Effect of Hydrated Calcium Aluminate Cement on the Chloride Immobilization of Portland Cement Paste

To improve the efficiency and stability of chloride immobilization of portland cement paste,hydrated calcium aluminate cement(HCAC)prepared by wet grinding of CAC was added into portland cement paste as an additive.The immobilized chloride ratio(ICR)was evaluated,and the mechanism of chloride immobilization was researched by XRD,DTG,NMR,and MIP tests.The analysis results demonstrated that HCAC could improve the chloride immobilization capacity of portland cement paste.The mechanism was attributed to the following aspects:chemical binding capacity was enhanced via producing more Kuzel’s salt;physical adsorption capacity was reduced by decreasing the C-S-H gel;migration resistance was enhanced through refining the pore structure.

Chloride Corrosion of Reinforced Calcium Aluminate Cement Mortar

This paper describes a study on the corrosion behavior of steel reinforcement in CAC mortars via electrochemical methods including corrosion potential,electrochemical impedance,and linear polarization evaluation.Results indicate that there is a non-linear relationship between the corrosion degree of steel reinforcement in CAC mortar and the concentration of NaCl solution.The electrochemical parameters of specimens immersed in 3%NaCl solution suddenly drop at 40 days,earlier than 60 days of the reference.And the charge transfer resistivity of the specimen has decreased by 11 orders of magnitude at 40 days,showing an evident corrosion on steel reinforcement.However,it is interesting to notice that the corrosion is delayed by high external chloride concentration.The specimens immersed in 9%and 15%NaCl solutions remain in a relatively stable state within 120 days with slight pitting.The great corrosion protection of CAC concrete to embedded steel bars enables its wide application in marine.

Consolidation Mechanism of Chloride Ion in Soda Residue from Ammonia Soda Process Method

Soda residue-magnesium oxychloride cement is prepared with soda residue from ammonia soda process method,magnesium oxide and magnesium sulfate heptahydrate as main raw materials,and its consolidation mechanism of chloride ion is studied.The results show that the hydration products of soda residue-magnesium oxychloride cement are mainly 5-phase,gypsum and brucite,which exist in the matrix in needle rod shape,long plate shape and hexagonal plate shape,respectively.When the molar ratio of MgO/MgCl_(2) is 8:1,the concentration of MgSO_(4) is 29%,and the mass ratio of soda residue:magnesium oxide:magnesium sulfate heptahydrate is 45.8:36.4:17.8.The chloride ion consolidation effect of the sample is the best,and the chloride ion consolidation content of the 7 d sample is about 93%.The chloride ion consolidation content of the 28 d sample is about 96%.

Influence of Recycled Concrete Fine Powder on Durability of Cement Mortar

In this paper,the durability of cement mortar prepared with a recycled-concrete fine powder(RFP)was examined;including the analysis of a variety of aspects,such as the carbonization,sulfate attack and chloride ion erosion resistance.The results indicate that the influence of RFP on these three aspects is different.The carbonization depth after 30 days and the chloride diffusion coefficient of mortar containing 10%RFP decreased by 13.3%and 28.19%.With a further increase in the RFP content,interconnected pores formed between the RFP particles,leading to an acceleration of the penetration rate of CO_(2)and Cl^(−).When the RFP content was less than 50%,the corrosion resistance coefficient of the compressive strength of the mortar was 0.84-1.05 after 90 days of sulfate attack.But the expansion and cracking of the mortar was effectively alleviated due to decrease of the gypsum production.Scanning electron microscope(SEM)analysis has confirmed that 10%RFP contributes to the formation of a dense microstructure in the cement mortar.

Modelling of Time Dependency of Chloride Diffusion Coefficient in Cement Paste

A computer-based model and method was presented to predict the time dependency of chloride diffusion coefficients in cement paste. The HYMOSTRUC3D model was applied to generate a 3D representative elementary volume （REV） of cement paste. In the simulation of microstructure, both of cement hydration and chloride binding were considered. With the simulated microstructure of cement paste, the finite element method was applied to simulate the diffusion process of chloride through the saturated cement paste. Based on the Fick’s first law, the chloride diffusion coefficient can be calculated. In this method, the influences of age and w/c ratio on the chloride diffusion coefficient were evaluated. The simulated chloride diffusivities with various w/c at different time were compared to experimental data obtained from the literature. The experimental results indicate that the chloride diffusion coefficient decreases with the increase of time and the decrease of w/c ratio. The trend of simulated relationship （diffusion coefficient vs time, diffusion coefficient vs w/c ratio） fits very well with the experiments.

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 （Ds/Do） of pore in the hardened pastes are n^3.2, Ds/Do=l.Ox 10-4 respectively 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.

A novel and promising engineering application of carbon dots:Enhancing the chloride binding performance of cement

Corrosion of reinforcement induced by chloride invasion is extensively considered as the dominating deterioration mechanism of reinforced concrete(RC)structures,leading to serious safety hazards and tremendous economic losses.However,it still lacks well dispersive and cost-efficient nanomaterials to improve the anti-chloride-corrosion ability of RC structures.Herein,specific carbon dots(CDs)with high dispersity and low cost are deliberately designed,successfully prepared by hydrothermal processing,and then firstly applied to immensely enhance chloride binding performance of cement,thereby contributing to suppressing the corrosion of reinforcement.Specifically,the tailored CDs are composed of the carbon core with highly crystalline sp^(2)C structures and oxygen-containing groups connecting on the carbon core;The typical equilibrium test confirms that with respect to that of the blank cement paste,the chloride binding capacity of cement paste involving 0.2 wt%(by weight of cement)CDs is increased by 109% after 14-day exposure to 3 mol/L Na Cl solution;according to comprehensive analyses of phase compositions,the chloride binding mechanism of CDs-modified cement is rationally attributed to the fact that the incorporation of CDs advances the formation of calcium silicate hydrate(C-S-H)gels and Friedel's salt(Fs),thus enormously enhancing the physically adsorbed and chemically bound chloride ions of cement pastes.This work not only firstly provides a novel high-dispersity and low-cost nanomaterial toward the durability enhancement of RC structures,but also broadens the application of CDs in the field of engineering,conducing to stimulating their industrialization development.

Experimental Study on Chloride Ion Penetration Resistance of Coal Gangue Concrete under Multi-Factor Comprehensive Action

In order to investigate the chloride ion penetration resistance of coal gangue concrete under multi-factor comprehensive action, the non-steady-state accelerated chloride ion migration test was used to test the chloride diffusion law of coal gangue concrete specimens by crack width, curing temperature and water-cement ratio. Three groups of crack width (0 mm, 0.05 - 0.12 mm, 0.12 - 0.2 mm), three curing temperatures (high temperature 45, medium temperature 25, low temperature 10), three water cement ratios (0.3, 0.4, 0.5) were set in the experiment. The results show that when the curing temperature and water cement ratio are constant, the crack width less than 0.12 mm has little effect on the chloride content and chloride diffusion coefficient. When the crack width is larger than 0.12 mm, the chloride penetration depth increases with the crack width. The resistance to chloride ion penetration of gangue concrete is greatly influenced by the water cement ratio. The influ-ence degree of three factors on chloride ion migration coefficient of gangue concrete is as follows: water cement ratio > crack width > curing temperature.

Mechanism and performance of a lithium chloride accelerator

To address present concerns about thickening time and high early-strength in deepwater cementing at low temperatures when using conventional accelerators, a new type of set-accelerating admixture comprising of lithium chloride, aluminium hydroxide and alkaline metal chlorides, named as LS-A, was studied in this paper. Mechanism analysis and performance tests show that the accelerator LS-A accelerated the hydration of tri- and dicalcium silicates （C3S and C2S） at low-temperatures by speeding up the breakdown of the protective hydration film and shortening the hydration induction period. Therefore, LS-A could shorten the low-temperature thickening time and the transition time of critical gel strength from 48 to 240 Pa of the Class-G cement slurry, and improve the early compressive strength of set cement at low-temperatures. It exhibited better performance than calcium chloride and had no effect on the type of hydration products, which remain the same as those of neat Class-G cement, i.e. the calcium silicate gel, Ca（OH）2 crystals and a small amount of ettringite AFt crystals. LS-A provides an effective way to guarantee the safety of cementing operations, and to solve the problems of low temperature and shallow water/gas flowing faced in deepwater cementing.

Chloride Binding Isotherm from Migration and Diffusion Tests

Chloride binding is often described by chloride binding isotherm, which is closely related to the service life of concrete structures in chloride environments. Many methods have been proposed to determine chloride binding isotherm. Compared to other methods, chloride binding isotherms obtained directly from non- steady-state diffusion tests seem closer to the reality. We studied the chloride binding isotherm from both non- steady-state electrical-accelerated migration and diffusion tests at different temperatures. Twelve concrete mixes with different supplementary cementing materials and water-to-binder ratios of 0.35, 0.48 and 0.6 were cast for study. The specimens after diffusion （or migration） tests were sliced layer by layer, and acid-soluble and free chloride contents of each layer were measured. A chloride binding isotherm was obtained from one specimen. Experimental results indicated that electrical voltage had a slight effect on the chloride binding isotherm of concrete. Temperature had a positive effect on chloride binding. The higher the water-to-binder ratio was, the higher the chloride binding was.

改性硫铝酸盐水泥砂浆力学性能及耐久性能研究

过快的流动度损失和后期不稳定的强度发展限制了硫铝酸盐水泥工程中的进一步应用。因此以葡萄糖酸钠作为缓凝剂,掺入硅灰制备成改性硫铝酸盐水泥砂浆,并对其工作性能、力学性能和抗氯离子渗透性能开展了系统研究。研究结果表明:葡萄糖酸钠掺量0.4%可有效延长硫铝酸盐水泥砂浆凝结时间,提高砂浆流动性和抗氯离子渗透性能;硫铝酸盐砂浆工作性能、力学性能和抗氯离子渗透性能均随硅灰掺量的增加先提高后降低,硅灰最优掺量为20%。

硅酸盐水泥氯离子固化机理及影响因素研究进展

氯离子侵蚀是滨海或除冰盐环境下钢筋混凝土结构耐久性劣化的首要原因。氯离子固化是提升钢筋混凝土结构抗氯离子侵蚀性能最为经济有效的手段之一。因此,大量文献对硅酸盐水泥(PC)氯离子固化机理、固化性能、影响因素等进行了广泛研究。然而,仍缺乏PC氯离子固化方面的系统性总结论述。因此,本文对PC氯离子固化机理及影响因素的研究进展进行了全面系统综述。首先详细介绍了两类氯离子固化机理;其次分析了氯离子固化性能影响因素;然后定义了氯离子固化贡献度并讨论了不同因素影响下的贡献度变化;最后概述了PC氯离子固化领域面临的主要挑战。本文有助于深化理解PC氯离子固化机理及影响因素,为钢筋混凝土结构寿命预测及耐久性提升提供参考。

机制砂与特细砂抗氯盐侵蚀混凝土的制备及性能研究

利用机制砂、特细砂等制备抗氯盐侵蚀混凝土对降低生产成本、提升桥梁服役寿命具有重要意义。本研究通过优化骨料设计与水泥浆体组成提升混凝土的抗氯离子侵蚀性能,研究了骨料级配、特细砂用量、砂率、水胶比、胶凝材料种类对混凝土性能的影响。结果表明:8~16 mm碎石质量分数由50%增加至80%时,氯离子扩散系数由4.0×10^(-12)m^(2)/s降低至2.9×10^(-12)m^(2)/s;特细砂替代50%(质量分数)机制砂时,氯离子扩散系数为3.6×10^(-12)m^(2)/s,仍可以满足设计要求;砂率增大,氯离子扩散系数先减小后增大。水胶比在0.36~0.38时,氯离子扩散系数低于3.5×10^(-12)m^(2)/s;矿渣组分水化改善了混凝土的孔结构,并形成了更多的AFm、C-S-H产物,提升了混凝土对氯离子的固化能力,矿渣水泥混凝土氯离子扩散系数较普通硅酸盐水泥混凝土降低了40%。

硫酸盐-氯盐环境下粉煤灰-水泥砂浆物相演变及定量分析

为了分析硫酸盐-氯盐环境下的水泥基材料的物相演变规律,本工作粉煤灰-水泥砂浆为研究对象,开展腐蚀溶液中的长期浸泡实验,并通过QXRD、SEM、MIP微观测试,分析砂浆中氯离子和硫酸根离子的交互作用、物相演变规律以及微结构变化。结果表明:在硫酸盐-氯盐侵蚀前、中期,离子间竞争作用导致侵蚀产物含量降低,延缓了试件中氯盐和硫酸盐的侵蚀进程;侵蚀后期,试件中侵蚀产物累积造成孔结构劣化,导致更多离子进入其内部,加速氯盐和硫酸盐侵蚀进程,此时,氯盐的存在不能再延缓硫酸盐的侵蚀。

阻锈剂对海砂硫铝酸盐混凝土中钢筋锈蚀的影响

利用海砂替代河砂制备海砂硫铝酸盐混凝土(SACS),以海砂普通硅酸盐混凝土(OPCS)为对照,研究SACS的氯离子固化性能,并采用硬化砂浆阳极极化法及钢筋失重率法研究不同阻锈剂(单氟磷酸钠、亚硝酸锂和三乙醇胺)及其掺量(0%、0.5%、1%、1.5%、2%,即阻锈剂与硫铝酸盐水泥的质量比)对海砂硫铝酸盐水泥砂浆(SS)中钢筋极化电位和钢筋锈蚀的影响。结果表明:标养28 d后SACS氯离子固化率较OPCS高约18%,SS中钢筋极化电位略高于海砂普通硅酸盐水泥砂浆(OPC_(S));在S_(S)中掺入阻锈剂能使钢筋极化电位正移,失重率降低;掺入1.5%三乙醇胺阻锈效果最佳,钢筋极化电位变化满足规范要求,钢筋失重率仅为0.0142%,与未掺入阻锈剂相比降低约91%。

不同煅烧温度下水泥窑协同处置危险固废中氯元素转化机理

采用高温水平管式炉模拟水泥窑协同处置危废过程,通过扫描电镜、X射线衍射、能谱仪、离子色谱等对样品进行表征分析,探讨在不同煅烧温度下氯的转化情况。结果表明,在水泥窑协同处置的过程中,煅烧温度对氯的分布有显著影响。在高温下,氯化物可与水泥熟料中的主要成分发生反应,包括钙、硅、铝和铁,形成氯化物固溶体,从而减少了氯的溶解度,且煅烧温度越高,进入烟气和飞灰的氯的比例越大。同时揭示了氯的迁移途径和其对水泥熟料结构的影响。

纳米增强水泥基复合材料抗氯离子迁移及固化性能综述

先进碳基纳米材料在水泥基材料领域的渗透突破了传统水泥基材料的性能使用局限,是未来高性能水泥基复合材料研究的热门领域。然而,虽然目前关于纳米增强水泥基复合材料水化、微观、力学等性能的研究相对较多,但是针对长期耐久性能,特别是对影响钢筋锈蚀的主要因素氯盐侵蚀方面的研究相对较少,且缺少一定的深度与广度。纳米增强水泥基材料抗氯盐侵蚀性能主要体现在两个方面:一是孔隙层面,通过密实孔隙降低整体孔隙率,增加纳微观毛细孔比例,从而降低外界氯离子侵入迁移速率;二是固化层面,通过孔隙固液交界处水泥水化产物氯离子置换固化作用,吸附并降低孔隙溶液中自由氯离子含量。它们的最终目的都是在结构服役寿命内保证钢筋表面侵入的自由氯离子浓度处于锈蚀临界浓度之下,降低锈蚀风险及维护成本。深入明晰纳米增强水泥基复合材料抗氯离子迁移和固化性能及机理,对建立有效预测模型,指导并推动高抗蚀纳米增强水泥基复合材料设计、制备及应用具有重要意义。本文总结了近年来纳米增强水泥基复合材料抗氯盐侵蚀性能的研究进展,对比分析了不同维度、不同制备手段、不同化学组成等各类纳米材料分别对抗氯离子迁移性能和氯离子固化性能的影响效果及机制,同时对未来纳米增强高抗蚀水泥基复合材料的发展及应用进行了展望。

镁铝水滑石对高铁低钙硅酸盐水泥性能的影响

采用成核晶化隔离法合成了镁铝水滑石(MgAl-LDH),并利用石灰石、砂岩和铁矿粉制备了高铁低钙硅酸盐水泥熟料(铁相质量分数为19.6%)。通过X射线衍射(XRD)、热重(TG)、扫描电镜(SEM)和化学分析法,研究了MgAl-LDH掺量对高铁低钙水泥氯离子固化能力、力学性能和微观结构的影响规律。结果表明:MgAl-LDH能显著提升高铁低钙水泥化学固化氯离子能力及力学性能;当MgAl-LDH掺量为3%(质量分数)时,与空白组相比,7 d氯离子固化率提升了46.4%,3、7、28 d抗压强度分别提高了27.5%、34.6%和17.1%。微观测试表明MgAl-LDH能促进高铁低钙水泥早期水化生成更多的水化产物,在一定程度上促进了水泥结构的致密化;并且能促进水化产物和氯离子发生反应生成Friedel′s盐,提升了高铁低钙水泥对氯离子的化学固化量。

自动电位滴定法测定水泥中氯离子含量不确定度评定

水泥材料中的氯离子含量对工程质量有着重要影响,因此需对其进行准确检测。自动电位滴定法相较于手动滴定方法,能够直接提升检测机构的工作效率和结果稳定度。依据标准检测方法和不确定度评定准则,检测机构开展自动电位滴定法测定水泥中氯离子含量不确定度评定工作能够提高氯离子检测结果准确性。通过建立数学模型、分析不确定度来源,并逐一开展各分项不确定度评估,从而计算出相对标准不确定度为0.0274,合成标准不确定度为0.0007946,扩展不确定度U95=0.002%。

内掺氯盐对粉煤灰系硬化水泥水化行为的影响

为分析水泥浆中氯离子对水泥水化过程的影响,研究了不同氯离子浓度条件下水泥试件微观形貌和孔隙结构的变化规律,探讨了水泥水化过程中氯离子与水化产物的结合机理。结果显示:氯离子与水泥反应生成Friedel’s盐,可促进水泥的水化速率和进程;增加氯盐浓度可以促进水化;氯离子参与水化后C-S-H凝胶数量增加,AFt数量减少,1.3%氯离子浓度水泥较未掺氯盐C-S-H凝胶生成量增加18.62%,AFt生成量减少32.67%;掺加氯盐后水化产物之间联系密切,可减小水泥试件的孔隙率,细化孔径,增加微观结构的致密性。