考虑界面过渡区及集料因素的水泥基材料碳化模型

集料和界面过渡区对水泥基材料抗碳化性能有显著影响。基于Fick第二定律,提出一种考虑集料和界面过渡区影响的CO₂扩散模型,模型中首次引入界面过渡区的扩散系数并给出相应参数的计算方法,并基于质量守恒定律给出CO₂反应模型,利用该模型研究荷载作用下C30和C50混凝土中CO₂的浓度分布。结果表明,考虑集料和界面过渡区影响能更准确地计算CO₂在水泥基材料中的扩散系数及碳化深度值。通过与文献中的试验数据进行比较,进一步验证了理论模型的可靠性。模型将集料和界面过渡区作为评价混凝土耐久性的重要因素,为钢筋混凝土结构的设计和寿命预测提供依据。     

Mechanical Properties and ITZ Microstructure of Recycled Aggregate Concrete Using Carbonated Recycled Coarse Aggregate

The effect of carbonation treatment and mixing method on the mechanical properties and interfacial transition zone (ITZ) properties of recycled aggregate concrete (RAC) was investigated. Properties of recycled concrete aggregate (RCA) were tested firstly. Then, five types of concretes were made and slump of fresh concrete was measured immediately after mixing. Compressive strength and splitting tensile strength of hardened concrete were measured at 28 d. Meanwhile, the microstructure of RAC was analyzed by backscattered electron (BSE) image. It was found that the water absorption ratio of carbonated recycled concrete aggregate (CRCA) was much lower when compared to the untreated RCA. Comparatively, the apparent density of CRCA was not significantly modified. The concrete strength results indicate that the mix CRAC-2 prepared with CRCA by adopting two-stage mixing approach shows the highest compressive strength value compared to the other mixes. The microstructural analysis demonstrate that the mix CRAC-2 has a much denser old ITZ than the untreated RAC because of the chemical reaction between CO₂ and the hydration products of RCA. This study confirms that the ITZ microstructure of RAC can be efficiently modified by carbonation treatment of RCA and encourages broadening the application of construction and demolition wastes.     

负载纳米TiO2石英砂功能集料-水泥石的界面粘结性能

光催化水泥基材料是先进建筑功能材料的研究热点之一,但传统内掺法制备的TiO₂-水泥基复合材料中TiO₂有效利用率低、经济效益差。针对此问题,采用负载法制备纳米TiO₂功能集料（QST）,并将QST集料负载于普通水泥砂浆表面。研究QST集料对水泥砂浆干缩性能的影响,通过拉拔法测试QST集料与水泥石的界面粘结力;采用SEM、EDS、MIP等微观测试方法,研究纳米TiO₂对集料-水泥石界面过渡区水化产物、孔结构的影响。结果表明：与普通石英砂集料相比,表面负载纳米TiO₂的石英砂功能集料可减少水泥砂浆的干缩,提高集料-水泥石的界面粘结力。在QST集料-水泥石的界面过渡区,纳米TiO₂促进了水泥浆体的水化,减少了界面过渡区Ca(OH)₂的富集,细化并改善了界面过渡区的孔结构。     

再生粗骨料混凝土碳化分析的多场耦合模型及验证

再生粗骨料混凝土的碳化是一个复杂的物理扩散和化学反应过程,其分析和预测较为困难。鉴于此,基于再生粗骨料混凝土的碳化机理,结合再生粗骨料混凝土中CO₂的扩散定律和可碳化物质的质量守恒定律,综合考虑再生粗骨料混凝土中CO₂的有效扩散系数、碳化反应速率系数、可碳化物质的量、再生粗骨料的表面附着砂浆等重要参数的影响,建立了再生粗骨料混凝土碳化分析的多场耦合模型,并通过试验数据验证了模型的有效性和适用性。     

Influence of compaction pressure on the accelerated carbonation of calcium hydroxide

Mineral carbonation using waste cement is a promising method to solve the problems caused by CO₂ emission and waste cement. Compaction pressure is an important parameter for mineral carbonation of calcium hydroxide, one of the most dominant composite of waste cement that can be carbonated. The carbonation degree, morphology of products and compressive strength of carbonated compacts are influenced by compaction pressure significantly. Results show that the carbonation degree of calcium hydroxide increases at first (0-8 MPa) and then decreases in the higher compaction pressure range (10-14 MPa). At the meantime, results also indicate that lower compaction pressure accelerates the early carbonation but hinder carbonation in the later stages. For the morphologies of carbonation products, calcium carbonate tends to form typical crystal morphology of calcite (rhombohedral) under lower compaction pressure, while it will become ellipsoid-like when compaction pressure reaches 8 MPa. TGA and water content results show that there is an optimal water content for the carbonation. In addition, lower water content is adverse to the carbonation at later stage and the CO₂ is difficult to penetrate into the inside of compacts when water content is high, which will hinder the carbonation. XRD and TGA results show that the carbonation products are calcite and small amount of amorphous calcium carbonate.     

Multi-scale modeling of the effective chloride ion diffusion coefficient in cement-based composite materials

N-layered spherical inclusions model was used to calculate the effective diffusion coefficient of chloride ion in cement-based materials by using multi-scale method and then to investigate the relationship between the diffusivity and the microstructure of cement-basted materials where the microstructure included the interfacial transition zone（ITZ）between the aggregates and the bulk cement pastes as well as the microstructure of the bulk cement paste itself.For the convenience of applications,the mortar and concrete were considered as a four-phase spherical model,consisting of cement continuous phase,dispersed aggregates phase,interface transition zone and their homogenized effective medium phase.A general effective medium equation was established to calculate the diffusion coefficient of the hardened cement paste by considering the microstructure.During calculation,the tortuosity（n）and constrictivity factors（D_s/D₀）of pore in the hardened pastes are n≈3.2,D_s/D₀=1.0×10^-4respectively from the test data.The calculated results using the n-layered spherical inclusions model are in good agreement with the experimental results;The effective diffusion coefficient of ITZ is 12 times that of the bulk cement for mortar and 17 times for concrete due to the difference between particle size distribution and the volume fraction of aggregates in mortar and concrete.     

Comparison of concrete carbonation process under natural condition and high CO2 concentration environments

To compare the results obtained under both natural and accelerated environments, the pH values of carbonated concrete were measured, the variation of pH values was determined, and the variations of Ca(OH)₂ and CaCO₃ contents in the carbonated concrete under natural condition and high CO₂ concentration accelerated climate environments were determined by microcosmic test methods such as DTA and X-ray diffraction. The experimental results showed that the overall variation trend of pH values and phase component of carbonation layer of concrete under accelerated climate environments with high CO₂ concentrations were the same as those under natural conditions. Therefore, the carbonation processes of concrete were considered consistent under both conditions. However there was a difference in the length of semi-carbonation zones. The one measured under high CO₂ concentration accelerated climate environments was shorter than that under natural condition. Experimental investigation showed that it was caused by the differences in climate condition (temperature and relative humidity) as well as the properties of the concrete. The concentration of CO₂ and the duration of the carbonation process have no effect on the length of semi-carbonation zone. Thus, it is acceptable to simulate the natural condition by applying the high CO₂ concentration artificial accelerated carbonation technique for the purpose of the study of carbonation process of concrete.     

再生细骨料及其混凝土的微观结构特征

采用扫描电镜、能谱分析、X射线衍射、显微硬度和氮吸附等微观测试方法研究了再生细骨料及其混凝土的微观结构特征.研究结果表明:再生细骨料是一种组成复杂的、具有一定水化活性的和高渗透性的人造骨料,其主要矿物相为SiO2、CaCO3以及少量的C2S.再生细骨料混凝土内部水泥石孔隙较多,结构密实性较差,同时其与再生细骨料间存在较为明显的界面过渡区,该界面过渡区宽度较大,且界面过渡区两侧的骨料和水泥石的显微硬度均较低.再生细骨料的多孔结构,以及再生细骨料混凝土内部水泥石和界面过渡区微观结构缺陷是导致其大孔增多的主要原因,大孔的增多会对混凝土抗渗性产生不利影响.     

超临界碳化对水泥基材料性能和孔径结构的影响

为了考察超临界碳化技术在水泥基材料改性方面的应用,研究超临界碳化对水泥基材料微观和宏观性能的影响,基于此设计了超临界二氧化碳碳化试验研究水泥砂浆、水泥净浆和混凝土试件的碳化深度、强度、孔径分布以及二氧化碳吸收量的变化。试验分析表明,超临界碳化可以快速实现水泥基材料的碳化,大幅提高试件强度,改善材料的孔径分布,使材料的中细径孔大幅降低,提高材料的抗渗透能力,为改善重金属等危害废物的水泥基固化效果提供了依据。同时超临界碳化可以将大量二氧化碳转化在碳酸钙中沉淀吸收,具有重要的环境保护意义。     

Numerical Simulation and Analysis of Migration Law of Gas Mixture Using Carbon Dioxide as Cushion Gas in Underground Gas Storage Reservoir

One of the major technical challenges in using carbon dioxide（ CO2） as part of the cushion gas of the underground gas storage reservoir（ UGSR） is the mixture of CO2and natural gas. To decrease the mixing extent and manage the migration of the mixed zone,an understanding of the mechanism of CO2and natural gas mixing and the diffusion of the mixed gas in aquifer is necessary. In this paper,a numerical model based on the three dimensional gas-water two-phase flow theory and gas diffusion theory is developed to understand this mechanism. This model is validated by the actual operational data in Dazhangtuo UGSR in Tianjin City,China.Using the validated model,the mixed characteristic of CO2and natural gas and the migration mechanism of the mixed zone in an underground porous reservoir is further studied. Particularly,the impacts of the following factors on the migration mechanism are studied： the ratio of CO2injection,the reservoir porosity and the initial operating pressure. Based on the results,the optimal CO2injection ratio and an optimal control strategy to manage the migration of the mixed zone are obtained. These results provide technical guides for using CO2as cushion gas for UGSR in real projects.     

CO2捕集技术的研究进展

化石燃料燃烧排放的CO_2是当今温室气体中CO_2污染的主要来源。介绍了CO_2捕集技术中吸附法、吸收法和膜分离法的研究进展,并分析了各种方法的特点,归纳总结了目前新型吸附/吸收剂中金属有机骨架、相变吸收剂、离子液体吸收剂和纳米流体吸收剂的研究成果。降低能耗和成本是今后研究工作的重点。开发新型吸附/吸收剂、优化反应过程和联合使用不同方法是未来CO_2捕集技术的发展方向。     

再生集料混凝土的微观结构特征

研究了两种不同类型的再生混凝土集料(RC集料)和一种花岗岩集料配制的混凝土的微观结构特征，尤其是集料和水泥浆体之间的界面过渡区的某些结构细节。结果表明，界面过渡区的微观结构特征与集料的密实程度有关，过于密实(天然集料)和过于疏松(低标号混凝土)的集料可引起界面区的多孔性或聚集粗大颗粒的水化产物，而适中密实程度的集料可形成较为密实的界面区。     

混凝土界面体积百分比的计算方法

讨论了混凝土界面体积百分比的计算.根据混凝土细观结构模拟技术,提出了混凝面体积百分比的计算方法.计算结果表明,对于给定的骨料体积百分比,界面体积百分比着骨料最小直径的减小而增大;对于给定的骨料最小直径,界面体积百分比随着骨料体积百分比的增大而增大.基于这些模拟结果,给出了界面体积百分比的计算公式.     

三峡库区低水位运行期库尾支流筑坝对其水体CO2分压及排放的影响

为探究三峡库区低水位运行期间库尾支流河坝对其上下游水体CO₂分压、水气界面扩散通量的影响,于2019年8月对三峡库区库尾典型筑坝支流御临河进行采样研究。原位测定关键理化环境因子,采用顶空平衡法,结合亨利定律和薄边界层理论计算水体CO₂分压（p_CO2）及水气界面扩散通量（Flux（CO₂））。结果表明,库区低水位运行期御临河水体表层p_CO2为54.55~336.73 Pa,均值为206.68 Pa,Flux（CO₂）为1.65~67.84 mmol/（m²·d）,均值为39.01 mmol/（m²·d）,表现为大气CO₂排放源排放水平显著高于三峡库区中下游支流。御临河受其下游筑坝影响,河坝上游较近河段表现为湖泊型水体（流速v<0.05 m/s）,上游较远河段和下游表现为过渡型水体（v=0.05~0.2 m/s）。湖泊型水体p_CO2和Flux（CO₂）显著低于过渡型水体,且p_CO2随水深增加迅速增大。相关性分析及多元逐步回归分析表明,库区低水位运行期御临河p_CO2及Flux（CO₂）受水温、pH、DO、Chl-a等理化环境指标影响显著,Chl-a是影响p_CO2的最主要因素,p_CO2是影响Flux（CO₂）的最主要因素。     

Determination of the apparent activation energy of concrete carbonation

Accelerated carbonation experiments about the development of carbonation rates of ordinary Portland cement concrete under different artificial climates were carried out. Six water cement ratios and six climate condition combinations of temperature and relative humidity were used. Results indicate that changes of concrete carbonation rate with environmental temperature agree the Arrhenius law well, which suggests concrete carbonation rate has obvious dependence on temperature. The higher the temperature is, the more quickly the concrete carbonates, and at the same time it is also affected by environmental relative humidity. Thereafter, the apparent activation energy E _a of concrete carbonation reaction was obtained, ranging from 16.8 to 20.6 kJ/mol corresponding 0.35–0.74 water cement ratio, and lower water cement ratio will cause the apparent activation energy increase. Concrete carbonation rates will increase 1.1–1.69 times as temperature increase every 10 °C at the temperature range of 10 to 60 °C.     

Corrosion rate of hydrogenation to C110 casing in high H2S environment

The corrosion behavior of C110 bushing at high temperature and high pressure with a high H₂S/CO₂ was studied, and a basis for the materials selection of sour gas well bushing was provided in H₂S, CO₂ and saline coexisting environment. Under acidic condiction, hydrogen atoms greatly entered into the material and caused the material properties changed. Weight loss method was used to study the corrosion rate of hydrogen charging samples and original untreated samples in simulated oil field environment. PAR2273 electrochemical workstation was used to examine the electrochemical performance of samples untreated, hydrogen charging after reacting in autoclave. The corrosion product film was observed through SEM. The experimental results show that sample with hydrogen charging has a much more obvious partial corrosion and pitting corrosion than the untreated blank sample even the downhole corrosion speed of bushing is increased after being used for a period of time. Polarization curve shows the corrosion tendency is the same between sample with or without hydrogen charging and corrosion tendency is reduced by corrosion product film. A layer of dense product film formed on the surface of samples provides a certain protective effect to the matrix, but cracked holes which will accelerate partial corrosion of the sample were also observed.     

开裂混凝土中氯离子扩散行为的细观数值模拟

为了研究氯离子在开裂混凝土中的扩散性能,考虑混凝土细观结构的非均质性,将含有裂纹的混凝土试件视为由砂浆基质、骨料、界面过渡区以及裂纹组成的四相复合材料,建立了带有单条裂纹的二维细观有限元模型.数值模型中,骨料不具有渗透能力,砂浆基质和界面中的氯离子扩散性能由水灰比确定,裂纹相中氯离子扩散系数与裂纹宽度的定量关系由文献中已有试验数据的拟合获得.研究了骨料分布形式对氯离子扩散行为的影响,分析并探讨了不同裂纹宽度下混凝土中的氯离子扩散行为.数值结果与试验数据的良好吻合说明了该方法的准确性.研究结果表明:骨料的随机分布形式基本不影响混凝土中氯离子的扩散行为;裂纹宽度小于50μm时,混凝土中氯离子的扩散行为基本不受裂纹的影响;裂纹宽度介于50~170μm时,氯离子的扩散随裂纹宽度增大而明显加剧;裂纹宽度大于170μm时,氯离子在裂纹附近的扩散趋于二维扩散形态.     

钙基吸附剂的改性

综合了6种钙基吸附剂的改性方法,分别为:加入有机酸;加入氯化氢;添加高熔点的金属氧化物,如MgO和MnO_2;在煅烧和碳酸化阶段加入蒸汽;通过高强度的声波脉;燃烧生物柴油的副产品。分析了6种方法的优缺点,研究了提高钙基吸附剂循环捕集CO_2能力的方法。     

Quantitative characterization and elastic properties of interfacial transition zone around coarse aggregate in concrete

Backscattered electron images (BSE) obtained by scanning electron microscope was used to quantitatively characterize the microstructure of interfacial transition zone (ITZ) in concrete. Influences of aggregate size (5, 10, 20, and 30 mm), water to cement ratio (0.23, 0.35 and 0.53) and curing time (from 3d to 90d) on the microstructure of interfacial transition zone between coarse aggregate and bulk cement matrix were investigated. The volume percentage of detectable porosity and unhydrated cement in ITZ was quantitatively analyzed and compared with that in the matrix of various concretes. Nanoindentation technology was applied to obtain the elastic properties of ITZ and matrix, and the elastic modulus of concrete was then calculated based on the Lu & Torquato model and self-consistence scheme by using the ITZ thickness and elastic modulus obtained from this investigation. The experimental results demonstrated that the microstructure and thickness of ITZ in concrete vary with a variety of factors, like aggregate size, water to cement ratio and curing time. The relative low elastic properties of ITZ should be paid attention to, especially for early age concrete.     

Comparisons of Fly Ash and Deposition Between Air and Oxy-FuelCombustion in Bench-Scale Fluidized Bed with Limestone Addition

In Oxy-fuel circulating fluidized bed, the residual CaO particles may react with high concentration of CO₂ in flue gas to form bonded deposit on heat transfer surfaces in backpass when limestone is used as a sorbent to capture SO₂.In this paper, experiments were designed on ash deposition in a bench-scale fluidized bed under oxy-fuel and air atmosphere. A novel ash deposit sampling probe was used to simulate the tubes of tail surfaces. The chemical composition of fly ash and ash deposit from both air-firing and oxy-fuel firing cases were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry(ICP-AES) and Scanning Electron Microscopy(SEM), respectively. The degrees of carbonation reaction of ash deposits were measured by Thermo Gravimetric Analysis. The results showed that there are distinct differences in fly ash deposition rate between oxy-fuel and air firing cases, and oxy-fuel combustion with limestone addition can affect chemical composition of fly ash and ash deposit, especially for elements of Ca, Na, K, and S. However, the carbonation reaction degree of ash deposits is found weak, which is due to the relatively low CaO content in ash deposit or not long enough of the sampling time.[JP]