蒸养制度对预制桥面板混凝土强度与抗渗性的影响

为了获得预制桥面板的最佳蒸养制度,研究了蒸养温度、蒸养时间及蒸养后补充养护方式对复掺粉煤灰和矿粉配制的C55混凝土的强度和氯离子扩散系数的影响,并采用X-射线衍射仪(XRD)和扫描电子显微镜(SEM)分析了不同蒸养制度下胶凝材料浆体的物相组成和微结构。结果表明,蒸养温度的提高和蒸养时间的延长,对水泥的水化和浆体微结构形成有促进作用,提高了混凝土的脱模强度,但降低了混凝土的后期强度和抗氯离子渗透性能。混凝土蒸养后的补充湿润养护为未水化水泥和矿物掺合料的持续水化提供水化用水,有利于混凝土的后期强度增进和抗渗性提高。预制桥面板在55 ℃温度下蒸养6 h,蒸养后补充覆盖洒水养护7 d具有较好的综合性能。     

Possibilities of using the ultrasonic wave transmission method to estimate initial setting time of cement paste

In this paper, the applicability of the ultrasonic wave transmission method to estimate the initial setting time of an arbitrary cement paste is discussed. Ultrasonic pulse velocity measurements were fully automated and measured continuously. The Vicat Needle Test was used in order to determine the initial setting time of cement pastes. Different cement pastes were prepared in order to check the influence of the water/cement ratio, type of cement, curing temperature, cement fineness, and some clinker compositions, on the relationship between the initial setting time and ultrasonic pulse velocity. It was found that the initial setting time of an arbitrary cement paste can be estimated very accurately by the time the first inflection point appears on the ultrasonic pulse velocity curve. Moreover, it can be estimated quite accurately by the time the ultrasonic pulse velocity reaches a fixed value, close to the value of the ultrasonic pulse velocity in water.     

X-ray absorption study of drying cement paste and mortar

X-ray absorption was used to observe water evaporation with hydration time in paste and mortar specimens, with the aim of studying the influence of water/cement (w/c) ratio, presence of aggregates, curing conditions on drying during early hydration. For the samples subjected to surface drying immediately after mixing, there exists a moisture gradient within the internal part of the specimen. However, obvious top-down drying only occurs within a small zone near the surface for early age cement pastes and mortars. The evaporation rate of water is very high in the first day after casting and is drastically reduced afterwards due to the formation of a microstructure that greatly improves specimens resistance to moisture loss. Mortars reveal a slightly lower evaporation rate since the aggregate increases the length of the transport route because of a larger tortuosity. However, the effect of sealed curing is much more important than the tortuosity effect of the aggregates.     

Microstructure of cement paste subject to early carbonation curing

Microstructure of Ordinary Portland Cement paste subjected to early age carbonation curing was studied to examine the effect of early carbonation on performance of paste at different ages. The study was intended to understand the mechanism of concrete carbonation at early age through the microstructure development of its cement paste. Early carbonation was carried out after 18-hour initial controlled air curing. The microstructure characterized by XRD, TGA, ²⁹Si NMR and SEM was correlated to strength gain, CO₂ uptake and pH change. It was found that early carbonation could accelerate early strength while allowing subsequent hydration. The short term carbonation created a microstructure with more strength-contributing solids than conventional hydration. Calcium hydroxide was converted to calcium carbonates, and calcium–silicate–hydrate became intermingled with carbonates, generating an amorphous calcium–silicate–hydrocarbonate binding phase. Carbonation modified C–S–H retained its original gel structure. The re-hydration procedure applied after carbonation was essential in increasing late strength and durability.     

矿物掺合料对蒸养水泥净浆性能的影响

研究了蒸养条件下粉煤灰、矿渣的掺量对水泥净浆化学结合水量和抗压强度的影响,揭示了矿物掺合料对蒸养水泥净浆水化性能和力学性能的影响。试验结果表明,与标准养护相比,蒸汽养护更有利于激发粉煤灰和矿渣的火山灰活性,促进水泥的早期水化,提高水泥浆体的早期强度;但无论是蒸汽养护还是标准养护,随着矿物掺合料掺量的增加,复合胶凝材料的水化性能和力学性能明显减弱,因此矿物掺合料掺量不宜太大。     

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.     

Early performances of cement paste in the presence of triethanolamine: Rheology,setting and microstructural development

The effect of triethanolamine (TEA) at various dosages on the early performance of cement paste was systematically evaluated through the techniques of rheological measurements, penetration tests, and ultrasonic pulse velocity. The correlation of early performance to the chemical hydration process was analyzed by calorimetry, zeta potential, in situ XRD, and pore solution analysis. It is found that the effect of TEA on the early performance of cement paste is strongly dependent on its dosage. With the TEA dosage below 0.1 wt%, the setting and microstructural development of cement paste are retarded. Meanwhile, the yield stress of fresh paste is decreased due to the increasing zeta potential of cement grains. The promoted formation of ettringite (AFt) and monosulfate (AFm) caused by TEA decreases the rheological retention ability. At dosages ≥0.2 wt%, the reaction of aluminate-containing phases is greatly accelerated and a flash setting is observed. Besides, the importance of ferric phase on the reaction of cement with TEA is highlighted. At a low dosage, TEA prefers to accelerate the dissolution of tetracalcium aluminoferrite (C₄AF) first and increases the [Fe] in the pore solution of cement paste. In cement without C₄AF, the retardation of TEA on silicate phase hydration is significantly alleviated.     

Modeling the linear elastic properties of Portland cement paste

The linear elastic moduli of cement paste are key parameters, along with the cement paste compressive and tensile strengths, for characterizing the mechanical response of mortar and concrete. Predicting these moduli is difficult, as these materials are random, complex, multi-scale composites. This paper describes how finite element procedures combined with knowledge of individual phase moduli are used, in combination with a cement paste microstructure development model, to quantitatively predict elastic moduli as a function of degree of hydration, as measured by loss on ignition. Comparison between model predictions and experimental results are good for degrees of hydration of 50% or greater, for a range of water : cement ratios. At early ages, the resolution of the typical 100³ digital microstructure is inadequate to give accurate results for the tenuous cement paste microstructure that exists at low degrees of hydration. Elastic computations were made on higher resolution microstructures, up to 400³, and compared to early age elastic moduli data. Increasing agreement with experiment was seen as the resolution increased, even when ignoring possible viscoelastic effects.     

Comparison between two ultrasonic methods in their ability to monitor the setting process of cement pastes

This paper presents the comparison between ultrasonic wave transmission (USWT) method and ultrasonic wave reflection (USWR) method in their ability to monitor the setting process of cement pastes. The velocity of ultrasonic longitudinal waves and shear wave reflection coefficient were measured simultaneously on cement pastes with different hydration kinetics. Even though both methods are able to reliably monitor the hydration process and formation of structure of an arbitrary cement paste, they monitor the setting process in different ways. The relationship between the velocity of longitudinal waves and shear wave reflection coefficient can be simplified into three characteristic phases and the end of the first phase can be used to define the beginning of the setting process of cement paste.     

New insights into the effects of sugar on the hydration and microstructure of cement pastes

The effects of adding sugar to cement paste on hydration and microstructure were observed. While 1% sugar delayed hydration as expected, the delay period was shortened by increased curing temperature. When samples containing sugar began to react, hydration progressed very quickly and the degree of hydration soon surpassed that of control samples. Sugar addition increased the surface area and altered the pore size distribution, as measured by nitrogen, of cement pastes. Results indicate that sugar not only alters the rate of cement paste hydration, but the microstructure of calcium-silicate-hydrate (C-S-H) as well.     

-3 ℃养护下考虑矿物掺合料影响的水泥水化程度计算模型

针对多年冻土地区工程施工时混凝土养护的问题,采用10%、20%、30%的矿粉和粉煤灰替代量等量替代水泥,测试了-3 ℃恒温养护条件下0.38水胶比水泥浆体在各个龄期的水泥水化热,计算了水泥水化程度;分析了龄期及矿物掺合料对水泥水化程度的影响规律,建立了综合考虑龄期和矿物掺合料替代量的水泥水化程度计算模型.结果表明:-3 ℃恒温养护下,矿物掺合料等量替代水泥,水泥浆体的水化程度会降低,粉煤灰降低水化程度的值要比矿粉高;在相同矿物掺合料替代量下,随着龄期的增长,矿物掺合料对水泥水化程度的影响逐渐减弱;同一龄期时,随着矿物掺合料的增加,矿物掺合料对水泥水化程度的影响逐渐增强;利用建立的模型计算了分别掺入15%矿粉和粉煤灰的水泥水化程度,与实测值相比,计算值偏离值较少,预测精度较高.     

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。     

Effects of copper sludge on cemented clay using ultrasonic pulse velocity

To evaluate the behavior of cemented clay treated reinforced with copper sludge, 63 ultrasonic pulse velocity tests were conducted. The copper sludge content, curing period and moisture content were all varied so that the behavior of the sample could be examined. The cement content was 11% by weight clay. The copper sludge contents were 0, 5, 10, 15, 25, 30 and 35% by weight of the cement and samples were cured for 7, 28, and 60 days. The results of this study can be used for estimating of the impacts of these variables on mechanical properties of cemented clay soil treated with copper sludge. The results of this study indicates that at early age of curing time inclusion of copper sludge decreases the ultrasonic pulse velocity. The ultrasonic pulse velocity of samples at 7 and 28 days decreases by increasing copper sludge content. The ultrasonic pulse velocity of sample on the wet side is higher than wet side of the dry unit weight-moisture content curve. Moreover an optimum percentage for the copper sludge (%15) in which the behavior of cemented clay improves significantly.     

5%乙烯-醋酸乙烯共聚物复合低热水泥浆体的碳化促进养护

采用碳化养护方法促进5%乙烯-醋酸乙烯共聚物复合低热水泥浆体的水化.通过X射线衍射定性及定量分析测定了养护后样品的组成及水泥中硅酸三钙(3CaO·SiO2,C3S)、硅酸二钙(2CaO·SiO2,C2S)的水化率;利用带固体样品测试装置的全有机碳素分析仪测定了样品所吸收的CO2量;采用水银压入法检测了样品的孔径分布.结果显示:碳化养护大大促进了低热水泥的水化,并有球霰石生成.样品中的总孔隙体积也明显减少,但有大量较大孔径的毛细孔隙存在.碳化养护前,对样品进行水中预养护可以减少这些毛细孔隙,但不利于水泥的水化并导致CO2吸收量及抗折强度显著降低.     

Influence of Cement Particle-Size Distribution on Early Age Autogenous Strains and Stresses in Cement-Based Materials

The influence of cement particle-size distribution on autogenous strains and stresses in cement pastes of identical water-to-cement ratios is examined for cement powders of four different finenesses. Experimental measurements include chemical shrinkage, to quantify degree of hydration; internal relative humidity development; autogenous deformation; and eigenstress development, using a novel embedded spherical stress sensor. Because the latter three measurements are conducted under sealed conditions, whereas chemical-shrinkage measurements are made under "saturated" conditions, the National Institute of Standards and Technology cement hydration and microstructure development model is used to separate the effects of differences in hydration rates (kinetics) from those caused by the different initial spatial arrangement of the cement particles. The initial arrangement of the cement particles controls the initial pore-size distribution of the cement paste, which, in turn, regulates the magnitude of the induced autogenous shrinkage stresses produced by the water/air menisci in the air-filled pores formed throughout the hydration process. The experimental results indicate that a small autogenous expansion (probably the result of ettringite formation), as opposed to a shrinkage, may be produced and early age cracking possibly avoided through the use of coarser cements.     

Early hydration and setting of oil well cement

A broad experimental study has been performed to characterize the early hydration and setting of cement pastes prepared with Class H oil well cement at water-to-cement ratios (w/c) from 0.25 to 0.40, cured at temperatures from 10 to 60 °C, and mixed with chemical additives. Chemical shrinkage during hydration was measured by a newly developed system, degree of hydration was determined by thermogravimetric analysis, and setting time was tested by Vicat and ultrasonic velocity measurements. A Boundary Nucleation and Growth model provides a good fit to the chemical shrinkage data.Temperature increase and accelerator additions expedite the rate of cement hydration by causing more rapid nucleation of hydration products, leading to earlier setting; conversely, retarder and viscosity modifying agents delay cement nucleation, causing later setting times. Lower w/c paste needs less hydration product to form a percolating solid network (i.e., to reach the initial setting point). However, for the systems evaluated, at a given w/c, the degree of hydration at setting is a constant, regardless of the effects of ambient temperature or the presence of additives.     

Early age microstructure of Portland cement mortar investigated by ultrasonic shear waves and numerical simulation

This paper investigates the ability of a shear wave reflection (WR) method to monitor microstructural changes of Portland cement mortar during hydration. The wave reflection method measures the reflection loss of shear waves at an interface between a steel plate and mortar. Mortars with water/cement ratios of 0.35, 0.5 and 0.6 were tested at isothermal curing conditions of 25 °C. The numerical model HYMOSTRUC3D was used to simulate the evolution of microstructural properties of the cement paste phase of the tested mortars. The parameters obtained from the simulations were the volume fraction of the total and connected solid phase and the specific contact area of the hydrated cement particles. The investigations have shown that the wave reflection measurements are governed primarily by the degree of the inter-particle bonding of the cement particles as calculated from the specific contact area of a simulated microstructure.     

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。     

The pozzolanic reactivity of monodispersed nanosilica hydrosols and their influence on the hydration characteristics of Portland cement

This study reveals that the nanosilica hydrosols with higher specific surface areas had faster pozzolanic reactivity, especially at early ages; moreover, the results are indicative of the accelerating influence of nanosilicas and silica fume on the hydration of cement. Faster initial and final setting times observed for cement pastes containing nanosilicas are consequence of these mechanisms. However, less hydration degree of cement compared to the plain paste was observed at age of 7 days and after. This can be attributed to the entrapment of some of mix water in the aggregates of nanosilicas formed in cement paste environment, making less water available for the progress of cement hydration. The same mechanism is believed to be responsible for the reduction of flowability of cement pastes.     

Influence of hydration on the fluidity of normal Portland cement pastes

The rheological properties of cement paste strongly influence the workability of concrete. It is known that early hydration processes alter phase composition and microstructure of cement pastes. These processes affect fluidity and setting behaviour of cement paste. While many studies tried to measure and model rheological properties of cement pastes, only a few studies assessed the influence of the hydrate morphology on the fluidity of cement pastes.Results of the present study compare the influence of long prismatic hydrates (i.e. syngenite, secondary gypsum) on the fluidity of cement pastes with the effect of other hydrates (AFm).To induce the formation of certain hydration products the cement composition was modified by addition of set regulators and alkali sulphates. Furthermore a combination of various analytical methods such as fluidity (viscometric) testing and microstructural analysis (phase quantification by XRD-Rietveld analysis, investigation by Environmental SEM, BET analysis etc.) was performed. Results are implemented into a fundamental discussion on the influence of various hydration products on the fluidity of the paste.