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.     

Sintering characteristics of low-rank coal ashes

The electrical resistance and compressive strength were measured to gain a better understanding of the sintering characteristics of low-rank coal ashes involved in deposit formation in combustion systems. Low-rank coal ashes were prepared by the standard ASTM ashing procedures at 750°C and then separated into three different particle size fractions. The sinter point determined by the electrical resistance method decreased with decreasing particle size at three different particle size fractions of each coal ash. The compressive strength lest was made as a function of temperature in the range 750–950°C. At a given sintering temperature, strength of the sintered ash was inversely proportional to particle size. For any given particle size of each coal ash, the strength increased with increasing sintering temperature. X-ray diffraction of the sintered coal ashes showed that, as sintering temperature increased, there was an inverse relationship between sinter strength and the amount of anhydrite in the sintered ash, and a direct relationship between strength and the amount of hauyne.     

The fluidity of fly ash-cement paste containing naphthalene sulfonate superplasticizer

The zeta potential measurement indicated that the surface potential of fly ash was different from ordinary Portland cement (OPC) in both sign and value. Hence, the Derjaguin-Landau-Verway-Overbeek (DLVO) theory for dispersion-flocculation of heterogeneous particles with different surface potentials was applied to explain the influence of fly ash on the rheology of cement paste containing naphthalene sulfonate superplasticizer. For the fly ash-cement paste without superplasticizer, the sign of zeta potential of fly ash was different from OPC. Thus, the extent of the potential energy barrier between particles was small or even showed negative value, and the change in the rheology of the fly ash-cement paste was mainly dependent on the bulk solid volume of fly ash, which was related to available free water for fluidizing paste. For the fly ash-cement paste with naphthalene sulfonate superplasticizer, fly ash and cement had the same sign and dispersed well due to higher potential barrier. The extent of potential energy barrier depended on the absolute value of surface potential, which was represented by a function of the amount of adsorbed superplasticizer. The bulk solid volume of fly ash also affected the change in flow ability, but the effect of potential energy barrier between particles was superior to that of the bulk solid volume of fly ash.     

Conversion of coal fly ash into zeolite and heavy metal removal characteristics of the products

Fly ash obtained from a power generation plant was used for synthesizing zeolite. Zeolites could be readily synthesized from the glassy combustion residues and showed potential for the removal of heavy metal ions. By the use of different temperatures and NaOH concentration, five different zeolites were obtained: Na-P1, faujasite, hydroxy sodalite, analcime, and cancrinite. The synthesized zeolites had greater adsorption capabilities for heavy metals than the original fly ash and natural zeolites. Na-P1 exhibited the highest adsorption capacity with a maximum value of about 1.29 mmole Pb g^-1 and had a strong affinity for Pb²⁺ ion. The metal ion selectivity of Na-P1 was determined as: Pb²⁺> Cu²⁺> Cd²⁺> Zn²⁺, consistent with the decreasing order of the radius of hydrated metal ion. The adsorption isotherm for lead by Na-P1 fitted the Freundlich rather than the Langmuir isotherm.     

Trapping characteristics of cesium in off-gas stream using fly ash filter

The cesium trapping characteristics with changing reaction temperature, carrier gas and gas velocity by the fly ash filter were analyzed. The SEM (Scanning electron microscope) on the pore structure of the fly ash filter showed that pores up to 0.1 mm in diameter were widely interconnected with each other throughout the whole structure of the filter. According to the XRD (X-ray diffraction) analysis for the cesium compound trapped on the fly ash filter, the thermally stable pollucite phase was formed. The cesium trapping quantity of the fly ash filter was increased with increasing reaction temperature, whereas it was decreased with increasing gas velocity. SEM showed that the fly ash filter after trapping gaseous cesium had mullite phase of needle-like crystals and pollucite phase of bulky crystals with rough surface. Presented at the Int’l Symp. on Chem. Eng. (Cheju, Feb. 8-10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

日本的粉煤灰综合利用

回顾了近年来日本粉煤灰发生量和利用量的推移情况,总结了日本粉煤灰利用领域和途径,举例介绍了为促进粉煤灰的综合利用,日本政府所实行的若干鼓励政策。旨在对中国的粉煤灰综合利用提供一些参考和借鉴。     

Performance characteristics of high-volume Class F fly ash concrete

More than 88 million tonnes of fly ash is generated in India each year. Most of the fly ash is of Class F type. The percentage utilization is around 10 to 15%. To increase its percentage utilization, an extensive investigation was carried out to use it in concrete. This article presents the results of an experimental investigation dealing with concrete incorporating high volumes of Class F fly ash. Portland cement was replaced with three percentages (40%, 45%, and 50%) of Class F fly ash. Tests were performed for fresh concrete properties: slump, air content, unit weight, and temperature. Compressive, splitting tensile, and flexural strengths, modulus of elasticity, and abrasion resistance were determined up to 365 days of testing.Test results indicated that the use of high volumes of Class F fly ash as a partial replacement of cement in concrete decreased its 28-day compressive, splitting tensile, and flexural strengths, modulus of elasticity, and abrasion resistance of the concrete. However, all these strength properties and abrasion resistance showed continuous and significant improvement at the ages of 91 and 365 days, which was most probably due to the pozzolanic reaction of fly ash. Based on the test results, it was concluded that Class F fly ash can be suitably used up to 50% level of cement replacement in concrete for use in precast elements and reinforced cement concrete construction.     

Influence of tragacanth resin on the thermal and mechanical properties of fly ash-cement composites

This study was aimed to search the possibility of usage of the thermal power plants fly ashes, cement and tragacanth composites in concrete or plaster by investigating their thermal insulation characteristics. The fly ash used in the experiments is supplied from Af?in Elbistan Thermal Power Station. Portland cement (KPC 325) with resin is used as binding and 24 specimens are prepared depending on the percentage of fly ash and tragacanth. In all fly ash, tragacanth and binding mixture, the weight percentages of fly ash are taken as 0, 10, 20, 30, 40 and 50%. The amount of the resin in the mixture is 0.5, 1 and 1.5% of the weight of the total cement and fly ash.

24 samples were prepared and tested to find out the effects of resin on thermal and mechanical properties of fly ash and cement composites. Whereas fly ash percentage increased from 0% to 50%, i) thermal conductivity and compressive strength decreased 19.37–28.62% and 7.66–16.55% respectively as the porosities of the samples increased 18.91–28.62% with the effect of artificial pores generated by 1.5% resin other than the pores generated by fly ash. ii) the new produced samples can be used as partition walls, floorings, ceiling concretes, briquettes or bricks and plaster.     

Microanalysis of alkali-activated fly ash-CH pastes

Samples of a Class F fly ash and calcium hydroxide (CH) hydrated in pH 13.2 sodium hydroxide solution were analyzed using backscattered electron, scanning Auger, and X-ray microanalysis. The Class F fly ash, composed mainly of aluminosilicate glass and silica, was reacted for 8, 14, and 78 days at various temperatures. These samples represent both long-term and early-age stages of hydration. Results show that a hydrate product with calcium to silica ratio near 1.4 and katoite are formed. X-ray and scanning Auger microanalysis show evidence of the formation of hydrate product on the surface of both fly ash and CH particles at early ages. This finding suggests a new mechanism to explain prior data that shows that the hydration rates increase with increasing CH-ash content in the starting mixture.     

A simplified model for prediction of pozzolanic characteristics of fly ash, based on chemical composition

The correlation between type and quantity of glassy phase and chemical composition of fly ash has been reviewed. A simplified model based on above has been proposed for assessment of pozzolanic reactivity of fly ash in terms of compressive strength of fly ash cement mortar. The model is fitted for 10%, 20%, 35% and 50% of fly ash replacement and for 28, 91 and 365 days of curing period using a least squares technique. The model is found to predict well for more than 20% fly ash replacement. The correlation coefficient (R²) between predicted and experimental values is maximum for 50% replacement. The model fit for 10% replacement of fly ash is poor.     

Automated foam index test: Quantifying air entraining agent addition and interactions with fly ash-cement admixtures

The use of a new automated foam index test (AFIT) instrument is discussed as a quantitative approach to observe air entraining agent (AEA) interactions with and to measure proper AEA dosage into cement-fly ash mixtures. Based on measuring acoustic emission from bubbles bursting on top of and in water-cement-ash mixtures after AEA addition, AFIT uses computer control to automate steps taken and quantities used during foam index testing, including: water addition; AEA titration; admixture agitation; and acoustic emission data acquisition. Variation of these steps is investigated relative to determining the dynamics of AEA adsorption onto the solid surfaces within water-cement-ash mixtures. Working within time durations needed for AEA equilibration, different water dilution and titration levels of AEA helped elucidate influences of equilibration on the AFIT-determined foam index values. A mathematical discussion about AFIT foam index curves is offered that relates changes in foam stability to surface tension and interactions with free calcia as AEA's are titrated into ash and cement-ash mixtures. Correlations between experimental surface tension data and the foam index curves are also presented. The potential of applying the AFIT to control air content in concrete with and without fly ash is also examined relative to using the C231 ASTM testing procedure on concrete mixes.     

碱渣和粉煤灰复合的强度特征

碱渣对粉煤灰的活性没有明显激发作用 ,各龄期的强度均随碱渣 /粉煤灰的增大而降低 ;加入半水石膏后 ,其强度随石膏掺量的增加而降低。     

The effect of porosity on the strength of foamed concrete

A study has been undertaken to investigate the effects of replacing large volumes of cement on the properties of foamed concrete (up to 75% by weight) with both classified and unclassified fly ash. This is the third paper in a series; it investigates the relationship between porosity and compressive strength and presents mathematical models that have been developed to describe this relationship. The compressive strength of the foamed concrete was shown to be a function of porosity and age, and a multiplicative model (such as the equation derived by Balshin) was found to best fit the results at all ages up to 1 year. In addition, it was concluded that the equation derived by Hoff could effectively be used to predict the compressive strength of foamed concrete mixtures containing high percentages of ash.     

Characterization of fly ash-pastes synthesized at different activator conditions

Compressive strength and mineralogical and microstructural characterization of the pastes prepared from alkali-activation of low calcium fly ash were investigated. XRD and FTIR were used to observe the mineralogical characterization; SEM, MIP, and NMR were used to observe the microstructural characterization of the pastes. The higher strength of paste was attributed to a more compact and continuous gelatinous matrix due to restructuring of reaction product. The lower values of both porosity (31.0%) and mean pore diameter (9.7 nm) of fly ash-pastes alkali-activated at 85 °C for 24 hr indicate higher reactivity of these pastes compared with other pastes, and result in the higher compressive strength.     

Ash content for optimum strength of foamed concrete

A study has been undertaken into the properties of foamed concrete in which large volumes of cement (up to 75 wt.%) have been replaced with both classified and unclassified fly ash. This fourth paper in the series examines the effects of increasing the levels of ash on the compressive strength of the concrete. Twenty-seven different mixtures were cast with varying ash contents and densities, and the results used to develop models that have been used to predict the compressive strength of the foamed concrete. The output from the models predicts an optimum ash content for maximum strength at a given porosity and age and shows that the optimum value increases with increasing age.     

An experimental study on strength development of concrete containing fly ash and optimum usage of fly ash in concrete

This paper presents a laboratory study on the strength development of concrete containing fly ash and optimum use of fly ash in concrete. Fly ash was added according to the partial replacement method in mixtures. A total of 28 mixtures with different mix designs were prepared. 4 of them were prepared as control mixtures with 250, 300, 350, and 400 kg/m³ cement content in order to calculate the Bolomey and Feret coefficients (K_B, K_F). Four groups of mixtures were prepared, each group containing six mix designs and using the cement content of one of the control mixture as the base for the mix design. In each group 20% of the cement content of the control mixture was removed, resulting in starting mixtures with 200, 240, 280, and 320 kg/m³ cement content. Fly ash in the amount of approximately 15%, 25%, 33%, 42%, 50%, and 58% of the rest of the cement content was added as partial cement replacement. All specimens were moist cured for 28 and 180 days before compressive strength testing. The efficiency and the maximum content of fly ash that gives the maximum compressive strength were obtained by using Bolomey and Feret strength equations. Hence, the maximum amount of usable fly ash amount with the optimum efficiency was determined.This study showed that strength increases with increasing amount of fly ash up to an optimum value, beyond which strength starts to decrease with further addition of fly ash. The optimum value of fly ash for the four test groups is about 40% of cement. Fly ash/cement ratio is an important factor determining the efficiency of fly ash.     

High-percentage replacement of cement with fly ash for reinforced concrete pipe

Fly ash is commonly used as a substitute for cement within concrete in various applications. Manufacturers of reinforced concrete products commonly limit the quantity of fly ash used to 25% or less by weight. Test cylinders with varying percentages of Class C (25-65%) and Class F (25-75%) fly ash and a water-reducing admixture (WRA) were created under field manufacturing conditions and tested for 7-day compressive strength. Seven-day compressive strength for the concrete/fly ash/WRA was found to be highest when the concrete mix included approximately 35% Class C or 25% Class F fly ash. However, substitution ratios of up to 65% Class C or 40% Class F fly ash for cement met or exceeded American Society for Testing and Materials (ASTM) strength requirements for manufacture of Class I, II and III reinforced concrete pipe (RCP).     

The effect of ionic contaminants on the early-age properties of alkali-activated fly ash-based cements

Alkali-activated fly ash-based cements are concrete binders that utilise fly ash as their major solid raw material. The solid particles are activated using concentrated silicate and hydroxide solution to produce high-strength products. Due to the highly alkaline nature of the solution, precipitation of the reactive species, both from the solids and from the solution, proceeds at a very fast rate. This renders short setting times, which can be advantageous or disadvantageous depending on the practical situation. The present work examines the effects of inorganic salt addition towards the setting and rheological characteristics of the early pastes. Compressive strength, Fourier transform infrared spectroscopy (FTIR) and X-ray diffractograms were collected to examine the hardened products. It was found that calcium (Ca) and magnesium (Mg) salts shortened the setting time by providing heterogeneous nucleation centers in the initial paste solution. Potassium salts retarded setting only to the cements, which used less sodium silicate in the initial solution for activation. Managed ionic contamination can be used to increase the product early strength. However, its long-term effects still need to be identified.