Effect of Carbonation on Alkali-Activated Slag Paste

Carbonation on waterglass- and NaOH-activated slag pastes was analyzed and compared with carbonation in Portland cement pastes to determine possible differences. Thermogravimetry-differential thermal analysis (TG/DTA), Fourier-transform infrared spectrometry, and nuclear magnetic resonance were used to determine the effects on the main reaction products. According to the TG/DTA results, carbonate precipitation following carbonation is much more intense in Portland cement pastes than in alkali-activated slag pastes. This may be attributed to the fact that in Portland cement paste both the portlandite and the C–S–H gel can be carbonated, whereas in alkali-activated slag pastes, only the C–S–H gel is carbonated directly. In both systems, carbonation leads to the formation of CaCO₃, Si-rich C–S–H gel, silica gel, and alumina. The carbonation of waterglass-activated slag pastes is not altered by the presence of either of the organic additives used in the study.     

Carbonation in the pore solution of metakaolin-based geopolymer

An experimental approach based on the study of the pore solution of metakaolin-based geopolymer has been established in this study to investigate the phenomenon of carbonation in these materials. The results obtained showed a very fast decrease in the pH, compared to Portland cement, and an almost total carbonation of the pore solution in natural condition after only 14 days. In natural CO₂ conditions, the formation of sodium carbonate did not lead to a decrease of pH below 10.5 at one year, thus limiting the risk of corrosion by depassivation of the reinforcement. Accelerated carbonation performed under 50% of CO₂ highlighted the formation of large amounts of sodium bicarbonate responsible for a lower pH of the pore solution, and a potential risk of corrosion by carbonation. That result also demonstrated that the accelerated carbonation tests made for Portland cement are not representative of the natural reaction in geopolymer cases.     

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.     

Carbonation rates of concretes containing high volume of pozzolanic materials

The project studies the influence of fly ash and slag replacement on the carbonation rate of the concrete. The experimental work includes samples of pure Portland cement concrete (CEM I 42,5 R), blast-furnace slag concrete (CEM III-B), and fly ash blended concrete. To reveal the effect of curing on carbonation rate, the concretes were exposed to various submerged curing periods during their early ages. After that, the samples were subsequently exposed in the climate room controlling 20 °C and 50% RH until the testing date when the samples had an age of 5 months. Then, the accelerated carbonation test controlling the carbon dioxide concentration of 3% by volume, with 65% relative humidity were started to perform. The depth of carbonation can be observed by spraying a phenolphthalein solution on the fresh broken concrete surface. Finally, according to Fick's law of diffusion theoretical equations are proposed as a guild for estimating the carbonation rate of fly ash and blast-furnace slag concretes exposed under natural conditions from the results from accelerated carbonation tests.     

Experimental research and prediction of the effect of chemical and biogenic sulfuric acid on different types of commercially produced concrete sewer pipes

New equipment and procedures for chemical and microbiological tests, simulating biogenic sulfuric acid corrosion in sewerage systems, are presented. Subsequent steps of immersion and drying, combined with mechanical abrasion, were applied to simulate events occurring in sewer systems. Both chemical and microbiological tests showed that the aggregate type had the largest effect on degradation. Concrete with limestone aggregates showed a smaller degradation depth than did the concrete with inert aggregates. The limestone aggregates locally created a buffering environment, protecting the cement paste. This was confirmed by microscopic analysis of the eroded surfaces. The production method of concrete pipes influenced durability through its effect on W/C ratio and water absorption values. In the microbiological tests, HSR Portland cement concrete performed slightly better than did the slag cement concrete. A possible explanation can be a more rapid colonisation by microorganisms of the surface of slag cement samples. A new method for degradation prediction was suggested based on the parameters alkalinity and water absorption (as a measure for concrete porosity).     

The influence of sulphates on chloride binding and pore solution chemistry

Ordinary Portland cement (OPC) and OPC/ground granulated blastfurnace slag (GGBS) 65% cements containing 2.0 to 9.0% sulphates derived from sodium sulphate and calcium sulphate were investigated in respect to their chloride binding properties and the concentrations of chloride and hydroxyl ions in the pore solutions. Chlorides derived from sodium and calcium chlorides were introduced at the time of mixing. The results indicate that calcium sulphate has a different effect on chloride binding and the pore solution chemistry than sodium sulphate. The slag cement has higher chloride binding capacities as a result of simple replacement for OPC, but at the same sulphate contents, the slag cement does not give the expected higher binding capacities, suggesting that the difference in sulphate content between the two cements may be the main reason for their different chloride binding behaviour.     

Effect of lime putty addition on structural and durability properties of concrete

The effect of lime putty addition on main structural and durability properties of concrete was studied. Different types of cement were used for concrete preparation: a Portland cement, a pozzolanic cement and a Portland cement with the addition of 20% fly ash. The measured concrete properties were compressive strength, setting times, length change, porosity, carbonation depth and degree of steel bar corrosion. It was found that the lime putty addition has a positive effect on the properties of concrete that contain pozzolans and a slightly negative effect on the properties of pure Portland cement. This behavior was correlated with the availability of active silica of cementitious materials. The active silica of pozzolans reacts with the added calcium hydroxide giving constituents, which improve the concrete stability and durability.     

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.     

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。     

阿利特高炉矿渣水泥的耐久性能研究

对矿渣掺加比例达到75%的阿利特高炉矿渣水泥耐久性进行了研究,其强度在28d至6个月期间继续增长;胶砂试体湿涨和干缩率略低于硅酸盐水泥样品数值;这种水泥有很高的抗硫酸盐侵蚀能力.     

基于不同胶凝材料钢筋混凝土耐腐蚀性试验研究

针对混凝土结构中钢筋的锈蚀问题,以普通硅酸盐水泥和镁水泥作为胶凝材料,制作了钢筋裸露和涂有环氧树脂涂层的4种钢筋混凝土试件,将其浸泡在氯盐、硫酸盐及镁盐的耦合溶液中,利用电化学方法得到极化曲线和交流阻抗谱,通过塔菲尔外推法进行非线性最小二乘法拟合计算得到腐蚀电流密度、腐蚀电位、腐蚀速率等相关评价指标来衡量混凝土中钢筋锈蚀情况.结果表明:长期浸泡在耦合盐溶液中,普通硅酸盐水泥和镁水泥钢筋混凝土试件都发生了较严重的腐蚀,普通硅酸盐水泥的抗腐蚀性优于镁水泥;在钢筋表面涂有GHT涂层可极大地降低钢筋的腐蚀速率;镁水泥GHT涂层混凝土试件的抗腐蚀性能劣于硅酸盐GHT涂层混凝土试件但优于硅酸盐裸露钢筋混凝土.     

硅酸盐水泥对钢渣活性激发的性能研究

试验研究了在硅酸盐水泥体系中通过碱性激发提高钢渣水化活性的方法.研究表明,钢渣掺入量<30%时,硅酸盐水泥对钢渣的活性激发效果最好;复掺矿渣对钢渣活性的激发效果优于粉煤灰,即使掺量为30%时,其早期强度也与相应龄期普通硅酸盐水泥强度持平,而后期强度逐渐超过纯水泥的强度;在普通硅酸盐水泥体系中掺入钢渣可以改善其硬化浆体的性能.     

Chloride resistance of high-performance concretes subjected to accelerated curing

The strengths and chloride penetration resistance of a series of high-performance concretes were measured after curing either at 23 °C or accelerated by heating to 65 °C. The results confirm that concretes containing silica fume (SF) or ternary blends of SF and ground granulated blast-furnace slag (GGBFS) exhibit improved chloride penetration resistance compared to those of plain Portland cement concretes. In addition, chloride penetration resistance of Portland cement concrete is adversely affected by accelerated curing. With the use of the ternary ordinary Portland cement (OPC)-SF-GGBFS binders, accelerated curing did not have detrimental effects on chloride penetration resistance and provided 18-h strengths in excess of 40 MPa.     

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.     

碳化过程中大水灰比水泥浆体孔结构的变化(英文)

使用特殊的增黏剂与聚羧酸减水剂,制备了掺加石灰石粉、高炉矿渣、硅灰等混合材的普通波特兰水泥浆体和和低热硅酸盐水泥浆体(水粉比为1.0)。这些水泥浆体在20℃的水中养护4年后基本完全水化。这些硬化水泥浆体在5%(质量分数)CO2、相对湿度66%和温度20℃条件下进行碳化,对比研究碳化前后水泥浆体孔结构的变化。结果显示:碳化浆体内孔直径大于10nm的孔体积明显减少;碳化浆体的孔径分布向大孔径范围偏移;掺加混合材的硬化水泥浆体结构明显趋于松散;与不掺加任何混合材的水泥浆体相比,掺加混合材的水泥浆体的孔径更大。     

养护措施和湿养护时间对掺与未掺矿渣混凝土性能的影响

研究了采用湿棉絮覆盖、喷养护剂、塑料薄膜密封3种养护措施对混凝土强度、收缩、中心温升、氯离子渗透性的影响,并通过不同龄期强度、氯离子渗透深度(5%NaCl溶液介质,质量分数)、钢筋腐蚀电位、加速腐蚀保护层开裂时间研究湿养护时间(1,7,14d和28d)对未掺和掺矿渣(等量取代水泥40%,质量分数)混凝土强度、抗渗性、护筋性的影响.结果表明:合适的养护措施能有效提高混凝土强度和抗渗性,降低水化温升和早期收缩.对未掺与掺矿渣的混凝土,更长的湿养护有助发展更高强度、耐久性和护筋性;而不充分养护导致混凝土较差的抗渗性和护筋性,该影响对掺矿渣混凝土尤为明显.7d湿养护对掺矿渣混凝土发展更高的潜在抗渗性是不够的.     

水泥组分对混凝土固化氯离子能力的影响

水泥中C3A的含量被广泛认为是衡量其固化氯离子的参数。经充分分析氯离子固化机理而提出新的衡量水泥固化氯离子的参数是有效铝酸盐AE,即AI2O3总含量减1．25倍的SO3含量;解释了产生碳化和硫酸盐侵蚀加速氯离子诱发钢筋锈蚀这两种现象的原因;强调采用C3A含量高的水泥以降低钢筋锈蚀的危险性不尽合理性;文献的试验结果能很好验证本文提出的新观点,在分析了影响水泥固化氯离子的因素以及决定氯离子诱发钢筋锈蚀的因素基础上,得出采用高性能混凝土是提高混凝土抗氯离子渗入的较佳措施。     

Studies using Al MAS NMR of AFm and AFt phases and the formation of Friedel's salt

This paper describes the application of the magic angle spinning (MAS) NMR spectroscopy to study the chemical environment of ²⁷Al-bearing phases in Portland cement-based concrete. A specific methodology is described that allows reliable spectra to be determined for combinations of different types of cements and fillers (in this case, Portland cement, fly ash, slag, silica fume, metakaolin and limestone filler). As well as the study of ‘molecular structure’ of cement matrix, the paper reviews the mechanism of Friedel's salt formation in cement systems. Mechanisms based on ion exchange of chloride for hydroxide in hydroxy-AF_m and on chloride absorption on formation are discussed. Finally, the nature of the chloride/hydrate binding phenomena are described to provide a reasonable robust and fundamental picture of the role different cements can play in the provision of overall concrete durability to chloride ingress from a chemical perspective.     

粉煤石改善石灰石硅酸盐水泥耐硫酸盐侵蚀的机理

探讨了石灰石硅酸盐水泥在低温和一定浓度MgSO4 溶液中的侵蚀过程。采用粉煤灰部分替代石灰石微粉或水泥来阻止侵蚀反应或延缓反应速度 ,并提高石灰石硅酸盐水泥的强度。对净浆试样定期目测观察 ,并用XRD物相鉴定方法对某些样品进行物相鉴定 ,证实侵蚀物质为水化碳硫硅酸钙 ,同时证实Ca(OH ) 2 参与了侵蚀反应 ,进一步明确了粉煤灰改善石灰石硅酸盐水泥耐硫酸盐溶液侵蚀的机理     

海工高性能混凝土用复合胶凝材料的试验研究

在调查分析海工混凝土工程实例的基础上，试验研究了硅酸盐水泥中掺入矿粉、粉煤灰、硅灰等混合材料对海工混凝土性能的影响。研究结果表明，在硅酸盐水泥中掺加矿粉、粉煤灰、硅灰等混合材料可以改善海工混凝土的综合性能。矿物混合材料的复合掺入比单独掺入能更好地改善混凝土抗Cl^-侵蚀性能。海工专用复合胶凝材料生产时宜尽可能地采用多种混合材料。