 有效弹性模量增大; 而当PST 夹杂颗粒直径a 与微极基体材料的特征长度lm 相比很大时, 微极理论对有效弹性模量预测的结果将趋近于采用传统Cauchy 介质理论预测的结果。     

具有开孔泡沫骨架的双连续相复合材料弹性模量的理论预测

基于具有开孔泡沫骨架的双连续相复合材料(IPC)的细观结构,提出了一种十四面体弹性地基梁力学模型,结合最小势能原理推导了该IPC的弹性模量预测公式。根据文献给出的实验材料参数进行算例分析,结果表明,理论估算结果与实验值吻合良好,证明了该模型的有效性。在此基础上,进一步讨论了不同骨架材料体积含量和支柱截面形状对IPC弹性模量的影响。本文给出的半经验理论模型为表征具有开孔泡沫骨架的IPC的弹性性能提供了新思路,也为进一步预测IPC的强度性能和热物理性能奠定了基础。     

0～20℃硅酸盐水泥的水化性能

采用X射线衍射仪(XRD)及环境扫描电子显微镜(ESEM)研究0℃、5℃、10℃及20℃时硅酸盐水泥的水化过程,并进行凝结时间及力学强度测试。结果表明,温度越低,硅酸盐水泥的初凝和终凝时间越长,水泥早期的水化程度和力学强度也越低;但水化后期,水泥水化产物C-S-H凝胶明显细长,水化产物间距较小,大孔减少且孔隙分布更均匀,水化程度和后期强度较高。     

低场核磁共振技术在水泥基材料研究中的应用及展望

阐述了低场核磁共振技术在水泥基材料研究中的应用现状,认为现有的研究主要集中于水泥水化进程和水在硬化浆体中的扩散特征,也包括对硬化水泥浆体孔结构和比表面积的测试。分析了低场核磁共振技术在实际应用中面临的挑战,展望了该技术在新拌水泥浆体结构性能研究中的应用前景。     

硬化水泥净浆基于纳米压痕的相态识别与水化程度计算

采用纳米原位压痕手段测量硬化水泥净浆中单一相态的代表性微观力学性能,并采用纳米点阵压痕研究各相态的含量。研究对象囊括水灰比为0.3、0.4、0.5的纯水泥净浆和水灰比为0.3情况下含50%、70%矿渣掺量复合体系,共5种配比,以表征它们的相态分布和微观力学性质的异同点。掺矿渣的试件中含有明显多的复合相,因此提出三相模型测算复合相中未水化物的体积分数。此外,提出基于纳米压痕技术计算纯水泥和掺矿渣水泥试件水化程度的方法,结果吻合于热重分析的结果,其中纯水泥净浆中复合相较少,计算得到的水化程度优于对掺矿渣水泥试件的计算。     

跨尺度模拟硬化水泥净浆力学性能:微观物相的影响

采用纳米压痕实验测量硬化水泥净浆中未水化物、水化产物和孔隙等微观物相的力学性能,并基于背散射电镜（BSE）图像的灰度分析计算各微观物相的含量。在得到各微观物相含量和力学性能的基础上,针对水泥净浆的弹性模量进行均一化建模,并讨论各微观物相及其模量的选取对跨尺度模拟硬化水泥净浆力学性能的影响。通过微米压痕的实测净浆模量验证模型及参数选取的可靠性。提出在硬化水泥净浆力学性能的多尺度模型中,需要选取12 GPa作为孔隙有效模量,并将水化产物划分为低密度的水化硅酸钙凝胶（LD-CSH）和高密度的水化硅酸钙凝胶（HD-CSH）两种物相,而不同种类的未水化物可被视作一种物相。在此基础上,使用Mori-Tanaka模型或自洽模型计算得到的净浆模量与实测净浆模量吻合。     

Multiscale characterization of carbonated wollastonite paste and application of homogenization schemes to predict its effective elastic modulus

This paper describes the multiscale characterization of the carbonated wollastonite paste using X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), and statistical nanoindentation (SNI, also known as ‘grid indentation’) methods as well as micromechanical homogenization models. Wollastonite (CaSiO₃) fibers are commonly used as filler in ceramics or plastics. However, wollastonite can also be regarded as non-hydraulic binder material since upon carbonation it forms a heterogeneous matrix with mechanical properties similar to those of the conventional hydrated cement pastes. Carbonation reaction of wollastonite results in the formation of two main products: calcium carbonate (CaCO₃) and amorphous silica gel (SiO₂). The SEM/EDS microanalysis performed on this system revealed that the average calcium to silica (Ca/Si) atomic ratio of the silica gel phase was around 0.40. Three individual carbonated wollastonite paste samples, each representing a different degree of carbonation were selected for nanoindentation tests. The obtained elastic moduli for silica gel, calcium carbonate, and unreacted wollastonite grains were, respectively, 41.7 GPa, 67.3 GPa, and 134.7 GPa. The micromechanical homogenization models were then utilized to predict the effective (also referred to as ‘homogenized’) elastic moduli of the carbonated wollastonite paste. The predicted values of the effective elastic moduli of carbonated wollastonite pastes were found to be in the range of corresponding values for hydrated high to ultra-high performance cement pastes. Additionally, the values of the effective elastic moduli of the carbonated wollastonite pastes were observed to increase with the increase in the degree of carbonation.     

Elastoplastic behavior of metal matrix composites based on incremental plasticity and the Mori-Tanaka averaging scheme

The applicability of the Mori-Tanaka averaging method for the prediction of the response of binary composites loaded in the plastic range is investigated. The applied loading is subdivided into small increments and the Eshelby solution for the inhomogeneity problem is used in conjunction with the Mori-Tanaka averaging scheme to obtain the load increments in the various phases. Since the Eshelby solution depends on the instantaneous matrix material properties and these are updated at the end of each load increment by using the backward difference scheme, an iterative procedure is necessary for the calculation of the correct load increments in the phases (concentration factors). The performance of the Mori-Tanaka method is compared with results obtained using the periodic hexagonal array (PHA) finite element model and experimental results for a B-Al unidirectional fibrous composite; it is also compared with numerical simulations obtained from the modified PHA model for a SiC _w -Al particulate composite.     

Prediction of the diffusivity of cement-based materials using a three-dimensional spatial distribution model

A three-dimensional image of hardened cement paste was reconstructed using a backscattered electron image (BEI) and used to predict the diffusion properties of hardened cement paste. After the BEI observations, an autocorrelation function (ACF) was calculated for each phase of the hardened cement paste, including the unhydrated cement, portlandite, and large pores. A three-dimensional image was reconstructed on the basis of the ACF based on random distributions. The dynamic elastic modulus and diffusion coefficient were calculated using a finite-element or finite difference method with the reconstructed three-dimensional images. The elastic modulus of the C-S-H phase was determined by micro-indentation, and the diffusivity of C-S-H was calculated using this elastic modulus based on previous reports. The resulting predicted dynamic elastic moduli and diffusion coefficients were in good agreement with the experimental results. Although, it was observed that the predicted values of the diffusivity of the blended cement pastes is different from the measured values, a new relationship between diffusivity and porosity of C-S-H in blended cement pastes was developed in this study.     

On the overall elastic moduli of polymer-clay nanocomposite materials using a self-consistent approach. Part II: Experimental verification

Polyamide-6 (PA6) based nanocomposites were prepared using a modified montmorillonite (MMT) Cloisite 20A as nanofillers. The silicate weight fraction of the prepared nanocomposites, determined by burning off the PA6 matrix, was ranged from 0.2 wt% up to 7.5 wt%. The thermomechanical properties of both the neat PA6 and the PA6 filled with MMT nanoclay were measured by means of uniaxial tension tests and dynamic mechanical thermoanalysis, their crystallinity analyzed by differential scanning calorimetry and their morphology observed by transmission electron microscopy. The elastic stiffness of PA6-clay nanocomposites was examined under two moisture levels and was analyzed with the theory formulated in the Part I of this work. Predicted results are found in good agreement with our experiments. The model capabilities are also critically discussed by comparisons with both experiments issued from the literature and the Mori-Tanaka approach widely used in recent literature. It is demonstrated that the proposed micromechanical model is more efficient than the Mori-Tanaka approach. Moreover, the obtained results support the idea that the elastic stiffness of polymer-clay nanocomposites is governed by the same mechanisms as microcomposites, the effects of particle dimension or constrained region being of a second order.     

Uncertainty propagation of a multiscale poromechanics-hydration model for poroelastic properties of cement paste at early-age

The durability of concrete materials with regard to early-age volume changes and cracking phenomena depends on the evolution of the poroelastic properties of cement paste. The ability of engineers to control the uncertainty of the percolation threshold and the evolution of the elastic modulus, the Biot–Willis parameter and the skeleton Biot modulus is key for minimizing the vulnerability of concrete structures at early-age. This work presents original results on the uncertainty propagation and the sensitivity analysis of a multiscale poromechanics-hydration model applied to cement pastes of water-to-cement ratio of 0.40, 0.50 and 0.60. Notably, the proposed approach provides poroelastic properties required to model the behavior of partially saturated aging cement pastes (e.g. autogenous shrinkage) and it predicts the percolation threshold and undrained elastic modulus in good agreement with experimental data. The development of a stochastic metamodel using polynomial chaos expansions allows to propagate the uncertainties of kinetic parameters of hydration, cement phase composition, elastic moduli and morphological parameters of the microstructure. The presented results show that the propagation does not magnify the uncertainty of the single poroelastic properties although, their correlation may amplify the variability of the estimates obtained from poroelastic state equations. In order to reduce the uncertainty of the percolation threshold and that of the poroelastic properties at early-age, engineers need to assess more accurately the apparent activation energy of calcium aluminate and, later on, of the elastic modulus of low density calcium-silicate-hydrate.     

Measurement of elastic properties of calcium silicate hydrate with atomic force microscopy

Atomic force microscopy (AFM) based indentation is compared to conventional nanoindentation for measuring mechanical properties of cement pastes. In evaluating AFM as a mechanical characterization tool, various analytical and numerical modeling approaches are compared. The disparities between the numerical self-consistent approach and analytical solutions are determined and reported. The measured elastic Young’s modulus determined from AFM indentation tests are compared to elastic Young’s modulus determined from nanoindentation tests of cement paste. These results indicate that the calcium silicate hydrate (C-S-H) phase of hydrated Portland cement has different properties on the different length scales probed by AFM versus nanoindenters. Packing density of C-S-H particles is proposed as an explanation for the disparity in the measured results.     

Calculated elastic constants of alumina-mullite ceramic particles

Using two theoretical models, we estimated the isotropic elastic constants of an alumina-mullite ceramic composite. The alumina phase, 20% by volume, consisted of brickshaped particles with a 4:1 aspect ratio embedded in a mullite matrix (mullite = 3Al₂O₃·2SiO₂). We took alumina elastic-constant values from the literature, and we measured mullite's elastic constants using a megahertz-frequency pulse-echo method. The two theoretical models, Datta-Ledbetter and Mori-Tanaka, proceed from very different viewpoints. The Datta-Ledbetter model uses the long-wavelength limit of a scattered plane wave ensemble-average approach. The model estimates the speed of a plane harmonic wave, averages the scattered field by the Waterman-Truell procedure and uses Lax's quasicrystalline approximation to sum over pairs. The Mori-Tanaka method proceeds by estimating the average matrix stress in a material containing ellipsoidal inclusions. For randomly oriented ellipsoids, it extends Eshelby's solution for a single ellipsoidal inclusion. Both models lack adjustable parameters. Surprisingly, the two models with different physical approaches give practically identical results. A rough check on our estimates is that they lead to correct predictions of the elastic constants of an alumina-mullite-particle aluminium-matrix composite.     

Elastic property prediction of single-walled carbon nanotube buckypaper/polymer nanocomposites: stochastic bulk response modeling

In this study, we investigated the statistical relationship between nanostructure variations of carbon nanotube buckypaper-polymer (BPP) composites and their resulting elastic properties. A statistical simulation was developed to predict the elastic properties of a single-layer BPP lamina and extrapolated to the resultant bulk composite part. The stochastic characteristics of BPP composite nanostructure were quantified from experimental observations and used to generate the input for each simulation set performed. The Mori-Tanaka method was used to calculate the stiffness tensor within the buckypaper-polymer region, and a Monte-Carlo simulation was applied to generate the probability distribution for the effective stiffness tensor within each BPP lamina. Classical laminate theory was then employed to predict the effective elastic response for a multi-layered BPP composite laminate. The theoretical predictions were compared with experimental data, and the resulting trends for the effective tensile modulus between experimental and theoretical corresponded well with each other.