Provis / Deventer Geopolymers

Structures, Processing, Properties and Industrial Applications
1. Auflage 2009
ISBN: 978-1-84569-638-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 464 Seiten

Reihe: Woodhead Publishing Series in Civil and Structural Engineering

A geopolymer is a solid aluminosilicate material usually formed by alkali hydroxide or alkali silicate activation of a solid precursor such as coal fly ash, calcined clay and/or metallurgical slag. Today the primary application of geopolymer technology is in the development of reduced-CO2 construction materials as an alternative to Portland-based cements. Geopolymers: structure, processing, properties and industrial applications reviews the latest research on and applications of these highly important materials.Part one discusses the synthesis and characterisation of geopolymers with chapters on topics such as fly ash chemistry and inorganic polymer cements, geopolymer precursor design, nanostructure/microstructure of metakaolin and fly ash geopolymers, and geopolymer synthesis kinetics. Part two reviews the manufacture and properties of geopolymers including accelerated ageing of geopolymers, chemical durability, engineering properties of geopolymer concrete, producing fire and heat-resistant geopolymers, utilisation of mining wastes and thermal properties of geopolymers. Part three covers applications of geopolymers with coverage of topics such as commercialisation of geopolymers for construction, as well as applications in waste management.With its distinguished editors and international team of contributors, Geopolymers: structure, processing, properties and industrial applications is a standard reference for scientists and engineers in industry and the academic sector, including practitioners in the cement and concrete industry as well as those involved in waste reduction and disposal. - Discusses the synthesis and characterisation of geopolymers with chapters covering fly ash chemistry and inorganic polymer cements - Assesses the application and commercialisation of geopolymers with particular focus on applications in waste management - Reviews the latest research on and applications of these highly important materials

Autoren/Hrsg.

Provis, J L

Deventer, J S J van

Leseproben

2 Fly ash glass chemistry and inorganic polymer cements
L.M. Keyte    University of Melbourne, Australia Abstract
In this chapter, the importance of developing a detailed understanding of coal fly ash as the basis for the analysis of inorganic polymer formation is outlined, and many of the factors controlling ash properties are discussed. The properties of fly ash are seen to depend very significantly on the composition of the coal burned, as well as the combustion conditions. The usual compositional ranges in which fly ashes fall display some very complex chemistry in the Al2O3-SiO2 system, which must be considered in analysis of ash-derived inorganic polymers. Key words fly ash glass chemistry inorganic polymer cement coal combustion aluminosilicate 2.1 Introduction
One of the main driving forces for the development of inorganic polymer technology is the potential of a viable alternative to Portland cement. Inorganic polymer cements (IPCs) can be synthesised by alkali-activation of a variety of materials including thermally activated clays, coal fly ash and blast furnace slag to produce a hardened material with mechanical properties potentially suitable for Portland cement replacement (Duxson et al., 2007). Inorganic polymer binders synthesised from the alkali-activation of metakaolin (thermally activated kaolinite) require large volumes of water to create workable pastes. Despite their water demand, hardened metakaolin based inorganic polymers can exhibit comparable or superior mechanical properties to Portland cement (Davidovits, 1991). IPCs based on coal fly ash are of particular interest as they can display superior paste workability with less than a quarter of the water required to produce a metakaolin inorganic polymer paste, which may result in improved mechanical properties (Lloyd and Keyte, 2007). In particular, coal fly ash with low calcium content (< 5 wt% CaO) is an abundant industrial by-product which is currently underutilised worldwide (Ingram and Crookes, 2005). The mechanical properties of inorganic polymers synthesised from coal fly ash can vary substantially (van Jaarsveld et al., 1997). To assess the potential of an IPC to be a replacement for Portland cement, its compressive strength is usually determined. This measurement gives a simple practical assessment of the extent of binder formation; an IPC binder can only be useful if a new phase with appreciable strength forms. The dissolution mechanisms of coal fly ash during inorganic polymer synthesis, and the influence of these mechanisms on the final hardened material, are still unclear. This is also, in part, due to most research investigations generally being performed on a single coal fly ash sample, rather than a selection of different ashes. A scientific understanding of coal fly ash glass chemistry is necessary to begin to understand the complex dissolution and reprecipitation processes occurring during IPC formation. This chapter will discuss the origin and history of low calcium coal fly ash, and how the coal from which the ash originates influences the nature and morphology of the phases present in the ash. The dissolution behaviour of coal fly ash during IPC formation will also be discussed in relation to the chemistry of the reactive phases present in the ash. 2.2 Origin and history of coal fly ash
One of the most primitive ways to generate heat is to ignite an organic material which will continue to generate heat in the presence of oxygen until all of the material has combusted. Coal, an organic sedimentary rock, is mined throughout the world and most of it used to generate electricity in power stations by utilising the heat generated when the coal is combusted. The formation of coal, known as coalification, occurs when a great deal of plant material is buried quickly before significant decay has occurred. The layer of sediment covering the plant material must be substantial enough that oxygen and sunlight are excluded and further decay cannot occur. Over millions of years, the weight of the sediment compresses the plant matter, squeezing out water and converting the dehydrated plant debris into the material known as coal (Cook, 2003). Coal deposits are usually formed in depressions in the Earth’s crust called basins. The coal deposits are usually found in layers of sedimentary rocks and are referred to as either coal beds or coal seams. A deposit may consist of very thick seams of almost pure coal or may have thinner seams separated by layers of shale, siltstones and sandstones. Coal is an organic sediment consisting of a complex mixture of substances; however, it is nearly always associated with incombustible inorganic material, some of which cannot be separated from the organic matter prior to combustion. This inorganic matter forms the majority of the waste stream from a coal-fired boiler in a power station, along with char and any other residues. There are three common types of coal-fired boilers used in power stations: dry-bottom boilers, wet-bottom boilers and cyclone furnaces (Tishmack, 1996). The most common type is the dry-bottom furnace and when pulverised coal is combusted in this boiler, around 80% of all the ash produced leaves the furnace entrained in the flue gas and is generally collected using electrostatic precipitation (Tishmack, 1996). This material, commonly referred to as coal fly ash, or fly ash, differs from other types of power station ash and byproducts with respect to particle size, composition and utilisation potential. Coal fly ash is the fine particulate residue of each individual coal particle after combustion. As the coal particles are carried in a gas stream and are only briefly exposed to high temperatures, there is little possibility for interaction between the fragments. As a consequence, the mineral matter present in each particle is also briefly exposed to high temperatures, which may cause physical and chemical transformation of the minerals, and carried out in the same stream as individual particles (Diamond, 1986). The particle size distribution and chemical composition of many coal fly ashes, as well as the generally spherical particle shape and low cost, make fly ash an ideal material for use as a supplementary cementitious material in concrete (Idorn, 1985). As coal seams can contain between 2 and 35% mineral matter, a constant supply of coal fly ash will be available as long as coal-fired power stations continue to operate. The addition of coal fly ash to cement has been well examined. It is generally agreed that the fly ash can be reactive in this system and improve properties in freshly mixed concrete such as (Idorn, 1985; Butler and Mearing, 1985; Hemmings and Berry, 1987): • Improved workability imparted by the spherical ash particles and the associated water reduction that minimises separation of water from the cement mixture. • Improved compressive strength and other mechanical properties as a result of the reduced water demand. • Reduced concrete cost as the value of coal fly ash is lower than that of cement. • Reduced CO2 emission as less cement is required. • Improved durability, and in some cases improved strength, in hardened concrete due to the pozzolanic reaction with calcium hydroxide generated during cement hydration increasing the volume of calcium silicate hydrate binder, which helps fill the reduced water voids and thus creates a more durable and less permeable concrete. The amount of coal fly ash that can be added to cement varies depending on its chemical composition. Coal fly ashes that contain high concentrations of calcium (>15% w/w CaO) can replace much more Portland cement than those with low concentrations (<5% w/w CaO). In some cases, up to 40% of the cement in concrete can be replaced with coal fly ash (Wei et al., 1992). This is due to the coal fly ash being able to contribute significantly to the reactions occurring during cement hydration, as well as the benefit of reduced water demand. Australia produces around 13 Mt of fly ash annually, but only around 4 Mt is utilised in a beneficial manner (Ingram and Crookes, 2005). The rest is simply disposed of in ponds or land-fill. The chemical composition of coal fly ash in Australia varies; however, all commercially available sources contain relatively low concentrations of calcium. As a result, less of the coal fly produced in Australia can be utilised in Portland cement concrete than is desirable. Coal fly ash with low calcium content is typically referred to as Class F fly ash, although the actual definition of a Class F fly ash is when the sum of the silica, alumina and iron oxide present is greater than 70% (ASTM C 618-05, 2005). Development of inorganic polymer cements that can utilise large amounts of Class F coal fly ash is of particular interest. Coal fly ash is the residue of the mineral matter present in the coal, and the composition of the ash will be related to the type and concentration of these minerals. Inorganic materials, known as mineral matter, occur in coal deposits as the plant materials themselves contain inorganic and organometallic complexes. Minerals may also be washed into the coal forming environment during...