The procedures that are currently being used for classification of fly ash are

Transcription

1 CHAPTER 2 PROPERTIES AND CLASSIFICATION OF FLY ASH 2.1 Introduction The procedures that are currently being used for classification of fly ash are reviewed. Some of the significant properties so as to classify the fly ash as per the current procedure are determined. The need of a separate classification procedure of fly ash for geotechnical engineering applications is established. As a geomaterial, index properties of fly ash are identified. Based on these index properties, a system for classification of fly ash in the light of soil classification system is proposed. 2.2 Existing Procedures for Classification of Fly Ash The classification procedures of fly ash are discussed under the following subsections Review of Specifications for Classification of Fly Ash Mehta [68] classified fly ash into two categories based on calcium content- (i) Fly ash resulting from combustion of anthracite and bituminous coal with CaO less than 5% (ii) Fly ash resulting from combustion of lignite and sub-bituminous coal with CaO up to 15-35% Currently the system defined by ASTM C 618 is the most commonly used classification system for fly ash. ASTM C618 specifications were developed for a specific application, the use of fly ash as a mineral admixture in Portland cement concrete. The chemical and physical requirements of fly ash defined by ASTM C 5

2 618 are given in Table 2.1 It distinguishes two classes o f fly ash, Class C and Class F. The distinction between Class F and Class C fly ash is based on the sum o f the total silicon, aluminum, and iron (Si02+Al203+Fe2C>3) in the ash. When the sum is greater than 70% an ash is classified as Class F. When the sum lies in the range o f 50% to 70% the ash is classified as Class C. Although, ASTM classification system does not classify the fly ash based on CaO content, the same is indirectly fulfilled by a requirement o f a minimum o f 70% o f major non calcium oxide for class F fly ash and 50% for class C fly ash. Thus fly ash o f class C and F o f ASTM classification represent high and low calcium ash respectively. Similar to the ASTM C618, IS: was developed for incorporation o f fly ash in (a) cement mortar and concrete (b) lime pozzolana mixture and (c) manufacture o f portland pozzolana cement. The chemical and physical requirements o f fly ash defined by IS: 3812 are given in Table 2.2 IS: distinguishes two grades o f fly ash, Grade I and Grade II depending upon the chemical and physical properties o f fly ash. While the sum total o f silicon, aluminum, and iron (SiC>2+Al203+Fe203) in the fly ash needs to be at least 70% for both o f fly ash types; physical properties such as fineness, lime reactivity, compressive strength, drying shrinkage and soundness are the determinant o f the grade o f the fly ash Chemical Composition of Fly ash Determination o f chemical composition o f fly ash is mandatory for classification o f fly ash as per existing standards. In general, the chemical composition o f fly ash is typically made up o f silicon, calcium, aluminum, iron, magnesium, and sulfur oxides along with carbon and various trace elements. These elements are 6

3 found in the ash because of their high melting points and the short duration of the ash particles actually remain in the furnace during combustion. The mineral quartz (SiC>2) survives the combustion process and remains as quartz in the coal ash. Other minerals decompose, depending on the temperature, and form new minerals. The clay minerals lose water and may melt, forming alumino-silicate crystalline and noncrystalline (glassy) materials. Elements such as Fe, Ca, and Mg combine with oxygen in the air to form oxide minerals, such as magnetite (FejOj), lime (CaO) and periclase (MgO). The chemical composition of the fly ash of the present study is given in Table 2.3 while the chemical composition of some of the Indian ashes are given in Table Physical Properties of Fly Ash Some of the physical properties of Salakati fly ash required for its classification by IS 3812 are summarized in Table 2.5 Fineness of the fly ash was determined in terms of specific surface by the Blaine s air permeability apparatus, as per IS: Portland cement with specific surface of 346 m2/kg was adopted as standard material. The specific surface was found to be 296 m2/kg. Lime reactivity of the fly ash was determined as per IS: and found to be 4.8 N/mm2. Lime reactivity of Salakati fly ash along with some other Indian fly ashes are provided in Table 2.6 Compressive strength of the fly ash was determined as per IS: 1712 and 7 and 28- days strength were found to be 6.1 and 19.6 N/mm2 respectively. 7

4 2.2.4 Type of Fly Ash as per Existing Standard Specifications One of the specifications of IS: 3812 for classification of fly ash is lime reactivity. It specifies the minimum lime reactivity for Grade I and Grade II fly ash as 4.0 and 3.0 N/mm2 respectively. As shown in Table 2.6 although fly ash of the ( Salakakiypower plant complies this requirement, fly ash of many other thermal plants do not comply with this requirement and can be graded neither as Grade I nor Grade II as lime reactivity is lower than the limit for prescribed against each grade. 2.3 Classification of Fly Ash as Geo-material As the fly ash of many power plants of Table 2.6 cannot be graded either as Grade I or Grade II, the fly ash of these power plants cannot be used in those purposes for which specifications of IS: 3812 are framed. In other words, fly ash of these plants of Table 2.6 cannot be incorporated in manufacture of Portland pozzolana cement or in cement mortar and concrete like applications. However the fly ashes of the power plants can be utilized as an artificial soil or geo-material for structural or non-structural fill or other geotechnical applications. Again fly ash of Salakati or other power plants that comply the specifications of IS: 3812 need not be classified with the stringent specifications for their applications as geo-material. This also calls for a separate classification system of fly ash for geotechnical applications, which provide a scope for bulk utilization of this abundantly available waste material. 8

5 2.3.1 Index Properties of Fly Ash as Geo-material The properties of soils that are indicative of the stress-deformation-time relationship in which the engineer is primarily interested are called index properties. The index properties are useful mainly for speedy comparison or grouping of soils. Thus, whenever the soil testing procedures are applied to characterise fly ash as a geo-material, attempt is made to evaluate the tests by looking what the tests are actually measuring, and their importance in the context of geotechnical applications. The identified index properties of fly ash and their significant features are discussed below Specific Gravity: Specific gravity is frequently required for finding out the degree of saturation, void ratio, unit weight of soil solids or moist soils. The unit weights "in turn are needed in pressure, settlement and stability problems in soil or geotechnical engineering. Therefore specific gravity is a very important physical-property of fly ash as a geo-material. In general, fly ash is characterised by low specific gravity. One explanation for this lower specific gravity is the feet that a high proportion of fly ash particles are cenospheres or hollow particles [28], The specific gravity of coal ash lays around 2.0 but can vary to a large extent, 1.6 to 3.1 [6], [66]. Variation of specific gravity of coal ash is due to the combination of many factors such as gradation, particle shape and chemical composition [28], Ashes with high iron contents have high specific gravity values [89]. Based on several data, Singh [92] has shown that specific gravity of fly ash depends on carbon and iron oxides contents. He found that a small increase in carbon content o f a fly ash results in a significant drop in 9

6 specific gravity. He found increase or decrease in specific gravity with decrease or increase in carbon or iron oxides content o f a fly ash respectively. The specific gravity of fly ash was determined by use of Le chatelier s flask as per IS and by the density bottle method as per IS: 2720 (part 3, section 1). Both the methods give same order of specific gravity and lie in the range of 2.06 to 2.10 Plate 3 depicts a Le chatelier s flask in operation for specific gravity test. Out of the two methods, specific gravity by the method of Le chatelier s flask is found much easier and can be conveniently used for geotechnical characterisation Grain size distribution Fig. 2.1 shows the range of grain size distributions of the fly ash generated by sieve and plummet balance analysis as per IS: 2720 (part 4). For the fraction of fly ash passing through 75 micron sieve, the plummet balance was used as it permits, at any time, to directly read percentage of particles in suspension with easy computation of particle size by Stoke s law. As an aid to solve the Stoke s equation IS: 2720(part 4) gives the co-efficient in the form of a chart applicable for specific gravity of particles in the range of 2.65 to 2.8. As specific gravity of fly ash is lower than soils, this chart needs to be modified. As such to conduct the grain size distribution of fly ash by Plummet balance, the chart was modified to cover the specific gravity of particles in the range 1.5 to 2.8 as shown in Fig. 2.2 The fraction of silt, sand and clay-sized particles and the salient grain size characteristics are shown in Fig. 2.3 and Table 2.7. Thus Salakati fly ash consists of predominantly silt-sized particles and low value of uniformity co-efficient indicates the uniform pattern of the ash particles. 10

7 Plasticity Characteristics of Fly Ash In soil mechanics, plasticity is defined as the property of a material, which allows it to be deformed rapidly, without rupture, without elastic rebound, and without volume change. Fly ash in a moist but unsaturated condition displays an apparent cohesion due to the tensile stresses of retained capillary water. However fly ash cannot be rolled into a thread as per the standard plastic limit test. Again, under dry condition the apparent cohesion in fly ash disappears with no dry strength, rather fly ash is prone to create considerable dust nuisance because o f its nonplasticity or lack of cohesion. As such fly ash is a non-plastic material. Liquid limit of fly ash could not be determined by use of Casagrande type apparatus of IS: 9259, as the sample disintegrates while cutting a groove. Also the grooved sample tends to slip rather than flow as per the underlying principle of the test. Attempt was made to determine liquid limit by cone penetration method using the penetrometer as given in IS: Difficulty for determination of liquid limit was also faced in this method due to occurrence of bleeding when the wet fly ash paste is transferred to the cylindrical cup of the cone penetrometer apparatus. Striking off the excess fly ash to get a smooth surface in the cup and without fatting up of the surface layer was also found difficult. All these lead for erratic results of liquid limit with the use of cone penetrometer, similar to the Casagrande type apparatus Unconfined Compressive Strength Cylindrical specimens of 50mm diameter and 100mm height were used for soaked and unsoaked unconfined compressive strength. Two sets of duplicate specimens extracted from compaction tests over the entire range of compactable water 11

8 content were used for strength tests. After curing for seven days, the two sets of specimens were used to determine the soaked and un-soaked unconfined compressive strength of the fly ash. The set of the specimens that were soaked collapsed completely. The effect of soaking or saturation of the compacted cylindrical specimens of 50mm diameter and 100mm height are depicted in strength in the order of 120kN/m2. unsoaked specimen yielded unconfined compressive ph The measurement of ph was carried out using Systronics expanded scale ph meter having a combined electrode with temperature setting arrangement. The instrument was standardized with two standard buffer solutions of ph 9.2 and 12 at 25 C as per standard procedure. 30 g of fly ash was taken in a 100 ml beaker. After adding 75 ml of distilled water, the suspension was stirred for few seconds. The beaker was covered with a glass cover and allowed to stand for one hour with occasional stirring. Finally ph was measured immediately after stirring. The measured ph of fly ash was found to be Proposed Classification System of Fly Ash as Geo-material The existing standards contain numerous physical and chemical requirements that are not necessarily useful for geotechnical applications. Also only the laboratories involved in research are usually equipped with comprehensive facilities for determination of chemical composition of soils. As such there is a need of a classification system of fly ash in the light of established principles of soil mechanics and consistent with the performance requirements of geotechnical 12

9 applications in one hand and easily executable by the user community on the other hand. Since more than 50% of the material passes through the 75 micron sieve fly ash can be considered as fine grained soil as per Indian Standard soil classification system. As per the code, subdivision of a fine-grained soil needs to be made based on liquid limit. The liquid limit of soils is dependent upon the clay minerals present in the soils, the intensity of the surface charges and the thickness of the attached water, and the ratio of surface area to volume or shape of the particles. The stronger the surface charge and thinner the particles the greater will be the proportion of attached viscous water and, therefore, the higher will be the liquid limit. Because the proportion of these thin scale-like particles affects the compressibility of the soil, the liquid limit is indicative of compressibility. However coal ashes do not contain clay and therefore exhibit no plasticity. As fly ash is non-plastic material further subdivision of fly ash based on liquid limit of cone penetration result would be misleading. Also, liquid limit for a cohesionless soil is meaningless, even though a value can be found for a fine-grained cohesionless soil [69], In other words liquid limit is not a relevant test for fly ash as far as classification of fly ash as geomaterial is concerned. Therefore the grain size distribution data by sieve analysis alone may be used as one of the criteria for classification of fly ash as geo-material. With the predominant fraction of particles being in the range of silt to fine sand, perhaps determination of its gradation in terms of co-efficient of uniformity or co-efficient of curvature is not necessarily required. Thus the combined fraction of silt and clay sized particles together with the fine sand fraction obtained by sieve analysis 13 JZ43I& 1

10 may be used for gradation of fly ash. Depending on the relative fraction of particles finer as well as coarser than 75 micron, fly ash may be described as nonplastic silt sized or fine sand sized ash. Strength, durability and deformation are important attributes of fly ash that depends on reactive silica, fineness, free lime content, loss of ignition etc. However since strength of fly ash would reflect the influence of these factors, strength based classification of fly ash would eliminate the need of ascertaining the chemical composition of fly ash. Thus soaked and unsoaked unconfined compressive strength of compacted and cured specimen of fly ash may be used as one of the index properties of fly ash for its classification as geo-material. In other words, classification of fly ash based on calcium content and in the spirit of the existing codes but without ascertaining the chemical composition can be assessed in terms of 7-day soaked unconfined compressive strength. In the present investigation, the specimens extracted from the compaction tests, covering the entire range of water were used for strength tests. However, for classification purpose, only two duplicate samples at optimum or maximum compactable water content may be prepared and cured for seven days. If the sample disintegrates on soaking the fly ash may be classified as non-self cementing fly ash. On the other hand if the specimen does not disintegrate and the soaked strength is at least 50% of the un-soaked strength, the fly ash may be considered as self-cementing fly ash. Based on the sieve analysis and strength tests fly ash may be classified as self- i cementing silt sized or fine sand sized ash. Coupled with the strength test, ph test may be conducted as a supplementary test to classify fly ash based on the calcium content in the fly ash. Fly ash showing neutral reaction is usually low calcium fly ash. On the other hand fly ash showing 14

11 sharply alkaline reaction (ph: 11-12) is high calcium fly ash having selfcementing property. The index properties that are relevant for classification of fly ash as a geo-material is provided in Table 2.8 and Salakati fly ash may be classified as silt-sized nonself cementing fly ash. 2.4 Conclusions: i. The numerous physical and chemical requirements laid out in the existing standards of fly ash are not necessarily useful for geotechnical applications. For promoting bulk utilization of fly ash as a geo-material, a separate identification and classification system is needed.. ii. As a geo-material, relevant index properties of fly ash are identified. Based on these index properties, a simple system for classification of fly ash in the light of soil classification system is proposed. 1

12 Table 2.1 Chemical and physical requirements defined by ASTM C 618 Chemical Requirements Class F Class C Si02+AI203+Fe203, min. % S 0 3, max. % Moisture content, max.% Loss of ignition (LOI), max % Available alkalies, as Na20, max % Physical Requirements Class F Class C Fineness, max.% Strength Activity Index with Portland cement, 7 days, min.% with Portland cement, 28 days, min.% with Portland cement, 7 days, min. psi Water requirements, max.% of control Soundness; autoclave expansion or contraction, max.% Uniformity Requirements Specific Gravity, max.% variation from average Percent retained on #325 sieve, max.% variation from average D o Table 2.2 Chemical and physical requirements defined by IS: 3812 Chemical Requirements Grade I Grade II S i0 2+Al20 3+Fe20 3, Min % 70.0 S i0 2, Max % 35.0 MgO, Max% 5.0 S 0 3, Max % 2.8 Loss of ignition (LOI), max.% 12.0 Available alkalies, as Na20, max.% 1.5 Physical Requirements Grade I Grade II Fineness in m2/kg by Blane's permeability, Min Lime reactivity, N/mm2 Min Compressive strength, 28 days, N/mm2 Min Not less than 80% of strength of corresponding plain cement mortar cubes Drying shrinkage, % Max Soundness; autoclave expansion or contraction, Max %

13 Table 2.3 Chemical composition of SFA Chemical Requirements Percentage (%) Silica (Si02) 65.0% Alumina (Al20 3t Feric Oxide (Fe20 3) 4.86 Calcium Oxide (CaO) 0.09 Free lime Below Detectable level Magnesium Oxide (MgO) 0.08 Sulpher(S04) 0.01 Potassium (K20) 0.01 Sodium (Na20) 0.01 Chloride (Cl) 0.01 Lead (Pb) Below Detectable level Zinc (Zn) Below Detectable level Copper (Cu) Below Detectable level Chromium (Cr) Below Detectable level Nical (Ni) Below Detectable level Cadmium (Cd) Below Detectable level Loss of ignition (LOI) 3.75 Table 2.4 Chemical composition of some Indian fly ashes Chemical Requirements Percentage Range(%) Silica (Si02) Alumina (Al20 3) Iron Oxide (Fe20 3) 4-10 Calcium Oxide (CaO) 1-4 Magnesium Oxide (MgO) Sulpher(S03) Loss of ignition (LOI) Source* Kumar et af [51] Table 2.5 Some physical properties of SFA as required by IS 3812 Fineness in term of specific surface Lime Reactivity Compressive Strength 7-days Compressive Strength 28-days 296 m2/kg 4.8 N/mm2 6.1 N/mm N/mm2 17

14 Table 2.6 Lime reactivity of some Indian fly ashes Raebareli Kobra Vijayawada Badarpur Ghaziabad Power plant Ramagundam Neyveli Farakka Vindyanagar Badarpur Kobra Ramagundam Singrauli Salakati Lime reactivity in N/mm Source [79] [91] present study Table 2.7 Grain size distribution and characteristics of SFA Grain Size Distribution Range Average Silt size (%) 68.5% Fine sand size (%) 28.9% Clay size Grain Size Characteristics (%) 2.6% ", o mm 0.014mm r> mm 0.06mm 1J (U mm 0.025mm (' (' Table 2.8 Index properties for classification of fly ash as geo-material Grain Size Distribution Range Average Silt and clay size (%) 71.1% Fine sand size (%) 28.9% Specific gravity Plasticity characteristics Plastic limit Non plastic Dry strength Nil Liquid limit Not needed Unconfined compressive strength at omc or near max compactable water content Unsoaked 120kN/m2 Soaked Nil ph 7.53 Type of Fly Ash Based on grain size distribution Non plastic silt sized Based on strength or hydration behaviour Non self-cementing 18

15 Figure 2.1 Range of grain size distribution for SFA clay size [2.5%] Dfine sand size: [28.9%] silt-size: [68.5%] Figure 2.2 Average fraction of particles in SFA by size G-2.6 G-2.8 * -G e -G : A -G Temperature of suspension in C GO CM o CM o o CM O o o o o o 00 - o m o o Figure 2.3 Chart for aid in solving Stake's equation 19