Hydration reactions of cement combinations containing vitrified incinerator fly ash

One treatment option for municipal solid waste incinerator fly ash (IFA) is vitrification. The process yields a material containing reduced levels of trace metals relative to the original ash. The material is glassy and potentially suitable as a cement component in concrete. This paper examines the vitrification of an IFA and studies the hydration reactions of combinations of this vitrified material and Portland cement (PC). Isothermal conduction calorimetry, powder X-ray diffraction (XRD), thermogravimetry (TG) and scanning electron microscopy were employed to study the hydration reactions. As the levels of vitrified ash increase, the quantities of AFt phase produced decrease, whilst quantities of AFm phase increase, due to the reduced levels of sulfate in the vitrified ash. The levels of calcium silicate hydrate (CSH) gel (inferred from estimates of quantities of gel-bound water) remain constant at 28 days regardless of vitrified ash content, indicating that the material is contributing toward the formation of this product.     

Reactions of fly ash with calcium aluminate cement and calcium sulphate

The hydration processes in the ternary system fly ash/calcium aluminate cement/calcium sulphate (FA/CAC/C$) at 20 °C were investigated; six compositions from the ternary system FA/CAC/C$ were selected for this study. The nature of the reaction products in these pastes were analysed by X-ray diffraction (XRD) and infrared spectroscopy (FTIR). At four days reaction time, the main hydration reaction product in these pastes was ettringite and the samples with major initial CAC presented minor ettringite but calcium aluminates hydrates. The amount of ettringite developed in the systems has no direct relation with the initial components.     

Effect of fly ash on autogenous shrinkage

The correlation between autogenous shrinkage and degree of hydration of fly ash was determined with the selective dissolution method. Then, the relationship between the degree of hydration of fly ash and autogenous shrinkage was examined. The results showed that the degree of hydration of fly ash increased as its Blaine surface area increased. The degree of hydration of fly ash increased with time, and autogenous shrinkage increased corresponding to the increase in the degree of hydration of fly ash. Moreover, it was found that the total quantity of Al₂O₃ in cement-fly ash samples affected autogenous shrinkage at early ages, but the long-term influence was very small.     

Studies on blended cement with a large amount of fly ash

The influence of the contents of the clinker, activators and fly ash on the properties of blended cement with high fly ash content was studied. Experimental data from X-ray diffraction and pore size distribution indicated that the main hydration product of the fly ash blended cement was C-S-H gel, ettringite and a small amount of Ca(OH)₂. The volume porosity of the pores with diameter bigger than 0.1 μm was lower than that of the micro pores and gel pores with diameter lower than 0.05 μm. The amount of chemical combined water has increased with the curing age duration, while the content of Ca(OH)₂ has reduced after 7 days.     

Activation of the fly ash pozzolanic reaction by hydrothermal conditions

The effect of hydrothermal treatment on the pozzolanic reaction of two kinds of Spanish fly ashes from coal combustion (ASTM class F) is discussed. Characterization of the compounds formed as a result of hydrothermal treatment and the changes provoked in the starting fly ashes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and infrared (IR) spectroscopy. α-C₂SH, CSH gel, different solid solutions of katoites (the cubic crystallographic variety of hydrogarnets series (C₃ASH₄)) and a mixed oxide (CaFe₂O₄) were formed depending on the kind of fly ash. The hydrated compounds are precursors of a new kind of low-energy cement called fly ash belite cement (FABC); besides, they have potential properties to intercalate toxic ions and therefore can be used as immobilization systems of these ions.     

Nano-crystal glass-ceramics obtained from high alumina coal fly ash

Glass has been obtained by melting high alumina coal fly ash with fluxing additives. A thermal treatment was employed to convert the obtained glass into nano-crystal glass-ceramics. X-ray diffraction (XRD) patterns show that the main crystalline phases in both the glass-ceramics are anorthite (CaAl₂Si₂O₈) and wollastonite (CaSiO₃). The crystals are homogeneously dispersed within the parent glass. The average crystal size is below 200 nm. Physical and mechanical properties, such as density, thermal expansion coefficient, hardness, and bending strength, of the glass have been examined and the corresponding microstructures are discussed. The results demonstrate that the glass-ceramics have potential for a wide range of construction application.     

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.     

The leachability of heavy metals in hardened fly ash cement and cement-solidified fly ash

The effect of mix proportion, leachant pH, curing age, carbonation and specimen making method etc. on the leaching of heavy metals and Cr(VI) in fly ash cement mortars and cement-solidified fly ashes has been investigated. In addition, a method for reducing the leaching of Cr(VI) from cement-solidified fly ashes is proposed. The results mainly indicate that: (1) either Portland cement or fly ash contains a certain amount of heavy and toxic metals, and the leaching of them from hardened fly ash incorporated specimens exists and is increased with fly ash addition and water to cement ratio; (2) the leachability of some heavy metals is greatly dependent on leachant pH; (3) when carbonation of cement mortars occurs the leaching of chromium ions is increased; (4) the amount of heavy metals leached from cement-solidified fly ashes depends more on the kind of fly ash than their contents in fly ash; and (5) with ground granulated blast furnace slag addition, the leaching of Cr(VI) from solidified fly ashes is decreased.     

Influence of a fine glass powder on cement hydration: Comparison to fly ash and modeling the degree of hydration

This paper reports the results of an investigation carried out to understand the influence of a fine glass powder on cement hydration. The pozzolanicity of the glass powder and a Class F fly ash for comparison was evaluated using strength activity index over a period of time, and a rapid electrical conductivity based method. Flame emission spectroscopy and electrical conductivity tests were used to quantify the alkali release from glass powder, and gain information on the rate of alkali release. It was found that the glass powder releases only a very small fraction of sodium ions into the solution. It was observed that the glass powder modified pastes show higher non-evaporable water contents than the plain paste and fly ash modified pastes, indicating that glass powder facilitates enhancement in cement hydration. An expression has been developed for the change in non-evaporable water content as a result of enhancement in cement hydration and the hydration of the cement replacement material. The efficiency of any cement replacement material with age in the paste system can be quantified using this parameter. Based on this parameter, a 5% cement replacement with glass powder was found to be effective at the chosen water-to-cementing materials ratio (w/cm), whereas at higher replacement levels, the dilution effect dominates. A model to predict the combined degree of hydration of cement pastes incorporating more than one cementing material is outlined. The measured and predicted combined degrees of hydration agree well.     

Effects of the incorporation of Municipal Solid Waste Incineration fly ash in cement pastes and mortars: I. Experimental study

This work falls within the scope of a general problem regarding the assessment of concrete manufactured from waste materials. The main objective is to study the long-term evolution of these materials during the leaching process, using the cellular automaton-based hydration model developed at the National Institute of Standards and Technology. The work is based on the analysis of mortars and cement pastes containing experimental waste: Municipal Solid Waste Incineration fly ash (MSWI fly ash). The study therefore aims to develop a methodology for assessing concrete manufactured from waste, and not to study a process or a formulation enabling the incorporation of the waste in concrete. The physical, chemical and mineralogical characteristics of MSWI fly ash were first analysed to introduce them into the model. A simplified quantitative mineralogical composition of the ash was proposed. The performance characteristics (setting times, compressive strengths, shrinkage, etc.) for mortars containing ash were then studied.     

Effect of fly ash on the kinetics of Portland cement hydration at different curing temperatures

This paper describes the effect of fly ash on the hydration kinetics of cement in low water to binder (w/b) fly ash-cement at different curing temperatures. The modified shrinking-core model was used to quantify the kinetic coefficients of the various hydration processes. The results show that the effect of fly ash on the hydration kinetics of cement depends on fly ash replacement ratios and curing temperatures. It was found that, at 20 °C and 35 °C, the fly ash retards the hydration of cement in the early period and accelerates the hydration of cement in the later period. Higher the fly ash replacement ratios lead to stronger effects. However, at 50 °C, the fly ash retards the hydration of the cement at later ages when it is used at high replacement ratios. This is because the pozzolanic reaction of the large volumes of fly ash is strongly accelerated from early in the aging, impeding the hydration of the cement.     

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).     

基于粉煤灰作混合材的水泥粉磨工艺的选择

串联磨技术是近几年发明的比较先进的水泥粉磨技术。在消化吸收串联磨技术的基础上进行工艺改进,利用粉煤灰可以作助磨剂和混合材的两种特性,在生产复合硅酸盐水泥时,将粉煤灰按一定比例分别在一级磨和二级磨中粉磨,使水泥磨产量、出磨水泥质量、粉煤灰掺量都有一定程度的提高。     

Geopolymeric materials prepared using Class F fly ash and elevated temperature curing

This paper reports the results of the study of the influence of elevated temperature curing on phase composition, microstructure and strength development in geopolymer materials prepared using Class F fly ash and sodium silicate and sodium hydroxide solutions. In particular, the effect of storage at room temperature before the application of heat on strength development and phase composition was studied. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and SEM were utilised in this study.Long precuring at room temperature before application of heat was beneficial for strength development in all studied materials, as strength comparable to 1 month of curing at elevated temperature can develop in this case only after 24 h of heat curing. The main product of reaction in the geopolymeric materials was amorphous alkali aluminosilicate gel. However, in the case of sodium hydroxide activator in addition to it, traces of chabazite, Linde Type A, Na-P1 (gismondine) zeolites and hydroxysodalite were also present. The type of zeolite present and composition of aluminosilicate gel were dependent on the curing history.     

Use of zeolite, coal bottom ash and fly ash as replacement materials in cement production

In this research, the effects of zeolite, coal bottom ash and fly ash as Portland cement replacement materials on the properties of cement are investigated through three different combinations of tests. These materials are substituted for Portland cement in different proportions, and physical properties such as setting time, volume expansion, compressive strength and water consistency of the mortar are determined. Then, these physical properties are compared with those of PC 42.5. The results showed that replacement materials have some effects on the mechanical properties of the cement. The inclusion of zeolite up to the level of 15% resulted in an increase in compressive strength at early ages, but resulted in a decrease in compressive strength when used in combination with fly ash. Also, setting time was decreased when zeolite was substituted. The results obtained were compared with Turkish Standards (TS), and it was found that they are above the minimum requirements.     

Pozzolanic properties of reject fly ash in blended cement pastes

Low-grade fly ash (reject fly ash, r-FA), a significant portion of the pulverized fuel ash (PFA) produced from coal-fired power plants and rejected from the ash classifying process, has remained unused due to its high carbon content and large particle size. But it may be used in certain areas, such as in solidification and stabilization processes of hazardous waste and materials for road base or subbase construction, which require relatively lower strength and reactivity. It is therefore necessary to extend research on the properties of r-FA and explore its possible applications. This paper presents experimental results of a study on the mechanical and hydration properties of cementitious materials prepared by blending r-FA with ordinary Portland cement (OPC). Parallel mixes were also prepared with the good ash [i.e., classified fine fly ash (f-FA)] for comparison. Selective chemical activators were added to the mix to study the effects of the activators on the properties of the blend system. The results show that r-FA generally has a lower rate of hydration than f-FA particularly at the early stage of hydration. Adding Ca(OH)₂ alone almost had no effect on accelerating the hydration of r-FA. But adding a small quantity of Na₂SO₄ or K₂SO₄ together with Ca(OH)₂ significantly accelerated the hydration reaction. The results of the compressive strength measurement correlated nicely with the degree of hydration results. It was also found that water-to-binder ratio (w/b) was an important factor in affecting the strength development and the hydration degree of r-FA pastes.     

Fly ash effects: I. The morphological effect of fly ash

The morphological effect is an important part of fly ash effects. The paper analyzes emphatically this effect and points out that it is composed of the filling role, surface role and lubricating role. For different fly ash, these roles are different. They must be considered synthetically when the morphological effect is analyzed. Analyzing result shows that the filling role is relative to the particle size, the surface role is relative to the specific surface area and the water affinity and the lubricating role is relative to the shape of particle. The morphological effect of fly ash is the synthetical embodiment of these roles.     

Ge extraction from gasification fly ash

Water-soluble germanium species (GeS₂, GeS and hexagonal-GeO₂) are generated during coal gasification and retained in fly ash. This fact together with the high market value of this element and the relatively high contents in the fly ashes of the Puertollano Integrated Gasification in Combined Cycle (IGCC) plant directed our research towards the development of an extraction process for this element. Major objectives of this research was to find a low cost and environmentally suitable process. Several water based extraction tests were carried out using different Puertollano IGCC fly ash samples, under different temperatures, water/fly ash ratios, and extraction times. High Ge extraction yields (up to 84%) were obtained at room temperature (25 °C) but also high proportions of other trace elements (impurities) were simultaneously extracted. Increasing the extraction temperature to 50, 90 and 150 °C, Ge extraction yields were kept at similar levels, while reducing the content of impurities, the water/fly ash ratio and extraction time. The experimental data point out the influence of chloride, calcium and sulphide dissolutions on the Ge extraction.     

Hydraulic conductivity of compacted cement-stabilized fly ash

When combined with portland cement and compacted, fly ash is a high-strength material. In some instances, it may also be desirable to control the hydraulic conductivity (k) of the compacted mixture. Therefore, a study was performed to assess the effects of water content (w), cement content, curing time, and compaction effort on the hydraulic conductivity of compacted cement-stabilized fly ash. When compacting relatively dry mixtures (w < 20%), k is independent of compaction effort, and is on the order of 10⁻⁵ cm/s. When compacting between w of 20% and optimum water content (w_opt), compaction effort affects k, and, at a given w, k decreases by about an order of magnitude when increasing from standard to modified proctor effort. When wet of w_opt, k is on the order of 10⁻⁶ cm/s regardless of compaction effort or water content. With respect to curing time, extended curing time has relatively little effect on k within a 60-day time frame. Based on the results of this study, an approach to construction quality assurance testing can be applied to estimate k based on in situ measurement of dry density (ρ_d) and w.