基于分子动力学水分和离子在水化硅酸钙纳米孔道中的传输特性研究

水化硅酸钙凝胶是水泥水化产物中最基本的粘结相,水分子和离子在凝胶孔中的传输从根本上决定着水泥混凝土材料的服役寿命.采用分子动力学方法系统地研究了水分子、氯离子和钠离子在1nm、2nm、3nm和4nm的水化硅酸钙凝胶孔中的传输过程.基于径向分布函数和均方位移的离子轨迹分析发现在纳米孔道中离子和水分子展现出异于毛细水的分子结构和动力学特性:水分子有序性排布、离子大量在界面吸附和扩散速度急剧下降.这种分子结构与动力学的特性是因为水化硅酸钙界面处硅链中的非桥接氧会与水分子形成稳定的氢键连接,而钠离子可以形成Na-O化学键,同时表面的钙离子也可以与氯离子形成CaCl2团簇体.此外,随着孔径的增大,离子和水分子的扩散系数逐渐由0.15×10-9m2/s、0.7×10-9m2/s增大到1.3×10-9m2/s、3×10-9m2/s,这很接近于实验测得的毛细水的扩散系数,说明在纳米尺度上,孔径的约束和界面化学键作用是决定离子和水分子传输的关键因素.     

Characterization of Pore Space of Cement-Based Materials by Combined Mercury and Wood's Metal Intrusion

Analysis of the pore space is crucial for a profound understanding of the transport and mechanical properties of porous materials. Mercury intrusion porosimetry (MIP) is an easy and widely applied method to determine the pore size distribution of mesoporous materials, but a principal problem makes data interpretation difficult. Large ink-bottle pores may be accessed by the intruding mercury through smaller, so-called neck pores only. This leads to significant under estimaion of pore sizes and to hysteresis effects between intrusion and extrusion in materials with a broad pore size distribution such as cement-based materials. More accurate pore space information is obtained when ink-bottle pores in the measurement are excluded from analysis. This may be achieved by repeated intrusion cycles or by impregnating the ink-bottle pore space with Wood's metal. The combination of Wood's metal impregnation (WMI) and mercury intrusion in mortars and cement pastes as presented allows a characterization of the pore space independent of accessibility considerations. Different special pore types are defined, analyzed, and quantified. In a cement paste, 50% of all pores are found to be ink-bottle type, of which 60% are accessible through neck entrances larger than 20 nm in diameter. A further 30% of all pores are nonink-bottle type but are connected to the surface through such ink-bottle pores only. Furthermore, hysteresis and contact angle alternation effects between intrusion and extrusion were studied. A contact angle shift of 26° between intrusion and extrusion is proposed.     

冻融循环作用下饱水砂浆孔结构的演变规律

为揭示水泥基材料低温冻融孔结构损伤的演变规律,以饱水白水泥砂浆为研究对象,利用低场磁共振技术测试了砂浆试件在连续冻融循环作用下的孔结构。设计了20℃常规养护和80℃高温养护制度以改变砂浆的初始孔隙结构,随后对这两种相同配比、不同孔结构的砂浆进行快速和慢速冻融循环试验。结果表明:80℃高温养护会显著降低砂浆抗冻性,增大凝胶孔并提高粗毛细孔体积分数,但不会显著改变介观毛细孔的分布强度;与高温养护作用相反,冻融循环作用不会改变凝胶孔和粗毛细孔的分布强度,但能显著提高微裂纹的分布强度;80℃高温养护砂浆在冻融作用下的微裂纹体积增长速率显著高于20℃常规养护砂浆;冻融速率不会显著影响砂浆的孔径分布变化规律;砂浆内部微裂纹体积增大和质量损失增加呈双线性关系。砂浆内部冻融破坏和外部剥落均主要源自于微裂纹的产生和发展,与凝胶孔和粗毛细孔无明显相关性。     

Microstructural analysis of plain and reinforced mortars under chloride-induced deterioration

This paper reports the results of microstructural analysis of plain and steel-reinforced mortar specimens deteriorated by chlorides that were admixed or introduced through chloride ingress. The electrical properties of mortars were measured and their microstructural characteristics were investigated using quantitative image analysis techniques. The influence of chloride ions on mortar microstructure are discussed in terms of hydration and corrosion products. The research reveals that chlorides will induce changes in the chemical compositions and morphology of cement hydration products, and thereby exert influence on ion transport in the mortar specimens. The electrical properties of plain and reinforced mortars are not only related to the presence of chlorides in the pore system but are also influenced by the pore structure characteristics. The cementitious matrix undergoes certain alterations in conditions of the combined effects of: cement hydration, chloride ion transport and chemical binding mechanisms. To this end the pore structure characteristics appeared to be a significantly contributing factor in the process of chloride-induced corrosion in reinforced cement-based materials.     

Multiscale modeling of ion transport in porous electrodes

Ion transport through nanoporous materials is of fundamental importance for the design and development of filtration membranes, electrocatalysts, and electrochemical devices. Recent experiments have shown that ion transport across porous materials is substantially different from that in individual pores. Here, we report a new theoretical framework for ion transport in porous materials by combining molecular dynamics (MD) simulations at nanopore levels with the effective medium approximation to include pore network properties. The ion transport is enhanced with the combination of strong confinement and dominating surface properties at the nanoscale. We find that the overlap of electric double layers and ion–water interaction have significant effects on the ionic distribution, flux, and conductance of electrolytes. We further evaluate the gap between individual nanopores and complex pore networks, focusing on pore size distribution and pore connectivity. This article highlights unique mechanisms of ion transport in porous materials important for practical applications.     

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.     

Pore structure in mortars applied on restoration: Effect on properties relevant to decay of granite buildings

Since mortars play a key role in buildings decay, their suitable choice is critical to the success of restoration projects. The focus of this paper is to characterise the pore structures of a set of mortars and correlate them with mechanical properties and vapour permeability, which are relevant to the decay of granite buildings. Water vapour transport was tested by means of a simple set-up developed in our laboratory. A good correlation was found between total porosity and the two parameters tested: strength and vapour diffusivity. Pore size distribution also showed a strong influence on diffusivity. A mix based on cement with a high sand proportion was considered as the most suitable for granite building restoration because it showed good mechanical properties and low free calcium content. A negative aspect was that this mix exhibited significantly lower vapour permeability than mortar containing lime; this could be explained by the smaller radius of its pores.     

Transport properties study of supplementary cementitious materials: case of tuff,limestone filler and granodiorite

The aim of this research is to determine the effects of replacing cement with tuffs on the transport properties and service life of cement-based materials. Specimens were prepared replacing 5% and 15% by weight of cement with both white and brown tuffs. Additional specimens were prepared replacing cement with 5% and 15% limestone filler and granodiorite to compare the effects of these types of materials. For these four types of specimen as well as a control mortar with no cement replacement transport properties (gas permeability and chloride ingress) were measured as well as porosity and pore diameter. The transport properties were compared the changes in these porosity and pore diameter. Based on the results, white tuff significantly improves the transport properties of the mortars due to its pozzolanic reaction and that white tuff may be an economically and functionally viable replacement for cement where concrete durability is important. Pore diameter and porosity did not correlate well with transport properties.     

Porengrössenverteilung von Mörteln nach lagerung im wasser und in einer chloridlösung

For the purpose of the investigation of relations between the chloride diffusion into concrete and the pore structure of the hardened cement paste, the porosity of cement mortars were examined. The mortar specimens of four cements with a graded blast-furnace slag content (0 to 79 % by mass) prepared at w/c ratios 0,50 and 0,70 were stored in a concentrated NaCl solution and water respectively up to 2 years. The pore size distributions of the mortars were determined by means of mercury intrusion porosimetry and water absorption. The samples stored in water showed a relationship between the percentage of small (gel) as well as great (capillary) pores and the slag content of the cement and the w/c ratio of the mortar. After 6 and 12 months, the storing in the chloride solution effected an alteration of the pore structure reducing the total and gel pore volume and increasing the capillar pore volume. Later, the gel pore fraction was near to that of the specimens stored in water.     

Modeling of diffusive mass transport in micropores in cement based materials

In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity.     

Three-dimensional microstructure analysis of numerically simulated cementitious materials

In order to predict the transport properties of porous media, such as permeability and electrical conductivity of cementitious materials, a better understanding of the microstructural characteristics, including the geometrical and topological properties, is required. In this contribution, the microstructure of cementitious materials is simulated by using the cement hydration model HYMOSTRUC. In this computer-based numerical model, the hydrating cement grains are modeled as gradually growing spheres, which become in contact while growing. The simulated porous medium can be described as a series of sections taken from three orthogonal directions, in which each unit (pixel) is filled either with a solid or a fluid phase (pores). Various algorithms based on a random walk process are utilized to determine the local geometrical information, such as gravity center's coordinate, perimeter and area of each individual pore. The percolating path of the fluid in three dimensions is traced by using an overlap algorithm. Both three-dimensional (3D) geometrical information and topological space characterization including branch node network and genus of the pores are derived. Calculation results of these algorithms are compared with results obtained by other microstructural models at various degree of hydration.     

Mechanical properties and microstructure evaluation of fly ash - Slag geopolymer foaming materials

The existing fly ash-slag foaming geopolymer materials generally have the shortcomings of low fly ash content and low porosity. It is urgent to develop geopolymer foaming materials with high fly ash content and high porosity. Using fly ash and slag as the main raw materials, geopolymer foaming materials were prepared by alkali activation. The effects of activator content and sodium silicate modulus on the macroscopic mechanical properties, pore structures and microstructures of geopolymer foaming materials were studied. The experimental results showed that when the activator content was 21% (wt.) and the modulus of sodium silicate was 1, the specimen exhibited the best performance. The compressive strength of the specimen reached 2.18 MPa at 28 d, the porosity was 63.07%, and the average pore sizes of macroscopic pores were 920 μm. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS) analysis showed that when the content of activator was 21% and the modulus of sodium silicate was 1, the reaction grade of the system was the highest, reached 55.12%, meanwhile the main product Sodium silicate hydrate (N-A-S-H) gel produced the largest amount. The fractal dimension calculations showed that the spatial complexity of a specimen with large pores was greater than that of a specimen with small pores. This study can provide a basis for the design of geopolymer foaming materials with high proportion of fly ash and high porosity.     

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.     

催化剂孔道结构设计及孔内反应-扩散耦合模拟

采用硬球-拟颗粒(HS-PPM)耦合方法并结合简化的集总反应模型,模拟了碳四(C4)烯烃催化裂解过程中反应物和产物在孔道内的反应?扩散耦合过程,提出了一种多级孔道结构可控设计方法,实现了对孔隙率、孔径和孔体积占比等参数的独立量化调控,建立了包含三种不同孔径分布的催化剂孔道模型并定义了量化参数对反应?扩散耦合进行表征。结果表明,各组分在复杂孔道内的扩散过程显著受限,反应?扩散耦合性能较差,导致催化剂总体性能降低,达不到本征动力学允许的高反应速率。提出的可控设计孔道模型结合HS-PPM耦合模拟方法有助于催化剂材料的孔尺度结构的设计。     

Effect of mixing proportions of concrete on its electrical conductivity and the rapid chloride permeability test (ASTM C1202 or ASSHTO T277) results

The rapid chloride permeability test (RCPT)—American Society of Testing and Materials (ASTM) test method C1202 or American Association of States Highway and Transportation Officials (AASHTO) test method T277—is virtually a measurement of electrical conductivity of concrete, which depends on both the pore structure characteristics and pore solution chemistry of concrete. This paper discusses the effects of several factors, such as cement composition, replacement of cement with supplementary cementing materials and inclusion of aggregate, on the electrical conductivity or RCPT results of hardened cement mortars and concrete. Analyses based on published results have indicated that all the three factors may have significant effects on the chemistry and specific conductivity of concrete pore solution, which has little to do with the transport of ions in the solution. Thus, RCPT is not a valid test for evaluation of permeability of concretes made with different materials or different proportions.     

Gelatin-based air-entraining agents for ordinary portland cement: Chelate Ca2+ and reduce surface tension

Introducing uniform, stable, and small pores by adding air-entraining agents (AEAs) is an effective strategy for improving the freeze–thaw resistance and workability of cement concrete materials. In this study, the gelatin with hydrophobic and hydrophilic functional groups was used as an AEA for cement concrete materials and systematically studied its effect on the performance of cement mortar specimens. The results showed that gelatin has a surfactant-like function and can effectively reduce the surface tension of the gas–liquid interface. In an alkaline environment, Ca²⁺ ions in cement pore solution chelate with carboxylate ions formed by carboxyl ionization, further enhancing the performance of gelatin-based AEAs. Thanks to the reduction in surface activity and carboxyl ionization, introducing gelatin increased the porosity of cement mortar specimens from 0.53% to 2.69%. Furthermore, introducing an appropriate amount (>0.1 wt%) of gelatin does not significantly impact the workability or mechanical properties of cement mortar. Based on these characteristics, gelatin has potential as an AEA for cement concrete materials.     

NMR relaxation of molecules confined inside the cement paste pores under partially saturated conditions

Nuclear Magnetic Resonance (NMR) relaxation studies of liquids confined inside cement samples mostly refer to water molecules and saturated pores. Here we extend these studies to the ethanol and cyclohexane molecules partially saturating a white cement paste. The two filling liquids were selected as representatives of polar and nonpolar molecules which should experience different interactions with the surface containing OH groups. The results on these two molecules are compared with those obtained on water filled sample. The transverse relaxation measurements, performed at different saturation degrees, clearly revealed three pore reservoirs in cement paste: intra-C-S-H sheet pores, inter-C-S-H gel pores and capillary pores. A two phase exchange model, describing the effective relaxation rate under partially saturated conditions, is also considered. The model allows finding of a relationship between the relaxation time distribution, the filling degree of the pores and the liquid morphology inside these pores.     

水泥-硅灰/粉煤灰体系强度、收缩性能与微观结构研究

为研究硅灰及粉煤灰对不同养护龄期的水泥浆体强度及收缩性能的影响,以水胶比为0.29的水泥浆体为基体,设计制备了五种硅灰及粉煤灰掺量的复合水泥浆体,借助量热仪和压汞仪测试表征了不同复合水泥浆体的水化放热特性以及孔结构组成,分析了水化放热量、孔隙率等参数随硅灰和粉煤灰掺量增加的变化规律,建立了复合浆体抗压强度与孔结构以及水化特性与收缩应变之间的量化关系。结果表明,掺入粉煤灰会大幅降低水泥净浆早期抗压强度,但对减小自收缩应变和干缩应变极为有利。掺入硅灰能明显提高净浆3 d抗压强度,但当硅灰掺量超过10%(质量分数)后,净浆3 d自收缩应变及28 d干缩应变增加极为明显。掺入硅灰会使水泥水化诱导期开始和结束的时间提前,还会增加水化反应级数和各阶段的反应速率常数值,导致水泥-硅灰复合浆体的水化放热总量和放热速率相较于水泥-粉煤灰体系大幅增加。粉煤灰和硅灰的掺入均能有效细化水泥浆体内部孔结构,提高凝胶孔比例,大幅降低大孔比例。复合浆体的72 h水化放热总量和3 d自收缩应变呈现正相关关系,而孔隙率和抗压强度呈现明显的负相关关系。     

水泥基材料氯离子结合机理及影响因素研究综述

水泥基材料的氯离子结合能力主要取决于水化硅酸钙(C-S-H)凝胶和Friedel's盐的含量及其稳定性,两者的含量越高、稳定性越好,水泥基材料的氯离子结合能力越强。对水泥基材料氯离子结合能力的分析需考虑多种因素的影响,如水泥种类、矿物掺合料种类、温度、氯离子浓度、阳离子类型、硫酸盐侵蚀和碳化等因素,它们会通过直接影响C-S-H凝胶和Friedel's盐的生成量,或间接影响孔隙液pH值和离子浓度改变C-S-H凝胶和Friedel's盐的稳定性,进而影响其物理吸附能力与化学结合能力,促使氯离子重新结合或释放,导致氯离子结合能力变化显著。本文综述了上述影响因素下,水泥基材料中C-S-H凝胶和Friedel's盐含量、稳定性的变化,及其对氯离子结合能力的影响,并为今后的研究方向提出了建议。