玻璃粉水泥浆的初期水化产物和浆体结构

采用SEM、XRD研究了玻璃粉水泥浆的初期水化产物、浆体结构.并用化学结合水量和有效结合水法来定性和定量分析玻璃粉对水化初期复合体系及水泥的促进或抑制作用以及作用程度.研究表明:在水化反应初期(1d内),因为玻璃粉的掺入既由此而产生的稀释作用使有效水灰比增加而产生的对水泥熟料水化的促进作用,因此,硅酸盐水泥熟料的水化程度较高,但从整体来看,大掺量(50%)的玻璃粉延缓了复合胶凝材料总水化程度;水化开始(6 h~1 d)时,水化反应开始加速进行,水化产物的数量迅速增加,主要为纤维状CSH凝胶、针棒状钙矾石晶体和Ca(OH)2,这些水化产物彼此间相互搭接、交错生长,部分未水化的水泥颗粒镶嵌其中,并将玻璃粉粘结成整体,构成体系骨架.     

超硫酸盐水泥净浆的酸性侵蚀劣化机制

本文对比研究了超硫酸盐水泥(SSC)与硅酸盐水泥在盐酸侵蚀条件下的力学性能变化规律,探讨了SSC的酸性侵蚀劣化机制。测试了SSC净浆试件正常养护后在盐酸溶液以及清水中抗压强度的变化,并计算了抗压强度保持率;采用X射线衍射(XRD)、扫描电子显微镜(SEM)和热重分析(TG-DTG)等微观测试方法分析了SSC在盐酸侵蚀下水化产物种类和数量的变化,并探究了其微观结构的演化过程,分析了劣化机理。结果表明：不同于硅酸盐体系,SSC体系主要水化产物为钙矾石和水化硅酸钙;与硅酸盐水泥相比,SSC在酸中具有较高的抗压强度保持率和更好的抗酸性侵蚀性能;盐酸侵蚀SSC后,SSC主要水化产物均被分解,体系中存在二水石膏和大量的二氧化硅胶体,而硅酸盐水泥体系中则存在大量的二氧化硅胶体。     

Experimental study and numerical simulation on the formation of microstructure in cementitious materials at early age

The formation of microstructure in early age cement paste and concrete was examined with an ultrasonic experimental set-up. Research parameters included the influence of curing temperature (isothermal curing at 20, 30 and 40 °C), water/cement ratio (0.40, 0.45 and 0.55) and amount of aggregate. In parallel with the experiments, the cement hydration model HYMOSTRUC was utilized to simulate the formation of the microstructure. In this study, the cement paste was considered as a four-phase system consisting of water, unhydrated cement, hydration products and that part of the hydration product that causes the contact between the hydrating cement grains (so called “bridge volume”). A correlation has been found between the growth of bridge volume calculated with the model and the changes in the pulse velocity. It is believed that ultrasonic pulse velocity (UPV) measurements can represent a valuable tool to investigate the development of the microstructure at early age.     

Modelling elasticity of a hydrating cement paste

Concrete is a complex multi-scale composite involving multi-physics processes. As it is the only evolving component of concrete, the cement paste has a major influence on the mechanical properties of concrete at early age. This paper focuses on the increase of the elastic properties of a cement paste during its hydration. The homogenization theory for disordered media is used in order to estimate the evolution of the effective elastic moduli of the hydrating paste. The morphological model refers to two types of C-S-H (calcium silicate hydrates, main hydration products of Portland cements) distinguished by many authors: inner products or high density C-S-H build up layers surrounding the anhydrous particles, while the outer products or low density C-S-H play the role of a porous matrix.The simulations of the effective Young's modulus at late age during hydration and at the end of hydration prove to be in excellent agreement with the experimental results available in the literature.     

缓凝剂对水泥浆中自由水状态演变的影响

用低场核磁共振技术考察了HN-1型缓凝剂对水泥浆体中可蒸发水的横向弛豫时间(T2)及状态演变过程的影响。结果表明:在初始水化阶段的150 min内,水泥浆GR(添加缓凝剂HN-1的水泥净浆)的弛豫峰峰形和峰顶位置均无明显变化,水泥浆G(纯水泥净浆)的弛豫峰峰形变窄且峰顶位置从2.15 ms迁移至0.95 ms,说明缓凝剂HN-1主要通过改变水泥浆体中不同状态水的存留时间来改变其水化进程。随养护时间的延长,硬化水泥石W0.44和WR0.5中可挥发水弛豫峰分布范围分别从0.11~4.75 ms变为0.08~0.58 ms、0.24~4.23 ms变为0.11~2.35 ms,总体趋向于短弛豫时间,表明水泥石中毛细水逐渐向凝胶水和物理结合水转化,水泥石养护龄期延长至25 d时,其内部凝胶结构水含量超过90%。利用XRD考察了缓凝剂对水泥浆水化产物的影响,结果表明:缓凝剂只改变水泥浆水化过程,对最终水化产物晶型及晶型结构不存在任何影响。     

Influence of solution chemistry on the hydration of polished clinker surfaces—a study of different types of polycarboxylic acid-based admixtures

The influence of two different types of polycarboxylic acid-based superplasticizers (homo- [HP] and copolymer [CP]) on the early hydration of Portland cement clinker is investigated. Polished clinker surfaces are hydrated in aqueous phase extracted from cement paste with and without superplasticizers as well as in saturated Ca(OH)₂/CaSO₄ solution and in deionized water. Scanning electron microscopy (SEM) is used to characterize the early hydration products after 30 min and after 24 h of hydration. Contrary to the hydration reaction with deionized water, in the presence of ionic solutions, the texture and morphology of the clinker surface are significantly altered, and amorphous and crystalline hydration products, mainly ettringite, are formed. The amount and type of ettringite formed are strongly related to solution chemistry and type of superplasticizer. In the presence of superplasticizers, the formation of hydration products and ettringite is reduced and the hydration of the surface is suppressed. Furthermore, the rate of hydration in mortar is studied in the presence of the two admixtures by measuring the temperature evolution as a function of time. The concentration-dependent delay in temperature evolution correlates well with the microscopic observations of different amounts of ettringite formation.     

The influence of admixed micelles on the microstructural properties and global performance of cement-based materials

In this study, the influence of the admixed micelles (poly(ethylene oxide)-block-polystyrene micelles) on the microstructure and hydration rate of cement paste and more global performance of mortar were investigated. In cement paste, the cement hydration rate was only slightly influenced by the admixed micelles; on the other hand, the micelles lead to a reduced porosity and a denser matrix. The nano indentation combined with MIP and microstrutural analyses proves that the admixed micelles lead to a more uniform distribution of hydration products. In mortar, the compressive strength of the micelle-containing mortar was almost unaffected; the coefficient of water permeability was 3 orders of magnitude lower for the micelle-containing specimens, compared to the control (micelle-free) specimens. All experimental results indicate that the micelles act as nucleation sites for the formation of new hydration products and possibly induce defloculation of un-hydrated cement particles, resulting in a more uniform and homogeneous cement matrix.     

硫酸盐侵蚀下硬化水泥浆体微结构演变及膨胀过程的数值模拟

根据硫酸盐侵蚀机理,利用改进的CEMHYD3D水化模型和随机概率方法,建立了硫酸盐侵蚀下硬化水泥浆体的微结构演变模型。在微观层次上,模拟了浆体孔溶液中硫酸根离子的自由扩散、随机碰撞和转化反应,分析了膨胀性侵蚀产物生长导致的微结构损伤和体积膨胀,计算了侵蚀过程中石膏和钙矾石的生成量及浆体的膨胀应变,并与已有试验结果对比分析验证了模型的合理性。在此基础上,数值模拟了硫酸盐侵蚀下不同水灰比水泥浆体的微结构演变及膨胀过程。结果表明：同一硫酸盐浓度下,硬化水泥浆体中氢氧化钙和含铝物相与孔隙的接触面积越小,浆体的膨胀应变越低;水灰比为0.25、0.30和0.35的硬化水泥浆体的孔隙填充程度分别达到9.09%、9.27%和9.41%时,浆体膨胀应变开始快速增大;硫酸盐侵蚀溶液浓度增大,浆体体积快速膨胀的时间提前。     

养护条件对EVA乳液改性水泥浆体水化行为和孔结构的影响

本文制备了不同乙烯-醋酸乙烯酯(EVA)乳液含量的聚合物改性硅酸盐水泥浆体,对比了其在相同水灰比,不同养护制度(标准养护、蒸汽养护)条件的水化性能与孔结构。通过X射线衍射分析、水化热分析、压汞分析、氮吸附测试,以及立方体抗压强度测试评价了EVA乳液在不同养护状态下对水泥水化产物、水化特性、孔结构以及抗压强度的影响。结果表明,与标准养护相比,蒸汽养护下EVA乳液没有改变水泥水化产物,在EVA乳液较高掺量下,降低了其延缓水泥水化的程度,降低了试样的孔隙率,并且增加了试样中小孔的含量,而蒸汽养护削弱了EVA乳液对水泥净浆强度的负面影响。     

Predicting Ca(OH)2 content and chemical shrinkage of hydrating cement pastes using analytical approach

A semiempirical model is proposed to predict the evolution of chemical shrinkage and Ca(OH)₂ content of cement paste at early age of hydration. The model is based on chemical equations and cement compound hydration rates. Chemical shrinkage and Ca(OH)₂ amount are computed using the stoichiometric results of the hydration reactions considered in the model and the density of hydration products and reactants. The model validation is conducted by comparison between computed and experimental results achieved on ordinary cement pastes with different water-to-cement (w/c) ratios (0.25, 0.30, 0.35 and 0.40) cured at 10, 20, 30, 40 and 50 °C, respectively. Hydration degree and Ca(OH)₂ content are determined using the thermogravimetric analysis (TGA) and chemical shrinkage evolution using a gravimetric method.The comparison reveals a good consistency between modelled and experimental data at early age of hydration.     

A new hydration kinetics model of composite cementitious materials,part 1: Hydration kinetic model of Portland cement

This work is the first part of an overall research project that aims to model the hydration kinetics of modern composite cementitious materials. The aim of this part is to build a hydration kinetic model to characterize the hydration behavior of cement at all ages. In this paper, the nucleation and growth of CSH and the diffusion of water are described by a modified BNG model and a modified Jander's model. The kinetic parameters are obtained by simulative fitting of the model to the experimental data. The linear functions between the nucleation and growth rates and the W/C ratios are given in this paper. The apparent activation energy of the nucleation rate constant, growth rate constant, and generalized diffusion constant are approximately 36.0, 37.5, and 42.0 kJ/mol, respectively. Finally, the phase compositions of hardened cement paste are calculated according to the kinetic model and the reaction formula of cement.     

己二酸对掺石灰石粉水泥浆性能的影响

为探究掺加石灰石粉的水泥浆中加入己二酸的作用效果,对添加己二酸的掺石灰石粉水泥浆体流动性、强度、水化热及化学结合水的影响规律进行了研究.利用X射线衍射和扫描电镜技术手段对其机理进行分析.结果表明:己二酸的加入提高了掺石灰石粉水泥浆体的流动度、3 d强度、早期水化温升值和早期水化结合水量,掺入己二酸在3d时生成更多的水化碳铝酸钙,水化产物结构更加致密.     

1H驰豫时间谱在水泥基材料研究中的应用

水泥水化、硬化体结构的形成及演化、水泥基材料内部不同水分之间的转化、吸水、干燥、水分在水泥基材料内部的扩散过程引起水分化学状态或所处环境物理状态的变化.这种变化可用1H核磁共振驰豫时间进行表征.已有的研究表明,1H驰豫时间谱可用于水泥水化过程、硬化体结构形成、孔结构、水分在水泥基材料内的传输过程等的表征,所得结果与其它方法所得结果有较好的一致性.且磁共振成像可直接观察水分在水泥基材料中的分布及传输,这是其它现代测试方法难以达到的.     

Detection of Failures around Main Cracks and Their Relation to the Fracture Toughness of Cement Pastes

Fluorescence microscopy examinations revealed the characteristics of crack path and the presence of local failures around the main cracks in hardened cement pastes. Reduction in the water: cement ratio and the addition of silica fume led to a decrease in locally damaged regions around the main cracks. However, the R - curve of the silica fume-free cement paste with the water: cement ratio of 0.55 was much different from that of the silica fume-bearing paste. The silica fume-bearing cement paste exhibited a relatively flat R -curve, whereas reduction in the water: cement ratio in the silica fume-free paste led to a rising R-curve. These results were related to the toughening mechanisms of cement pastes derived from the characteristics of the crack path and the areas of local failures around main cracks that were revealed by fluoresence microscopy. Presumably, these differences in fracture toughness resulted from the nature of hydration products and/or their inherent microstructure.     

内掺硅烷乳液憎水剂对混凝土性能的影响

通过强度测试、水化热实验、热重分析、毛细吸水实验以及压汞实验研究了内掺硅烷乳液憎水剂对混凝土强度以及毛细吸水性能的影响规律及其机理.结果表明:高掺量硅烷乳液一定程度上抑制水泥水化,从而导致混凝土早龄期抗压强度和劈裂抗拉强度下降以及28 d养护后水泥净浆试块中毛细孔的体积增大.毛细吸水实验表明:28 d龄期混凝土毛细吸水...     

环氧乳液改性水泥基材水化过程的硬化机理

极端环境和复杂荷载条件对混凝土结构的材料性能提出了更高的要求,聚合物通过改性水泥基材提高混凝土性能的方法已经得到了广泛应用。本研究为揭示环氧乳液改性水泥基材水化过程的硬化机理,通过等温放热试验分析环氧乳液对水泥水化放热过程的影响,结合原位XRD技术跟踪水泥主要矿物熟料和水化产物在水化反应早期的相含量发展。研究结果表明,环氧乳液对水泥水化的阻滞作用与环氧颗粒、水泥矿物熟料和水化产物之间的相互作用有关,并随着水化时间的延长,相互作用效果越明显。在水泥胶凝体系中,环氧乳液会减缓水化放热速率,降低水化放热峰值,减少累积放热量。环氧乳液通过抑制水泥矿物(C₃S、C₃A、石膏)的溶解和水化产物(钙矾石、氢氧化钙)的析出,延缓硅酸盐反应和铝酸盐反应;环氧乳液对水泥水化的影响随着其掺量的增加而增强。     

Influence of a fine glass powder on cement hydration: Comparison to fly ash and modeling the degree of hydration

This paper reports the results of an investigation carried out to understand the influence of a fine glass powder on cement hydration. The pozzolanicity of the glass powder and a Class F fly ash for comparison was evaluated using strength activity index over a period of time, and a rapid electrical conductivity based method. Flame emission spectroscopy and electrical conductivity tests were used to quantify the alkali release from glass powder, and gain information on the rate of alkali release. It was found that the glass powder releases only a very small fraction of sodium ions into the solution. It was observed that the glass powder modified pastes show higher non-evaporable water contents than the plain paste and fly ash modified pastes, indicating that glass powder facilitates enhancement in cement hydration. An expression has been developed for the change in non-evaporable water content as a result of enhancement in cement hydration and the hydration of the cement replacement material. The efficiency of any cement replacement material with age in the paste system can be quantified using this parameter. Based on this parameter, a 5% cement replacement with glass powder was found to be effective at the chosen water-to-cementing materials ratio (w/cm), whereas at higher replacement levels, the dilution effect dominates. A model to predict the combined degree of hydration of cement pastes incorporating more than one cementing material is outlined. The measured and predicted combined degrees of hydration agree well.     

缓凝剂对补偿收缩胶凝体系水化放热的影响

采用等温量热法,研究了酒石酸和硼酸这2种缓凝剂对掺氧化钙型膨胀剂与氧化钙-硫铝酸钙复合型膨胀剂的水泥净浆的诱导期结束时间、主放热峰出现时间及其最大放热速率、不同时间累计放热量等方面的影响.结果表明:与掺膨胀剂水泥净浆相比,缓凝剂均明显推迟补偿收缩胶凝材料体系的诱导期结束时间和主放热峰出现时间,减少最大放热速率和24 h内累计水化热,72 h总水化热略低.对氧化钙型膨胀剂水泥体系,硼酸比酒石酸缓凝效果好.     

Determining the water–cement ratio, cement content, water content and degree of hydration of hardened cement paste: Method development and validation on paste samples

We propose a new method to estimate the initial cement content, water content and free water/cement ratio (w/c) of hardened cement-based materials made with Portland cements that have unknown mixture proportions and degree of hydration. This method first quantifies the composition of the hardened cement paste, i.e. the volumetric fractions of capillary pores, hydration products and unreacted cement, using high-resolution field emission scanning electron microscopy (FE-SEM) in the backscattered electron (BSE) mode and image analysis. From the obtained data and the volumetric increase of solids during cement hydration, we compute the initial free water content and cement content, hence the free w/c ratio. The same method can also be used to calculate the degree of hydration. The proposed method has the advantage that it is quantitative and does not require comparison with calibration graphs or reference samples made with the same materials and cured to the same degree of hydration as the tested sample. This paper reports the development, assumptions and limitations of the proposed method, and preliminary results from Portland cement pastes with a range of w/c ratios (0.25–0.50) and curing ages (3–90 days). We also discuss the extension of the technique to mortars and concretes, and samples made with blended cements.     

减水剂对硫铝酸盐水泥早期水化影响

研究了聚羧酸系高效减水剂(PCE)和萘系减水剂(FDN)对硫铝酸盐水泥净浆工作性能及力学性能影响,通过XRD和SEM检测手段对水化产物进行表征.结果表明:两种减水剂对硫铝酸盐水泥净浆流动度的影响存在饱和点;相比于FDN型减水剂,PCE型减水剂对硫铝酸盐水泥净浆具有更好的减水效率及分散能力.PCE型减水剂阻碍硫铝酸盐水泥净浆早期水化,并降低硫铝酸盐水泥净浆1 d抗压强度;FDN型减水剂能够加速硫铝酸盐水泥净浆早期水化,缩短初凝和终凝时间,提高硫铝酸盐水泥净浆1d抗压强度.两种减水剂对硫铝酸盐水泥净浆3d后抗压强度及水化产物种类均没有影响.