Colorimetric evaluation of admixture adsorption by fly ash for use in air-entrained concrete

A colorimetric absorption test for evaluating fly ash/air-entraining admixture (AEA) behaviour with regard to their use in concrete was investigated. Acid blue 80 (AB80) standard dye sorbate (typically used to examine activated carbon) was considered. Initial experiments established a calibration between the concentration of AB80 in solution and absorbance; and a suitable test procedure to determine the AB80 adsorption (i.e. difference in initial and final concentrations of AB80 solution after exposure to fly ash). In general, a 2.0?g fly ash sample in 100?ml of 100?mg/l AB80 and a contact time of 30?min were satisfactory test conditions. Factors including dye adsorption during filtration and the effect of high/low adsorption fly ashes were examined and methods for dealing with these described. To evaluate the test, 15 fly ashes were considered and their AB80 adsorptions compared with several markers of fly ash/AEA behaviour. This gave reasonable agreement with loss-on-ignition, and very good correlations with specific surface area (measured by N₂ adsorption) and foam index. In general, 1?mg of AB80 was adsorbed per m2 surface area of fly ash. The study also investigated relationships between AB80 adsorption and admixture doses required (using several AEAs) to achieve target air contents in mortar and concrete, which gave very good correlations. The practical implications of the study were considered and it is suggested that the AB80 method has potential both as a test for characterising fly ash and in air-entrained concrete production with the material.     

Air void system in concrete containing circulating fluidized bed combustion fly ash

The increased use of advanced coal-burning technologies for power generation, such as circulating fluidized bed combustion (CFBC), results in new waste products. The potential for using CFBC fly ash in air-entrained concrete was investigated in order to assess the influence of CFBC fly ash on the microstructure of air voids in hardened concrete. A special specimen surface preparation technique for contrasting the image and enabling measurements of air voids size and distribution using an automated image analysis procedure was used. The microstructure of air voids was evaluated on the basis of the total air content, the spacing factor, and the specific surface of air voids. It was found that a satisfactory air void system in concrete could be produced when using CFBC fly ash for partial replacement of cement. The air-void system was characterized by a decreased specific surface of voids and an increased spacing factor.     

The effect of processed fly ashes on the durability and the corrosion of steel rebars embedded in cement–modified fly ash mortars

This paper deals with the study of corrosion level of reinforcing steel bars embedded in Portland cement mortars containing different types of fly ash. Fly ashes used were obtained by physico-chemical treatments of an original F class fly ash to modify their magnetic properties and reduce their particle size. An original fly ash (T0) and three types of modified ashes were tested according to treatment duration and magnetic properties (T60, ground fly ash; TNM, non-magnetic fraction; TM, magnetic fraction). Corrosion tests on reinforced mortar specimens with and without different types of fly ashes, cured at 40 °C, and under accelerated carbonation conditions and seawater immersion, have been performed in order to obtain conclusions on durability. From the corrosion point of view the addition of TNM in mortars showed to be much more effective than addition of the original T0 fly ash.     

Models for strength prediction of foam concrete

There are several strength prediction relations developed for plain cement paste, mortar and concrete. In concrete where air voids contribute significantly to volume of voids (like aerated and foam concrete), more general expressions including the volume of air voids is to be developed as the better alternative. The objective of this paper is to propose prediction relations for the compressive strength of foam concrete by extending two of the well-known relations available for cement paste, mortar and normal concrete, viz., Balshin’s strength-porosity model and Power’s gel-space ratio equation. For this, theoretical equations were derived for porosity and gel-space ratio relating it to the density, proportion of ingredients in the mix and material characteristics like specific gravity. Foam concrete with fly ash showed lesser dependency on pore parameters than cement-sand mixes. As both the prediction relations developed in this study consider the effect of composition on the strength, it can serve as a simple tool for predicting the strength of foam concrete. But strength-porosity model stands out compared to gel-space model as it correlates well with the measured strength and also because of its ease in application since it employs the composition of constituents and easily measurable parameters.     

Fracture mechanics of air-entrained concrete subjected to compression

Air voids are entrained in concrete for protection of constructed elements, especially highway pavements, against freeze-thaw damage. Entrained air void systems inadvertently reduce the compressive strength of the concrete. The present study describes development of an analytical model for evaluation of the effects of entrained air void system on the compressive strength of concrete. The model developed here will assist in predicting the compressive strength of concrete for specified mix designs. The constitutive relationships for air-entrained concrete were established by considering a micro cracked porous material with randomly distributed circular air voids and uniformly oriented cracks from the air voids. Linear elastic fracture mechanics was employed to explain the evolution of damage due to the individual voids and cracks that emanate from such voids. The damage model considers the interactions among the voids and cracks during various stages of loading. The analytical results from this study were evaluated through an experimental program for comparison of the computed and measured compressive strengths. A wide range of samples were examined that included concretes with air contents ranging from 2% to 13% air by volume of concrete. The experiments involved microscopic determination of air content and spacing factors as well as compressive strength tests for all the concrete samples.     

Portland cement-fly ash-silica fume systems in concrete

Laboratory flow, strength, and ultrasnic pulse velocity tests were performed on mortars made with 70% (by weight) of portland cement and 30% of pozzolanic materials where the pozzolanic materials consisted of various combinations of fly ash and silica fume. In addition to these ternary systems, binary blends, such as Portland cement and fly ash, and Portland cement and silica fume, along with 100% Portland cement mortars, were investigated for comparison. The purpose of the investigation, preliminary in nature, was to see under what circumstances, if any, would be a synergistic action when a ternary system of Portland cement-fly ash-silica fume is used in a mortar or concrete.Mortars were made with two cements of type I and two cements of type III along with class F and class C fly ashes. One silica fume was used. Standard flow tests were performed on the fresh mortars, and compressive strength as well as ultrasonic pulse velocity tests were performed with each hardened mortar at various ages up to 28 days. It is expected that the results and conclusions obtained here on mortars will be transferable to concretes.There are several novel, or at least lesser known, results of the investigation. For instance, a new explanation is offered for the plasticizing effect of fly ash which is based on the optimum particle-size distribution concept. Another such result is that ground fly ash produced greater flow increases with type I cement than with type III. A third finding is that the superplasticizer is more effective in increasing the flow as well as strength when the mortars contain fly ash and/or silica fume than in the case of mortars without mineral admixture. Also, it appears that when type I cement is used, the silica fume in the quantity of 5% of the weight of the cement produces relatively greater strength increase in the presence of fly ash than without fly ash.These promising results are preliminary in nature. Therefore, further research is justified with ternary systems in concrete. The presented work is a portion of a larger investigation.     

Research on the influence of the fly ash on the concrete carbonation

The influence of fly ash on the fresh properties, mechanical properties and carbonation properties were studied in this paper. The performance of a kind of curing agent which was applied to the hardened concrete surface was evaluated. Incorporating large volume of fly ash will risk the concrete carbonation. And the curing agent could prevent the concrete carbonation. And the mechanism was explained.     

Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures

This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 °C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO₂ for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)₂ lowered stress from rapid recrystallization after exposure to 600 °C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance.     

Investigating the role of reactive silica in the hydration mechanisms of high-calcium fly ash/cement systems

High-calcium fly ashes (ASTM Class C) are being widely used as a replacement of cement in normal and high strength concrete. In Greece such fly ashes represent the majority of the industrial by-products that possess pozzolanic properties. Even thought the contribution of factors, such as fineness and water/binder ratio, on the performance of fly ash/cement (FC) systems has been a common research topic, little work has been done on examining whether and to what extent reactive silica of fly ashes affects the mechanisms occurring during their hydration.The work presented herein describes a laboratory scale study on the influence of active silica of two high-lime fly ashes on their behavior during hydration. Volumes up to 30% of Greek high-calcium fly ashes, diversified both on their reactive silica content and silicon/calcium oxides ratio, were used to prepare mixes with Portland cement. The new blends were examined in terms of compressive strength, remaining calcium hydroxide, generation of hydration products and microstructural development. It was found that soluble silica of fly ashes holds a predominant role especially after the first month of the hardening process. At this stage, silica is increasingly dissolved in the matrix forming additional cementitious compounds with binding properties, principally a second generation C–S–H. The rate however, that fly ashes react in FC systems seems to be independent of their active silica content, indicating that additional factors such as glass content and fineness should be taken into account for predicting the contribution of fly ashes in the final performance of pozzolanic cementitious systems.     

Use of mineral admixtures to prevent thaumasite formation in limestone cement mortar

Concrete made from limestone cement may exhibit a lack of durability due to the formation of thaumasite. The addition of minerals that improve the concrete durability is expected to slow down the formation of thaumasite. In this work the effect of natural pozzolana, fly ash, ground granulated blastfurnace slag (ggbs) and metakaolin on the thaumasite formation in limestone cement mortar is examined. A limestone cement containing 15% w/w limestone was used. Mortar specimens were prepared by replacing a varying part of the limestone cement with the above minerals. Siliceous and calcareous sand was used in order to study the effect of the sand type on the thaumasite formation. The specimens were immersed in a 1.8% MgSO₄ solution and cured at 5 and 25 °C. The formation of thaumasite was checked and confirmed by visual inspection, strength tests, ultrasonic pulse velocity measurements, XRD and TGA. It is concluded that the use of specific minerals, as partial replacement of cement, inhibits the thaumasite formation in limestone cement mortar.     

High-calcium fly ash as the fourth constituent in concrete: problems,solutions and perspectives

Even though Hellenic high-calcium fly ashes of different origin are widely used by the cement industry for the production of several CEM II types of cements according to EN 197-1, their systematic use in concrete still presents some difficulties. This inhibits the establishment of specifications for their addition. Main problems concerning the quality, are focused on variations in chemical and mineralogical composition, necessity for supplementary grinding, high proportion of free-CaO and periodically high proportion of SO₃ content.These problems as well as the solutions, for every day use by the concrete industry, applied during the construction of a dam, are discussed, in this paper. To overcome these problems, untreated fly ash was cheaply upgraded by grinding at a specially designed ball mill, with simultaneously hydration, for the reduction of free-CaO.Details also (i) for fly ash variations in relation to their origin, (ii) the grinding plant and (iii) the industrial production of fly ash, are given. Finally, in a separate chapter of this paper, aiming to explain the treatment of fly ashes followed during their industrial production, data of the mechanical strength of mixtures of cements incorporating fly ashes with different treatment, concerning their free-CaO and their fineness, are given.     

An explanation for the negative effect of elevated temperature at early ages on the late-age strength of concrete

Elevated curing temperature at early ages usually has a negative effect on the late-age strength of concrete. This article aims to study the mechanism of this phenomenon. The results show that elevated curing temperature at early ages has a negative effect on the late-age strength of hardened cement paste, but it has a greater negative effect on the late-age strength of cement mortar. After elevated temperature curing at early ages, the late hydration of cement is hindered, but the late reaction of fly ash is not influenced. Owing to the continuous reaction of fly ash, the late-age pore structure of cement–fly ash paste under elevated curing temperature is finer than that under standard curing temperature, and the late-age strength of cement–fly ash paste under elevated curing temperature is higher. However, the late-age strength of cement–fly ash mortar under elevated curing temperature is lower. Apparently, there are differences between the effects of elevated curing temperature on hardened paste and mortar. It is the deterioration of transition zone between hardened paste and aggregate that makes the negative effect of elevated curing temperature on the mortar (or concrete) be greater than the hardened paste. As the water-to-binder ratio decreases, the negative effect of elevated curing temperature on the transition zone tends to be less.     

Pore structure of mortars with cellulose ether additions – Mercury intrusion porosimetry study

In this study, multi-cycle mercury intrusion porosimetry (MIP) was employed to study the pore structure of model tile adhesive mortars containing different types (Methyl Cellulose, Hydroxyethyl- and Hydroxypropyl-Methyl Cellulose) and different dosages (0.3%, 0.8%) of cellulose ethers (CE). The two cycles of mercury intrusion and extrusion allowed resolving not only the total porosity of the mortar, but also the interconnected porosity of the matrix. Different application scenarios were tested: application on a substrate with no water absorption (plastic mold), on a moderately absorbing substrate (cement–fiber board) and on a substrate with high absorption (concrete slabs). These conditions allowed for determining the influence of different moisture regimes on the pore structure of the mortars. Finally, tensile adhesion strength tests with different open times were performed. It was observed that the addition of CE did not considerably affect the capillary and gel pores of the matrix. At the same time, it led to a substantial increase of the total porosity due to the entrainment of air, with slightly increasing size of entrained air voids at higher CE addition rates. A very pronounced coarsening of the porosity could be observed when mortars where applied on a substrate with high water absorption. An improvement in tensile adhesion strength respect to the reference mortar was observed thanks to CE addition, in particular for longer open times.     

Corrosion behaviour of rebars in fly ash mortar exposed to carbon dioxide and chlorides

Addition of fly ash has beneficial effects on some mechanical properties of concrete, as well as on the corrosion process induced by the chloride ion. The aim of this study was to investigate the effect of fly ash addition on the corrosion process occurring in reinforced concrete exposed simultaneously to carbon dioxide and chloride. The corrosion process of steel rebars embedded in mortar with 15% and 30% of fly ash was tested under carbon dioxide and sodium chloride contamination. Monitoring of open circuit potential and electrochemical impedance spectroscopy (EIS) were used to follow the corrosion process. Results have shown that under accelerated carbonation fly ash mortar shows higher corrosion rates. The chloride content in mortar exposed to accelerated carbonation increases with the amount of fly ash. However, under natural carbonation it decreases with the addition of fly ash.     

The importance of the network-modifying element content in fly ash as a simple measure to predict its strength potential for alkali-activation

Six different Class F fly ashes were examined to identify the material properties that determine the strength development in geopolymerization. All of the fly ashes displayed typical features of Class F fly ash in chemical and mineral composition, amorphous phase (glass) content, and X-ray diffraction pattern profile; however, the strength developments of the ashes were quite different from each other. The results suggest that the strength is higher under the following configuration: greater content of network modifying elements, greater glass content, lower silicon content, lower intensity of quartz-related ²⁹Si NMR peak, and higher fraction of Al(IV) in the ²⁷Al NMR spectrum. Among the possibilities, the network-modifying elemental content may be the simplest and most accurate measure for strength development of alkali activation of fly ash.     

Effects of fly ash fineness on the mechanical properties of concrete

The present study reviews the effects of fly ash fineness on the compressive and splitting tensile strength of the concretes. A fly ash of lignite origin with Blaine fineness of 2351?cm²/g was ground in a ball mill. As a consequence of the grinding process, fly ashes with fineness of 3849?cm²/g and 5239?cm²/g were obtained. Fly ashes with three different fineness were used instead of cement of 0%, 5%, 10%, and 15% and ten different types of concrete mixture were produced. In the concrete mixtures, the dosage of binder and water/cement ratio were fixed at 350?kg/m³ and 0.50, respectively. Slump values for the concretes were adjusted to be 100 ± 20?mm. Cubic samples were cast with edges of 100?mm. The specimens were cured in water at 20°C. At the end of curing process, compressive and splitting tensile strengths of the concrete samples were determined at 7, 28, 56, 90, 120 and 180?days. It was observed that compressive and splitting tensile strength of the concretes was affected by fineness of fly ash in short-and long-terms. It was found that compressive and tensile strength of the concretes increased as fly ash fineness increased. It was concluded that Blaine fineness value should be above 3849?cm²/g fineness of fly ash to have positive impact on mechanical properties of concrete. The effects of fly ash fineness on the compressive and splitting tensile strength of the concretes were remarkably seen in the fly ash with FAC code with fineness of 5235?cm²/g.     

Frost salt scaling resistance of concrete containing CFBC fly ash

The possibility for using coal combustion by-products in concrete exposed to frost-salt aggression was investigated. The research was aimed to assess an influence addition of circulating fluidized bed combustion (CFBC) fly ash on frost-salt scaling of air-entrained concrete. For evaluation of the resistance of concrete to frost salt scaling the test called “depth sensing indentation” (DSI) was applied. The DSI test method was implemented on a universal testing frame using a standard Vickers indenter. Experimental tests were performed on cement paste specimens and concrete specimens designed with partial replacement of cement with coal combustion by-products. The mass of scaled material in standard frost salt scaling resistance tests on concrete was inversely proportional to the Vickers hardness of the paste containing CFBC fly ash; the best-fit arithmetic relationship is provided.     

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.     

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.     

Effect of magnetic field treated water on mortar and concrete containing fly ash

This research investigates the workability and compressive strength of mortar and concrete, which were mixed with magnetic field treated water (MFTW) and contained fly ash. MFTW was obtained by passing tap water through a magnetic field. Test variables included the magnetic strength of water, fly ash content in place of cement, water-to-cementitious material ratio (W/CM) and curing age.Results show that the compressive strength of mortar samples mixed with MFTW is higher than those prepared with tap water. The best compressive strength increase of concrete is achieved when the magnetic strength of treated water is of 0.8 and 1.2 T. The compressive strength increase of concrete prepared with MFTW is more significant at early age.