硝酸中微量氯离子测定方法探讨

进行了莫尔法、佛尔哈德法、硫氰酸汞分光光度法测定实际硝酸样品和标样中微量Cl^-含量的对比试验,结果表明：莫尔法不能准确测定硝酸中微量Cl^-含量,硫氰酸汞分光光度法和佛尔哈德法能准确测定硝酸中微量Cl^-含量。通过试验,还确定了硫氰酸汞分光光度法和佛尔哈德法测定硝酸中微量Cl^-含量的条件。     

食品中微量硒测定方法对比研究

硒是人体中重要的微量元素,能提高人的免疫力,还能对于一些癌症、心脑血管疾病等起到预防作用。目前人体对于微量硒元素的吸收大都是来自食品,人体中硒含量过多会导致硒中毒,含量过少,又会导致人体器官的病变,所以对食品中微量硒的测定极为重要。本文深入探讨了测定食品中微量硒含量常用的方法,并分析每种测定方法的优缺点,希望通过本文的研究结果能为相关工作人员提供多样化的测定方法,用于有效测定食品中微量硒的含量。     

铁矿粉中微量硅含量测定方法研究

报道了一种精铁矿粉中微量二氧化硅含量测定的新方法,重点探讨了操作中各步骤的最佳工艺及其对硅含量分析精确度的影响。本方法分析精确度高,适合于精铁矿粉中微量二氧化硅含量(SiO2:0.01%～1%)的测定,比传统方法测定极限下延了10倍。     

乙氧基镁中微量四氯化碳的测定

乙氧基镁的合成,是用四氯化碳为催化剂。为了控制系统中有机氯的含量(四氯化碳在固体中的含量,总氯的含量不大于0.1%),以保证产品质量,所以测定乙氧基镁中微量有机氯含量是必要的。根据工作需要,我们采用色谱法对乙氧基镁中微量四氯化碳含量进行了测定。方法介绍如下:     

电感耦合等离子体发射光谱法测定钢中微量铝分析方法研究

本文针对电感耦合等离子体发射光谱法测定钢中微量铝含量检测过程中容易出现的超出标准范围要求问题进行研究,主要从仪器分析参数、试剂与其它材料、分析谱线、观测方向、标准样品的选择这几方面进行研究,最终确定适合测定钢中微量铝含量的分析方法,通过准确度与精密度试验,RSD小于5%(n=8),回收率在102.1%～103.6%之间,该方法的准确度较高,精密度较好,能够准确、快速测定钢中微量铝含量。     

电量法测定聚乙烯粒料中水含量

介绍了电量法测定取消乙烯粒料等固体样品中微量水含量的分析方法.样品中水分在水分汽化器中汽化,由载气导入水分测定仪中进行检测,可以快速、准确地测定聚乙烯树脂等固体样品中微量水含量.结果显示方法准确性和重复性都很好.     

阿司匹林药片中乙酰水杨酸含量测定的微型化研究

本文以常量、微量两种方法做对比,对阿司匹林药片中乙酰水杨酸的含量进行了测定和讨论,微量滴定的精密度达到常量测定水平,相对标准偏差小于2‰,微量测定简便快捷,节约经费,减少污染,效果良好,可满足分析化学的要求。     

工业硅酸盐中Al_2O_3含量的常量和微量测定法研究

本文对工业硅酸盐中Al2O3含量的常量和微量测定法进行了比较研究。对于常量Al2O3含量的测定,采用化学分析法即配位滴定法,简便快速,效果较好;而对于微量Al2O3含量的测定,采用分光光度法,灵敏度高,重现性好;这两种测定法都可以得到满意的测定结果。     

用溴离子选择电极测定磷矿中的微量溴

基于溴离子选择电极的线性范围,溴质量分数在0.002%～0.2%时,应选择1μg/mL标准溶液系列并建立工作曲线,用于测定磷矿中微量溴的含量。该法便捷、快速,适用于磷矿中微量溴的测定。     

磷矿石中微量钼的测定—结晶紫分光光度法

测定微量钼的分光光度法很多,但都是用于含量在10~(-3)%以上的样品,对于复杂矿物中含量在10~(-4)～10~(-5)%范围内的分光光度法尚未见有报导.本文研究根据文献[2]的显色反应和条件采用结晶紫分光光度法测定磷矿石中微量     

An investigation of microstructure evolution in cement paste through setting using ultrasonic and rheological measurements

The response of hydrating cement paste through setting are monitored using rheological measurements and ultrasonic reflection measurements. Increases in the elastic modulus and yield stress of cement paste with time are obtained from the rheological measurements. Ultrasonic measurements are performed using horizontally polarized shear waves (SH) reflected off of the hydrating cement paste. Changes in the ultrasonic signal through setting are related with changes in the porosity and stiffness of an equivalent water-filled poroelastic material, which provides identical acoustic impedance. The measured changes in the shear modulus obtained from ultrasonic measurement are shown to correlate well with increase in elastic modulus obtained from rheological measurements. The increase in the shear modulus of the porous material obtained from the ultrasonic measurements is shown to correspond well with the observed increase in the yield stress of the cement paste. By combining the information from rheological and ultrasonic measurements, it is found that even in the fluid stage there is sufficient structural integrity in the arrangement of cement grains to support low-amplitude shear stress and the evolution of a continuously connected network of cement particles within the paste is coincident with a rapid increase in the shear modulus of the porous skeleton.     

A method based on isothermal calorimetry to quantify the influence of moisture on the hydration rate of young cement pastes

Cement hydration needs water to proceed and if water is lost by drying, the hydration rate will decrease. This can be of importance in cases when concrete surfaces are exposed to drying so that their strength development will be retarded. We describe a method based on isothermal calorimetry to assess how the rate of cement hydration is influenced by removal of water (drying) at different times up to three days after mixing. Thin samples of cement pastes are hydrated in a calorimeter and at different times exposed to one hour drying periods. The resulting decrease in thermal power following the removal of water is quantified as a measure of the reduction in hydration rate. The mass loss is found by weighing the samples before and after a measurement, and the change in water activity of a sample during drying can be found from the slope of the thermal power during the drying period.     

Alkali binding in hydrated Portland cement paste

The alkali-binding capacity of C-S-H in hydrated Portland cement pastes is addressed in this study. The amount of bound alkalis in C-S-H is computed based on the alkali partition theories firstly proposed by Taylor (1987) and later further developed by Brouwers and Van Eijk (2003). Experimental data reported in literatures concerning thirteen different recipes are analyzed and used as references. A three-dimensional computer-based cement hydration model (CEMHYD3D) is used to simulate the hydration of Portland cement pastes. These model predictions are used as inputs for deriving the alkali-binding capacity of the hydration product C-S-H in hydrated Portland cement pastes. It is found that the relation of Na⁺ between the moles bound in C-S-H and its concentration in the pore solution is linear, while the binding of K⁺ in C-S-H complies with the Freundlich isotherm. New models are proposed for determining the alkali-binding capacities of C-S-H in hydrated Portland cement paste. An updated method for predicting the alkali concentrations in the pore solution of hydrated Portland cement pastes is developed. It is also used to investigate the effects of various factors (such as the water to cement ratio, clinker composition and alkali types) on the alkali concentrations.     

Interactions between shrinkage reducing admixtures (SRA) and cement paste's pore solution

Shrinkage reducing admixtures (SRA) have been developed to combat shrinkage cracking in concrete elements. While SRA has been shown to have significant benefits in reducing the magnitude of drying and autogenous shrinkage, it has been reported that SRA may cause a negative side effect as it reduces the rate of cement hydration and strength development in concrete. To examine the influence of SRA on cement hydration, this study explores the interactions between SRA and cement paste's pore solution. It is described that SRA is mainly composed of amphiphilic (i.e., surfactant) molecules that when added to an aqueous solution, accumulate at the solution-air interface and can significantly reduce the interfacial tension. However, these surfactants can also self-aggregate in the bulk solution (i.e., micellation) and this may limit the surface tension reduction capacity of SRA. In synthetic pore solutions, SRA is observed to form an oil-water-surfactant emulsion that may or may not be stable. Specifically, at concentrations above a critical threshold, the mixture of SRA and pore fluid is unstable and can separate into two distinct phases (an SRA-rich phase and an SRA-dilute phase). Further, chemical analysis of extracted pore solutions shows that addition of SRA to the mixing water depresses the dissolution of alkalis in the pore fluid. This results in a pore fluid with lower alkalinity which causes a reduction in the rate of cement hydration. This may explain why concrete containing SRA shows a delayed setting and a slower strength development.     

Linear and non-linear analysis of desorption processes in cement mortar

Based on the gathered experimental data concerning adsorption/desorption processes in cement mortar, it has been stated that the rate of these processes changes in time even if they proceed in stable conditions. In this paper an attempt is made to describe such processes by applying linear and non-linear diffusion theories for comparison. The main aim of these studies is to determine the diffusion coefficient by correlating the theoretically determined desorption isotherms with the experimental ones. The validation of the diffusion coefficient was accomplished through comparison of the theoretical desorption curves with the experimental data for narrow and broad ranges of the air humidity changes. The final conclusion is that the moisture transfer in hygroscopic porous materials for broad ranges of the air humidity changes should be modeled by the non-linear diffusion theory, in which the diffusion coefficient is a function of moisture content. The new material in this paper concerns very long time measurements in desiccators, and evaluation of the diffusion coefficient by an advanced optimization algorithm.     

Corrosion process and structural performance of a 17 year old reinforced concrete beam stored in chloride environment

The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel-concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel-concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.     

Toward a better comprehension and modeling of hysteresis cycles in the water sorption–desorption process for cement based materials

The aim of this work is to describe a method based on a simple representation of the pore size distribution, which is able to predict hysteresis phenomena encountered in water sorption–desorption isotherms, particularly for cementitious materials. The hysteresis effect due to network constrictivity is taken into account in order to extend models of transfer in porous media to situations involving wetting–drying cycles. This is not achieved in earlier models and their performance in terms of prediction in such conditions is thus limited. The present modeling is based on an idealized pore size distribution. This has three modes, associated with C–S–H pores, medium capillary pores, and large capillary pores including consideration of cracks. The distribution is assessed from the chemical composition of the cement, the formulation of the material, the degree of hydration, the total water porosity and the intrinsic permeability.     

The role of pH in thaumasite sulfate attack

The thaumasite form of sulfate attack (TSA) has been recognised in recent years as a distinct mechanism by which degradation of buried concrete can occur in the presence of an external source of sulfate ions. There is, however, disagreement about the role of pH. It has been proposed that attack by sulfuric acid, produced by oxidation of pyrite, is sometimes the primary cause of deterioration. Others believe that the acid is rapidly neutralised giving a higher concentration of sulfate ions in the ground, hence increasing the extent of attack. The aim of the laboratory study reported here was to understand the role of sulfuric acid in TSA by examining concrete cubes, made from three types of cement and two types of aggregate, immersed at low temperature in two solutions, one alkaline corresponding to BRE Design Sulfate Class DS-3 and the other acidic. It is concluded that the presence of acid does not promote the formation of thaumasite. Although degradation of the concrete was observed in acid conditions, the mechanism was not TSA as observed in alkaline conditions.     

Assessment of the long-term stability of cementitious barriers of radioactive waste repositories by using digital-image-based microstructure generation and reactive transport modelling

Cement-based grout plays a significant role in the design and performance of nuclear waste repositories: used correctly, it can enhance their safety. However, the high water-to-binder ratios, which are required to meet the desired workability and injection ability at early age, lead to high porosity that may affect the durability of this material and undermine its long-term geochemical performance.In this paper, a new methodology is presented in order to help the process of mix design which best meets the compromise between these two conflicting requirements. It involves the combined use of the computer programs CEMHYD3D for the generation of digital-image-based microstructures and CrunchFlow, for the reactive transport calculations affecting the materials so simulated. This approach is exemplified with two grout types, namely, the so-called Standard mix 5/5, used in the upper parts of the structure, and the “low-pH” P308B, to be injected at higher depths.The results of the digital reconstruction of the mineralogical composition of the hardened paste are entirely logical, as the microstructures display high degrees of hydration, large porosities and low or nil contents of aluminium compounds.Diffusion of solutes in the pore solution was considered to be the dominant transport process. A single scenario was studied for both mix designs and their performances were compared. The reactive transport model adequately reproduces the process of decalcification of the C-S-H and the precipitation of calcite, which is corroborated by empirical observations. It was found that the evolution of the deterioration process is sensitive to the chemical composition of groundwater, its effects being more severe when grout is set under continuous exposure to poorly mineralized groundwater. Results obtained appear to indicate that a correct conceptualization of the problem was accomplished and support the assumption that, in absence of more reliable empirical data, it might constitute a useful tool to estimate the durability of cement-based structures.