Fly Ash Mortars Research Articles

Alkali–aggregate reaction in activated fly ash systems

Certain aspects of the durability of a new cementitious material, alkali activated fly ash, are addressed in this article; specifically, a series of findings relating to the alkali-silica reaction are reported. The approach adopted in the study was to compare the new cementitious systems to analogous Portland cement mortars using aggregates of differing reactivity and a procedure based on the test described in ASTM standard C 1260. The results of SEM/EDX and XRD analysis of the materials showed that activated fly ash mortars performed better than the Portland cement equivalents.

알카리 활성화에 의한 플라이애쉬 모르타르의 강도 발현 및 경화 메커니즘

The discharge of fly ash that is produced by coal-fired electric power plants is rapidly increasing in Korea. The utilization of fly ash in the raw materials would contribute to the elimination of an environmental problem and to the development of new high-performance materials. So it is needed to study the binder obtained by chemically activation of pozzolanic materials by means of a substitute for the cement. Fly ash consists of a glass phase. As it is produced from high temperature, it is a chemically stable material. Fly ash mostly consists of , and it assumes the form of an oxide in the inside of fly ash. Because this reaction has not broken out by itself, it is need to supply it with additional through alkali activators. Alkali activators were used for supplying it with additional . This paper concentrated on the strength development according to the kind of chemical activators, the curing temperature, the heat curing time. Also, according to scanning electron microscopy and X-Ray diffraction, the main reaction product in the alkali activated fly ash mortar is Zeolite of type.

Efficiency of fly ash in cementitious materials

A quantitative understanding of the efficiency of fly ash as a mineral admixture in cement-based materials is essential for its effective utilisation. The present paper is directed towards a specific understanding of the efficiency of fly ash in cementitious materials systems by considering the independency of fly ash and its dependency on the characteristics of the cementitious materials system. A new method of quantitatively evaluating the strength effect of fly ash is proposed, in which two parameters, the strength-effect index and the strengthening factor, are employed to study strength development in fly ash mortars and concretes and to further analyse the influences of water to binder ratio (w/b) and the replacement amount of fly ash on strength. Results indicate that the strength effect of fly ash in mortar systems is different from that in concrete; the strength effect of fly ash varies with both amount of fly ash and w/b. Furthermore, in a concrete system, two different optimal w/b ratios are used to maximise the strength-effect index and the strengthening factor of fly ash, respectively. An optimum amount of fly ash exists for an optimal unit strength-effect index in concrete. It is shown that the method presented in this paper is reasonable and effective in assessing the efficiency of fly ash. This information will strengthen the effective utilisation of fly ash in cementitious materials and design of fly ash concrete.

Composition and microstructure of alkali activated fly ash binder: Effect of the activator

The alkali activation of fly ashes is a chemical process by which the glassy component of these powdered materials is transformed into very well-compacted cement. In the present work the relationship between the mineralogical and microstructural characteristics of alkaline activated fly ash mortars (activated with NaOH, Na 2CO 3 and waterglass solutions) and its mechanical properties has been established. The results of the investigation show that in all cases (whatever the activator used) the main reaction product formed is an alkaline aluminosilicate gel, with low-ordered crystalline structure. This product is responsible for the excellent mechanical-cementitious properties of the activated fly ash. However the microstructure as well as the Si/Al and Na/Al ratios of the aluminosilicate gel change as a function of the activator type used in the system. As a secondary reaction product some zeolites are formed. The nature and composition of these zeolites also depend on the type of activator used.

Packing effect and pozzolanic reaction of fly ash in mortar

This research is to study the effect of particle size of fly ash on packing effect and pozzolanic reaction of mortar when 20% of fly ash is used to replace Portland cement type I. Both effects can be determined by using fly ash and insoluble material which have almost the same particle size to replace Portland cement type I. Normally, the compressive strength of fly ash mortar is contributed from hydration reaction, packing effect, and pozzolanic reaction. For mortar mixed with insoluble material, the compressive strength is due to hydration reaction and packing effect. Thus, compressive strength due to pozzolanic reaction can be determined from the difference in compressive strength between fly ash mortar and insoluble material mortar. The results show that the strength activity index of fly ash mortar depends on the median particle size of fly ash and curing ages of mortar samples. At early ages, the strength activity index of fly ash mortar due to packing effect is higher than that due to pozzolanic reaction. At the ages of 3 to 90 days, the difference in strength activity index due to packing effect of fly ashes with median particle size of 2.7 and 160 μm is almost constant about 22% of the strength of standard mortar (STD). The differences in strength activity index due to pozzolanic reaction of fly ashes with median particle size of 2.7 and 160 μm are 3%, 20%, and 27%, respectively, at the ages of 3, 28, and 90 days.

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.

A study of disposed fly ash from landfill to replace Portland cement

The landfills of fly ash are the problem of all power plants because this disposed fly ash is not used in any work. This research studies the potential of using disposed fly ashes which have disposal time of 6–24 months from the landfill of Mae Moh power plants in Thailand to replace Portland cement type I. Median particle sizes of disposed fly ashes between 55.4 and 99.3 μm were ground to reduce the sizes to about 7.1–8.4 μm. Both original and ground disposed fly ashes were investigated on physical and chemical properties. Compressive strengths of disposed fly ash mortars were determined when Portland cement type I was replaced by disposed fly ashes at the rate of 10%, 20%, and 30% by weight of cementitious material (Portland cement type I and disposed fly ash). The results presented that most particles of original disposed fly ashes were solid and sphere with some irregular shape while those of ground disposed fly ashes were solid and irregular shape. CaO and LOI contents of disposed fly ashes with different disposal times had high variation. The compressive strengths of original disposed fly ash mortars were low but those of ground disposed fly ash mortars at the age of 7 days were higher than 75% of the standard mortar and increased to be higher than 100% after 60 days. From the results, it could be concluded that ground disposed fly ashes were excellent pozzolanic materials and could be used as a partial replacement of cement in concrete, even though they were exposed to the weather for 24 months.

ASR Potentials of Glass Aggregates in Water-Glass Activated Fly Ash and Portland Cement Mortars

The purpose of the present study was to use two solid wastes—fly ash and mixed color waste glass—as constituents in concrete materials. The fly ash with water glass as activation chemical was used as a binder in the concrete, and the waste glass particles were used as aggregate to replace nature aggregate. The present study focused on the potential of alkali-silica reaction (ASR) of glass aggregates used in portland cement mortar and in water-glass activated fly ash (WAFA) mortar. The ASR potential was examined using the accelerated testing method, ASTM C-1260. The results showed that with 10% substitution of natural aggregate by glass particles, the ASR expansion of portland cement mortar depends on the size of glass aggregate. The ASR expansion of WAFA mortar does not depend on the sizes of glass aggregate but on the content of glass aggregate. The ASR expansion of WAFA mortar was below the critical ASR expansion up to 100% of replacement by glass aggregate. The color of glass does not affect the ASR potential of WAFA mortar.

A new way to increase the long-term bond strength of new-to-old concrete by the use of fly ash

The short-term and long-term bond strengths of new-to-old concrete were experimentally investigated with an emphasis on the influence of new concretes and binders. These new concretes included ordinary Portland cement concrete, expansive concrete and high-volume fly ash concrete, while the binders included pure cement paste (C-binder), expansive binder (E-binder) and fly ash mortar (F-binder). The results showed that the short-term bond strength of all specimens with fly ash concrete was lower than that with ordinary Portland cement concrete, which in turn was lower than that with expansive concrete. The bond strength of the specimens with F-binder was the lowest at the age of 7 days. However, the long-term bond strength of all specimens with added fly ash was the highest and strength losses were observed in the specimens repaired with expansive concrete or E-binder at the age of 3 years. The microstructure of the transition zone with F-binder was also studied by using both scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) at the ages of 28 days and 1 year, respectively.

Strength and microstructural characteristics of chemically activated fly ash–cement systems

The use of fly ash as a cement replacement material increases the long-term strength and durability of concrete. Despite these great benefits, the use of fly ash is limited due to the low early strength of fly ash concrete. To eliminate this problem, many studies have been conducted on accelerating the pozzolanic properties of fly ash. The study reported below investigated the strength and microstructural characteristics of fly ash–cement systems containing three kinds of activators—Na2SO4, K2SO4, and triethanolamine—to accelerate the early strength of fly ash mortars. Through the use of thermal gravity analysis, it was demonstrated that the activators not only decreased or maintained the amount of Ca(OH)2 products, but also increased the production of ettringite at early ages. X-ray diffraction, scanning electron microcopy, and mercury intrusion porosimetry also confirmed that in the early curing stages of fly ash–cement pastes containing activators, large amounts of ettringite were formed, resulting in a reduction in the pore size ranging from 0.01 to 5 μm. The research results support the supposition that the addition of small amounts of activators is a viable solution for increasing the early-age compressive strength of fly ash concrete.

Long term mechanical strength behaviour in fly ash/Portland cement mortars prepared using processed ashes

AbstractIt is well‐known that some treatments of fly ash may improve its reactivity and consequently the strength of mortars containing these processed ashes. In this paper the effects of treatments on fly ashes are compared: sieving, grinding, and magnetic extractions from aqueous suspensions. The influence of these processed ashes on compressive strength development of mortars containing processed fly ashes for long curing periods was studied. The gain or loss of compressive strength with respect to mortar that contains original ash has been calculated; most mortars present positive values, although for the longest curing times (more than 90 weeks), these values tend towards unity except for ground fly ash mortars. The grinding treatment has proved to be an effective treatment for improving the compressive strength of mortars containing fly ashes over long curing times. The high activity index values versus curing time for mortars containing 60% replacement demonstrate that for high fly ash volume mortars the processing of original ash has very favourable effect on the strength. Moreover, the morphology of the cement/fly ash hydrated matrix of old mortars was analysed by SEM and also thermogravimetric methods have been applied for evaluation of lime content and hydrates. For old mortars containing a high volume of ground fly ashes (60% replacement) residual lime due to hydration of Portland cement has not been detected, suggesting that the optimum replacement in order to produce the maximum quantity of pozzolanic reaction products would be lower than 60%.© 2002 Society of Chemical Industry

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.

Mechanical treatment of fly ashes: Part IV. Strength development of ground fly ash-cement mortars cured at different temperatures

The influence of fly ash grinding on the strength development of fly ash/cement mortars cured at different temperatures was studied. A significant increase of compressive strength ( R c) for fly ash mortars was found at early age (3 days) when curing temperature is raised. However, the highest R c values at 28 days' curing time were obtained for fly ash mortars cured at 40°C. The higher reactivity of fly ashes was found for ground fly ashes (GFA) compared to unground fly ash (T0). When R c vs. curing time (in logarithmic scale) is represented, the slope values from regression analysis data obtained for the 20°C curing temperature series and origin ordinate from regression analysis data obtained for the 40°C or 60°C curing temperature series become good parameters for evaluating pozzolanic activity. An equivalence among R c values obtained at curing temperatures of 20°C and 40°C and different curing times was proposed from strength gain data for mortars containing 15%, 30%, 45% and 60% replacing percentages. Finally, the compressive ( R c) and flexural ( R f) strength values for early ages and different curing temperatures were measured. A mathematical model has been proposed for the mechanical properties at early age of mortars containing fly ashes in 15–60% replacement range and cured in 20–80°C range.

Effects of fly ash particle size on strength of portland cement fly ash mortars

Fly ashes do not have the same properties for different size fractions. It can be accepted that the effect of a fly ash on mortar strength is a combined effect of its size fractions. Therefore, it was concluded that by separating the size fractions and replacing cement with them, the combined bulk effect of a fly ash on strength can be better analysed. In this study, different size fractions of fly ash were used to replace cement partially in standard compressive strength mortars. We attempted to interpret the strength of Portland cement-fly ash mortars in terms of the chemical, minerological, morphological, and physical properties of different fly ash size fractions used. Strengths of the mortars were compared at 2, 7, 28, and 90 days. Also strength of mortars with all-in ash (original ash containing all the fractions) were estimated by using strength of mortars with size fractions and the suitability of this estimation was discussed.

97/03802 Microstructural alterations in fly ash mortars: study on phenomena affecting particle and pore size

97/03802 Microstructural alterations in fly ash mortars: study on phenomena affecting particle and pore size

97/02800 Comparisons among magnetic and nonmagnetic fly ash fractions: Strength development of cement fly ash mortars

97/02800 Comparisons among magnetic and nonmagnetic fly ash fractions: Strength development of cement fly ash mortars

Microstructural alterations in fly ash mortars: Study on phenomena affecting particle and pore size

In the last years, relation among porosity, permeability and durability of mortar has been investigated, and a direct relation among them has been verified. However, these properties (mainly in the case of fly ashes) are influenced by vibration and stirring processes to which cementitious mass is subjected during its elaboration (mixing and moulding). These processes could provoke the breaking of fly ash plenospheres. This fact modifies the behaviour of mortars and concretes with this industrial by-product. In the present work a laboratory study is carried out. Fly ash granulometric variations provoked by vibration and stirring processes and their influence on mortar properties is shown.

Effects of Curing Conditions and Age on Chloride Permeability of Fly Ash Mortar

Effects of fly ash type and content on the permeability characteristics of mortar materials subjected to two different curing conditions and two ages of testing were studied. Three different fly ash contents and three different fly ash types were examined in order to provide sufficient data for statistical analysis of results. Fly ash was observed to be capable of reducing the permeability of mortar, except for the Class C fly ash at lower cement substitution levels. Selection of fly ash type, the level of cement substitution with fly ash, curing conditions, and age of testing had important effects on the permeability characteristics of fly ash mortar; the interactions between these factors were also generally important. Results showed that as the moist-curing duration increased the fly ash mortar became less permeable. The interruption of moist-curing and exposure to a low-humidity environment led to increased permeability, except for the Class F fly ashes after longer moist-curing durations, for which permeability stayed almost constant. One may attribute this effect of air-drying to the formation of shrinkage microcracks, with particularly adverse effects on permeability that more than compensate for any positive effects of the slow progress of hydration or pozzolanic reaction in an environment with low humidity. In conclusion, assessment of mortar permeability based on a 28-day testing age after an initial moist-curing can be misleading.

Influence of mineral admixtures on expansive cement mortars

The influence of mineral admixtures on the properties of expansive cement mortars was studied. Fly ash, condensed silica fume, and rice hull ash were used aspartial replacements (15% by wt.) for ASTM Type K expensive cement in mortars with a water to cementitious materials ratio of 0.5. Properties measured included compressive strength, rapid chloride permeability (AASHTO T 227), restrained expansion, and drying shrinkage. X-ray diffraction analysis was performed on similar pastes to monitor the relative amounts of calcium hydroxide and ettringite at various ages. In addition, the effects of expansive cement storage were investigated. The results indicate that when expansive cement was replaced by any of the three mineral admixtures, there was a significant decrease in chloride permeability though the total expansion was slightly reduced. The mortars containing silica fume or rice hull ash showed a marked increase in compressive strength, whereas the fly ash mortar showed a lower strength than the reference mortar, especially at early ages. There was also a significant reduction in restrained expansion of mortars when the cement was stored for a period of one year.

Curing requirements of silica fume and fly ash mortars

The curing requirements of silica fume and fly ash mortars were investigated in this study. Silica fume and fly ash mortar specimens were moist cured for periods of 0, 3, 7, 14 and 28 days. After each of the five periods, the moist curing was interrupted by oven-drying the specimens at a temperature of 110°C for 3 days. The specimens were later tested for compressive strength and absorptivity. In this study, it was also determined whether the losses in strength and impermeability of silica fume and fly ash mortars due to an interruption in curing could be regained by recuring. The test results clearly indicate that the curing requirement of silica fume mortar is less than that of plain cement mortar, while in the case of fly ash mortar it is hogher than that of plain cement mortar.