Appl Note 1010

Copyright © XRD.US

Powder Diffraction of Fly Ash

 

Introduction

For crystalline phases of Fly  Ash, quantitative X-ray diffraction [11] would appear to be the most straightforward means for obtaining phase quantification. This would be particularly useful for the fly ash hydration products such as Freidel's salt and stratlingite.

Common Phases

Sample Preparation

Phase Identification

Phase Quantification

Other Application Notes also available:

 

Note No

Title

Author

Link to ApplNote

AN-101 Powder diffraction of Asbestos XRD.US http://www.xrd.us/applnote/asbestos.htm
AN-102 Powder diffraction of Catalysts XRD.US www.xrd.us/applnote/catalysts.htm
AN-103 Powder diffraction of Ceramics XRD.US www.xrd.us/applnote/ceramics.htm
AN-104 Powder diffraction of Chemicals XRD.US www.xrd.us/applnote/chemicals.htm
AN-105 Powder diffraction of Clays and clay minerals XRD.US www.xrd.us/applnote/clays.htm
AN-106 Powder diffraction of Clinker and cement XRD.US http://www.xrd.us/applnote/clinker and cement.htm
AN-107 Powder diffraction of Composite materials XRD.US http://www.xrd.us/applnote/composite materials.htm
AN-108 Powder diffraction of Corrosion products XRD.US http://www.xrd.us/applnote/corrosion products.htm
AN-109 Powder diffraction for Environmental Appl XRD.US http://www.xrd.us/applnote/environmental.htm
AN-1010 Powder diffraction of Fly ash XRD.US www.xrd.us/applnote/fly ash.htm
AN-1011 Powder diffraction of Forensics XRD.US http://www.xrd.us/applnote/forensics.htm
AN-1012 Powder diffraction of Industrial wastes XRD.US http://www.xrd.us/applnote/industrial wastes.htm
AN-1013 Powder diffraction of Metals and alloys XRD.US http://www.xrd.us/applnote/metals and alloys.htm
AN-1014 Powder diffraction for  Mineral studies XRD.US http://www.xrd.us/applnote/minerals.htm
AN-1015 Powder diffraction for Mining XRD.US http://www.xrd.us/applnote/mining.htm
AN-1016 Powder diffraction of Nanomaterials XRD.US http://www.xrd.us/applnote/nanomaterials.htm
AN-1017 Powder diffraction for Oil drilling XRD.US http://www.xrd.us/applnote/oil drilling.htm
AN-1018 Powder diffraction in Patent process XRD.US http://www.xrd.us/applnote/patent infringment.htm
AN-1019 Powder diffraction of Pharmaceuticals XRD.US http://www.xrd.us/applnote/pharmaceuticals.htm
AN-1020 Powder diffraction of Plastics, fibers & textiles XRD.US http://www.xrd.us/applnote/plastic fiber textile.htm
AN-1021 Powder diffraction of Raw materials XRD.US http://www.xrd.us/applnote/raw materials.htm
AN-1022 Powder diffraction of Refractory materials XRD.US http://www.xrd.us/applnote/refractory materials.htm
AN-1023 Powder diffraction of Rocks XRD.US http://www.xrd.us/applnote/rocks.htm
AN-1024 Powder diffraction of Semiconductors XRD.US http://www.xrd.us/applnote/semiconductors.htm
AN-1025 Powder diffraction of Soils XRD.US http://www.xrd.us/applnote/soils.htm

 

 

Application of Powder diffraction for study of Fly ash

 

It consists of: 1. ash from minerals and 2. coke from coal

 

 

 

 

 Mostly porous coke or censpheres from coal

Combustion ash residues from coal minerals such as clay, other silicates, carbonates, quartz pyrite.

 

FLY ASH

 Fly ash consists of inorganic matter present in the coal that has been fused during coal combustion. This material is solidified while suspended in the exhaust gases and is collected from the exhaust gases by electrostatic precipitators. Since the particles solidify while suspended in the exhaust gases, fly ash particles are generally spherical in shape (Ferguson et. al., 1999). Fly ash particles those are collected in electrostatic precipitators are usually silt size (0.074 - 0.005 mm).

 


Fly Ash Classification

Fly ash is a pozzolanic material and has been classified into two classes, F and C, based on the chemical composition of the fly ash.  According to ASTM C 618, the chemical requirements to classify any fly ash are shown in Table 3.1.

 

Table 3.1. Chemical Requirements for Fly Ash Classification

Properties

Fly Ash Class

 

 

Class F

Class C

 

Silicon dioxide (SiO2) plus aluminum oxide (Al2O3) plus iron oxide (Fe2O3), min, %

70.0

50.0

 

Sulfur trioxide (SO3), max, %

5.0

5.0

 

Moisture Content, max, %

3.0

3.0

 

Loss on ignition, max, %

6.0*

6.0

 

* The use of class F fly ash containing up to 12% loss of ignition may be approved by the user if acceptable performance results are available

 
 

Class F fly ash is produced from burning anthracite and bituminous coals. This fly ash has siliceous or siliceous and aluminous material, which itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperature to form cementitious compounds (Chu et. al., 1993). Class C fly ash is produced normally from lignite and sub-bituminous coals and usually contains significant amount of Calcium Hydroxide (CaO) or lime (Cockrell et. al., 1970). This class of fly ash, in addition to having pozzolanic properties, also has some cementitious properties (ASTM C 618-99).

Color is one of the important physical properties of fly ash in terms of estimating the lime content qualitatively. It is suggested that lighter color indicate the presence of high calcium oxide and darker colors suggest high organic content (Cockrell et. al., 1970).

 


Fly Ash Chemistry

Chemical constituents of fly ash mainly depend on the chemical composition of the coal. However, fly ash that are produced from the same source and which have very similar chemical composition, can have significantly different ash mineralogies depending on the coal combustion technology used. Because of this, the ash hydration properties as well as the leaching characteristic can vary significantly between generating facilities.

The amount of crystalline material versus glassy phase material depends largely on the combustion and glassification process used at a particular power plant. When the maximum temperature of the combustion process is above approximately 12000 C and the cooling time is short, the ash produced is mostly glassy phase material (McCarthy et. al., 1987). Where boiler design or operation allows a more gradual cooling of the ash particles, crystalline phase calcium compounds are formed.

The relative proportion of the spherical glassy phase and crystalline materials, the size distribution of the ash, the chemical nature of glass phase, the type of crystalline material, and the nature and the percentage of unburned carbon are the factors that can affect the hydration and leaching properties of fly ash (Roy et. al., 1985).  The primary factors that influence the mineralogy of a coal fly ash are (Baker, 1987):

1.     Chemical composition of the coal

2.     Coal combustion process including coal pulvarization, combustion, flue gas clean up, and fly ash collection operations

3.     Additives used, including oil additives for flame stabilization and corrosion control additives.

The minerals present in the coal dictates the elemental composition of the fly ash. But the mineralogy and crystallinity of the ash is dictated by the boiler design and operation.

 


Hydration of Fly Ash

Formation of cementitious material by the reaction of free lime (CaO) with the pozzolans (AlO3, SiO2, Fe2O3) in the presence of water is known as hydration. The hydrated calcium silicate gel or calcium aluminate gel (cementitious material) can bind inert material together. For class C fly ash, the calcium oxide (lime) of the fly ash can react with the siliceous and aluminous materials (pozzolans) of the fly ash itself. Since the lime content of class F fly ash is relatively low, addition of lime is necessary for hydration reaction with the pozzolans of the fly ash. For lime stabilization of soils, pozzolanic reactions depend on the siliceous and aluminous materials provided by the soil. The pozzolanic reactions are as follows:

  Ca(OH)2  => Ca++  +  2[OH]-

 Ca++  +  2[OH]-  +  SiO2   =>     CSH

                                            (silica)           (gel)

 Ca++  +  2[OH]- +  Al2O3   =>    CAH

 (alumina)         (gel)

          

Hydration of tricalcium aluminate in the ash provides one of the primary cementitious products in many ashes. The rapid rate at which hydration of the tricalcium aluminate occurs results in the rapid set of these materials, and is the reason why delays in compaction result in lower strengths of the stabilized materials.

The hydration chemistry of fly ash is very complex in nature. So the stabilization application must be based on the physical properties of the ash treated stabilized soil and cannot be predicted based on the chemical composition of the fly ash.

 


Leaching from Fly Ash

The total metals content for a specific ash source depends on the composition of the coal. The potential for leaching of these metals not only depends on the total metals content but also influenced by the crystallinity of the fly ash, as this would dictate whether the metals are incorporated within the glasseous phase or within crystalline compounds, which will hydrate (ACAA). The metals in the glasseous phase are expected to leach at much lower rate than that from the crystalline phase.

Since the degree of crystallinity is a function of boiler design and remains relatively constant for a given source, leachable materials remain relatively constant for a given ash source. A number of state regulatory agencies have issued source approval for specific generating facilities after the consistency of these materials had been demonstrated.

For stabilized soil, the leachability of metals not only depends on the property of the fly ash but also the soil that are used for stabilized soil. Some part of these metals leached from the fly ash will be adsorbed on the clay minerals of the soil.

 

References:

  1. Quantitative XRD Analysis of Coal Combustion By-Products by the Rietveld Method. Test Mixtures,” R.S. Winburn, S.L. Lerach, B.R. Jarabek, M. Wisdom, D.G. Grier, and G.J. McCarthy, Adv. X-Ray Anal., 2000, 42, 387.

  2. Rietveld Quantitative X-Ray Diffraction of NIST Fly Ash Standard Reference Materials,” R.S. Winburn, D.G. Grier, G.J. McCarthy, and R.B. Peterson, Powd. Diff., 2000, 15, 163.

  3. “Coal Combustion By-Product Diagenesis,” D.G. Grier, G.J. McCarthy, R.S. Winburn, and R.D. Butler, Proceedings of the 1997 International Ash Utilization Symposium, 1997.

 

 

 

Contact XRD.US


Home  Services  Facilities  Order  Courses  Software  Vendors  ApplNotes  TechNotes  Sitemap  

© www.xrd.us - All rights reserved. This page was last updated on 03/03/2005

Please address all questions related to this site to webmaster

XRD.US    52 Papania Drive, Mahopac, New York 10541  USA. (Ph  845 661 1421. Fax 845 628 8461)