Effects of slag content,fineness and additive to cement pastes on the corrosion behaviour of embedded reinforcing steel

Previous studies proved that the slag content of the blast furnace slag-Portland cement mix could be increased, while retaining the engineering properties of the produced slag cement pastes within the normal range and increasing the fineness of the mix. The corrosion behaviour of reinforcing steel embedded in the cement pastes giving the optimum mechanical properties was studied using the galvanostatic polarisation technique. The most corrosion-resistant mix has been determined. The effect of adding CaCl₂ to a paste of this mix on the corrosion behaviour of embedded reinforcing steel has been investigated. For the purpose of comparison, anodic polarisation tests were carried out using pastes having the composition of the most corrosion-resistant mix but at the Blaine area of the ordinary slag cement. Portland and normal slag cements were also tested. The threshold concentration of CaCl₂, below which breakdown of steel passivity did not occur, has been determined. The results of this study might have practical implications in the field.     

Hardened portland blast-furnace slag cement pastes: I. Effects of additives on surface area and on pore structure

Portland blast-furnace slag cement pastes were prepared with various water/cement ratios. Specific surface areas and pore structures of the hardened pastes were investigated by nitrogen adsorption. The “accessibility” of the nitrogen molecules to the pore structure is discussed in terms of degree of hydration and total porosities of the pastes. Effect of presence of CaCl₂, a typical steel reinforcement corrosive agent, was also studied, and results indicated that it alters the area and pore structure extensively, to a more “open structure,” thus facilitating its own accessibility. Lime and gypsum addition was also studied in presence and in absence of CaCl₂, and the effect of the Blaine surface area of the unhydrated cement is particularly emphasized in this investigation.     

Corrosion rates of steel embedded in cement pastes

Dynamic polarization techniques were used to estimate the corrosion rates of steel embedded in cement paste made with different binder systems. A corrosion model is proposed. The corrosion rates were determined by employing the Evans diagrams constructed from cathodic polarization curve of steel embedded in chloride free cement paste and anodic polarization curve of steel embedded in cement pastes containing chloride. Within the limitations of the adopted experimental conditions and sample configuration, the preliminary results indicate that corrosion rates of steel embedded in blended cement pastes generally fall within the range shown by steel embedded in portland cement pastes.     

Hardened portland blast-furnace slag cement pastes III. Corrosion of steel reinforcement versus pore structure of the paste matrix

The pore structure of the slag cement paste matrix seems to affect to a sensible extent the corrosion behavior of embedded steel. For both additive free, and additive containing slag cement pastes, the pore structure data were discussed in paper I of this series, whereas the corrosion behavior of embedded steel was discussed in paper II. In this paper III, the correlation between papers I and II is established, and the concluding remarks presented.     

Compressive strength versus residual porosity in polymer impregnated cement pastes

Impregnated cement pastes were prepared from fine cement of Blaine area 600 m²/kg using various initial W/C ratios in the range 0.25–0.70, cured for various durations, and by using styrene or methyl methacrylate, which were polymerized $\text{in situ}$ by thermal treatment. The polymer effect on compressive strength was found to be variable and depends on pore size. Upon impregnation, low porosity samples show measurable reduction in zero strength porosity, whereas high porosity samples show measurable reduction in the strength at zero porosity. Enhancement of compressive strength takes place at an optimum range of porosity and gel/space ratio.     

转炉钢渣粉磨动力学的实验研究

针对冷却方式不同的二种转炉铜渣（水淬和自然冷却），采用水泥试验球磨机进行微细粉制备，分析比较了两者的粉磨动力学特性．导出了各自的粉磨速度方程。研究结果表明：两种钢渣粉体的比表面积S（m^2／kg）与粉磨时间t（min）均呈一阶指数衰减关系，水淬钢渣的易磨性低于慢冷钢渣。慢冷钢渣的粉磨速度方程为dS／dt=14．01exp（-t／34．80），水淬钢渣的粉磨速度方程为dS／dt=11．55exp（-t／39．83）。     

The influence of different cements on chloride-induced corrosion of reinforcing steel

Corrosion of embedded steel in concrete may occur as a result of the depassivating effects of chloride ions. Two important parameters governing the risk of chloride-induced corrosion in cement matrices of varied compositions are believed to be: (i) the relative concentrations of chloride and hydroxyl ions in the pore electrolyte and (ii) the diffusivities of chloride ions. Measurements of these parameters for cement pastes of constant water/cement ratio and fixed total chloride content have been used to rank a series of Portland cements, slag blended cement and fly-ash blended cement in terms of their expected levels of corrosion protection. The validity of the predicted rank orders has been independently assessed by electrochemical monitoring of the corrosion rates of embedded steel electrodes by means of the method of linear polarisation.     

An experimental study on corrosion resistance of concrete with ground granulate blast-furnace slag

This paper presents experimental test results on corrosion resistance of concrete containing ground granulate blast-furnace slag (GGBS) and ASTM Type I or ASTM Type V cement. To investigate the problem, a series of tests were performed. First, rapid chloride permeability tests were executed in accordance with ASTM C 1202 to determine the qualitative terms of chloride-ion penetrability. Second, accelerated chloride-ion diffusion tests were done to calculate diffusion coefficients of chloride-ions permeated through concrete specimens. Third, accelerated steel corrosion tests were carried out by using the repeated wetting and drying technique. Fourth, half-cell potential tests were implemented in accordance with ASTM C 876 to evaluate the probability of steel corrosion. Finally, the surface area of corrosion on embedded steel in concrete specimens was measured to confirm half-cell test results. Test results showed that the coefficient of permeability of Type I cement concrete was lower than that of Type V cement concrete. All the concrete mixed with GGBS exhibited lower diffusion coefficient, compared to GGBS-free concrete. Moreover, the corrosion probability of steel bar in Type V cement concrete was higher than that of steel bar in Type I cement concrete. Based on the test results, it is suggested that much stronger corrosion resistance can be achieved, if higher volume of GGBS is added in Type I cement rather than Type V cement.     

Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes

The paper investigates the possibility of using a shear wave reflection technique to monitor the viscoelastic behavior (represented by storage shear modulus and viscosity) of Portland cement paste at very early age. Three cement pastes with water/cement ratios equal to 0.4, 0.5 and 0.6 cured under water at a constant temperature of 25 °C were studied. By measuring the wave reflection coefficients and the phase angles of reflected ultrasonic waves, the dynamic storage shear moduli and the viscosity of the cement paste can be calculated. The calculated results of the storage modulus were compared with the results obtained directly from the oscillatory rheometric measurement. In addition, the viscosity calculated from the wave reflection measurements was compared with results obtained directly from the step rheometric method and a qualitative agreement was found. The results show that as a non-destructive method, the ultrasonic wave reflection method provides useful information about both the elastic and viscous behavior of cement pastes at very early age.     

Hardened slag-cement pastes of various porosities I. Compressive strength,degree of hydration and total porosity

Hardened blast-furnace slag-cement pastes were prepared from cements of different Blaine areas, and mixed with various water/cement ratios in the range 0.20–0.70. The pastes were cured for various periods ranging from 1 to 365 days, and the degree of hydration, total porosities, and compressive strengths were determined. It is recommended in this investigation that the compressive strength values be compared at either constant total porosities or constant degree of hydration. The results obtained could indicate that the total porosity plays a more dominant role in affecting the strength than the degree of hydration.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

改善石灰石硅酸盐水泥耐腐蚀性能的研究

探讨了在一定浓度的MgSO4溶液中，一定温度环境下石灰石硅酸盐水泥受侵过程，采用矿渣微粉部分替代石灰石微粉或水泥可以延迟或阻止侵蚀反应，对水泥石定期目测观察，并对某些样品进行XRD和DSC分析，证实侵蚀产物为水化碳硫硅酸钙，Ca(OH)2参与侵蚀反应，揭示了矿渣微粉提高石灰石硅盐盐水泥耐硫酸盐溶液侵蚀的机理。     

Corrosion initiation of reinforced concretes based on Portland or GGBS cements: Chloride contents and electrochemical characterizations versus time

The ingress of chlorides in cylindrical reinforced concretes based on ordinary Portland cement (OPC) or ground granulated blast furnace slag cement (GGBS) has been investigated together with the corrosion behaviour of the steel rebar. Chloride exposure was obtained by wetting and drying cycles during one to two years. The evolution of total and free chloride contents versus time of exposure shows that GGBS concretes induced a delay in chloride ingress. Corrosion initiation of steel was evaluated through nondestructive electrochemical measurements (half cell potential and linear polarization resistance) versus time of exposure. For GGBS concretes, corrosion assessment was not reliable based on the ASTM standard or RILEM recommendations. For OPC concretes, the transition from passive to active corrosion was studied considering a drop of potential or a corrosion current threshold value. Considering this latter, total and free chloride contents larger than 2.2% or 0.7% by weight of cement were estimated.     

Pore solution composition and reinforcement corrosion characteristics of microsilica blended cement concrete

Plain and microsilica blended cement pastes with water-cement ratio of 0.6 were prepared using a 14% C₃A cement. Two levels of chloride from NaCl corresponding to 0.6% and 1.2% by weight of cement were added through mix water. The pastes were allowed to hydrate in sealed containers for 180 days and then subjected to pore solution expression. The expressed pore fluids were analyzed for chloride and hydroxyl ion concentrations. The results show that the OH⁻ ion concentration in the pore solutions of both chloride-free and chloride-bearing pastes drop steeply with increasing cement replacement by microsilica. For 10% microsilica cement pastes the pH for both 0.6% and 1.2% chloride addition was found to be around 13.30. However, the pH drops to a level below that of saturated Ca(OH)₂ solution when cement replacement by microsilica is increased from 10% to 20%. This is ascribable to the consumption of Ca(OH)₂ by microsilica as shown by the DTA/TGA results. 10% and 20% microsilica blending more than doubles the free chloride ion concentration in the pore solutions of the chloride-bearing pastes. 10% microsilica replacement raises the Cl⁻/OH⁻ ratio 4 to 5 fold, whereas for 20% microsilica replacement, the Cl⁻/OH⁻ ratio is increased to 77 and 39 folds over the corresponding values for the plain cement pastes for 0.6% and 1.2% chloride additions respectively. Accelerated corrosion monitoring tests carried out on steel bars embedded in plain and microsilica blended cement concretes exposed to 5% NaCl solution show a 3 fold superior performance of microsilica blended cement concretes in terms of corrosion initiation time. This corrosion behaviour is contrary to the prediction from the increased aggressivity of pore solution composition in terms of highly elevated Cl⁻/OH⁻ ratios. This is attributable to the densification of cement matrix by the pozzolanic reaction between microsilica and calcium hydroxide. No discernable advantage in terms of corrosion initiation time is evident by increasing microsilica blending from 10% to 20%.     

Synthesis, characterization and corrosion protection properties of poly(N-(methacryloyloxymethyl) benzotriazole-co-glycidylmethacrylate) coatings on mild steel

The synthesis of copolymers from different feed ratios of N-(methacryloyloxymethyl) benzotriazole (MMBT) and glycidyl methacrylate (GMA) was achieved by using free radical solution polymerization technique and characterized using FT-IR and ¹³C NMR spectroscopy. The thermal stability of the synthesized copolymers was studied using thermogravimetric analysis (TGA). The corrosion performances of mild steel specimens dip coated with different composition of copolymers were investigated in 0.1 M HCl using potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) method. The polarization and impedance measurements showed different corrosion protection efficiency with change in composition of the copolymers. It was found that the corrosion protection properties are owing to the barrier effect of the polymer layer covered on the mild steel surfaces. However, it was observed that the copolymer obtained from 1:1 mole ratio of MMBT and GMA exhibited better protection efficiency than other combinations.     

Electrical conductivity and rheological properties of ordinary Portland cement-silica fume and calcium hydroxide-silica fume pastes

Two groups of solids mixtures were prepared: (i) the first group includes four mixes having different ordinary Portland cement/silica fume (OPC/SF) weight ratios and (ii) the second group consists of four blends having different Ca(OH)₂/SF molar ratios. Electrical conductivity measurements were carried out on the pastes of the first group mixes using two initial water-to-solid (W/S) ratios of 0.55 and 0.70 by weight; while the W/S ratios used for the second group mixes were 1.00 and 1.20 by weight. The measurements were done at 25 and 45 °C for each paste during setting and hardening processes after gauging with deionized water. Rheological properties were studied at room temperature for all mixes using various W/S ratios. The results obtained indicate clearly the effect of SF and W/S ratio on the rheological properties and electrical conductivity of all pastes under investigation. The relation between the electrical conductivity and rheological properties for different mixes were discussed based on the chemical nature and physical state of the hydration products formed at early ages of hydration.     

磷渣对水泥浆体水化性能和孔结构的影响

通过对水泥浆体凝结性能、水化放热、力学性能和孔结构的测定,以及扫描电镜分析和差热-热重分析,研究了不同掺量磷渣对水泥浆体水化性能和微观结构的影响.结果表明:随着磷渣掺量的增加,浆体的凝结时间延长,水化热减少,早期抗压强度下降.但掺磷渣水泥浆体的后期抗压强度已接近或超过了纯水泥浆体的,磷渣掺量的增加对水泥浆体的后期抗压强度影响不显著.浆体中的Ca(OH)2量随龄期的延长而增加并随磷渣掺量的增加而降低.磷渣的活性效应和填充效应的发挥有效地改善了浆体水化后期的微观结构和孔结构,从而使浆体的力学性能有所提高.     

碱-磷渣-粉煤灰胶凝材料的性能与硬化浆体结构

为充分利用磷渣和粉煤灰两种工业废渣生产高性能胶凝材料,研究了不同磷渣/粉煤灰配合比的碱-磷渣-粉煤灰胶凝材料性能,并用扫描电子显微镜和压汞仪分析了硬化浆体的细观结构和孔结构.结果表明:碱-磷渣-粉煤灰胶凝材料的凝结时间正常,在粉煤灰掺量为0～30 %(质量分数)范围内,随粉煤灰的掺量的增加,碱-磷渣-粉煤灰胶凝材料的凝结时间略有延长.与普通硅酸盐水泥相比,碱-磷渣胶凝材料的抗压强度较高,其3d和28d抗压强度分别可达到30.9MPa和98.8MPa,但其抗折强度相对较低.掺加粉煤灰后碱胶凝材料的抗压强度降低,而抗折强度提高.碱-磷渣-粉煤灰胶凝材料的耐蚀性和抗冻性能均显著优于硅酸盐水泥,其干缩比硅酸盐水泥的大.用部分粉煤灰取代磷渣粉可一定程度减小干缩.碱-磷渣-粉煤灰胶凝材料硬化浆体的结构非常致密,其孔隙率和平均孔径均小于普通硅酸盐水泥硬化浆体.     

Retardation of Cs+ and Cl− Diffusion Using Blended Cement Admixtures

Diffusion of cesium chloride through thin plates of hardened cement pastes was studied. Blast-furnace slag and condensed silica fume were used as blending admixtures in an attempt to retard the diffusion of cesium and chloride ions. The curing and diffusion temperatures were varied from 27° to 60°C, and the water/solid ratio was varied from 0.30 to 0.40. Results indicate that the cesium ion diffuses more slowly than the chloride ion in hardened cement paste systems. Blending admixtures caused a further reduction in diffusivity for both ions, which is important for preventing corrosion or restricting radionuclide transport.     

Estimation of the degree of hydration of blended cement pastes by a scanning electron microscope point-counting procedure

A scanning electron microscope (SEM) point-counting technique was employed to study the hydration of plain portland and blended cement pastes containing fly ash or slag. For plain portland cement pastes, the results for the degree of cement hydration obtained by the SEM point-counting technique were consistent with the results from the traditional loss-on-ignition (LOI) of nonevaporable water-content measurements; agreement was within ±10%. The standard deviation in the determination of the degree of cement hydration via point counting ranged from ±1.5% to ±1.8% (one operator, one sample). For the blended cement pastes, it is the first time that the degree of hydration of cement in blended systems has been studied directly. The standard deviation for the degree of hydration of cement in the blended cement pastes ranged from ±1.4% to ±2.2%. Additionally, the degrees of reaction of the mineral admixtures (MAs) were also measured. The standard deviation for the degree of fly ash reaction was ±4.6% to ±5.0% and ±3.6% to ±4.3% for slag. All of the analyses suggest that the SEM point-counting technique can be a reliable and effective analysis tool for use in studies of the hydration of blended cement pastes.