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Fly Ash, Slag, Silica Fume, and Natural Pozzolans, Chapter 3

Chapter 3 Fly Ash, Slag, Silica Fume, and Natural PozzolansFly ash, ground granulated blast- furnace slag, Silica Fume, and Natural pozzolans, such as calcined shale, calcined clayor metakaolin, are materials that, when used in conjunc-tion with portland or blended cement, contribute to theproperties of the hardened concrete through hydraulic orpozzolanic activity or both (Fig. 3-1). A pozzolan is asiliceous or aluminosiliceous material that, in finelydivided form and in the presence of moisture, chemicallyreacts with the calcium hydroxide released by the hydra-tion of portland cement to form calcium silicate hydrateand other cementitious compounds. Pozzolans and slagsare generally catergorized as supplementary cementitiousmaterials or mineral admixtures.

processed materials, which are heat treated in a kiln and then ground to a fine powder (Figs. 3-10, 3-11, and 3-12); they include calcined clay, calcined shale, and metakaolin. ground granulated blast furnace slag and ACI 233 (1995) provides an extensive review of slag. SILICA FUME Silica fume, also referred to as microsilica or condensed

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Transcription of Fly Ash, Slag, Silica Fume, and Natural Pozzolans, Chapter 3

1 Chapter 3 Fly Ash, Slag, Silica Fume, and Natural PozzolansFly ash, ground granulated blast- furnace slag, Silica Fume, and Natural pozzolans, such as calcined shale, calcined clayor metakaolin, are materials that, when used in conjunc-tion with portland or blended cement, contribute to theproperties of the hardened concrete through hydraulic orpozzolanic activity or both (Fig. 3-1). A pozzolan is asiliceous or aluminosiliceous material that, in finelydivided form and in the presence of moisture, chemicallyreacts with the calcium hydroxide released by the hydra-tion of portland cement to form calcium silicate hydrateand other cementitious compounds. Pozzolans and slagsare generally catergorized as supplementary cementitiousmaterials or mineral admixtures.

2 Table 3-1 lists the appli-cable specifications these materials meet. The use of thesematerials in blended cements is discussed in Chapter 2 andby Detwiler, Bhatty, and Bhattacharja (1996).The practice of using supplementary cementitiousmaterials in concrete mixtures has been growing in NorthAmerica since the 1970s. There are similarities betweenmany of these materials in that most are byproducts ofother industrial processes; their judicious use is desirablenot only from the national environmental and energyconservation standpoint but also for the technical benefitsthey provide concrete. Supplementary cementitious materials are added toconcrete as part of the total cementitious system. They maybe used in addition to or as a partial replacement of portlandcement or blended cement in concrete, depending on theproperties of the materials and the desired effect on cementitious materials are used toimprove a particular concrete property, such as resistance toalkali-aggregate reactivity.

3 The optimum amount to useshould be established by testing to determine (1) whether thematerial is indeed improving the property, and (2) the correctdosage rate, as an overdose or underdose can be harmful ornot achieve the desired effect. Supplementary cementitiousmaterials also react differently with different , fly ash, slag, calcined clay, calcined shale,and Silica fume were used in concrete individually. Today,due to improved access to these materials, concrete produc-ers can combine two or more of these materials to optimizeconcrete properties. Mixtures using three cementitiousmaterials, called ternary mixtures, are becoming moreFig. 3-1. Supplementary cementitious materials. From leftto right, fly ash (Class C), metakaolin (calcined clay), silicafume, fly ash (Class F), slag, and calcined shale.

4 (69794)Fig. 3-2. Scanning electron microscope (SEM) micrographof fly ash particles at 1000X. Although most fly ashspheres are solid, some particles, called cenospheres,are hollow (as shown in the micrograph). (54048)57 HOMEPAGE area is typically 300 to 500 m2/kg, although some fly ashescan have surface areas as low as 200 m2/kg and as high as700 m2/kg. For fly ash without close compaction, the bulkdensity (mass per unit volume including air betweenparticles) can vary from 540 to 860 kg/m3(34 to 54 lb/ft3),whereas with close packed storage or vibration, the rangecan be 1120 to 1500 kg/m3(70 to 94 lb/ft3).Fly ash is primarily silicate glass containing Silica ,alumina, iron, and calcium. Minor constituents are magne-sium, sulfur, sodium, potassium, and carbon.

5 Crystallinecompounds are present in small amounts. The relativedensity (specific gravity) of fly ash generally rangesbetween and and the color is generally gray or C 618 (AASHTO M 295) Class F and Class C flyashes are commonly used as pozzolanic admixtures forgeneral purpose concrete (Fig. 3-4). Class F materials aregenerally low-calcium (less than 10% CaO) fly ashes withcommon. Supplementary cementitious materials are used inat least 60% of ready mixed concrete (PCA 2000). ASTM C311 provides test methods for fly ash and Natural pozzolansfor use as supplementary cementitious material in ASH Fly ash, the most widely used supplementary cementitiousmaterial in concrete, is a byproduct of the combustion ofpulverized coal in electric power generating plants.

6 Uponignition in the furnace , most of the volatile matter andcarbon in the coal are burned off. During combustion, thecoal s mineral impurities (such as clay, feldspar, quartz,and shale) fuse in suspension and are carried away fromthe combustion chamber by the exhaust gases. In theprocess, the fused material cools and solidifies into spheri-cal glassy particles called fly ash (Fig. 3-2). The fly ash isthen collected from the exhaust gases by electrostaticprecipitators or bag filters. Fly ash is a finely dividedpowder resembling portland cement (Fig. 3-3).Most of the fly ash particles are solid spheres and someare hollow cenospheres. Also present are plerospheres,which are spheres containing smaller spheres.

7 Groundmaterials, such as portland cement, have solid angularparticles. The particle sizes in fly ash vary from less than1 m (micrometer) to more than 100 m with the typicalparticle size measuring under 20 m. Only 10% to 30% ofthe particles by mass are larger than 45 m. The surface58 Design and Control of Concrete Mixtures EB001 Fig. 3-4. Fly ash, slag, and calcined clay or calcined shale are used in general purpose construction, such as (left to right)walls for residential buildings, pavements, high-rise towers, and dams. (67279, 48177, 69554, 69555)Fig. 3-3. Fly ash, a powder resembling cement, has beenused in concrete since the 1930s. (69799)Table 3-1. Specifications and Classes ofSupplementary Cementitious MaterialsGround granulated iron blast- furnace slags ASTM C 989(AASHTO M 302)Grade 80 Slags with a low activity indexGrade 100 Slags with a moderate activity indexGrade 120 Slags with a high activity indexFly ash and Natural pozzolans ASTM C 618 (AASHTO M 295)Class NRaw or calcined Natural pozzolans including:Diatomaceous earthsOpaline cherts and shalesTuffs and volcanic ashes or pumicitesCalcined clays, including metakaolin, and shalesClass FFly ash with pozzolanic propertiesClass CFly ash with pozzolanic and cementitious propertiesSilica fume ASTM C 1240carbon contents usually less than 5%, but some may be ashigh as 10%.

8 Class C materials are often high-calcium (10%to 30% CaO) fly ashes with carbon contents less than 2%.Many Class C ashes when exposed to water will hydrateand harden in less than 45 minutes. Some fly ashes meetboth Class F and Class C ash is used in about 50% of ready mixed concrete(PCA 2000). Class F fly ash is often used at dosages of 15%to 25% by mass of cementitious material and Class C flyash is used at dosages of 15% to 40% by mass of cementi-tious material. Dosage varies with the reactivity of the ashand the desired effects on the concrete (Helmuth 1987 andACI 232 1996). SLAGG round granulated blast- furnace slag (Fig. 3-5), also calledslag cement, is made from iron blast- furnace slag; it is anonmetallic hydraulic cement consisting essentially of sili-cates and aluminosilicates of calcium developed in amolten condition simultaneously with iron in a blastfurnace.

9 The molten slag at a temperature of about 1500 C(2730 F) is rapidly chilled by quenching in water to form aglassy sandlike granulated material. The granulated mate-rial, which is ground to less than 45 microns, has a surfacearea fineness of about 400 to 600 m2/kg Blaine. The relativedensity (specific gravity) for ground granulated blast- furnace slag is in the range of to The bulk densityvaries from 1050 to 1375 kg/m3(66 to 86 lb/ft3).The rough and angular-shaped ground slag (Fig. 3-6)in the presence of water and an activator, NaOH or CaOH,both supplied by portland cement, hydrates and sets in amanner similar to portland cement. However, air-cooledslag does not have the hydraulic properties of water-cooled slag. Granulated blast furnace slag was first developed inGermany in 1853 (Malhotra 1996).

10 Ground slag has beenused as a cementitious material in concrete since the begin-ning of the 1900s (Abrams 1925). Ground granulated blast- furnace slag, when used in general purpose concrete inNorth America, commonly constitutes between 30% and45% of the cementing material in the mix (Fig. 3-4) (PCA2000). Some slag concretes have a slag component of 70%or more of the cementitious material. ASTM C 989(AASHTO M 302) classifies slag by its increasing level ofreactivity as Grade 80, 100, or 120 (Table 3-1). ASTM C 1073covers a rapid determination of hydraulic activity of59 Chapter 3 Fly Ash, Slag, Silica Fume, and Natural PozzolansFig. 3-5. Ground granulated blast- furnace slag. (69800)Fig. 3-7. Silica fume powder. (69801)Fig.


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