Calculation of chloride ion concentration in expressed pore solution of cement-based materials exposed to a chloride salt solution

To investigate the relationship between the chloride ion concentration of the expressed pore solution (C_e) and the exposure solution (C_es) of cement-based materials exposed to a chloride salt solution, a calculation of the C_e, based on an electrical double layer (EDL) model was proposed. The C_e values were calculated using the proposed equations, applying the measured parameters: relative potential (δψ₀), dielectric constant (ε), porosity (P), bulk density (D), volume percentage (p_i) of pores with diameter d_i. The calculations explain why C_e does not always equal to C_es. The results indicate that the expression method does not test the concentration of free chloride ions in the bulk solution (C_Cl⁻_b) or the average concentration of the transportable chloride ions (C_ttr.a) in the pores. However, the ratio (C_r) of C_e to C_Cl⁻_b and the relationship between C_e and C_ttr.a can be calculated to more accurately measure chloride ion migration in cement-based materials.     

The effects of fly ash composition on the chemistry of pore solution in hydrated cement pastes

This paper reports the findings of an investigation to determine the influence of fly ash composition on the evolution of the pore solution chemistry in Portland cement/fly ash systems. Twelve fly ashes, selected to represent the wide range of composition of North American ashes, were used in the study. In addition to pore solution expression and analysis, inner hydration products were analyzed using energy-dispersive X-ray analysis. The study shows that the alkalinity of pore solution increases as the calcium and alkali content of the fly ash increase, and decreases as the silica content of the ash increases. However, there is no consistent trend between the composition of the inner calcium-silicate hydrate and fly ash composition.     

Methods of obtaining pore solution from cement pastes and mortars for chloride analysis

Two techniques for the recovery of pore solution from cement mortars are examined: pore solution expression and miscible displacement using a high pressure permeameter. In the former, the pore solution is expressed from the mortar by crushing; in the latter, it is eluted from the mortar over 30 min by miscible displacement with water. Experimental results are presented for a range of cement pastes and mortars into which known amounts of chloride ion have been incorporated by using sodium chloride solution as the mix water. The results show that both eluted and expressed solutions exhibit a decrease in chloride ion concentration as the cement matrix ages, with the elution method showing a greater sensitivity to mix composition. Both methods show a decrease in chloride concentration as the water: cement ratio of the mix is increased. Overall, the high pressure elution method is capable of recovering a significantly higher proportion of the incorporated chloride. The application of these techniques to pore solution analysis is discussed.     

Influence of lithium-based products proposed for counteracting ASR on the chemistry of pore solution and cement hydrates

Low- and high-alkali cement pastes were made with or without LiNO₃ or a Li-bearing glass. The [Li]/[Na+K] molar ratio was kept constant to 0.74. The specimens were stored at 23, 38, and 60 °C in sealed containers. After 3, 7, 28, and 91 days, their pore solutions were extracted and analysed, and their residual water contents were obtained by drying. The Li glass was found to react quite slowly, and the corresponding [Li⁺] in solution progressively increased with time, temperature, fineness (as-received glass vs. ground glass), and the [Na⁺+K⁺] concentration in solution. This glass increased the pH by about 0.1, and by about 0.2 after it was finely ground. In contrast, LiNO₃ decreased the pH by about 0.1, despite significantly increasing the [Na⁺+K⁺] in the pore solution. The higher the total %Na₂O_e content (including Li) in the original mixtures, the higher the total alkali content incorporated in the cement hydrates. The [Li⁺]-[Na⁺+K⁺] ratio in solution was about half of the initial ratio (0.74), while this ratio in the cement hydrates was always over 1.1. Li is the alkali most preferentially incorporated into the cement hydrates, while K is the least.     

Accelerated carbonation testing of alkali-activated binders significantly underestimates service life: The role of pore solution chemistry

The carbonation resistance of alkali-activated binders is often tested via accelerated test protocols designed for Portland cements, without questioning whether the tests replicate the mechanisms observed in service. Thus, validation of accelerated methods is required to enable realistic prediction of material performance. Changes in pore solution equilibria cause the formation of sodium bicarbonates during accelerated carbonation, compared with hydrous sodium carbonates in natural carbonation. This shifts the carbonation mechanism to favour more rapid reaction progress, to give a higher apparent degree of acceleration (compared to natural conditions) than in Portland cements. The pore solution pH under accelerated carbonation is significantly lower than at natural CO₂ concentrations, leading to a falsely short predicted service life (time to expected corrosion of embedded steel), as natural CO₂ concentrations appear not to reduce the pH below 10. Thus, accelerated carbonation testing is unduly aggressive towards alkali-activated binders, and test results must be cautiously interpreted.     

Effects of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution

The objective of this research was to develop activated carbon selection criteria that assure the effective removal of trace organic contaminants from aqueous solution and to base the selection criteria on physical and chemical adsorbent characteristics. To systematically evaluate pore structure and surface chemistry effects, a matrix of activated carbon fibers (ACFs) with three activation levels and four surface chemistry levels was prepared and characterized. In addition, three granular activated carbons (GACs) were studied. Two common drinking water contaminants, relatively polar methyl tertiary-butyl ether (MTBE) and relatively nonpolar trichloroethene (TCE), served as adsorbate probes. TCE adsorbed primarily in micropores in the 7-10 Å width range while MTBE adsorbed primarily in micropores in the 8-11 Å width range. These results suggest that effective adsorbents should exhibit a large volume of micropores with widths that are about 1.3 to 1.8 times larger than the kinetic diameter of the target adsorbate. Hydrophobic adsorbents more effectively removed both TCE and MTBE from aqueous solution than hydrophilic adsorbents, a result that was explained by enhanced water adsorption on hydrophilic surfaces. To assure sufficient adsorbent hydrophobicity, the oxygen and nitrogen contents of an activated carbon should therefore sum to no more than about 2 to 3 mmol/g.     

Changes in structure and chemistry of cement mortars stressed by a sodium chloride solution

Cement mortars, one-sidedly ctressed by a sodium chloride solution of 20 % for 140 days, show a marked area at a distance from the surface of nearly 30 mm. Its position is reflected in concentration profiles of the water soluble and nitric acid soluble parts of chloride, sulphate, sodium, potassium, and aluminium, in the mineral content as well as in the pore size distribution. This could serve as the starting point for deteriorations caused by cycles of freezing and thawing in connection with deicing salts, because destroying mechanisms could easily attack here.Further studies must be carried out and the observation of stressed cement mortars and concretes has to be intensified to get a better insight into the principles and the mechanisms of such processes.     

The pore solution phase of carbonated cement pastes

Samples of hydrated cement pastes were exposed to atmospheres with various carbon dioxide concentrations at relative humidities controlled by different saturated salt solutions. When carbonated throughout their thickness, as indicated by the phenolphthalein test, they were resaturated with water and subjected to pore solution expression and analysis. The effects of the various carbonating environments on the pore solution composition and on aspects of the pore structure and mineralogy of the carbonated products are reported. Implications regarding the likely effects of different accelerated carbonation regimes on the corrosion behaviour of steel in concrete are discussed. In particular, it is shown that the use of saturated sodium nitrite solution to control the relative humidity of atmospheres with high concentrations of carbon dioxide may cause an evolution of gaseous oxides of nitrogen, which can result in the contamination of the pore solution with nitrite and nitrate salts.     

Pore solution chemistry of the hydrated system tricalcium silicate/sodium chloride/water

The relative tendency of different Portland cements to remove chloride ions from concrete mix water by forming insoluble complexes is an important determinant of the corrosion behaviourod steel in concrete. Whilst the C₃A phase plays a dominant role in binding chloride ions, other cement minerals may be of secondary importance but their effects are not well established. The reported investigation is an attempt to elucidate the extent to which chloride binding occurs within the hydration products of the C₃S (alite) phase of Portland cement when sodium chloride is present in the mix water.     

Alkali–silica reaction: Kinetics of chemistry of pore solution and calcium hydroxide content in cementitious system

This paper presents the results of the investigations on the chemistry of pore solutions, the contents of calcium hydroxide, and the expansions in mortars containing both reactive and non-reactive aggregates. In order to examine the effect of the temperature, experiments were performed at three different temperatures (23 °C, 38 °C and 55 °C).The compositions of the pore solution were measured at short time intervals for a period of up to 130 days in order to capture the kinetics of the chemistry of pore solution. The results showed that the changes in the concentrations of alkali ions can be best explained by the first order reaction. In addition, the proposed rate equation could reasonably simulate the changes in the actual concentrations of alkalis. Finally, the results in this paper suggest that the rate of the alkali–silica reaction in cementitious system containing highly reactive aggregate can be also expressed as the first order reaction.     

The influence of substrate absorbency on coating surface chemistry

The composition of the top surface of a coating layer can influence its functional properties or subsequent processing steps. The effect of the substrate absorbency on the coating surface chemistry is reported. Different coating systems containing a kaolin clay pigment, fine or coarse precipitated calcium carbonates, and a common latex binder were examined. The influence of a soluble polymer added into the coating was characterized. The surface chemistry was measured with attenuated total internal reflectance (ATR) and X-ray photoelectron spectroscopy (XPS).

Absorbent substrates generate bulky coatings with high voids and low gloss. Rapid dewatering by the absorbent substrate pulls the small particles, like latex binder, away from the top layers causing a low latex concentration at the surface. On non-absorbent substrates, the addition of the soluble polymer generates coating layers with higher void volume, lower gloss, and lower latex concentrations at the coating surface. However, on absorbent substrates, polymer addition causes coatings with lower void volumes and higher gloss. In this case, the rapid dewatering and mobility of particles is reduced by the polymer, which helps to retain the small particles at the surface. As a result, latex concentration at the surface increases with polymer addition on absorbent substrates.     

Immobilized penicillin G acylase on mesoporous silica: The influence of pore size, pore volume and mesophases

Penicillin G acylase (PGA) was immobilized on KIT-6 and SBA-15 with different pore sizes to study the influence of pore size, pore volume and mesophases on the immobilized enzyme (IME) activity. The results show that the pore size, pore volume, mesophase and surface area have an obvious influence on the IME activity, of which the pore size is the most important factor. The activity of PGA immobilized on KIT-6/130 reaches to 3522 IU/g by dry support, which is higher than that ever reported for carrier-bound penicillin G acylase [A.I. Kallenberg, F. van Rantwijk, R.A. Sheldon, Adv. Synth. Catal. 347 (2005) 905–926]. Because of the covalent immobilization, the operational stability of IME increases.     

The influence of preparation methods on the pore structure of alumina

Structural characterization of eleven kinds of aluminas prepared by different methods was carried out by measurement of the adsorption of nitrogen, X-ray diffraction and scanning electron microscopy. Aluminas prepared using aqueous ammonia as precipitation reagent exhibited a single pore size distribution, the maximum being at a radius of about 20Å, whereas aluminas prepared with urea exhibited a twin peaked pore size distribution, with maxima at radii of about 20Åand 50Å. The pore structures of aluminas prepared from aluminium nitrate by pyrolysis and from sodium aluminate with carbon dioxide were similar to the former, while aluminas prepared by pyrolyses of aluminium isopropoxide and of aluminium trichloride had a pore structure similar to the latter.Based on scanning electron microscopy and nitrogen adsorption-desorption hysteresis loop observations, the crystallite structures of aluminas and pore structures were discussed in connection with the preparation methods employed.     

Aspects of the pore solution chemistry of blended cements related to the control of alkali silica reaction

The effects of four pulverised fuel ashes (PFA) and three ground blastfurnace slags (GBFS) on the alkalinity of the pore solution phase of hardened cement pastes has been studied. It has been found that the total alkali content of a PFA is an important factor, but not the only one, determining its effectiveness in reducing the hydroxyl ion concentration of the pore solution. For GBFS, however, there was found to be no direct correlation between the total alkali content of the slag and the composition of the pore solution. Implications regarding the roles of PFA and GBFS in reducing expansion associated with alkali silica reaction (ASR) are discussed.     

氯离子对硝铵磷水溶液热安全性影响研究

利用自制临界爆炸装置对80%硝铵磷水溶液的热安全性进行测试实验。结果表明,含氯离子硝铵磷水溶液比纯硝铵磷水溶液的临界爆炸温度要低;一定浓度的氯离子有促进硝铵磷水溶液爆炸的作用,硝铵磷水溶液的临界爆炸温度随氯离子浓度的增大呈先降低后升高的趋势。研究结果对使用硝铵磷生产复合肥安全生产具有重要指导意义。     

Effect of supplementary cementitious materials (binder type) on the pore solution chemistry and the corrosion of steel in alkaline environments

The pore solution compositions of paste samples produced with Ordinary Portland Cement (PC), slag 25%, 50% and 75%, fly ash 30%, condensed silica fume (SF) 7%, and a ternary blend of 50% PC, 43% slag and 7% SF were determined. Not only are there significant variations in the concentration of the major cations and anions but also, and equally important from the perspective of development of the passivity of steel in solution, in the level of dissolved oxygen and redox potential.Further, the impact of changes in the pore solution chemistry of cement pastes with SCMs on the passivation and corrosion of steel was investigated with mild steel in simulated pore solutions (SPS). Sulphides and thiosulphates, typically found in slag bearing pastes, appeared to reduce the chloride threshold concentration and increase the rate of corrosion in SPS, which has potential implication for the long term performance of reinforced concrete structures.     

聚丙烯腈纤维孔隙结构对预氧化的影响

采用湿法纺丝,通过不同的牵伸条件,制备具有相同的化学组分和不同孔隙率(体密度)的聚丙烯腈纤维,然后在相同的条件下预氧化。借助红外吸收光谱(FT-IR)、场发射扫描电子显微镜(FESEM)、差示扫描量热(DSC)等表征手段对原丝和预氧丝的环化度、皮芯结构及预氧丝横截面的氧径向分布进行分析对比。实验结果表明,随着孔隙率降低(体密度增加),纤维径向的氧含量梯度变大,皮芯结构趋于严重。     

The role of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration

This study investigates the effect of hydrodynamic conditions and solution chemistry on protein fouling during ultrafiltration. Drastic flux reduction was observed at high initial flux and/or low cross-flow velocity. A limiting flux existed during BSA filtration, beyond which membrane flux cannot be sustained. Further increase in pressure over the limiting value did not enhance the stable flux. The rate and extent of BSA fouling were also strongly dependent on the feedwater composition, such as BSA concentration, pH, and ionic strength. Foulant concentration had no effect on the stable flux, although the rate approaching to the stable flux increased proportionally with increasing foulant concentration. Fouling was most severe at the isoelectric point of BSA (pH 4.7), where the electrostatic repulsion between foulant molecules is negligible. Membrane fouling became less severe at pHs away from the isoelectric point. Increasing ionic strength at pH 3.0 promoted severe fouling likely due to electric double layer (EDL) compression. On the other hand, the flux behavior was insensitive to salt concentration at pH 4.7 due to the lack of electrostatic interaction. At a solution pH of 5.8, effect of ionic strength on long-term flux behavior was directly opposite to that on the transient behavior. While the long-term flux was lower at higher ionic strength due to EDL compression, the transient behavior was also affected by the BSA retention of the membrane.     

氯离子对硫铵结晶系统的影响及对策

研究了硫铵结晶系统中氯离子的来源以及对硫铵结晶系统的影响,并制定了控制氯离子含量的对策。硫铵结晶系统由于氯离子进出严重不平衡,导致母液中的氯离子含量高,造成设备腐蚀,影响产品质量和系统安全,因而必须控制氯离子的含量。     

The influence of chloride ions on the kinetics of iron dissolution

The anodic dissolution of pure iron was studied in oxygen-free solutions at high concentrations of chloride and hydrogen ions at 25°C under potentiostatic steady-state conditions. In the range c_H⁺ ? 1 M the following kinetic data were obtained: Tafel slope b₊ ? +100 mV, electrochemical reaction order related to C_H⁺, n_+.H⁺ = +1·1, and electrochemical reaction order related to the chloride ion concentration, n_+,Cl^? = +0·6. These values cannot be correlated to the currently proposed mechanisms of iron dissolution, another mechanism is suggested for the described conditions. The correlations to known mechanisms are discussed.