Effectiveness of corrosion inhibiting admixture combinations in structural concrete

A long-term corrosion study was conducted to determine the effectiveness of calcium nitrite, silica fume, fly ash, ground granulated blast furnace slag, and DSS in reducing corrosion of reinforcing steel in concrete. Mixture proportions included single, double, and triple combinations of these admixtures. Non-cracked and pre-cracked slab specimens were evaluated by visual inspections, macrocell readings, half-cell potentials, and autopsies. Triple combinations of calcium nitrite, silica fume, and either fly ash or ground granulated blast furnace slag, as well as a double combination of calcium nitrite and ground granulated blast furnace slag, performed very well and are recommended in concrete mixtures exposed to severe corrosive environments. DSS outperformed the other admixtures for corrosion prevention in this study; however, it resulted in somewhat lower compressive strengths and had not been fully tested for effects on other concrete properties.     

矿物掺合料对聚羧酸减水剂与水泥相容性的影响

高性能减水剂与水泥适应性差会导致混凝土流动性和坍落度损失过快,矿物掺合料将影响高性能减水剂与水泥的相容性。对比研究矿物掺合料种类和掺量对水泥净浆、砂浆和混凝土流动性的影响;采用TOC法测试了矿物掺合料对聚羧酸减水剂吸附量的影响;分析了矿物掺合料影响聚羧酸减水剂与水泥相容性的机理。结果表明,粉煤灰和矿渣对提高水泥净浆流动性具有一定的叠加效应,可用胶砂减水率的加权平均值进行量化;硅灰对水泥浆体流动性的不利影响远大于粉煤灰和矿渣的辅助减水分散作用,不利于改善聚羧酸减水剂与水泥的相容性;粉煤灰和矿渣增加聚羧酸减水剂在水泥体系中的吸附量;粉煤灰和矿渣对聚羧酸减水剂在混凝土中的减水分散效果有改善作用但不显著。     

Effects of mineral admixtures on fresh and hardened properties of self-compacting concretes: binary,ternary and quaternary systems

The paper presented herein investigates the effects of using supplementary cementitious materials in binary, ternary, and quaternary blends on the fresh and hardened properties of self-compacting concretes (SCCs). A total of 22 concrete mixtures were designed having a constant water/binder ratio of 0.32 and total binder content of 550 kg/m³. The control mixture contained only portland cement (PC) as the binder while the remaining mixtures incorporated binary, ternary, and quaternary cementitious blends of PC, fly ash (FA), ground granulated blast furnace slag (GGBFS), and silica fume (SF). After mixing, the fresh properties of the concretes were tested for slump flow time, L-box height ratio, V-funnel flow time, setting time, and viscosity. Moreover, compressive strength, ultrasonic pulse velocity, and electrical resistivity of the hardened concretes were measured. Test results have revealed that incorporating the mineral admixtures improved the fresh properties and rheology of the concrete mixtures. The compressive strength and electrical resistivity of the concretes with SF and GGBFS were much higher than those of the control concrete.     

Simulation of a temperature rise in concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are among the industrial by-products and have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, compared with Portland cement, the hydration of concrete containing fly ash or slag is much more complex. In this paper, by considering the producing of calcium hydroxide in cement hydration and the consumption of it in the reaction of mineral admixtures, a numerical model is proposed to simulate the hydration of concrete containing fly ash or slag. The heat evolution rate of fly ash or slag blended concrete is determined from the contribution of both cement hydration and the reaction of mineral admixtures. Furthermore, a temperature rise in blended concrete is evaluated based on the degree of hydration of cement and mineral admixtures. The proposed model is verified with experimental data on the concrete with different water-to-cement ratios and mineral admixtures substitution ratios.     

Property investigation of individual phases in cementitious composites containing silica fume and fly ash

In this study, nanoindentation was used to investigate the microstructures of cementitious composites containing silica fume and fly ash. With the help of scanning electron microscope, the mechanical properties (elastic modulus and hardness) of individual phases (like outer product, inner product, calcium hydroxide, remained fly ash particles, residual cement grains) in cementitious composites containing silica fume and fly ash were investigated and analyzed. Additionally, this study examined the differences between the ‘C–S–H’ phases in the different cementitious composites and provided an insight into the influence of mineral admixtures (silica fume and fly ash) on the properties of the ‘C–S–H’ phase.     

The effect of mineral admixtures on the properties of high-performance concrete

The paper presents a laboratory study on the influence of two mineral admixtures, silica fume (SF) and fly ash (FA), on the properties of superplasticised high-performance concrete. Assessment of the concrete mixes was based on short- and long-term testing techniques used for the purpose of designing and controlling the quality of high-performance concrete. These include compressive strength, porosity, oxygen permeability, oxygen diffusion and chloride migration. Measurements were carried out after curing at 20% and 65% relative humidity up to the age of 1 yr. The results, in general, showed that mineral admixtures improved the properties of high-performance concretes, but at different rates depending on the binder type. While SF contributed to both short- and long-term properties of concrete, FA required a relatively longer time to get its beneficial effect. In the long term, both mineral admixtures slightly increased compressive strength by about 10%, but contributed more to the improvement of transport properties of concretes.     

Effect of admixtures on the setting times of high-strength concrete

The effect of silica fume (SF), metakaolin (MK), fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the setting times of high-strength concrete has been investigated using the penetration resistance method (ASTM C 403). In addition, the effect of a shrinkage-reducing admixture (SRA) on the setting times of normal and high-strength concrete was also studied. The setting times of the high-strength concrete were generally retarded when the mineral admixtures replaced part of the cement. While the SRA was found to have negligible effect on the setting times of normal strength concrete, it exhibited a rather significant retarding effect when used in combination with superplasticiser in high-strength concrete. The inclusion of GGBS at replacement levels of 40% and greater resulted in significant retardation in setting times. In general, as replacement levels of the mineral admixtures were increased, there was greater retardation in setting times. However, for the concrete containing MK, this was only observed up to a replacement level of 10%.     

Evaluating and forecasting the initial and final setting times of self-compacting concretes containing mineral admixtures by neural network

An experimental study was conducted to investigate the effects of using binary, ternary, and quaternary cementitious blends of portland cement (PC), fly ash (FA), ground granulated blast furnace slag (GBS), silica fume (SF), and metakaolin (MK) on initial and final setting times of self-compacting concretes (SCCs). For this purpose, a total of 65 SCC mixtures were prepared at two different water binder ratios. Furthermore, based on the experimental results, neural network (NN) model-based explicit formulations were developed to predict the initial and final setting times of SCCs in terms of the amount of concrete constituents, namely mixing water, PC, FA, GBS, SF, MK, fine (fa) and coarse (ca) aggregates, and superplasticizer (SP). The test results have revealed that the mineral admixtures were very effective on the initial and final setting times of SCCs. Besides, it was found that the model developed by using NN seemed to have a high prediction capability of initial and final setting times of SCCs.     

Nanoscale mechanical properties of concrete containing blast furnace slag and fly ash before and after thermal damage

Portland cement blended with waste products such as blast furnace slag and fly ash are frequently used to create more sustainable concrete, but their nanoscale mechanical behavior, particularly after thermal damage, has not been well-studied. Here, nanoindentation experiments confirm that concrete produced with blended cements contains hydration products with nearly identical nanoscale mechanical properties to the hydration products found in concretes produced with ordinary Portland cement. The volume fractions of the hydration products, particularly calcium-silicate-hydrate (C-S-H) phases, are formed in different proportions with the addition of fly ash and blast furnace slag. After exposure to fire damage, the nanoscale behavior of concretes produced with fly ash and slag also matches the nanoscale behavior of conventional concretes. This suggests that any macroscopic differences between fire damage behavior of blended cement concrete and ordinary Portland cement concrete must have origins in a larger length scale.     

炭纤维增强轻质矿粉混凝土的热电行为

炭纤维增强混凝土能用来感知温度,其因在于短炭纤维的P-型传导性引起的塞贝克(Seebeck)效应所致.通过测量添加炭纤维或矿质掺和物(飞灰、硅土粉)前后六种波特兰水泥基混凝土的热电功率,研究了炭纤维增强轻质混凝土热敏的能力及其矿质掺合物对Seebeck效应的影响.结果表明: 炭纤维增强轻质混凝土具有类似于炭纤维增强标准混凝土的Seebeck效应,只是Seebeck系数因掺合了矿粉而减低.掺有矿粉的炭纤维增强轻质混凝土可用作建筑物的热传感器.     

Optimization of ternary cementitious mortar blends using factorial experimental plans

Producing cements incorporating high-volume replacement of ordinary portland cement (OPC) by recycled industrial by-products is perceived as the most promising venture for the cement and concrete industry to meet its environmental obligations. However, the two-component (binary) cements thus produced are often associated with shortcomings such as the need for extended moist-curing, increased use of chemical admixtures, low early age strength, increased cracking tendency due to drying shrinkage, and de-icing salt scaling problems. There is need for research to investigate whether high-volume replacement multi-component (ternary and quaternary) cements could be optimized with synergistic effects allowing component ingredients to compensate for any mutual shortcomings. This study uses factorial experimental plans to investigate the performance of OPC-silica fume (SF)-class F fly ash (FA) and OPC-SF-ground granulated blast furnace slag (GBFS) ternary cementitious blends. Response surfaces for the superplasticizer requirement to achieve a constant flow, setting time, drying shrinkage up to 112 days, compressive strength at 1, 7, 28 and 56 days, and for the sulfate expansion up to 9-months were obtained for up to 20%, 60%, and 60% replacement levels of OPC by SF, FA and GBFS, respectively. A multiparametric optimization is used to establish response surfaces for a desirability function, which is used to rate ternary cementitious blends. Results indicate that when rheological, mechanical, durability and cost requirements are combined; the use of costly mineral admixtures such as silica fume is not economic in ternary OPC-SF-FA or OPC-SF-GBFS blends beyond levels of about 3 to 5% Moreover, it is shown that the major hurdle for high-volume replacement of OPC with class F fly ash is compromising the early age performance. Results also indicate that a good quality high-fineness GBFS can be used at replacement levels of OPC up to 60% without major disadvantages.     

Structure formation of slag ash concrete on the basis of high-calcium fly ash and silica fume

Cementless slag ash concrete may be manufactured using high-calcium fly ash and silica fume as replacements for a binder and a microfiller, and incorporating slag sand from thermal power plants (TPP) as an aggregate. This concrete consists of waste products from TPP (fly ash and slag) and ferro-alloy plants (silica fume) and contains neither natural nor artificial aggregates for lightweight and heavy concretes. Silica fume (10–20% by weight of ash) and hot water together with subsequent heat treatment of concrete products or of castin situ structures binds the excess free calcium oxide present in the ash, and thus prevents deterioration of the concrete. The processes of concrete structure formation were investigated after 24 hours, 28 days, 3 and 6 months and the physico-mechanical, deformation and special properties (frost resistance, heat conductivity, protection of reinforcement from corrosion) were studied. This concrete conforms to the Russian Federation GOST requirement for use in single, two-storey buildings. The cost of the concrete is reduced by a factor of 3 compared with that of ordinary concrete.     

Fiber reinforced dry-mix shotcrete with metakaolin

Both material and fiber rebounds in shotcrete are high, and the use of an appropriate mineral admixture is known to alleviate the problem. However, the exact influence of the particle size gradation and shape on the rebound and the ultimate mechanical properties of shotcrete are poorly understood. In the same context, high reactivity metakaolin (HRM), which has shown a great deal of promise in cast concrete, has not been investigated in shotcrete. This study is divided into two parts: in the first part, the effectiveness of four mineral admixtures – fly ash, silica fume, HRM and carbon black – with varying particle size gradations and shapes was investigated from a rebound reduction point of view. In the second part, HRM and silica fume were compared on the basis of the hardened mechanical properties with special emphasis on flexural toughness in the presence of fiber reinforcement. It was found that so far as rebound reduction is concerned, particle size of the mineral admixture is a more governing factor than its shape. Silica fume, due to its ultra-fine particle size, is superior to HRM in reducing rebound, but a blend of silica fume and HRM achieves better overall properties.     

Use of natural zeolite as a supplementary cementitious material

Natural zeolite, a type of frame-structured hydrated aluminosilicate mineral, is used abundantly as a type of natural pozzolanic material in some regions of the world. In this work, the effectiveness of a locally quarried zeolite in enhancing mechanical and durability properties of concrete is evaluated and is also compared with other pozzolanic admixtures. The experimental tests included three parts: In the first part, the pozzolanic reactivity of natural zeolite and silica fume were examined by a thermogravimetric method. In this case, the results indicated that natural zeolite was not as reactive as silica fume but it showed a good pozzolanic reactivity. In the second part, zeolite and silica fume were substituted for cement in different proportions in concrete mixtures, and several physical and durability tests of concrete were performed. These experimental tests included slump, compressive strength, water absorption, oxygen permeability, chloride diffusion, and electrical resistivity of concrete. Based on these results, the performance of concretes containing different contents of zeolite improved and even were comparable to or better than that of concretes prepared with silica fume replacements in some cases. Finally, a comparative study on effect of zeolite and fly ash on limiting ASR expansion of mortar was performed according to ASTM C 1260 and ASTM C 1567. Expansion tests on mortar prisms showed that zeolite is as effective as fly ash to prevent deleterious expansion due to ASR.     

复合盐浸下多元外掺剂-混凝土抗干湿-冻融循环性能

通过对粉煤灰、硅灰、矿渣、膨胀剂、引气减水剂（Air-Entrained Water Reduce Agent,AEWRA）和水泥基自愈合防水材料（Cementitious Capillary Crystalline Waterproofing Material,CCCWM）等多元外掺剂进行组合搭配掺入混凝土中,设计了5组混凝土配比。分析了复合盐（氯盐、硫酸盐和碳酸盐）浸-干湿-冻融循环等多种因素共同作用下多元外掺剂-混凝土的腐蚀破坏现象、质量损失率、相对动弹性模量衰减规律和抗侵蚀系数变化规律。采用SEM、EDS和XRD,研究了多元外掺剂-混凝土腐蚀的微观结构变化规律。研究结果表明,双掺粉煤灰和硅灰混凝土提高混凝土的抗侵蚀性能作用有限;在双掺粉煤灰和硅灰基础上加入适量的膨胀剂能够较大幅度提高混凝土的抗侵蚀性能,经11次复合盐浸-干湿-冻融循环后,其相对动弹性模量仍然在80%以上,抗侵蚀系数在0.9以上;CCCWM作为一种外掺剂加入混凝土会降低混凝土的耐侵蚀性,经4次复合盐浸-干湿-冻融循环后,相对动弹性模量就降到了60%以下,抗侵蚀系数从1.0降到了0.3。微观机制研究也表明,在复合盐浸-干湿-冻融循环作用下,腐蚀产物钙矾石和方解石共同作用会加速混凝土的腐蚀破坏。     

矿物掺合料早期水化活性的测试和分析

本文采用环境扫描电子显微镜(ESEM)和热重-差热(TG-DTA)分析仪对磨细矿渣微粉、高钙粉煤灰、低钙粉煤灰的早期水化活性进行了系统测试和分析.理论和试验结果分析表明,掺合料取代水泥时,浆体早期抗压强度的提高取决于掺合料自身参与水化反应的速度和水化产物的数量.水化产物在掺合料颗粒表面沉积的速度和浆体中硅酸盐、铝酸盐水化产物的非蒸发水量随掺合料活性的提高而提高.掺合料活性按磨细矿渣微粉、高钙粉煤灰、低钙粉煤灰的顺序降低,将磨细矿渣微粉或高钙粉煤灰与低钙粉煤灰复合,可以克服低钙粉煤灰大掺量取代水泥时混凝土早期强度降低的缺陷,这是提高低钙粉煤灰在高强高性能混凝土中掺量的一个有效措施.     

Effects of different curing regimes on the compressive strength properties of self compacting concrete incorporating fly ash and silica fume

This paper presents the effect of air curing, water curing and steam curing on the compressive strength of Self Compacting Concrete (SCC). For experimental study, SCC is produced with using silica fume (SF) instead of cement by weight, by the ratios of 5%, 10% and 15%, and fly ash (FA) with the ratios of 25%, 40% and 55%. It is observed that mineral admixtures have positive effects on the self settlement properties. The highest compressive strength was observed in the concrete specimens with using 15% SF and for 28 days water curing. Air curing caused compressive strength losses in all groups. Relative strengths of concretes with mineral admixtures were determined higher than concretes without admixtures at steam curing conditions.     

Interdependence of microstructure and strength of structural lightweight aggregate concrete

The demand for lightweight concrete is steadily increasing because of economic and practical considerations. Hence, the inherent internal and external features of lightweight aggregates have been a subject of intense research in recent years. This study provides new insight into the micro-structural and chemical factors which influence the strength properties of structural lightweight aggregate concrete. These are described with respect to four expanded clay lightweight aggregates used in nine concrete compositions containing various types and proportions of dispersing agents such as water-reducing admixtures and superplasticizers, with silica fume and ground granulated blast-furnace slag as optional mineral admixtures. The microstructural characteristics of the paste-aggregate interface and the paste porosity of these concretes are discussed. The methods used include scanning electron microscopy-energy-dispersive X-ray analysis, X-ray diffraction analysis, optical microscopy and compressive strength testing.     

Mechanical properties of ultra-high-performance fiber-reinforced concrete: A review

A comprehensive investigation into the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC), considering various influential factors, is imperative in order to obtain fundamental information for its practical utilization. Therefore, this paper reviewed the early-age strength (or setting) development and mechanical properties of hardened UHPFRC. In connection with the latter, the effects of the curing conditions, coarse aggregate, mineral admixtures, fiber properties, specimen size, and strain-rate on the mechanical performance of UHPFRC were specifically investigated. It was obvious that (1) heat treatment accelerates the hydration process, leading to higher strength; (2) a portion of the silica fume can be replaced by fly ash, slag, and rice husk ash in mechanical perspective; (3) the use of deformed (hooked and twisted) or long straight steel fibers improves the mechanical properties at a static rate; and (4) high rate loading provides a noticeable increase in the mechanical properties. Alternatively, there are some disagreements between the results from various ‘size effect’ tests and the effectiveness of using twisted steel fibers at static and high rate loadings. Further research to reduce the production cost of UHPFRC is also addressed in an attempt to make its widespread use more practical.     

The relationship between chloride migration rate for concrete and electrical current in steady state using the accelerated chloride migration test

In this study the electrochemical technique is applied to accelerate chloride ion migration in concrete to determine the chloride ions in anode cell. This paper presents a new method for determining the chloride migration rate in concrete from steady state migration test by measuring the electrical current. The plain ordinary Portland cement concrete and concrete containing different type of mineral admixtures (fly ash and slag) with w/b ratios of 0.35, 0.45, 0.55, and 0.65 were used.For a given charge passed in steady state, the current corresponding to the given charge passed was correlated with the chloride migration rate. The results for all mixtures show that the chloride migration rate and the current corresponding to a given charge passed in steady state is linearly correlated.