Simulation of water permeability and water vapor diffusion through hardened cement paste

The transport properties of cement-based materials significantly affect their durability. This results from the fact that most of the damaging reagents are transported, often solved in water, through the open pore space into the microstructure. This paper focuses on simulating water permeation (movement under a gradient of pressure) and water vapor diffusion (movement under a gradient of concentration) through hardened cement paste (hcp). The main goal is to derive the water permeability and the water vapor diffusion coefficient directly from the morphology of the 3D microstructure. For this purpose microtomographic images of a hcp made of ordinary Portland cement are used to represent the microstructure and especially the pore space through which the moisture transport will occur. With the use of a skeletonization algorithm, also known as “thinning algorithm”, the skeleton or centerline of the pore space is extracted. This skeleton is in a second step converted into a transportation network of cylindrical tubes. Bernoulli's law is applied to every tube for simulating water permeation. The permeability coefficient is then calculated by using Darcy's law. In the case of water vapor diffusion the diffusion coefficient is calculated using Fick's law. An erratum to this article is available at .     

Micromachining of hardened Portland cement pastes using femtosecond laser pulses

Femtosecond laser pulses (30 fs in length) of various energies were utilised for production of single and multiple overlapping ablation sites on flat polished surfaces of hardened Portland cement pastes. In order to assess the sizes of the ablation sites and possible subsurface laser-induced damage, the ablation sites were investigated using environmental scanning electron microscopy (ESEM) – both from normal top–down view and in cross-sections. Furthermore, approximately 10-μm wide notches were produced using femtosecond pulses on cylindrical microspecimens (150 μm in diameter) of hardened Portland cement pastes. In addition to electron microscopy observations, several microspecimens were investigated using synchrotron-based X-ray computed microtomography (SRμCT). The results suggest that production of “damage-free” samples for micromechanical testing of hardened Portland cements pastes is possible.

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Résumé Des impulsions laser (de durée 30 fs) et d’énergie variable ont été utilisées pour produire une ablation ponctuelle ou linéique à la surface d’un ciment durci de type Portland et préalablement polie. Pour déterminer la taille des impacts et d’éventuels dommages causés par le laser sous la surface, les points d’impact ont été visualisés à l’aide de la technique de l’ESEM (microscope électronique à balayage environnemental). De plus, des piliers de 10 micromètres de diamètre environ ont été réalisés par ablation femtoseconde sur des échantillons cylindriques de 150 microns de diamètre. Pour compléter les observations faites au microscope électronique, certains échantillons ont été observés grace à la technique SRμCT (synchrotron-based X-ray computed microtomography). Les résultats montrent que la production d’échantillons non-endommagés de ciment de type Portland pour des tests micro-mécaniques ultérieurs est possible.

     

氧化石墨烯对水泥净浆流动度及水泥石结构和性能的影响

采用Hummers法及超声破碎分散法制备了氧化石墨烯(GO)纳米相分散液,研究了GO对水泥净浆流动度和水泥石微观结构的影响,用FT-IR、AFM、XRD及SEM对GO及水泥石结构进行了表征。结果表明GO的掺入降低了净浆流动性,每增加0.01%的GO需要增加0.02%的聚羧酸系减水剂(PCs)以保持水泥净浆流动度在3 h内在200 mm以上,GO的掺入能够使水泥石的微观结构发生明显的变化,当GO/PCs掺量为0.01%/0.24%~0.03%/0.28%时,水化龄期7 d的水泥石出现了大量分散均匀的花状微晶体,当GO/PCs掺量为0.05%/0.32%~0.07%/0.36%时,同龄期水泥石中出现大量的片状晶体,在水化龄期延长到28 d时有转化为密实结构的趋势,结果说明GO具有调控水泥水化产物形貌的能力及增强增韧的作用,此研究结果对于提高水泥基材料的力学性能具有重要意义。     

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。     

The microstructure of cement paste and concrete––a visual primer

This paper represents an attempt to provide an introduction to the microstructure of cement paste and concrete as seen in backscatter-mode scanning electron microscopy (SEM). Illustrations are provided of the ‘internal architecture' underlying the microstructure of hardened cement paste and paste in concrete, at the size scales accessible to this instrument. Concrete is a uniquely complex engineering material. The aim of this paper is to provide researchers and practitioners who deal with it an acquaintance with its internal structure.     

基于土体isotache模型的硬化水泥净浆微观徐变预测

水泥基材料是一种应变率敏感性材料,充分掌握水泥基材料的应变率效应对其力学性能和变形的影响可保障混凝土结构的安全设计。采用连续刚度测试（CSM）研究不同应变率下硬化水泥净浆微观徐变特征,试样的水灰比为0.3、0.4和0.5,CSM的最大压入深度为30 μm,应变率分别为0.01 s^-1、0.05 s^-1、0.1 s^-1和0.5 s^-1,并讨论土体的isotache模型表征硬化水泥净浆微观徐变行为的适用性。结果表明,在0.01~0.5 s^-1应变率范围内,微观尺度上的硬化水泥净浆存在唯一的h_vp-P-ε关系,与土体isotache模型的ε_vp-σ-ε关系类似,该关系可由归一化后的h_vp-P/（P_N/P_N0.05）曲线和P_N/P_N0.05-ε曲线来表征。采用h_vp-P/（P_N/P_N0.05）曲线和P_N/P_N0.05-ε曲线可预测不同应变率下硬化水泥净浆加载阶段的h_vp-P曲线,预测结果与实测结果基本吻合,表明可以采用土体的isotache模型来预测硬化水泥净浆微观尺度上的徐变行为。     

Pore structure of hardened cement paste and mortar

The paper describes an experimental investigation into the pore structure of hardened cement paste and mortar. The pore structure was studied using mercury porosimeter. Ordinary portland cement and natural river sand were used. Pore structure determination was carried out for both the cement paste and mortar mixes over four hydration periods and five water-cement ratios. The threshold radius which was found to be prominent in the hardened cement paste, flattens out as the fine aggregate content increased.     

Prediction of chloride ingress and binding in cement paste

This paper summarizes recent work on an analytical model for predicting the ingress rate of chlorides in cement-based materials. An integral part of this is a thermodynamic model for predicting the phase equilibria in hydrated Portland cement. The model’s ability to predict chloride binding in Portland cement pastes at any content of chloride, alkalis, sulfates and carbonate was verified experimentally and found to be equally valid when applied to other data in the literature. The thermodynamic model for predicting the phase equilibria in hydrated Portland cement was introduced into an existing Finite Difference Model for the ingress of chlorides into concrete which takes into account its multi-component nature. The “composite theory” was then used to predict the diffusivity of each ion based on the phase assemblage present in the hydrated Portland cement paste. Agreement was found between profiles for the Cl/Ca ratio predicted by the model and those determined experimentally on 0.45 water/powder ratio Portland cement pastes exposed to 650 mM NaCl for 70 days. This confirms the assumption of essentially instantaneous binding where quasi-equilibrium is established locally. This does not imply steady state diffusion however. It simply implies that incremental increases in the concentration of diffusing ions in the pore solution will rapidly re-equilibrate with the hydrates present locally, where, the greater the ratio of bound to free ions, the greater the buffering effect which slows down the rate of ingress. In the case of chlorides, this buffering effect is greatest at high contents of AFm (AFm is aluminium ferrite compounds with a single (mono) formula unit CaX.) phase and low alkali metal contents.     

掺石墨/碳纤维电磁屏蔽砂浆的研究

以石墨和碳纤维为电磁屏蔽功能基元材料,将其添加到砂浆中,部分取代砂浆中的砂,调节电磁屏蔽功能材料的含量,使砂浆对电磁波具有屏蔽和吸收功能。研究了电磁屏蔽功能基元材料含量对砂浆电磁屏蔽效能的影响。结果表明添加石墨和碳纤维电磁屏蔽功能基元材料的电磁屏蔽砂浆,在30～200MHz频率范围内,电磁屏蔽效能SE较大,随着频率增加,屏蔽效能SE迅速降低。在200MHz～1.8GHz频率范围内,电磁屏蔽效能SE值趋于平缓,屏蔽效能较低。在1.8GHz附近,碳纤维电磁屏蔽砂浆的SEmin最大,为10dB。     

Properties of 3D-printed fiber-reinforced Portland cement paste

First insights into a 3D-printed composite of Portland cement paste and reinforcing short fibers (carbon, glass and basalt fibers, 3–6 mm) are presented, resulting in novel materials that exhibit high flexural (up to 30 MPa) and compressive strength (up to 80 MPa). Alignment of the fibers, caused by the 3D-printing process is observed, opening up the possibility to use the print path direction as a means to control fiber orientation within the printed structures. Apart from completely dense cementitious bodies, hierarchically structured bodies, displaying precisely adjusted macroporosity, are presented, the latter exhibiting a unique combination of strength and materials efficiency.