Effect of drying-rewetting on the alkali concentration of the concrete pore solution

Experience has shown that the pore solution alkalinity of resaturated concrete was lower than expected. Laboratory-concrete specimens were made with reactive aggregates, stored over water at 38 °C, and then broken into two pieces. The pore solution of half-specimens was extracted by high-pressure squeezing. The other half-specimens were dried at ambient air and rewetted in humid air to their initial weight. The pore solution was then extracted and compared with the composition results of the first extraction. The results obtained from this investigation confirmed that a certain part of the alkali ions in pore solution that had become fixed by drying are not subsequently extracted after rewetting. The alkali concentration [Na+K] was reduced from 34% to 61% by the drying and rewetting treatments.     

In situ leaching method for determination of chloride in concrete pore water

In the in situ leaching (ISL) method, pore water ionic content is determined in small cavities drilled in mortar/concrete specimens. Prior investigations have demonstrated the ISL applicability to obtain pH and nitrite ion concentration in concrete/mortar pore water. The application of the method is extended here to determine pore water chloride ion concentration (and pH) within practical test times in mortars and concretes prepared in the laboratory and in concrete cores extracted from a bridge deck in deicing salt service. Spatial resolution for the determination of composition profiles is also illustrated. Modeling of the ISL process indicates that chloride binding accelerates the approach toward a terminal cavity concentration, reducing test duration to practical lengths for moderate permeability concretes. This acceleration can be attributed to maintaining a higher gradient of free chloride near the cavity wall due to the release, during leaching, of previously bound chloride. Consequently, there is a faster chloride buildup in the cavity water compared with the no-binding case. Experimental chloride and pH results obtained by the ISL test in mortar samples show good agreement with those from the pore water expression (PWE) method. Also, examples are presented of application of ISL data to obtain chloride binding isotherms, and pore water chloride to hydroxide ratio relevant to assessing conditions for corrosion of steel reinforcement. The ISL method presents a less costly and less disruptive alternative to PWE for pore water analysis. It is noted, however, that in a few instances ISL could not be implemented because of excessive absorption of cavity water by the surrounding medium.     

Corrosion of zinc in simulated carbonated concrete pore solutions

The electrochemical behaviour of pure Zn and galvanized steel in solutions simulating the pore solution of carbonated concrete has been studied by means of potentiodynamic polarization tests and polarization resistance measurements. Pure Zn was chosen because it simulates well the behaviour of galvanized steel, yielding more reproducible results. The effect of different degrees of carbonation and the presence of different chloride contents in the simulated pore solutions was investigated. Results show that at a given pH (about 9.5) the corrosion susceptibility of Zn depends on anions concentration (carbonate and bicarbonate). The results obtained in simulated carbonated concrete pore solutions show that with low anion concentration Zn does not passivate while in presence of high levels of carbonate and bicarbonate the corrosion resistance is improved. Besides, the presence of chloride increases the corrosion susceptibility.     

Corrosion behavior of reinforcing steel in simulated concrete pore solutions: A scanning micro-reference electrode study

The scanning micro-reference electrode (SMRE) technique was used to study the corrosion behavior of reinforcing steel in simulated concrete pore (SCP) solutions with different pH values. The early stage as well as the propagation of the localized corrosion of reinforcing steel in different solutions was explored. The results indicated that the potential distribution on the reinforcing steel surface changed in homeostasis and the steel remained passive in the pure simulated concrete pore solution. The solution pH had a significant effect on the localized corrosion of reinforcing steel, and the critical pH value for localized corrosion of reinforcing steel in SCP solutions was between 11.46 and 11.31.     

The role of activating solution concentration on alkali–silica reaction in alkali-activated fly ash concrete

To enable commercial use of alkali-activated fly ash concrete, its durability must be better understood. Alkali–silica reaction is a primary concern since highly alkaline solutions are generally used for activation. This study investigated the effect of NaOH activating solution concentration on pore solution alkalinity and subsequent alkali–silica reaction in alkali-activated fly ash concrete. It was found that pore solution alkalinity increased with increasing activating solution NaOH concentration, and this effect was amplified at concentrations above an optimum, defined as the concentration that resulted in the highest mortar compressive strength. Expansion of concrete prisms containing highly reactive fine aggregate and activating solution concentrations above the optimum concentration was approximately three times that of concrete with optimum activating solution concentrations, but only about 5% of the expansion observed in the ordinary portland cement control. The low expansion may be attributed to the low calcium levels in the alkali-activated fly ash concrete.     

由辣椒粉浸出液萃取辣椒碱

以市售干辣椒为原料、95%乙醇为浸出剂,实验研究了溶剂萃取法由辣椒粉浸出液中萃取纯化辣椒碱的工艺条件. 在模拟体系和实际体系中均以正己烷和乙酸丁酯为萃取剂,考察了pH值和有机相/水相体积比对辣椒碱收率的影响. 结果表明酸化萃取步骤为工艺控制步骤. 模拟体系中pH值为11、体积比为7:1时乙酸丁酯对辣椒碱的萃取收率均大于90%,表明以乙酸丁酯为萃取剂,可以提高辣椒碱的浓缩度. 实际体系萃取结果与模拟体系趋同,但约20%的辣椒碱不能被萃取. 实际体系优化的工艺条件为辣椒粉用量50 g,250 mL 95%乙醇2次浸出,辣椒碱收率为97.0%;浸出液浓缩为浸膏并溶解于NaOH溶液中,在pH值13, O/W体积比1:1时萃取,再用盐酸调节水相pH<11,O/W体积比为1:5时用乙酸丁酯萃取,辣椒碱的收率为浸出量的80%左右.     

用脱镁海泡石制备介孔分子筛MCM-41

以脱镁海泡石为硅源,在水热条件下合成介孔分子筛MCM-41.考察了晶化时间、晶化温度、pH值等因素对介孔分子筛MCM-41的影响.用X射线衍射分析、Brumauer-Emmett-Teller法和红外分析表征MCM-41.结果表明:在pH值为12,晶化温度为100℃,晶化时间为24h时,所得MCM-41的结晶度最好,比表面积为860m2/g,孔体积为0.77 cm3/g,孔径为2.9nm,孔壁厚为1 nm.红外分析表明:脱镁海泡石在合成介孔分子筛MCM-41过程中生成了≡SiO-CTA 络合物.     

Threshold chloride concentration for stainless steels activation in concrete pore solutions

The threshold chloride concentration for stainless steels activation in concrete environments depends not only on chemical composition of the steel and on pH level of concrete pore solution but is also considerably affected by the superficial state of the steel. The presence of scales leads to significant decrease of corrosion resistance. This study evaluates the corrosion resistance of various stainless steels exposed to pore solutions of fresh concrete by means of electrochemical methods. All tested materials with bare surface, except for martensitic chromium steel FeCr12, proved to be completely resistant to fresh concrete pore solution containing chlorides. If the surface is scaled, the identically high resistance cannot be expected even for the most alloyed stainless steels.     

Change of pore size in concrete due to electrochemical chloride extraction and possible implications for the migration of ions

Electrochemical chloride extraction (ECE) is used for the rehabilitation of chloride-contaminated concrete. Anions such as chloride and hydroxide are pushed away from the cathode (reinforcement), and cations such as sodium, potassium and calcium are attracted to the cathode. During ECE an increase of the concrete resistance can be observed. The results of a petrography study on ECE-treated concrete are presented in this paper. It also investigates the influence of pore size on ion migration using a concrete substitute model with known pore size. Findings showed that the pore size and pore size distribution of concrete are altered due to ECE. It is therefore suggested that concrete acts as active migration medium in the migration process by releasing ions into the pore solution. Moreover, small pores hinder the migration of ions, which may partially be responsible for changes in concrete resistance.     

Electrochemical behavior of mild steel in concrete: Influence of pH and carbonate content of concrete pore solution

The increase of the rebars corrosion rate due to the concrete carbonation is the major cause of reinforced concrete degradation. The aim of this study was to investigate the transition from passive to active corrosion of mild steel rebars in carbonated concrete. For this purpose, electrochemical techniques (polarization curves, free corrosion potential measurements) and surface analyses (EDS, XRD, XPS) were used. Five different electrolytes, with pH ranging from 13 to 8.3, were chosen to simulate the interstitial concrete pore water at various degrees of carbonation. The results indicate that the transition pH is between 10 and 9.4. XPS results indicate a passivation of mild steel for pH values ranging from 13 to 10 due to the formation of a thin iron III oxide layer. Immersion tests highlight the importance of the buffering effect of the carbonate content. At the free corrosion potential in an aerated solution, a decrease of the carbonate content increases the corrosion rate. On the opposite, at low electrode potential, the kinetics of oxidation increases with the carbonate content.     

Low temperature direct methylation of lipids in biological materials

The procedure for low temperature methylation of fatty acids in lipids by sulfuric acid-methanol has been adapted to direct methylation of fatty acids of lipids in biological materials without prior extraction of the lipids. Successful application requires a solution or a suspension of fine particles of the lipid bearing material in ether. Concentrated sulfuric acid is added to the solution or suspension at low temperatures followed by addition of absolute methanol. The acid is neutralized by methanolic KOH and the esters extracted. Application of the method to prepare methyl esters of lipids in cream, blood serum, swine liver and kidney tissue, and cells of yeast onStaphylococcus aureus show that fatty acid composition based on this method compares with that determined by methylation of extracted lipids. Michigan Agricultural Experiment Station Publication No. 3803. Presented in part at the AOCS Meeting in Chicago, 1964.     

Interaction of bentonite with supercritically carbonated concrete

Bentonite and concrete are essential components in construction of a geological high level nuclear waste (HLNW) repository. Conventional Ordinary Portland Cement (OPC) used for concretes gives a pore water leachate with a pH as high as 13.5 in contact with ground water. This alkaline plume of leaching waters might perturb the engineered barrier system, which might include bentonite buffer, backfill material or the near-field host rock. The accepted solution to maintain the bentonite stability, which is controlled by the pH, is to develop cementitious materials with pore water pH around 11. Four lixiviation experiments representative of long-term interaction of solids and pore fluids at the concrete/bentonite interface were performed with two types of cement paste, Portland and calcium aluminate cement, before and after being carbonated under supercritical conditions, with granite water at 80 °C. The evolution of the pH indicates that the supercritical carbonation reduced the alkalinity of the cement pastes and calcite likely controls the equilibrium of Ca at the end of the experiments. The bentonite helps to buffer the alkalinity of concrete leachates through several reactions such as dissolution of montmorillonite and precipitation of secondary products as trioctahedral smectite, zeolites (gismondine), and presumably Mg hydroxides and amorphous gels. Carbonation may reduce propagation of the alkaline plume and enhance the barrier performance.     

Synergistic extraction of Nd(III) with mixture of 8-hydroxyquinoline and its derivative with di-2-ethyl hexyl phosphoric acid in different diluents

Solvent extraction of neodymium (III) from chloride medium with synergistic mixture of 8-hydroxyquinoline or 2-methyl-8-hydroxyquinoline and di-2-ethyl hexyl phosphoric acid was investigated in different diluents. Effect of various experimental parameters such as pH, extractant concentration, diluents and temperature was studied. Role of diluents on extraction of neodymium is explained in terms of activity coefficient adopting Hildebrand regular solution theory. The negative value of enthalpy change suggests that the extraction of ternary system is exothermic. The IR data for the extracted species also substantiated the nature of extracted species as observed by slope analysis method.     

A novel fabrication method for glass foams with small pore size and controllable pore structure

In this paper, a novel fabrication method for glass foams has been introduced based on colloidal suspension foaming method using waste glass as starting materials. It is demonstrated for the first time that foam stabilization via in‐situ hydrophobization of waste glass particles is possible. The fabrication conditions for stable glass particle‐stabilized foams are optimized by investigating the influences of pH value, concentration of propyl gallate and ball milling time, respectively. It is demonstrated that this is a versatile method for fabrication of stable foamed glass suspension and final glass foams with small pore size of dozens of microns, which is much smaller than that of most glass foams in previous literatures. This novel method enables both closed pore structure and open pore structure simply by tailoring solid loading of glass suspension. A “sieve‐like” hierarchical pore structure can be achieved by adjusting sintering temperature. The glass foams with controllable structure could be applied in thermal insulation fields for closed pores, and in catalyst loading, filtration, and separation fields for open pores and hierarchical pore structure.     

Potentiostatic transient technique, a simple approach to estimate the corrosion current density and Stern-Geary constant of reinforcing steel in concrete

The linear polarization resistance (LPR) method, introduced by Stern and Geary, is widely used in determining corrosion rates of steel bars in concrete or synthetic pore solution. The major limitation of this method is that the value of the Tafel slopes, and consequently the Stern-Geary constant (B), is necessary for accurate calculation of the corrosion rate. This paper proposes a simple method for determining the corrosion current density and the Stern-Geary constant, using the results of the potentiostatic transient technique. The effect of the Stern-Geary constant on the corrosion rate of steel bar in concrete is also discussed. Results of this study show that potentiostatic transient method can be used successfully to determine the corrosion rate of steel bars in concrete without using the pre-assumed value of B.     

Experimental and numerical study of electrochemical chloride removal from brick and concrete specimens

The electrochemical technique for chloride extraction (desalination) was applied in galvanostatic mode to cylindrical brick and concrete specimens with a steel bar as reinforcement placed in the centre. The specimens were initially contaminated by immersion in a solution of 35 g/l NaCl. Based on the Nernst-Planck equations, a numerical model was developed considering the interactions between the various ionic species in the pore solution. The model makes it possible to predict the evolution of the chloride profile with time. The numerical and experimental results are compared and the model parameters discussed.     

碱-白云石反应机理与预防

根据颗粒堆积原理计算了碱白云石反应引起的固相框架体积的变化,并用图象分析技术研究了反应产物间的孔隙体积。研究了碱白云石反应机理,从热力学和通过实验分析了溶液ｐＨ值对碱白云石反应程度与速率的影响。通过监测白云岩粉末样压实体和基本不含粘土及石英的白云质灰岩在碱溶液中的膨胀行为,研究了碱白云石反应本身的膨胀性。评估了低碱度的硫铝酸盐水泥对碱白云石反应的抑制作用。结果表明：球形产物如呈最紧密堆积,则固体产物的孔隙率为２５．９５％,碱白云石反应后固相框架体积将增加２９．２３％。实际上,固体产物的孔隙率为１５％,反应后固相框架体积将增加１２．５％,这为反应产生膨胀提供了前提条件。白云石与碱作用生成水镁石、方解石和碳酸碱,该过程能直接引起膨胀。反应和膨胀的速率取决于溶液的ｐＨ值,ｐＨ值越高,反应和膨胀越快,当ｐＨ值低于某一值后,碱白云石反应将不发生,相应地,岩石不产生膨胀。碱白云石反应膨胀的驱动力为去白云化反应生成的方解石和水镁石晶体在受限空间生长产生的结晶压力。硫铝酸盐水泥能有效地抑制碱白云石反应膨胀,从而能防止混凝土的开裂破坏。     

硅酸盐与硫铝酸盐复合水泥孔隙液相pH值变化研究

为了解硅酸盐水泥与硫铝酸盐水泥复合之后性能变化的原因,通过对复合水泥孔隙溶液的pH值进行研究,探讨了pH值变化与性能之间的内在联系。研究表明,孔隙溶液pH值变化与强度有良好相关性,水化3h后复合水泥pH值在SAC掺量为20%-80%之间出现波谷,水化剧烈并开始大量放热,后期强度也出现明显波谷区,其中以其掺量为60%时pH值和后期强度最低;当SAC或PC掺量为0%-20%时,复合水泥孔隙溶液pH值和强度相对于纯的SAC或PC而言基本没有变化,甚至还会稍微提高,这对降低水泥生产成本和提升水泥性能具有积极意义。     

Air‐void characterisation of foam concrete

The pore structure of cementitious material, predetermined by its porosity, permeability and pore size distribution, is a very important characteristic as it influence the properties of the material such as strength and durability. The pore parameter could therefore be a primary factor influencing the material properties of foam concrete and an in depth look into this aspect is required to establish relationships between this and material properties. In order to evaluate these relationships it was necessary to develop parameters to explain and quantify the air‐void structure of foam concrete. This paper discusses the investigations done to characterise the air‐void structure of foam concrete by identifying few parameters and influence of these parameters on density and strength. A camera connected to an optical microscope and computer with image analysis software were used to develop these parameters. It is found that out of the air‐void parameters investigated, volume, size and spacing of air voids have influence on strength and density. Mixes with a narrower air‐void size distribution showed higher strength. At higher foam volume merging of bubbles seems to produce larger voids, results in wide distribution of void sizes and lower strength. Air‐void shape has no influence on the properties of foam concrete.     

Effect of mixing proportions of concrete on its electrical conductivity and the rapid chloride permeability test (ASTM C1202 or ASSHTO T277) results

The rapid chloride permeability test (RCPT)—American Society of Testing and Materials (ASTM) test method C1202 or American Association of States Highway and Transportation Officials (AASHTO) test method T277—is virtually a measurement of electrical conductivity of concrete, which depends on both the pore structure characteristics and pore solution chemistry of concrete. This paper discusses the effects of several factors, such as cement composition, replacement of cement with supplementary cementing materials and inclusion of aggregate, on the electrical conductivity or RCPT results of hardened cement mortars and concrete. Analyses based on published results have indicated that all the three factors may have significant effects on the chemistry and specific conductivity of concrete pore solution, which has little to do with the transport of ions in the solution. Thus, RCPT is not a valid test for evaluation of permeability of concretes made with different materials or different proportions.