Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Chloride-induced corrosion of reinforcement in low-calcium fly ash-based geopolymer concrete

Geopolymer concrete (GPC) has significant potential as a more sustainable alternative for ordinary Portland cement concrete (PCC). However; as a rather new engineering material, there are some concerns over the durability aspects of geopolymer-based binders. In this study, the performance of chloride-contaminated reinforced GPC specimens manufactured using a blended low-calcium fly ash and slag cement is investigated by long-term monitoring of corrosion parameters such as open circuit corrosion potential, polarization resistance and Tafel slopes. The electrochemical results are validated by contrasting the electrochemical mass losses with the mass losses obtained from the gravimetric measurements. The investigated low-calcium fly ash-based GPC exhibit a comparable electrochemical performance to a similar strength PCC during the propagation phase of corrosion. Additionally, some of the conventional classifications which are commonly used to assess the severity of corrosion in Portland cement-based corroding systems might need some recalibration to be used for low-calcium fly ash-based corroding systems.     

粉煤灰掺量对氯氧镁水泥混凝土物理力学性能的影响

为了拓展氯氧镁水泥(MOC)材料的应用领域,以盐湖提钾肥副产物水氯镁石、轻烧氧化镁和粉煤灰为胶凝材料,制备了不同粉煤灰掺量的氯氧镁水泥混凝土(MOCC)。研究了粉煤灰掺量对MOCC抗压强度、物相组成、微观形貌和孔结构的影响。结果表明:随着粉煤灰掺量的增加,MOCC的抗压强度逐渐降低,当粉煤灰掺量为40%(质量分数)时,其300 d抗压强度降低至39.99 MPa,降低了22.52%。MOCC的主要水化产物为5Mg(OH)₂·MgCl₂·8H₂O(5·1·8)和Mg(OH)₂,掺加粉煤灰并没有产生新的晶相。掺入粉煤灰增加了MOCC的孔隙率和有害孔体积,从而降低了其抗压强度。采用相同水灰比制备了普通硅酸盐水泥混凝土,抗压强度对比测试结果表明:掺40%的粉煤灰MOCC的抗压强度虽然比未掺粉煤灰MOCC抗压强度低,但仍比普通硅酸盐水泥混凝土300 d龄期的抗压强度(33.42 MPa)高出19.66%,说明MOCC比普通硅酸盐水泥混凝土具有较高的抗压强度。     

Microstructure and mechanical properties of ternary mortars with brick powder,glass powder,slag, fly ash,and limestone

The objective of this research is to study the effects produced by ternary binders which combine the addition of waste brick powder with fly ash, limestone, ground granulated blast furnace slag or waste glass powder in the microstructure and mechanical properties of mortars. In these ternary binders, the ordinary Portland cement was partially replaced by 10% of waste brick powder and 10% of another of the abovementioned additions. Mortars prepared with ordinary Portland cement without additions were also prepared. The microstructure was characterized with mercury intrusion porosimetry, electrical resistivity, and thermogravimetric analyses. Ultrasonic pulse velocity, compressive and flexural strengths were also determined. Mortars made using ternary binders with two active additions showed higher pore refinement and higher electrical resistivity at 250 days. Furthermore, their compressive strength and ultrasonic pulse velocity were relatively similar or even higher than that noted for reference specimens.     

掺Ⅱ级粉煤灰高性能混凝土在短龄期养护条件下的抗侵蚀性能初讨

按高性能混凝土配比方法配制不同掺量的Ⅱ级粉煤灰胶砂试件,分别与普通水泥胶砂试件和高抗硫酸盐水泥胶砂试件进行抗硫酸盐侵蚀的对比试验。试验结果发现,在短龄期养护条件下,经过高浓度硫酸盐溶液的侵蚀浸泡,不掺粉煤灰的普通胶砂试件和高抗硫水泥胶砂试件的抗蚀系数随侵蚀浸泡时间下降很快,在较短时间内就丧失了抗蚀能力;而掺有Ⅱ级粉煤灰的胶砂试件则维持较高的抗蚀系数,特别当Ⅱ级粉煤灰掺量达到60%时,其抵抗高浓度侵蚀溶液侵蚀的效果显著。     

Properties of high-volume fly ash concrete incorporating nano-SiO2

This paper presents a laboratory study on the properties of high-volume fly ash high-strength concrete incorporating nano-SiO₂ (SHFAC). The results were compared with those of control Portland cement concrete (PCC) and of high-volume fly ash high-strength concrete (HFAC). Assessments of these concrete mixes were based on short- and long-term performance. These included compressive strength and pore size distribution. Significant strength increases of SHFAC compared to the high-volume fly ash high-strength were observed as early as after 3 days curing, and improvements in the pore size distribution of SHFAC were also observed. In this work, the hydration heat of nano-SiO₂ fly ash cement systems was also studied in comparison to the fly ash-cement systems and to the pure cement systems. In addition, the weight change of fly ash incorporating nano-SiO₂, fly ash, and nano-SiO₂ alone after immersed in saturated lime solution was also studied.     

Effect of nano-silica on the chemical durability and mechanical performance of fly ash based geopolymer concrete

In this study, the effect of nano silica on the short term severe durability performance of fly ash based geopolymer concrete (GPC) specimens was investigated. Four types of GPC were produced with two types of low calcium fly ashes (FAI and FAII) with and without nano silica, and ordinary Portland cement concrete (OPC) concrete was also cast for reference. For the geopolymerization process, the alkaline activator has selected a mixture of sodium silicate solution (Na₂SiO₃) and sodium hydroxide solution (NaOH) with a ratio (Na₂SiO₃/ NaOH) of 2.5. Main objectives of the study were to investigate the effect of usability or replaceability of nano silica-based low calcium fly ash based geopolymer concretes instead of OPC concrete in structural applications and make a contribution to standardization process of the fly ash based geopolymer concrete. To achieve the goals, four types of geopolymer and OPC concretes were subjected to sulfuric acid (H₂SO₄), magnesium sulfate (MgSO₄) and seawater (NaCl) solutions with concentrations of 5%, 5%, and 3.5%, respectively. Visual appearances and weight changes of the concretes under chemical environments were utilized for durability aspects. Compressive, splitting tensile and flexural strength tests were also performed on specimens to evaluate the mechanical performance under chemical environments. Results indicated that FAGPC concretes showed superior performance than OPC concrete under chemical attacks due to low calcium content. Amongst the chemical environments, sulfuric acid (H₂SO₄) was found to be the most dangerous environment for all concrete types. In addition, nano silica (NS) addition to FAGPC specimens improved both durability and residual mechanical strength due to the lower porosity and more dense structure. The FAIIGPC specimens including nano silica showed the superior mechanical performance under chemical environment.     

Combined effects of mineral admixtures and curing conditions on the sorptivity coefficient of concrete

The effect of mineral admixture and curing condition on the sorptivity of concrete are investigated. In the present work, the maximum particle size and the grading of coarse aggregate, the cement content and water/cement ratio of the concrete are kept constant. Then, in the ordinary Portland cement (OPC) 42.5 concrete, a portion of the sand is replaced by a mineral admixture such as fly ash (FA), limestone filler, sandstone filler or silica fume (SF). This paper presents the results of both the sorptivity coefficient and the compressive strength of OPC 42.5 concretes with these mineral admixtures, and concretes with OPC 32.5, blended cement (BC) or trass cement (TC). The results obtained indicate that the sorptivity coefficient of concrete decreases as the compressive strength of concrete increases. It is also shown that the sorptivity coefficient of concrete is very sensitive to the curing condition. The effect of curing condition on the sorptivity coefficient of concrete seems to be higher in low-strength concretes.     

Effect of cooling methods on residual compressive strength and cracking behavior of fly ash concretes exposed at elevated temperatures

This paper presents the effects of cooling methods on residual compressive strength and cracking behavior of concretes containing four different class F fly ash contents of 10%, 20%, 30% and 40% as partial replacement of cement at various elevated temperatures. The residual compressive strength of the aforementioned fly ash concretes is measured after being exposed to 200, 400, 600 and 800 °C temperatures and two different cooling methods, for example, slow cooling and rapid water cooling. Results show that the residual compressive strengths of all fly ash concretes decrease with increase in temperatures irrespective of cooling regimes, which is similar to that of ordinary concrete. Generally, control ordinary concrete and all fly ash concretes exhibited between 10% and 35% more reduction in residual compressive strength because of rapid cooling than slow cooling except few cases. Cracks are observed over concrete specimens after being exposed to temperatures ranging from 400 to 800 °C. Samples that are slowly cooled developed smaller cracks than those rapidly cooled. At 800 °C, all fly ash concretes that are exposed to rapid cooling showed the most severe cracking. X‐ray diffraction analysis shows reduction of Ca(OH)₂ peak and formation of new calcium silicate peak in concretes containing 20% and 40% fly ash when subjected to 800 °C in both cooling methods. Thermo gravimetric analysis and differential thermal analysis results show increase in thermal stability of concrete with increase in fly ash contents. The existing Eurocode also predicted the compressive strength of fly ash concretes with reasonable accuracy when subjected to the aforementioned elevated temperatures and cooling methods. Copyright © 2014 John Wiley & Sons, Ltd.     

Effect of fine aggregate replacement with Class F fly ash on the abrasion resistance of concrete

This paper presents the abrasion resistance of concrete proportioned to have four levels of fine aggregate replacement (10%, 20%, 30%, and 40%) with Class F fly ash. A control mixture with ordinary Portland cement was designed to have 28 days compressive strength of 26 MPa. Specimens were subjected to abrasion testing in accordance with Indian Standard Specifications (IS: 1237). Tests were also performed for fresh concrete properties and compressive strength. Tests on compressive strength and abrasion were performed up to 365 days.Test results indicated that abrasion resistance and compressive strength of concrete mixtures increased with the increase in percentage of fine aggregate replacement with fly ash. Abrasion resistance of concrete was improved approximately by 40% over control mixture with 40% replacement of fine aggregate with fly ash, and concrete with fine aggregate replacement could be suitably used.     

Use of ground coarse fly ash as a replacement of condensed silica fume in producing high-strength concrete

This paper presents a method of improving coarse fly ash in order to replace condensed silica fume in making high-strength concrete. The coarse fly ash, having the average median diameter about 90-100 μm, yields a very low pozzolanic reaction and should not be used in concrete. In order to improve its quality, the coarse fly ash was ground until the average particle size was reduced to 3.8 μm. Then, it was used to replace Portland cement type I by weights of 0%, 15%, 25%, 35%, and 50% to produce high-strength concrete. It was found that concrete containing the ground coarse fly ash (FAG) replacement between 15% and 50% can produce high-strength concrete and 25% cement replacement gave the highest compressive strength. In addition, the concrete containing FAG of 15-35% as cement replacement exhibited equal or higher compressive strengths after 60 days than those of condensed silica fume concretes. The results, therefore, suggest that the FAG with high fineness is suitable to use to replace condensed silica fume in producing high-strength concrete.     

Effects of fly ash on the properties of environmentally friendly dam concrete

The utilization of a solid waste – fly ash (FA) in the construction of concrete dams was investigated in this paper, which contained its effects on the strength, shrinkage and expansion strain of dam concrete with and without 8% of a novel MgO-bearing expansive agent. The results are shown a relationship between the content of fly ash replacing cement and the above properties of dam concrete.The compressive strengths of dam concrete with 50% fly ash in 90 d are higher than that of dam concrete with 30% fly ash or without fly ash slightly. Fly ash may decrease the deformation of dam concrete in that with 50% fly ash, and the shrinkage and expansive strain was reduced significantly – about 33% and 40% less than that of the specimens without fly ash respectively.     

Using limestone aggregates and different cements for enhancing resistance of concrete to sulphuric acid attack

A research program was undertaken to improve concrete's resistance against sulphuric acid attack. Six concretes were investigated, four using calcareous limestone aggregates and two using silicious aggregates. Cements used in these concretes included a portland cement, a binary cement containing ground granulated blast furnace slag, and two ternary cements containing slag and silica fume or fly ash and silica fume. All the concretes had the same water/cement ratio of 0.4, with compressive strengths in the range of 45 MPa and 58 MPa at the age of 28 days. In the experiment, concrete cylinders were immersed in 1% sulphuric acid solution and they were periodically examined for appearance, measured for mass change and tested in compression up to 168 days. The concrete using limestone aggregates and the ternary cement containing silica fume and fly ash performed the best.     

The influence of initial water curing on the strength development of ordinary portland and pozzolanic cement concretes

The effect of initial water-curing period on the strength properties of concretes was investigated. Three types of cement, one ordinary Portland cement (OPC) and two natural pozzolanic cements (blended and trass cements), were used in the concrete mixtures. Six different curing regimes were applied to the specimens, the first of which was continuous water storing, and the second continuous air storing. In the remaining four regimes, the specimens were stored under varying initial water-curing periods of 3, 7, 14, and 28 days, respectively. The compressive strength tests were carried out on the cubic specimens at the ages of 7, 14, 28, 90, and 180 days. The variation of compressive strength with time was evaluated by using a semilogarithmic function and the strength-gaining rates were calculated by using this equation for different curing conditions. It was found that poor curing conditions are more adversely effective on the strength of concretes made by pozzolanic cements than that of OPC, and it is necessary to apply water curing to the former concretes at least for the initial 7 days to expose the pozzolanic activity. However, when the pozzolanic cement concretes have sufficient initial curing, they can reach the strength of OPC concretes in reasonable periods of time.     

掺粉煤灰和矿粉对混凝土开裂性能的影响

选用粉煤灰、矿粉作为掺合料以单掺和复掺的形式掺入到硅酸盐水泥混凝土中,检测其坍落度和3d、28d抗压强度以及早期抗开裂性能。研究结果表明:矿粉、粉煤灰均会降低混凝土3d强度,矿粉可以提高混凝土28d强度,粉煤灰掺量过大会降低混凝土28d强度。随着矿粉掺量的增加,混凝土坍落度随之降低,粉煤灰刚加入时会使混凝土坍落度降低,但是随着掺量的升高,坍落度会逐渐增大;开裂方面:单掺时,粉煤灰与矿粉都在掺量为37%时,抗开裂效果最佳,其中粉煤灰效果最好,复掺时,随着矿粉相对掺量的变大,裂缝的面积和数目都在增大。     

等强度条件下粉煤灰对补偿收缩混凝土变形性能的影响

通过测定水中限制膨胀率和水养7d转干空后的限制膨胀率,计算干燥收缩落差,讨论粉煤灰在等强度条件下对补偿收缩混凝土变形性能的影响.结果表明:等强度条件下掺加适量粉煤灰可以明显地促进限制膨胀率的增长;掺量过大容易导致早期约束不足,降低对限制膨胀的促进作用.粉煤灰可以明显降低混凝土转干空后早期的干燥收缩落差.随着粉煤灰掺量增加,干燥收缩落差进一步降低,空气中放置28 d后均表现为膨胀.对于硫铝酸钙-氧化钙类膨胀剂和粉煤灰复掺的补偿收缩混凝土,膨胀剂最小用量的要求可以适当降低.     

Corrosion resistance in activated fly ash mortars

The question of whether reinforcing steel can be protected with activated fly ash cement as effectively as with Portland cement is explored in this study. Corrosion potential (E_corr) and polarisation resistance (R_p) values for steel electrodes embedded in Portland cement mortar and two fly ash mortars, respectively activated with NaOH and waterglass+NaOH solutions, are monitored. Chloride-free activated fly ash mortars are found to passivate steel reinforcement as speedily and effectively as Portland cement mortars, giving no cause to fear that corrosion may limit the durability of reinforced concrete structures built with these new types of activated fly ash cement. The polarisation curves and the response to short-term anodic current pulses (galvanostatic pulse technique) obtained further corroborate the full and stable passivation of the steel by the concrete manufactured with these binders.     

利用超细粉煤灰及钢渣配制用于道路的复合硅酸盐水泥的试验研究

通过大量试验在水泥熟料中复合掺入超细粉煤灰及磨细钢渣粉,配制了用于公路路面水泥混凝土工程的复合硅酸盐水泥,重点改善道路水泥的抗折强度、耐磨性能以及收缩抗裂性能。结果表明,随着超细粉煤灰及磨细钢渣粉的掺入,所配制的水泥胶砂强度及耐磨性均满足425号道路硅酸盐水泥要求,与基准水泥相比,规定龄期的收缩变形均显著降低,圆环法抗裂试验结果也表明水泥抗裂性能得到大幅度增强。     

Influence of natural pozzolan, colemanite ore waste, bottom ash, and fly ash on the properties of Portland cement

This study has examined the effect of natural pozzolan (NP), colemanite ore waste (CW), coal fly ash (FA), and coal bottom ash (BA) on the properties of cement and concrete. The parameters studied included compressive strength, bending strength, volume expansion, and setting time. A number of cements were prepared (in the presence of fixed quantity of 10% FA, 10% BA, and 4% CW) by the replacement of Portland cement (PC) with NP in range of 5-30%. The results showed that the final setting time of cement pastes were generally accelerated when the NP replaced part of the cement. However, NP exhibited a significant retarding effect when used in combination with CW. The results also showed that the inclusion of NP at replacement levels of 5% resulted in an increase in compressive strength of the specimens compared with that of the control concrete. The replacement of PC by 10-15% of NP in the presence of fixed quantity of CW improves the bending strength of the specimens compared with control specimens after 60 days of curing age.     

利用高钙粉煤灰生产膨胀水泥的初步研究

通过对掺加游离氧化钙含量不同的高钙粉煤灰水泥的研究,表明掺入合适的添加物不仅可提高该水泥的强度,而且可调节水泥硬化浆体的膨胀值,使掺高钙粉煤灰水泥的基本性能满足一些膨胀水泥的技术指标,可利用高钙粉煤灰生产膨胀水泥。