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.     

Use of ternary blends containing silica fume and fly ash to suppress expansion due to alkali-silica reaction in concrete

This paper investigates the effects of cementitious systems containing Portland cement (PC), silica fume (SF) and fly ash (FA) on the expansion due to alkali-silica reaction (ASR). Concrete prisms were prepared and tested in accordance with the Canadian Standards Association (CSA A23.2-14A). Paste samples were cast using the same or similar cementitious materials and proportions that were used in the concrete prism test. Pore solution chemistry and portlandite content of the paste samples are reported. It was found that practical levels of SF with low-, moderate- or high-calcium FA are effective in maintaining the expansion below 0.04% after 2 years. Pore solution chemistry shows that while pastes containing SF yield pore solutions of increasing alkalinity at ages beyond 28 days, pastes containing ternary blends maintain the low alkalinity of the pore solution throughout the testing period (3 years).     

Mechanical performance and microstructure of the calcium carbonate binders produced by carbonating steel slag paste under CO2 curing

Calcium carbonate binders were prepared via carbonating the paste specimens cast with steel slag alone or the steel slag blends incorporating 20% of Portland cement (PC) under CO₂ curing (0.1 MPa gas pressure) for up to 14 d. The carbonate products, mechanical strengths, and microstructures were quantitatively investigated. Results showed that, after accelerated carbonation, the compressive strengths of both steel slag pastes and slag-PC pastes were increased remarkably, being 44.1 and 72.0 MPa respectively after 14 d of CO₂ curing. The longer carbonation duration, the greater quantity of calcium carbonates formed and hence the higher compressive strength gained. The mechanical strength augments were mainly attributed to the formation of calcium carbonate, which caused microstructure densification associated with reducing pore size and pore volume in the carbonated pastes. In addition, the aggregated calcium carbonates exhibited good micromechanical properties with a mean nanoindentation modulus of 38.9 GPa and a mean hardness of 1.79 GPa.     

Effect of temperature on pore solution composition in plain cements

Cement pastes with a water-cement ratio of 0.6 were prepared using three ordinary portland cements with C₃A contents of 2.43, 7.59 and 14%. Three levels of chlorides 0.3, 0.6 and 1.2% by weight of cement, derived from sodium chloride, were added through mix water. The pastes were allowed to cure in sealed containers at 20 and 70°C for 180 days and then subjected to pore solution extraction. The expressed pore solutions were analyzed for chloride and hydroxyl ion concentrations. Results show that increase in temperature from 20 to 70°C increased unbound chlorides and decreased hydroxyl ion concentration of pore solutions for all the three cements. The simultaneous increase in unbound chlorides and decrease in hydroxyl ion concentration drastically increased Cl⁻/OH⁻ ratio of the pore solution, thereby indicating an increase in corrosion risk. This adverse effect of increase in the Cl⁻/OH⁻ ratio of the pore solution with increase in temperature is higher in the high 14% C₃A cement than in the low C₃A cements, and is also higher for the low 0.3% chloride treatment level than the higher chloride inductions. Increase in temperature is also expected to cause an increase in ionic diffusion to steel embedded in concrete as well as in the rate of corrosion reaction. All these factors tend to increase corrosion risk of steel reinforcement in concrete with an increase in temperature.     

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%.     

Effect of shrinkage-reducing admixtures on the properties of alkali-activated slag mortars and pastes

The effect of a shrinkage-reducing admixture (SRA) based on polypropylenglycol on the dimensional stability of waterglass-activated slag mortars was studied. The analysis also showed the effect of the admixture on pore structure of the mortars as well as on the mineralogical composition and microstructure of the alkali-activated slag pastes.The SRA reduced the shrinkage by up to 85 and 50% when the alkali-activated slag mortar specimens were cured at relative humidities of 99 and 50%, respectively. The mechanism primarily involved in shrinkage reduction is the decrease in the surface tension of pore water prompted by the admixture. The SRA also modified the pore structure - under both curing conditions - increasing the percentage of pores with diameters ranging from 1.0 to 0.1 μm. Capillary stress is much lower in these pores than in the smaller capillaries prevailing in mortars prepared without admixtures.Microstructurally, the SRA occasioned a slight increase in the proportion of Si units Q² in the CSH gel and a decrease in the percentage of Al replacing the Si in the gel structure. The admixture did not, however, modify the mineralogical composition of the pastes.Finally, the SRA admixture retarded the alkaline activation of the slag, more intensely at higher admixture dosages. While the admixture did not significantly alter the degree of reaction in pastes cured for 7 days at RH = 99%, the value of this parameter dropped by 7% in the presence of the admixture in pastes cured at 50% relative humidity.     

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.     

高抗蚀胶凝材料的制备及其抗硫酸盐侵蚀性能研究

为了提高在城市污水处理系统服役的混凝土管道的抗硫酸盐侵蚀性能,本文对高抗蚀胶凝材料(HCRC)展开了系列研究。基于正交试验,以抗蚀系数为考核指标,优化矿粉、粉煤灰、硅灰和脱硫石膏替代水泥的比例,获得高抗蚀胶凝材料的最优配合比(HCRC1)。采用模拟污水浸泡法研究了HCRC1的抗硫酸盐侵蚀性能,并利用FTIR、XRD、TG-DSC、压汞法(MIP)和氮气吸附法(BJH)等测试分析了水化产物和孔结构的变化。结果表明,HCRC1由26%水泥、50%矿粉、15%粉煤灰、6%硅灰和3%脱硫石膏(均为质量分数)组成。随着在污水中浸泡时间的增加,试件抗压强度比(K_f)下降,将HCRC1浸泡在污水中100 d后,其K_f值比普通胶凝材料(NC)的高37.94%。此外,微观分析表明浸泡于污水的浆体中的Ca(OH)₂和C-S-H凝胶被腐蚀性离子部分消耗,侵蚀产物主要为石膏,其中HCRC1生成石膏比NC少。同时,与NC浆体相比,HCRC1浆体孔径更为细小,其中有害孔、少害孔向更小孔径转变,这有助于提高其抵抗腐蚀性离子侵蚀的能力。因此,所研制的高抗蚀胶凝材料具有高抗硫酸盐侵蚀性能,可用于混凝土污水管道。     

Hardened Portland blast-furnace slag cement pastes II. The corrosion behavior of steel reinforcement

The corrosion behavior of reinforcing steel embedded in various slag cement pastes was studied using the galvanostatic polarization technique. The corrosion resistance is appreciably affected by the degree of fineness of the dry slag cement. In pastes produced from high Blaine area cement, the behavior of embedded steel was very close to that in normal or type I portland cement paste, and is much better than a low Blaine area cement. W/C ratios of 0.25 and 0.40 provided a better passivating medium as compared with W/C ratios of 0.18 and 0.70. Effects of lime or gypsum addition were also investigated and comparatively studied for their action on the corrosion of embedded steel. The results obtained were supported by corrosion rates obtained using the linear polarization technique.     

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.     

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。     

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.     

Slag Corrosion Resistance of MgO-ZrO2 Refractories

Slag corrosion resistance of MgO ZrO2 refractories was investigated in this work. The results indicate that in a non-oriented electric steel slag system with a high ratio of calcia to silica,the slag resistance of MgO ZrO2 refractories can be described as follows: ZrO2 reacts with CaO forming calcium zirconate compound which strengthens the material and blocks the channel of the slag infiltration; however,in an oriented electric steel slag system with a high concentration of silica and the low ratio of calcia to silica,the slag corrosion resistance of MgO ZrO2 refractories is different; ZrO2 reacts with CaO forming the calcium zirconate and simultaneously one more product C2S as well; C2S can improve corrosion resistance by blinding pore and enhancing slag viscosity; therefore,it is expected to be the major reason for the enhanced corrosion resistance observed for MgO ZrO2 refractories.     

The corrosion resistance of coated steel dowels determined by impedance spectroscopy

The corrosion resistance of coated smooth steel dowels in simulated pore solution with and without 3.5% sodium chloride solution is reported. The dowels are coated with a double-layer structure composed of a 180 μm of Nickel-Chromium-Boron (referred to as NiCrB) that is overlaid by a 20 μm inorganic layer made of silicon powder, silica fume, blast furnace slag, and combinations thereof. The composition of the synthetic pore solution is based on the composition of pore fluids expressed from Type I portland cement that is characterized by pH of about 13.4 and an ionic strength of 0.4.Alternating current impedance spectroscopy measurements conducted in the range of 10 mHz to 10 kHz were found to be instrumental in detecting any change in the resistance of the coatings. It was found that the NiCrB sub-layer provides a good corrosion resistance that outperforms that of the inorganic coating. Of the five inorganic compositions tested, the combination of slag and silica fume has the best performance. The protection against corrosion is attributed mostly to particle packing rather than densification of the matrix due to pozzolanic reaction.     

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.     

Electrochemical behaviour of steel in mortar and in simulated pore solutions: Analogies and differences

On evaluating the corrosion resistance of concrete, it is frequent to perform electrochemical tests in the so called simulated pore solutions (SPS) to replace tests performed in concrete specimens. Besides, to study the effect of the chloride content in concrete, chloride ions are added to the SPS. However, it is not obvious whether the SPS simulate the electrochemical behaviour of steel in concrete. Another concern is related to the relationship between the chloride content in concrete and the chloride content of a SPS. To investigate this issue a comparison between the polarization curves of steel in mortar and in SPS was performed. It was found that the SPS is not fully representative of the corrosion behaviour of steel in mortar but it yields conservative results. The relationship between the chloride content in mortar and in SPS that yields similar behaviour is not straightforward but depends on the electrochemical parameter considered.     

The role of alumina on performance of alkali-activated slag paste exposed to 50 °C

The strength and microstructural evolution of two alkali-activated slags, with distinct alumina content, exposed to 50 °C have been investigated. These two slags are ground-granulated blast furnace slag (containing 13% (wt.) alumina) and phosphorous slag (containing 3% (wt.) alumina). They were hydrated in the presence of a combination of sodium hydroxide and sodium silicate solution at different ratios. The microstructure of the resultant slag pastes was assessed by X-ray diffraction, differential thermogravimetric analysis, and scanning electron microscopy. The results obtained from these techniques reveal the presence of hexagonal hydrates: CAH₁₀ and C₄AH₁₃ in all alkali-activated ground-granulated blast-furnace slag pastes (AAGBS). These hydrates are not observed in pastes formed by alkali-activated ground phosphorous slag (AAGPS). Upon exposure to 50 °C, the aforementioned hydration products of AAGBS pastes convert to C₃AH₆, leading to a rapid deterioration in the strength of the paste. In contrast, no strength loss was detected in AAGPS pastes following exposure to 50 °C.     

Effective monitoring of corrosion in reinforcing steel in concrete constructions by a multifunctional sensor

A novel multifunctional sensor is developed for in situ and non-destructive monitoring of the corrosion current and open circuit potential of reinforcing steel, as well as the pH and Cl⁻ concentration of concrete. The pH and Cl⁻ sensors show good responses to the pH and Cl⁻ concentration of concrete pore solutions, respectively, and are able to monitor both the carbonization process of concrete and the ingress of Cl⁻ in concrete. Combined with measurements of the corrosion potential and corrosion current density, as well as the EIS spectra of reinforcing steel in concrete, this study demonstrates that the pH and the Cl⁻ concentration of concrete are two of the most crucial factors that determine the corrosion of reinforcing steel in concrete. The corrosion tendency and corrosion rate of reinforcing steel largely depend on the chemical environment in the concrete. The multifunctional sensor is a powerful tool for in situ monitoring corrosion of steel in concrete, and provides accurate details of the chemical condition of the concrete pore solution and the corrosion status of the reinforcing steel in concrete. These are essential for corrosion predictions and service life evaluations of concrete constructions.     

复合型阻锈剂阻锈性能的研究

文章采用线形极化法和动电位扫描法电化学测试方法,快速评定了二乙烯三胺分别与钼酸钠和亚硝酸钠复合,对钢筋在含氯例子的混凝土模拟液的阻锈效果。结果表明一定量的钼酸钠与二乙烯三胺复合,能很好的提高单一阻锈剂的阻锈效果。     

Evaluation on steel bar corrosion embedded in antiwashout underwater concrete containing mineral admixtures

This study aims the evaluation of the corrosion of steel bar embedded in antiwashout underwater concrete, which has rather been neglected to date. To that goal, accelerated steel bar corrosion tests have been performed for three series of steel bar-reinforced antiwashout underwater concrete specimens manufactured with different admixtures and under different environments. The three series of antiwashout underwater concrete were: concrete constituted exclusively by ordinary portland cement (OPC), concrete composed by ordinary portland cement mixed with fly-ash in 20% ratio (FA20) and concrete with ground granulated blast furnace slag is mixed in 50% ratio (BFS50). And, the three different environments were: manufacture in the air, in tap water, and in artificial seawater.Measurement results using half-cell potential surveyor showed that, among all the specimens, steel bar in OPC manufactured in artificial seawater was the first one that exceeded the threshold value proposed by ASTM C 876 with a potential value below − 350 mV after 14 cycles. And, the corresponding corrosion current density and concentration of water soluble chloride were measured as 0.3 μA/cm² and 0.258%. On the other hand, for the other specimens that are FA20 and BFS50, potential values below − 350 mV were observed later at 18 and 20 cycles, respectively.Results confirmed the expectation that mineral admixtures may be more effective in delaying the development of steel bar corrosion in antiwashout underwater concrete.