Properties of concrete incorporating fly ash and ground granulated blast-furnace slag

This paper presents a laboratory study on the influence of combination of fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the properties of high-strength concrete. A contrast study was carried out for the concrete (GGFAC) incorporating FA and GGBS, control Portland cement concrete and high-volume FA high-strength concrete (HFAC). Assessments of the concrete mixes were based on short- and long-term performance of concrete. These included compressive strength and resistance to H₂SO₄ attack. The microstructure of the concretes at the age of 7 days and 360 days was also studied by using scanning electron microscope. The results show that the combination of FA and GGBS can improve both short- and long-term properties of concrete, while HFAC requires a relatively longer time to get its beneficial effect.     

The anticorrosive effect of fly ash, slag and a Greek pozzolan in reinforced concrete

The influence of the addition of 15% and 30% fly ash, 15% and 30% of a Greek natural pozzolan and 50% granulated blastfurnace slag to ordinary Portland cement on the corrosion resistance of the reinforcing bars was studied in a program of long-term exposure to seawater. The use of blended cements resulted in a decrease in the corrosion rate, especially after long exposure times. The most effective protection was rendered by the 30% fly ash mix. This performance was related to the chloride content and the chloride binding capacity of the blended cements.     

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.     

Effect of fly ash and slag on concrete: Properties and emission analyses

Recycled concrete is a material with the potential to create a sustainable construction industry. However, recycled concrete presents heterogeneous properties, thereby reducing its applications for some structural purposes and enhancing its application in pavements. This paper provides an insight into a solution in the deformation control for recycled concrete by adding supplementary cementitious materials fly ash and blast furnace slag. Results of this study indicated that the 50% fly ash replacement of Portland cement increased the rupture modulus of the recycled concrete. Conversely, a mixture with over 50% cement replacement by either fly ash or slag or a combination of both exhibited detrimental effect on the compressive strength, rupture modulus, and drying shrinkage. The combined analysis of environmental impacts and mechanical properties of recycled concrete demonstrated the possibility of optimizing the selection of recycled concrete because the best scenario in this study was obtained with the concrete mixture M8 (50% of fly ash+ 100% recycled coarse aggregate).     

Properties of fresh concrete incorporating a high volume of fly ash as partial fine sand replacement

The use of fly ash in concrete is very common nowadays, mainly as a partial replacement for cement. However, the amount actually used in many countries is only between 15 to 25 percent. Disposal of unusable fly ash raises severe ecological problems and is quite expensive, not to mention the difficulty of finding dumping sites. Increased utilization of fly ash is thus, in many countries, in the national interest. A research program was initiated on the utilization of large quantities of fly ash Class F of marginal-quality in concrete as partial fine-sand replacement. The present paper studies the effect of such replacement on the properties of fresh concrete. The mechanical properties of the hardened concrete will be presented in another paper. The workability of most fly-ash mixtures was better than that of the reference mix (without fly ash). The water requirement of the fly-ash mixtures was either the same, or higher by about 9 percent, as compared with the reference mix. The rate and volume of bleeding were rather similar for the fly ash and the reference mixes. However, a significant reduction in bleeding by the fly-ash was found in the concrete mixtures with the chemical admixtures water reducer and retarder and high-range water reducer. Setting was delayed by the fly-ash, but the additional delay, that beyond the effect of the chemical admixture proper, was much less in the mixtures with the water reducer and retarder and negligible in the mixtures with the high-range water reducer.     

Efficiency of fly ash in concrete

Earlier efforts towards an understanding of the efficiency of fly ash in concrete has led to the introduction of rational methods. Based on the results available on some of the more recent pulverised fuel ashes, the authors evaluated the efficiency of fly ash in concrete over a wide range of percentage replacements (15–75%). It was clearly shown that the overall efficiency of fly ash cannot be adequately predicted using a single efficiency factor at all percentages of replacements. The overall efficiency factor (k) has been evaluated at all percentages of replacements considering the general efficiency factor (k_e) and the percentage efficiency factor (k_p). This study resulted in a quantitative assessment of the behaviour of fly ash in concrete, especially for the 28 day compressive strength at different percentages of replacement.     

新疆地区商品混凝土中粉煤灰和矿粉双掺的试验研究

本文根据混凝土设计强度的不断提高和混凝土行业可持续发展的要求,通过对粉煤灰、矿粉的单掺及粉煤灰和矿粉的双掺,对混凝土的性能进行比较研究,双掺混凝土不论是拌合物的和易性,还是混凝土的力学性能、耐久性都优于单掺混凝土。实现混凝士优化配置,并且降低成本,可以采用双掺技术,有利于环保和可持续发展。     

Durability of concrete incorporating high-volume of low-calcium (ASTM Class F) fly ash

Research on structural concrete incorporating high volumes of low-calcium (ASTM Class F) fly ash has been in progress at CANMET since 1985. In this type of concrete, the cement content is kept at about 150 kg/m³. The water-to-cementitious materials ratio is of the order of 0·30, and fly ash varies from 54 to 58% of the total cementitious material. A large dosage of a superplasticizer is used to achieve high workability.

This paper presents data on the durability of this new type of concrete. The durability aspects considered are: freezing and thawing cycling; resistance to chloride ion permeability; and the expansion of concrete specimens when highly reactive aggregates are used in the concrete.

The investigations performed at CANMET indicate that concrete incorporating high volumes of low-calcium fly ash has excellent durability with regard to frost action, has very low permeability to chloride ions and shows no adverse expansion when highly reactive aggregates are incorporated into the concrete.     

Effect of fly ash and slag on the fracture characteristics of high performance concrete

The premature deterioration of concrete structures in aggressive environments has necessitated the development of high performance concrete (HPC). The major difference between conventional concrete and HPC is essentially the use of chemical and mineral admixtures. The improved pore structure of HPC achieved by the use of chemical and mineral admixtures causes densification of paste-aggregate transition zone, which in turn affects the fracture characteristics. Hence, studies were taken up to investigate the effect of fly ash and slag on the fracture characteristics of HPC. Beam specimens (geometrically similar and single size variable notch) with locally available fly ash (25%) and slag (50%) as cement replacement materials were prepared and tested in a servo-controlled Universal Testing Machine (UTM) under displacement control. From the value of the peak load for each beam, various fracture parameters were calculated. The results show that there is a reduction in the fracture energy due to addition of fly ash or slag, which can be attributed to the presence of unhydrated particles of size larger than that of normal flaws in concrete. Also due to densification, the post peak behaviour is steeper for the fly ash or slag based HPC mixes. The results of the investigation are presented in this paper.     

Heat deformations of fly ash concrete

When dealing with concrete resistance to high temperatures it is important for design purposes to know the elastic parameters, such as the temperature–strain curves and the modulus of elasticity.Concretes containing a high volume of fly ash differ from conventional mixes in the cementitious phase. This results in a different behaviour under heating compared to plain Portland cement concretes. To find the elastic response of fly ash concrete four series of concrete mixtures were manufactured: one with cement only, another with 30% by mass partial replacement of cement by fly ash, and two with 30% and 40% by mass replacement of cement by ground fly ash. Tests were carried out on cylinders (150 × 300 mm). A high-calcium fly ash was used.The conditions were selected so that the applied level of stress corresponded to 25% or to 40% of the ultimate compressive strength of concrete, and a transient type of temperature regime was followed. Based on the experiments the critical temperature, the residual deformation and the modulus of elasticity were determined.The results indicate that concretes containing a high volume of fly ash are more sensitive to high temperatures, since they developed greater deformations. The fineness of the fly ash used also seems to influence the degree of deformation in an adverse way.     

The permeability of fly ash concrete

Oxygen permeability tests were carried out on plain ordinary Portland cement (OPC) and fly ash concretes at three nominal strength grades. Prior to testing the concretes were subjected to a wide range of curing and exposure conditions. The results emphasize the importance of adequate curing to achieve concrete of low permeability, especially when the ambient relative humidity is low. In addition, the results demonstrate the considerable benefit that can be achieved by the use of fly ash in concrete. Even under conditions of poor curing, fly ash concrete is significantly less permeable than equal-grade OPC concrete, the differences being more marked for higher-grade concretes. Attempts were made to correlate strength parameters with permeability but it is concluded that neither the strength at the end of curing nor the 28-day strength provides a reliable indicator of concrete permeability. A reliable correlation was established between the water to total cementitious material ratio [w/(c+f)] and the permeability of concretes subjected to a given curing and exposure regime.     

Compressive strength and microstructure of alkali-activated fly ash/slag binders at high temperature

This paper reports the results of the compressive strength and microstructure of various alkali-activated binders at elevated temperatures of 300 and 600 °C. The binders were prepared by alkali-activated low calcium fly ash/ground granulated blast-furnace slag at ratios of 100/0, 50/50, 10/90 and 0/100 wt.%. Specimens free of loading were heated to a pre-fixed temperature by keeping the furnace temperature constant until the specimens reached a steady state. Then the specimen was loaded to failure while hot. XRD, SEM and FTIR techniques were used to investigate the microstructural changes after the thermal exposure. The fly ash-based specimen shows an increase in strength at 600 °C. On the other hand, the slag-based specimen gives the worst high-temperature performance particularly at a temperature of 300 °C as compared to ordinary Portland cement binder. This contrasting behaviour of binders is due to their different binder formulation which gives rise to various phase transformations at elevated temperatures. The effects of these transformations on the compressive strength are discussed on the basis of experimental results.     

Transport properties of self compacting concrete with limestone filler or fly ash

The durability of a cementitious material is greatly influenced by the permeability of the material for potentially aggressive substances. As the pore structure of self compacting concrete (SCC) might be different in comparison with traditional concrete (TC), some changes in durability behaviour may occur. At this moment however, it is unclear how significant these differences will be with regard to the concrete practice. In this paper, the gas and water transport in SCC with limestone filler or fly ash is investigated experimentally. Nine different concrete compositions are considered: one TC and eight SCC mixtures. Some important parameters like the water/cement (W/C) and cement/powder ratio (C/P), type of filler (limestone filler and fly ash), type of aggregate and type of cement are considered. The results of the gas and water transport are discussed and linked to experimental data concerning pore volume. Lower transport properties can be obtained by using fly ash instead of limestone as filler material, by lowering the W/C ratio, decreasing the C/P ratio at a constant W/C ratio or using blast furnace slag cement instead of portland cement. The effect of changing from gravel to crushed limestone is small. SCC is differing strongly of TC with respect to the apparent gas permeability. This difference is probably due to the differences in pore volume, as seen from MIP results.     

Behaviour of concrete-filled steel tubular columns incorporating fly ash

The subject of this work is to investigate the effect of fly ash on the strength of concrete filled steel tubular columns from 28 to 365 days. A contrast study was carried out on concrete filled steel tubular columns incorporating 10–40 wt% fly ash, and for control Portland cement concrete filled steel tubular columns. The effect of pre-coating the inner surface of steel tubes with a thin layer of fly ash was also studied. Assessments of the concrete mixes were based on the compressive strength and the bond strength. The results show that a lower replacement with fly ash can improve both bond strength and compressive strength, while a higher replacement with fly ash requires a relatively longer time to achieve similar beneficial effects. Pre-coating the inner surface of steel tubes with a thin layer of fly ash can notably improve the bond strength. The microstructure of the interface between concrete and steel tube was also studied by using scanning electron microscopy analyzer.     

Mechanical properties of structural concrete incorporating a high volume of Class F fly ash as partial fine sand replacement

A research program was carried out on the utilization of large quantities of Class F fly ash of marginal quality in structural concrete as partial fine sand replacement. The present paper studies the effect of such replacement on the properties of hardened concrete. The effect on the properties of the fresh concrete was presented in another paper [1]. The properties studied were: compressive strength, modulus of elasticity, drying shrinkage and water penetration under pressure. The cement content of the concrete mixtures was maintained constant. Also, the consistency and slump were kept constant by adjusting the amount of the mixing water. The test results show clearly that fly ash of marginal quality, as partial fine sand replacement, has a beneficial effect on the compressive strength of structural concrete, particularly at later ages, and also on the modulus of elasticity. The drying shrinkage of the fly ash mixtures was similar or somewhat lower than that of the reference mix. The maximum penetration depth of water under pressure of the fly ash concrete mixtures was somewhat smaller than that of the reference mix.

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Résumé Un programme de recherches a été entrepris sur l'utilisation de grandes quantités de cendres volantes de qualité marginale (Classe F) en remplacement partiel de sable fin. Cet article étudie les effets de tels remplacements sur les propriétés du béton durci. Les effets sur les propriétés du béton frais ont été présentés dans un autre article [1]. Les propriétés étudiées étaient la résistance à la compression, le module d'élasticité, le retrait de séchage et la pénétration de l'eau sous pression. La teneur en ciment des formules de béton a été maintenue constante. La consistance et l'affaissement ont également été maintenus constants par ajustement de la quantité d'eau. Les résultats montrent clairement que les cendres volantes de qualité marginale, en tant que remplacement partiel de sable fin, ont un effet bénéfique sur la résistance à la compression du béton, en particulier à des ages avancés, ainsi que sur le module d'élasticité. Le retrait de séchage des formules contenant des cendres volantes était égal ou légèrement inférieur à celui du mélange de référence. Dans des formules contenant des cendres volantes, la profondeur maximale de pénétration de l'eau sous pression était légèrement inférieure à celui de la formule de référence.

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Editorial note Dr. D. Ravina works at the Faculty of Civil Engineering and the National Building Research Institute of the Technion Israel Institute of Technology, a RILEM Titular Member. He is a corresponding member of TC 094-CHC on Concrete for Hot Countries.     

Valorization of co-combustion fly ash in concrete production

The purpose of this paper is to compare the effects of two different Supplementary Cementing Materials (SCMs) on mechanical and durability-related properties of structural concrete. Three mixes were produced, where coal and co-combustion fly ashes were used as partial substitute of cement (20% in volume) and compared with a control/reference concrete. Performances investigated included fresh concrete properties, compressive and tensile strength, elastic modulus, permeability, capillarity and drying/wetting resistance. Results indicate that both the SCMs can be classified as low-carbon fly ashes, and their use in concrete improves the workability of the mixes. A slight reduction of mechanical strength was observed for the concretes including both the SCMs. In addition, concrete transport properties were also slightly reduced when co-combustion fly ash was used. Wetting-drying cycles affected significantly the durability of all the mixes: compressive strength after these cycles was significantly lowered, and the cracks occurred due to the thermal stress applied, appeared to be filled by needle-shape crystals of ettringite.     

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.     

粉煤灰陶粒混凝土的抗盐冻性能

为了评价桥面铺装中应用的轻骨料混凝土长期耐久性,利用R ILEM推荐混凝土抗盐冻性能试验标准(CDF)研究了粉煤灰陶粒混凝土与普通混凝土的抗盐冻性能.结果表明:相同强度等级的掺加矿物掺合料的粉煤灰陶粒混凝土的抗盐冻性能明显优于普通混凝土的抗盐冻性能.复掺矿物掺合料混凝土的抗盐冻性能优于单掺矿物掺合料混凝土的抗盐冻性能.与普通混凝土相比,通过SEM-EDXA的分析,掺与未掺矿物掺合料陶粒与水泥石之间的界面过渡区的范围明显变小,这是粉煤灰陶粒混凝土抗除冰盐性能优良的重要原因.