非饱和及饱和状态下水化硅酸钙孔道中水分和离子传输的分子动力学研究

水分子和离子在纳米孔道中的传输是混凝土材料的耐久性的重要影响因素.利用分子动力学方法可以模拟水分子和离子在非饱和及饱和状态下的毛细传输过程,进而探究纳米孔道中水分和离子的传输规律以及与C-S-H界面之间的相互作用特性.非饱和状态下,水分子和离子的传输主要受到毛细作用的驱使,以半月板形界面沿3.4 nm的水化硅酸钙孔道向上侵入;饱和状态下的水分子的传输主要是上下部水分子互相扩散而形成的交互传输过程,使得水分子沿孔道的扩散速度减慢.且饱和状态下,孔道界面处水化层更为紧密稳定,易形成离子簇,直接阻碍了后续水分子和离子的进入.因此,非饱和状态下水分子携带离子在纳米孔道中的传输速度远快于饱和状态.     

基于暴露试验的大掺量粉煤灰混凝土耐久性试验研究

为研究海洋环境水位变动区大掺量粉煤灰混凝土的长期耐久性,利用华南地区12年现场暴露试验,研究了大掺量粉煤灰混凝土的抗氯离子渗透性,分析了长龄期时混凝土水化产物的微观产物与形貌及孔结构的演变过程,以及粉煤灰对混凝土耐久性的影响机理.结果 表明:大掺量粉煤灰可显著提升混凝土的抗氯离子渗透性能,延缓海水中氯离子的侵蚀速度,降低混凝土氯离子扩散系数,12年暴露龄期时30％ ～ 40％大掺量粉煤灰混凝土的氯离子侵蚀深度约为27 mm,氯离子扩散系数仅为0.24×10-12 ～0.41×10-12 m2/s,比空白混凝土降低了6倍以上;大掺量粉煤灰显著改善混凝土中孔结构的分布,降低了混凝土的孔隙率和平均孔径,粉煤灰中活性SiO2及Al2O3与水化产物CH反应生成C-A-S-H凝胶产物,提高了水化产物基团的聚合度,进而提升了凝胶产物的密实度,有效提升了混凝土的长期耐久性.     

水分子和离子在地聚合物胶凝材料中的分子动力学研究

地聚合物是一种具有三维网状结构的水泥基胶凝材料,其主要水化产物是水合硅酸铝钠凝胶(NASH).在工程应用中,氯盐侵蚀与硫酸盐侵蚀是影响耐久性的主要原因.运用分子动力学方法,研究NaCl溶液与Na2SO4溶液在NASH凝胶纳米孔道中的传输特性.结果表明,NaCl溶液与Na2SO4溶液在凝胶孔道的非饱和传输过程中,呈现出明显的毛细吸附现象,NASH凝胶界面呈现亲水性与电负性.Na2SO4溶液的侵入速率明显低于NaCl溶液,硫酸根离子对水分子侵入的抑制作用强于氯离子.     

水质中的离子对三元体系界面张力的影响

研究了水质中的钠离子、钾离子、碳酸氢根离子、氯离子和溴离子对三元体系界面张力的影响,结果表明,向蒸馏水配制的强碱三元体系中加入一定浓度的钠离子可以明显改善三元体系的油水界面张力,并使界面张力达到超低;加入钾离子和碳酸氢根离子都会使三元体系的界面张力由10~(-3)数量级迅速回缩至10~(-2);加入氯离子和溴离子则对三元体系的界面张力影响很小。     

基于分子动力学理论研究离子在C-A-S-H纳米孔道中的吸附与传输特性

运用分子动力学理论研究了氯化钠溶液在不同铝掺量[Al/(Al+Si)]的水化硅铝酸钙(C-A-S-H)凝胶孔模型中的基本特性,通过密度分布、吸附比例、径向分布函数(RDF)、时间相关函数(TCF)和均方位移(MSD)等手段进行表征.离子的密度在界面处出现了明显的峰值,其吸附比例亦随着铝掺量的增加有所增加.水化硅铝酸钙孔道界面对离子的动力学特性也有较大的影响,随着铝掺量的增加,钠离子与界面结构氧原子(Os)的吸附稳定性逐渐增强,阴阳离子的均方位移减小,即离子运动变慢,使得在水化硅酸钙孔道中的传输受到限制.从分子动力学的角度揭示了C-A-S-H界面对离子的吸附机理,对富含铝相水泥基材料的耐久性研究提供了重要的依据.     

碱金属掺杂对硫硅酸钙水化性能的影响机理

碱金属离子作为水泥原料中常见的杂质离子，其存在会影响熟料矿物结构与性能。采用分析纯试剂合成碱金属掺杂的硫硅酸钙单矿物，借助等温量热仪、综合热分析、扫描电镜和²⁹Si核磁共振等手段，研究了碱金属离子对硫硅酸钙水化活性及力学性能的影响。结果表明：碱金属离子在硫硅酸钙晶体结构中的固溶，能够降低晶体结晶度，形成晶体缺陷，有效提升硫硅酸钙早期水化活性，促进其早期力学性能快速发展。同时，碱金属的掺杂能够改变水化硅酸钙(C–S–H)凝胶等产物的微观形貌及结构。其中，Li₂O掺杂能够稳定絮状形态的C–S–H凝胶，而Na₂O和K₂O掺杂能够诱导C–S–H凝胶纤维状生长。经碱金属掺杂影响，C–S–H凝胶聚合度有所增加，平均硅链长增长。     

混凝土孔结构及其分形维数与氯离子扩散性能的关系

氯离子扩散系数是研究海洋环境下混凝土结构耐久性的重要参数之一。通过开展不同水胶比混凝土的压汞试验和盐雾扩散试验,研究了混凝土内部孔隙率、孔径分布及临界孔径对氯离子扩散系数的影响规律。结合Menger海绵体模型,建立孔体积分形维数与氯离子扩散系数的关系。结果表明:孔隙率和临界孔径与无量纲化氯离子扩散系数的相关性很高,可作为反映混凝土氯离子扩散性能的重要参数;通过数学分析计算得到的孔表面分形维数分布在2.56~3.86之间,孔体积分形维数分布在2.85~2.98之间;基于压汞法和分形理论计算得到的孔体积分形维数可以作为评价氯离子扩散系数的指标,在孔径小于10 nm、10～100 nm、100～1 000 nm以及大于1 000 nm四类区间,氯离子扩散系数随孔体积分形维数的增加而下降。     

海水与矿物掺合料对水泥基材料微结构和力学性能的影响

海水中多元离子会通过参与水化反应改变水泥水化产物组分和微结构,从而影响其力学性能和耐久性。本工作通过对孔溶液离子成分测试和水化产物矿相表征以揭示海水中多元离子与矿物掺合料耦合作用下,水泥基材料微结构和力学性能演化机制。采用离子色谱仪及电感耦合等离子发射光谱仪测得压滤法获得的净浆孔溶液的离子成分,同时结合X射线衍射、热重分析等微观表征研究水化产物矿相的演变规律。结果表明：海水中多元离子会促进水泥中硅酸三钙和硅酸二钙的水化,提高海水净浆孔溶液碱度,加快矿物掺合料与氢氧化钙的二次水化反应。硅灰对于孔溶液碱度的降低效果显著优于粉煤灰,也能通过生成更多水化硅酸钙凝胶实现钠离子和钾离子的有效固化。粉煤灰可以通过促进钙矾石、Friedel’s盐等铝相产物生成,实现对氯离子的有效固化,同时固化效果优于硅灰。本工作可以为海水海砂混凝土微结构和力学性能调控提供参考。     

水分在托贝莫来石中运动特性的分子动力学模拟研究

水和离子会通过水泥基材料的孔隙进入基体内部,导致一系列有害反应的发生.分子动力学方法(Molecular Dynamics,MD)可以模拟水分和托贝莫来石界面的相互作用,研究水泥基材料内部水分的运动特性.通过计算均方位移函数(Mean Squared Displacement,MSD)发现,托贝莫来石层间水分子的运动速率远小于溶液水分子的.此外,通过径向分布函数(Radial Distribution Function,RDF)和氢键数量计算发现水分子会和托贝莫来石表面形成大量氢键,使水化硅酸钙凝胶层间的水分子排列十分有序.     

NEPE推进剂中硝酸酯扩散的分子动力学模拟及实验研究

针对硝酸酯增塑聚醚(NEPE)推进剂中硝酸酯迁移扩散影响装药服役寿命的问题,采用分子动力学模拟与液相色谱实验相结合,开展硝酸酯(NG/BTTN)在NEPE推进剂中扩散机理及扩散系数的理论计算研究,并探讨了温度对NG和BTTN在NEPE推进剂中扩散行为的影响。结果表明,NG/PEG体系中NG分子的空间位置对初始位置的偏离程度相比于BTTN/PEG体系中BTTN分子较大,且BTTN分子在两个温度条件下的扩散系数均小于NG分子;实验计算的NG和BTTN在55℃和65℃下扩散系数的数量级与分子动力学模拟计算结果相同,通过模拟计算得到两种温度下NG分子在共混体系中的扩散系数分别为3.42×10-13 m2/s(65℃)和4.81×10-14 m2/s(55℃),BTTN分子在共混体系中的扩散系数分别为2.93×10-13 m2/s(65℃)和4.25×10-14 m2/s(55℃);随温度升高,硝酸酯分子的扩散系数增大,即迁移性增大,这与分子动力学模拟结果一致。     

Variation in the Composition of C-S-H Gel in Portland Cement Pastes Cured at Various Temperatures

Variation in the composition of C-S-H gel, the main hydration product of portland cement, cured at various temperatures, was studied using scanning electron microscopy with energy dispersive X-ray spectroscopy. Samples of two cement pastes were cured isothermally under water at 10°, 30°, and 60°C for one year. For the inner-product C-S-H gel of both cement pastes, increased curing temperature decreased the Ca/Si and Al/Ca ratios, but increased the S/Ca ratio. The Al/S and (Al + Fe)/S ratios decreased with increased curing temperature. Sulfur, probably in the form of sulfate ions, could be sorbed by the C-S-H gel at higher temperatures.     

Deconstructing water sorption isotherms in cement pastes by lattice density functional theory simulations

The moisture content in cement pastes influences their mechanical properties and durability. However, the complex, multiscale nature of cement pastes makes it challenging to isolate the contributions of each scale to their macroscopic water sorption isotherms. In particular, the contribution of the calcium–silicate–hydrate gel (the binding phase of cement pastes) remains only partially understood. Here, we introduce a density functional theory lattice model describing water sorption in calcium–silicate–hydrate, which properly reproduces experimental water sorption isotherms in cement pastes. Based on this model, we deconstruct the contribution of each pore scale (interlayer spacing, gel pores, and capillary pores) to the total sorption isotherm. We show that, when the relative humidity is below 80%, the calcium–silicate–hydrate gel accounts for more than 90% of the moisture content adsorbed in cement pastes. In turn, we find that the contribution of the interlayer space within the calcium–silicate–hydrate grains is governed by the competition between the rate of interlayer space opening and the increasing propensity for water to fill larger pores upon increasing relative humidity. Overall, our results highlight the key role played by the calcium–silicate–hydrate in governing the sorption isotherms of cement pastes.     

Studies on blended cement with a large amount of fly ash

The influence of the contents of the clinker, activators and fly ash on the properties of blended cement with high fly ash content was studied. Experimental data from X-ray diffraction and pore size distribution indicated that the main hydration product of the fly ash blended cement was C-S-H gel, ettringite and a small amount of Ca(OH)₂. The volume porosity of the pores with diameter bigger than 0.1 μm was lower than that of the micro pores and gel pores with diameter lower than 0.05 μm. The amount of chemical combined water has increased with the curing age duration, while the content of Ca(OH)₂ has reduced after 7 days.     

Implications of the structure of cementitious wastes containing Pb(II), Cd(II), As(V), and Cr(VI) on the leaching of metals

Cement stabilisation has been widely applied for the immobilisation of heavy metal ions before their disposal in landfills. This paper investigated the microstructure of cementitious wastes containing Pb, Cd, As, and Cr using an electron probe microanalyser and examined the implications of the microstructure on the leaching of the metal ions. From the microstructure analysis, it was proposed that Pb, As, and Cr ions were homogeneously dispersed in the calcium silicate hydrate (C-S-H) matrix by adsorption or precipitation with calcium or silicate compounds present in the cement. However, Cd formed discrete Cd(OH)₂ precipitates believed to be contained within the cement pores or adsorbed on the C-S-H matrix. The leaching of metals in the pH region of 6 to 8 decreased in the following order: Cr(VI)>Cd(II)>Pb(II)>As(V). This leaching trend was found to be influenced by the manner in which the metal ions were incorporated into the cement matrix.     

State of Water in Hydrating Tricalcium Silicate and Portland Cement Pastes as Measured by Quasi-Elastic Neutron Scattering

Quasi-elastic neutron scattering (QENS) was used to monitor the state of water in portland cement and tricalcium silicate pastes during the first 2 days of hydration at three different temperatures. By applying a double-Lorentzian rather than a single-Lorentzian fitting function, the QENS signal from water at a given hydration time was divided into three separate populations arising from liquid water, chemically bound water, and constrained water. The constrained water population consisted of water adsorbed on surfaces and contained in very small (<10 nm) pores, and could be associated primarily with the calcium-silicate-hydrate (C-S-H) phase. The rate of increase in the chemically bound water population closely followed the exothermic heat output, while the constrained water population increased more rapidly during the first several hours of hydration and then leveled off.     

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C₃S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C-S-H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C-S-H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C-S-H at Ca/Si ∼ 1.2 and eventually to the decomposition of C-S-H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C-S-H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.     

Calcium-silicate-hydrates/polycarboxylate ether nanocomposites doped by magnesium: Enhanced stability and accelerating effect on cement hydration

C-S-H/PCE (calcium-silicate-hydrates/polycarboxylate ether) nanocomposites have a significant accelerating effect on early hydration and early strength development of cementitious materials. In this study, magnesium ions are doped into C-S-H/PCE nanocomposites, and it is found that C-S-H/PCE nanocomposites doped by appropriate magnesium ions have much better stability. Besides, the accelerating effects of the modified nanocomposites on early cement hydration also get improved. The doped magnesium ions displace calcium ions and promote the absorption of PCE. Moreover, the incorporation of magnesium ions results in two unique morphologies of nanocomposites, that is, globule and gel. The globules are semi-crystalline similar to C-S-H, while the gels are amorphous and have more branched silicate chains.     

钢渣水化产物的特性(英文)

用X射线衍射分析、水化热的测量、化学结合水量的测定、孔结构的测定、扫描电镜观察及强度测试研究了钢渣的水化产物的特性。结果表明:钢渣硬化浆体中主要含有水化硅酸钙(C–S–H)凝胶、Ca(OH)2、惰性组分[RO相、铁酸二钙(C2F)和Fe3O4]和未水化的胶凝相[硅酸三钙(C3S)和硅酸二钙(C2S)];总体而言,钢渣的水化过程与水泥的水化过程相似;钢渣早期的水化速率远低于水泥,但钢渣后期,尤其是90d之后的水化速率高于水泥的。钢渣水化产生的C–S–H凝胶不具有良好的胶凝性能,凝胶之间的相互黏结也不牢固,因此钢渣砂浆的强度很低。     

基于X-CT的非饱和水泥基材料水分传输与渗透系数计算

利用X射线计算机断层扫描(X-CT)联合Cs离子增强技术连续监测水分在非饱和水泥基材料中的动态传输过程,建立水分传输距离与时间的关系,获得水分传输的毛细吸水系数,在此基础上提出了计算水泥基材料渗透系数的理论模型。系统研究了水灰比、粉煤灰掺量、矿渣掺量和砂体积掺量对水泥基材料毛细吸水系数和渗透系数的影响,结果表明:当水灰比从0.35增大到0.55时,硬化水泥浆体的毛细吸水系数从2.07×10^-4 m/s^1/2增大到3.22×10^-4 m/s^1/2,而固有渗透系数增大1个数量级;粉煤灰的掺入能有效降低浆体的水分传输性能,且粉煤灰的最佳掺量为30%(质量分数),当矿渣掺量为30%(质量分数)时,硬化浆体的固有渗透系数比掺同等质量粉煤灰的高1个数量级;当砂体积掺量从0%增加到40%时,砂浆的毛细吸水系数和固有渗透系数均下降,当砂体积掺量大于42.4%时,砂浆的界面过渡区(ITZ)连通,砂浆的毛细吸水系数增大。