Fly ash is a by-product which is generated during the burning of pulverized coal in a thermal power plant. Specifically, it is the unburned residue which is carried away from the burning zone in the boiler by the flue gases and then collected by electrostatic separators or by any other means (Fig 1). The heavier unburned material drops to the bottom of the boiler and is termed bottom ash which is not generally suitable for use as a cementitious material for concrete, but is used in the manufacture of concrete masonry block.
Fig 1 Generation of fly ash in a power plant
For the generation of fly ash, coal is first pulverized in grinding mills before being blown with air into the burning zone of the boiler. In this zone, the coal burns producing heat with temperatures reaching around 1500 deg C. At this temperature the non-combustible inorganic minerals (such as quartz, calcite, gypsum, pyrite, feldspar and clay minerals etc.) melt in the furnace and fuse together as tiny molten droplets. These droplets are carried from the combustion chamber of the boiler by exhaust or flue gases. Once free of the burning zone, the droplets cool to form spherical glassy particles called fly ash (Fig 2). The fly ash is collected from the exhaust gases in the dust-collection systems which remove particles from the exhaust gases.
Fig 2 Fly ash
Fly ash is generally finer than Portland cement and consists mostly of small spheres of glass of complex composition involving silica, ferric oxide, and alumina. The composition of fly ashes varies with the source of coal. Presently, two major classes of fly ash are related to the type of coal burned. These are designated Class ‘F’ and Class ‘C’ by the American Society for Testing and Materials (ASTM) and this differentiation is normally now widely used.
Class F is defined in ASTM specification C 618 as the fly ash normally produced from burning anthracite or bituminous coal. Under the present conditions no appreciable amount of anthracite coal is used for power generation. Thus essentially all Class F fly ash now available is derived from bituminous coal. Class F fly ashes are not self-hardening but normally have pozzolanic properties. This means that in the presence of water, the fly ash particles react with calcium hydroxide (lime) to form cementitious products. The cementitious products so formed are chemically very similar to those present in hydrated Portland cement. The pozzolanic reactions occur slowly at normal atmospheric temperatures.
Class F fly ash, is available in larger quantities, which is normally low in lime, less than 15 %, and contains higher combination of silica (SiO2), alumina (Al2O3) and iron oxide (Fe2O3) (more than 70 %) compared to Class C fly ash. Class F is a solution to a wide range of summer concreting problems and it is often recommended for using where concrete can be exposed to sulphate ions in soil and ground water.
Class C fly ashes normally result from the burning of sub-bituminous coal and lignite coal. They have pozzolanic properties but can also be self-hardening. That is, when mixed with water they harden by hydration much the same way as the Portland cement hardens. In most cases, this initial hardening occurs relatively fast. These materials are referred to as being cementitious and the degree of cementitiousness normally varies with the calcium oxide (CaO) content of the fly ash. Higher values of CaO denote higher cementitiousness.
Class C fly ash normally comes from coal which produces an ash with higher lime content, normally more than 15 %, often as high as 30 %. Also, high CaO gives Class C unique self hardening characteristics. Class C is mostly used in situations where higher early strengths are important.
The general classification of fly ash by the type of coal burned does not adequately define the type of behaviour to be expected when this material is used in concrete. There are wide differences in the characteristics within each class. Despite the reference in ASTM C 618 to the classes of coal from which Class F and Class C fly ashes are derived; there is no requirement that a given class of fly ash is to come from a specific type of coal. For example, Class F ash can be produced from the coals which are not bituminous and bituminous coal can produce ash which is not Class F. Moreover, Class C fly ash is not required to have any CaO. Consideration is now being given in ASTM and other standard organizations to reclassify fly ash in a manner more closely related to the characteristics of the ash itself and its effect on the properties of concrete, but as yet no agreement has been reached as to the basis of such classification.
Fly ash is a pozzolanic material. It is a finely divided amorphous alumino-silicate with varying amounts of calcium, which when mixed with Portland cement and water, reacts with the calcium hydroxide released by the hydration of Portland cement to produce various calcium-silicate hydrates (C-S-H) and calcium-aluminate hydrates. Some fly ashes with higher amounts of calcium also display cementitious behaviour by reacting with water to produce hydrates in the absence of a source of calcium hydroxide. These pozzolanic reactions are beneficial to the concrete in that they increase the quantity of the cementitious binder phase (C-S-H) and, to a lesser extent, calcium-aluminate hydrates, improving the long term strength and reducing the permeability of the concrete system. Both of these mechanisms enhance the durability of the concrete.
The performance of fly ash in concrete is strongly influenced by its physical, mineralogical and chemical properties. The mineralogical and chemical composition are dependent to a large extent on the composition of the coal and since a wide range of domestic and imported coals (anthracite, bituminous, sub-bituminous and lignite) are burned in different power plants, the properties of the fly ash can be very different between sources and collection methods. The burning conditions within a power plant can also affect the properties of the fly ash. The calcium content of the fly ash is perhaps the best indicator of how the fly ash is going to behave in concrete, although other compounds such as the alkalis (Na2O and K2O), carbon (normally measured as LOI), and sulphate (SO3) can also affect the performance of the fly ash.
Low-calcium fly ashes (less than 8 % CaO) are invariably produced from anthracite or bituminous coals and are mainly composed of alumino-silicate glasses with varying amounts of crystalline quartz, mullite, hematite and magnetite. These crystalline phases are essentially inert in concrete and the glass requires a source of alkali or lime, for example, Ca(OH)2 to react and form cementitious hydrates. Such fly ashes are pozzolanic and display no considerable hydraulic behaviour.
High-calcium fly ashes (higher than 20 % CaO) can be produced from lignite or sub-bituminous coals and are comprised of calcium-alumino-silicate glass and a wide variety of crystalline phases in addition to those found in low-calcium fly ash. Some of these crystalline phases react with water and this, coupled with the more reactive nature of the calcium-bearing glass, makes these fly ashes react more rapidly than low-calcium fly ashes and renders the fly ash both pozzolanic and hydraulic in nature. These fly ashes react and harden when mixed with water due to the formation of cementitious hydration products.
If the calcium content of the fly ash is high enough, it is possible to make concrete with moderate strength using the fly ash as the sole cementing material. In addition to providing an indication of the mineralogy and reactivity of the fly ash, the calcium content is also useful in predicting how effective the fly ash is in terms of reducing the heat of hydration, controlling expansion due to alkali-silica reaction, and providing resistance to sulphate attack. The relationship between CaO content and the sum of SiO2 + Al2O3 + Fe2O3 content in several ash samples is given in Fig 3.
Fig 3 Relationship between CaO content and the sum of SiO2 + Al2O3 + Fe2O3 content in several ash samples
Physical and chemical properties of fly ash
Classification and selection of fly ash depend on its properties in order to utilize in an efficient way. Selection involves mainly monitoring the fineness and loss on ignition (LOI) of the fly ash. In addition, there are other physical and chemical properties which are involved in identifying different categories of fly ash.
Fly ash can be tan to dark gray in colour, depending on its chemical and mineral constituents. Tan and light colours are typically associated with high lime content. A brownish colour is typically associated with the iron content. A dark gray to black colour is typically attributed to a high level of unburned carbon content. Fly ash colour is normally very consistent for each power plant and coal source.
The physical properties of fly ash depend on (i) the nature of coal, (ii) mineral matter chemistry and mineralogy, (iii) boiler design, operation, and method of particulate control such as sulphur oxides (SOx) and nitrogen oxides (NOx) control technologies. The fly ash particles are normally glassy, solid or hollow and spherical in shape. The hollow spherical particles are called as cenospheres. The fineness of individual fly ash particle ranges from 0.5 micrometers to 1000 micrometers size. The fineness of fly ash particles has a significant influence on its performance in cement concrete. The fineness of particles is measured by measuring specific surface area of fly ash by Blaine’s specific area technique. Greater is the surface area more is the fineness of the fly ash. The other method used for measuring fineness of fly ash is dry and wet sieving. The specific gravity of fly ash varies over a wide range of 1.9 to 2.55. The physical properties of certain types of fly ash are given in Tab 1. These properties influence the properties of the final product such as workability, pumpability, water requirement as well as permeability.
|tab 1 Physical properties of certain types of fly ash|
|Sl. No.||Property||Unit||Type 1||Type 2||Type 3||Type 4|
|3||Fineness||mm||13.8 % in no. 325||0.6-0.001|
|5||Maximum dry density||g/cc||1.65||0.9-1.6||1.53|
|8||Angle of internal friction||degrees||30-40||23-41|
|11||Coefficient of consolidation||sqm/year||0.1-0.5|
The main constituents of the fly ash are SiO2, Al2O3, Fe2O3 and CaO. The other minor constituent of the fly ash are MgO, Na2O, K2O, SO2, MnO, TiO2 and unburnt carbon. There is wide range of variation in the principal constituents such as SiO2 (25 % to 60 %), Al2O3 (10 % to 30 %) and Fe2O3 (5 % to 25 %). It can be seen that the main chemical compounds of the fly ash namely SiO2, Al2O3, Fe2O3, and CaO are the same as the main compounds found in Portland cement. However the proportion of each compound differs considerably when comparing fly ash (Class F and Class C) with the Portland cement. It can be seen that Portland cement is rich in lime (CaO) while fly ash is low. Fly ash is high in reactive silicates while Portland cement has smaller amounts.
The quantity of alkalis in fly ash can range from less than 1 % Na2Oeq (Na2Oeq = Na2O + 0.658 x K2O) upto 10 % Na2Oeq. The fly ash tends to be very reactive as the alkalis raise the pH of the pore solution when it is mixed in concrete and the high pH accelerates the dissolution of the glass in the fly ash. Particular attention is to be paid to the (alkali-silica) reactivity of the aggregates when high-alkali fly ash is used in the concrete. There is no limit placed on the alkali content of fly ash by ASTM C618.
The sulphate content of fly ash normally ranges from less than 0.1 % to 5 % SO3. In exceptional cases the sulphate content can exceed 5 % SO3. ASTM C618 limits the sulphate content of fly ash to 5 % SO3 when the material is to be used in concrete
The only other limit placed on the composition of the fly ash by ASTM specification is a maximum allowable loss-on-ignition (LOI). The LOI limit in ASTM C618 is 6 % for Class F and Class C fly ash. However, the specification allows Class F fly ashes with upto 12 % LOI to be approved by the user if either acceptable performance records or laboratory test results are made available.
Excessive amounts of magnesia (MgO) or free lime (CaO) in cementitious materials can cause unsoundness (undesirable volume change) when these materials are used in concrete. ASTM C618 requires fly ash to pass an autoclave expansion test to demonstrate soundness.
Fly ash contains environmental toxins in significant amounts. These toxins include arsenic (45 ppm), barium (805 ppm), beryllium (5 ppm), boron (310 ppm), cadmium (3 ppm), chromium (135 ppm), chromium 6 (90 ppm), cobalt (35 ppm), copper (110 ppm), fluorine (30 ppm), lead (55 ppm), manganese (250 ppm), nickel (75 ppm), selenium (8 ppm), strontium (775 ppm), thallium (10 ppm).vanadium (250 ppm), and zinc (180 ppm).
Effect on the properties of fresh concrete
Fly ash can also be introduced to concrete through the use of blended hydraulic cement consisting of Portland cement, fly ash, and possibly other cementitious components. The effects of fly ash on the concrete properties are given below.
Workability – The use of good quality fly ash with a high fineness and low carbon content reduces the water demand of concrete and, consequently, the use of fly ash permits the concrete to be produced at lower water content when compared to a Portland cement concrete of the same workability. Although the exact amount of water reduction varies widely with the nature of the fly ash and other parameters of the mix, a gross approximation is that each 10 % of fly ash allows a water reduction of at least 3 %. A well-proportioned fly ash concrete mixture has improved workability when compared with a Portland cement concrete of the same slump. This means that, at a given slump, fly ash concrete flows and consolidates better than a conventional Portland cement concrete when vibrated. The use of fly ash also improves the cohesiveness and reduces segregation of concrete. The spherical particle shape lubricates the mix rendering it easier to pump and reducing wear on equipment.
It is to be noted that these benefits are realized in well-proportioned concrete. The fresh properties of concrete are strongly influenced by the mixture proportions including the type and amount of cementing material, the water content, the grading of the aggregate, the presence of entrained air, and the use of chemical admixtures. The improved rheological properties of high-volume fly ash concrete make it suitable for use in self-consolidating concrete.
Coarser fly ashes or those with high levels of carbon normally produce a smaller reduction in water demand and some can even increase water demand. Careful consideration is to be given before using the fly ash in concrete especially at higher levels of replacement in structural concrete.
Bleeding – Normally fly ash reduces the rate and amount of bleeding primarily due to the reduced water demand. Particular care is needed to determine when the bleeding process has finished before any final finishing of exposed slabs. High levels of fly ash used in concrete with low water contents can almost eliminate bleeding. Hence, the freshly placed concrete is to be finished as quickly as possible and immediately protected to prevent plastic shrinkage cracking when the ambient conditions are such that rapid evaporation of surface moisture is likely. An exception to this condition is when fly ash is used without an appropriate water reduction, in which case bleeding (and segregation) increases in comparison to Portland cement concrete.
Air entrainment – Concrete containing low-calcium (Class F) fly ash normally needs a higher amount of air-entraining admixture to achieve a satisfactory air-void system. This is mainly due to the presence of unburned carbon which absorbs the admixture. Hence, higher amounts of air-entraining admixture are needed as either the fly ash content of the concrete increases or the carbon content of the fly ash increases. The carbon content of fly ash is normally measured indirectly by determining its loss-on-ignition (LOI).
The increased demand for air entraining admixture does not present a significant problem to the concrete production provided the carbon content of the fly ash does not vary significantly. It has been seen that as the admixture amount needed for a specific air content increases, the rate of air loss also increases. Normally, high-calcium fly ash needs a smaller increase in the air-entrainment amount compared to low-calcium fly ash. Some Class C fly ash high in water-soluble alkali can even need lesser admixture than those mixes without fly ash.
Setting time – The impact of fly ash on the setting behaviour of concrete is dependent not only on the composition and quantity of the fly ash used, but also on the type and amount of the cement, the water-to-cementitious materials ratio, the type and amount of chemical admixtures, and the concrete temperature. It is fairly well-established that low-calcium fly ash extends both the initial and final set of concrete. During hot weather the amount of retardation due to fly ash tends to be small and is likely to be a benefit in many cases.
During cold weather, the use of fly ash, especially at high levels of replacement, can lead to very significant delays in both the initial and final set. These delays can result in placement difficulties especially with regards to the timing of finishing operations for floor slabs and pavements or the provision of protection to prevent freezing of the plastic concrete. Practical considerations can need that the fly ash content is limited during cold-weather concreting. The use of set-accelerating admixtures can wholly or partially offset the retarding effect of the fly ash.
Higher-calcium fly ash normally retards setting to a lesser degree than low-calcium fly ash, probably because the hydraulic reactivity of fly ash increases with increasing calcium content. However, the effect of high-calcium fly ash is more difficult to predict since the use of some of these ashes with certain cement-admixture combinations can lead to either rapid (or even flash) setting or to severely retarded setting.
Heat of hydration – The reduction in the rate of the heat produced and hence the internal temperature rise of the concrete has long been an incentive for using fly ash in mass concrete construction. One of the first full-scale field trials indicated that the use of fly ash reduces the maximum temperature rise over ambient from 47 deg C to 32 deg C. In massive concrete pours where the rate of heat loss is small, the maximum temperature rise in fly ash concrete is primarily a function of the amount and composition of the Portland cement and fly ash used, together with the temperature of the concrete at the time of placing. Concrete with low Portland cement contents and high fly ash contents are particularly suitable for minimizing autogenous temperature rises. The high-volume fly ash concrete mixes (with around 55 % Class F fly ash) is effective in reducing both the rate of heat development and the maximum temperature reached within the concrete block.
This property can be very advantageous when early-age strength is not important. Higher early-age strengths can be achieved by increasing the cementitious material content of the high-volume fly ash concrete system, although this does result in an increase in the autogenous temperature rise. High-volume fly ash concrete systems have been successfully used in commercial applications to control the temperature rise in large placements.
The rate of heat development normally increases with the calcium content of the ash. Fly ash high in calcium can produce little or no decrease in the heat of hydration (compared to plain Portland cement) when used at normal replacement levels.
However, high-calcium fly ashes can be used to meet performance criteria when used at a sufficient replacement level. High levels of high-calcium (Class C) fly ash have been used to control the temperature rise in mass concrete foundations. Concrete with 50 % Class C fly ash has been used to control temperature while thermocouples have been used to determine when thermal blankets can be removed without causing thermal shock.
Finishing and curing – The use of fly ash can lead to considerable retardation of the setting time, which means that finishing operations are to be delayed. At normal temperatures, the rate of the pozzolanic reaction is slower than the rate of cement hydration, and fly ash concrete needs to be properly cured if the full benefits of its incorporation are to be realized. When high levels of fly ash are used, it is normally advised that the concrete is moist cured for a minimum period of 7 days. If adequate curing cannot be provided in practice, the amount of fly ash used in the concrete is to be limited.
Effect of on the properties of hardened concrete
Compressive strength development – Normally the early-age strength of the concrete decreases when the level of replacement of a certain mass of Portland cement with low-calcium (Class F) fly ash increases. However, long-term strength development is improved when fly ash is used and at some age the strength of the fly ash concrete is equal that of the Portland cement concrete so long as sufficient curing is provided. The age at which strength parity with the control (Portland cement) concrete is achieved is greater with the higher levels of fly ash. The ultimate strength achieved by the concrete increases with increasing fly ash content, at least with replacement levels upto 50 %. Normally the differences in the early-age strength of Portland cement and fly ash concrete are less for fly ash with higher levels of calcium, but this is not always the case.
In many cases, concrete is proportioned to achieve a certain minimum strength at a specified age (typically 28 days). This can be achieved by selecting the appropriate water-to-cementitious materials (w/cm) ratio for the blend of cement and fly ash being used. The w/cm ratio needed varies depending on the level of fly ash replacement, the composition of the ash, and the age and strength specified. If the specified strength is needed at 28 days or earlier this normally needs lower values of w/cm ratio when using higher levels of fly ash. A lower w/cm ratio can be achieved by a combination of (i) reducing the water content by either taking advantage of the lower demand in the presence of fly ash, or by using a water-reducing admixture, or both, and (ii) increasing the total cementitious content of the mix.
The rate of early-age strength development is strongly influenced by temperature, and this is especially the case for fly ash concrete as the pozzolanic reaction is more sensitive to temperature than is the hydration of Portland cement.
Other mechanical properties -The relationships between the tensile strength, flexural strength and elastic modulus, and the compressive strength of concrete are not considerably affected by the presence of fly ash at low and moderate levels of replacement. The long-term flexural and tensile strength of high-volume fly ash concrete can be much improved due to the continuing pozzolanic reaction strengthening the bond between paste and the aggregate. The elastic modulus of high-volume fly ash concrete can be increased due to the presence of significant amounts of unreacted fly ash particles which act as fine aggregate and because of the very low porosity of the interfacial zone.
Creep – The creep of the concrete is influenced by a large number of parameters and the effect of fly ash on creep depends to some extent on how the effect is measured. For example, if loaded at an early age, fly ash concrete can show higher amounts of creep than the Portland cement concrete because it has a lower compressive strength. However, if concretes are loaded at an age when they have attained the some strength, fly ash concrete shows less creep because of its continued strength gain. The creep of high-volume fly ash concrete tends to be lower than Portland cement concrete of the same strength and this has been attributed to the presence of unreacted fly ash. It is also likely that the very low water and paste contents attainable in high-volume fly ash concrete (and concurrently high aggregate content) play an important role in reducing the creep of concrete with high levels of fly ash.
Drying shrinkage – For concrete with dimensionally-stable aggregates, the key parameters affecting drying shrinkage are (i) the amount of water in the mix, (ii) the w/cm ratio, and (iii) the fractional volume of aggregate. In well-cured and properly-proportioned fly ash concrete, where a reduction in the mixing water content is made to take advantage of the reduced water demand resulting from the use of the fly ash, the amount of shrinkage is to be equal to or less than an equivalent Portland cement concrete mix. It has been reported that the drying shrinkage of high volume fly ash concrete is normally less than conventional concrete and this is undoubtedly due to the low amounts of water used in producing such concrete.
Effect of on the durability of concrete
Abrasion resistance – It has been demonstrated that the abrasion resistance of properly finished and cured concrete is primarily a function of the properties of the aggregate and the strength of concrete regardless of the presence of fly ash. This appears to hold true at higher levels of fly ash.
Permeability and resistance to the penetration of chlorides – Fly ash reduces the permeability of concrete to water and gas provided the concrete is adequately cured. This has been attributed to a refinement in the pore structure. It is now more common to use indirect measurement of concrete permeability by rapid chloride permeability test (RCPT). Despite the known limitations of this test (it measures electrical conductivity, not permeability) it does provide a reasonable indication of the ability of concrete to resist chloride penetration. Various parameters (materials, mixture proportions, curing, maturity, etc.) affect the outcome of this test. The data for concretes (w/cm ratio = 0.40) with various levels of fly ash ( around 13 % CaO) shows that at 28 days, the charge passed increases with fly ash content, with the chloride permeability of the concrete containing 56 % fly ash being almost double that of the control concrete without fly ash. However, there is a rapid decrease in the charge passed with time for fly ash concretes, and by 180 days, there is a reversal in the trend with chloride permeability decreasing with increasing fly ash content. After, around 7 years the concretes with 25 %, 40 % and 56 % fly ash are 4 times, 14 times, and 29 times less electrically conductive than the control concrete, respectively. The various test results indicate that the concretes have very low to negligible chloride penetrability. The data from mature high-volume fly ash structures seem to indicate that the concrete becomes nearly impermeable to chlorides.
Steady-state diffusion tests conducted on cement pastes indicate that fly ash reduces the chloride diffusion coefficient, the magnitude of the reduction in short-term laboratory tests with 20 % to 30 % fly ash being anywhere from 2.5 times to 10 times. Testing of concrete exposed to marine environments show that the beneficial effects of fly ash become more significant with time as the concrete containing fly ash shows substantial reductions in chloride penetrability with time.
The concretes are proportioned to provide the same strength at 28 days and contain zero and 50 % fly ash. The concretes are unsaturated at the time of first exposure and there is a fairly rapid penetration of chlorides into both concretes. However, beyond this time there are clearly substantial differences between the two concretes in terms of the resistance to chloride ion penetration. The concrete without fly ash offered little resistance, whereas there is very little increase in the chloride content of the fly ash concrete with time, especially at depth.
Alkali-silica reaction – It is well established that low-calcium (Class F) fly ash is capable of controlling damaging alkali-silica reaction (ASR) in concrete at moderate levels of replacement (20 % to 30 %) and the effect has been ascribed to the reduced concentration of alkali hydroxides in the pore solution when fly ash is present. High-calcium Class C fly ash is less effective in this role. It is seen that most fly ashes with low to moderate calcium oxide and alkali contents (less than 20 % CaO and less than 4 % Na2Oeq) are effective in controlling damaging expansion (for instance, expansion less than 0.04 % at 2 years) when used at a 25 % level of replacement with this aggregate. High-alkali / high-calcium Class C fly ash (higher than 20 % CaO) are less effective and the expansion at 2 years normally increases with the CaO content of the fly ash. Some of the fly ash may have to be used at replacement levels above 50 %.
Normally, the level of fly ash required to suppress deleterious expansion of concrete increases with the (i) increased calcium and alkali content of fly ash, (ii) decreased silica content of fly ash, (iii) increased aggregate reactivity, (iv) increased alkali availability from Portland cement (and other components of the concrete), and (v) increased alkali in the environment (for example, from de-icing or anti-icing salts).
There is a low risk of ASR expansion occurring in the field when very high-volume fly ash concrete with 50 % or more fly ash is used, however, testing is desired when high-calcium fly ash is used. High-alkali fly ash (higher than 5 % Na2Oeq) is not recommended for use with potentially reactive aggregates.
Sulphate resistance – A number of studies have shown that the use of sufficient quantities of low-calcium Class F fly ash can increase the resistance of concrete to chemical attack when the concrete is exposed to sulphate-bearing soils or ground water. However, these studies have also shown that high-calcium Class C fly ash is are normally not effective in this role and can even increase the rate and extent of the sulphate attack. It has been shown that blends of high-C3A Type I Portland cement and high calcium fly ash (higher than 20 % CaO) cannot meet the requirements for moderate sulphate resistance, even when the level of fly ash is increased to 40 % by mass of the total cementitious material.
Studies on concrete exposed to wetting and drying cycles have shown that the principle mechanism of deterioration in this environment is physical sulphate attack due to the formation and crystallization of sodium sulphate. Under these conditions, fly ash does not lead to any significant improvement in the performance even when upto 40 % of a Class F fly ash is used.
Carbonation – The rate of carbonation of properly proportioned and well cured concrete is slow. Provided adequate cover is given to embedded steel reinforcement, carbonation-induced corrosion of the steel is unlikely to occur during the typical service life of a reinforced concrete structure. However, problems with steel corrosion initiated by carbonation are occasionally encountered in concrete structures due to a combination of either poor quality concrete, inadequate curing, or insufficient cover.
It has been documented that concrete containing fly ash carbonates at a similar rate compared with Portland cement concrete of the same 28-day strength. This means that fly ash increases the carbonation rate provided that the basis for comparison is an equal w/cm ratio. It has also been shown that the increase due to fly ash is more pronounced at higher levels of replacement and in poorly-cured concrete of low strength. Even when concretes are compared on the basis of equal strength, concrete with fly ash (especially at high levels of replacement) can carbonate more rapidly in poorly-cured, low strength concrete.
In various studies, the data show that, within a single strength grade, concretes containing fly ash carbonate at a faster rate (especially for lower strength concrete with higher levels of fly ash) after only 1 day moist curing. To achieve similar performance as concrete without fly ash, concrete containing 50 % fly ash is to be moist-cured for an extended period of time or else be designed to have a higher strength.
It has been seen that the use of high levels of fly ash results in much increased carbonation rates. However, the carbonation is not an issue for well-cured high-volume fly ash concrete based on the calculated time-to-corrosion (higher than 200 years) for reinforcing steel with a depth of cover of 40 mm in concrete exposed outdoors. This conclusion is only valid if some conditions are met namely (i) high-volume fly ash concrete is proportioned with a very low w/cm ratio (less than 0.32), (ii) concrete is moist-cured for at least 7 days, (iii) concrete is directly exposed to moisture during service (for example, not protected from precipitation), and (iv) the specified minimum cover requirements (for example, 40 mm) are met. If there are changes in the mixture proportions, if the concrete is not directly exposed to moisture (for example, it is protected from rainfall), and if 7-days curing and 40 mm cover are specified, but not achieved in practice, the time-to-corrosion reduces substantially.
Resistance to cyclic freezing and thawing, and deicer salt scaling – Concrete can be resistant to cyclic freezing and thawing provided it has sufficient strength and an adequate air-void system, and the aggregates are frost-resistant. This holds true for fly ash concrete regardless of the fly ash content. However, a number of laboratory studies have shown that concrete containing fly ash can be less resistant to scaling when subjected to freezing and thawing in the presence of deicer salts and the lower scaling resistance of fly ash concrete is more pronounced in lean concretes (low cementitious material content) or concretes with high levels of cement replaced with fly ash. However, some studies have shown satisfactory performance at levels of fly ash upto 30 % and, in some cases, even higher.
Based on a review of data from laboratory tests and a survey of fly ash concrete structures exposed to de-icing salts the observation are (i) scaling increases as the w/cm ratio increases, (ii) scaling mass loss generally increases with fly ash content, especially at high levels of replacement (higher than 40 % to 50 %), (iii) results from concrete containing fly ash tend to be more variable, and (iv) the use of curing compounds (membranes) reduces scaling which is particularly noticeable for fly ash concrete.
Fly ash concrete is likely to provide satisfactory scaling performance provided the w/cm ratio does not exceed 0.45 and the level of fly ash does not exceed 20 % to 30 %. This, of course, assumes an adequate air-void system is present in the concrete and that proper construction practices are adhered to. High-volume fly ash concrete invariably performs poorly in laboratory scaling tests even when the w/cm ratio is maintained at very low values. The field performance of high-volume fly ash concrete with regards to deicer salt scaling is varied.
Although concrete containing moderate to high volumes of fly ash can be produced to be resistant to freeze-thaw action in the presence of deicer salts, it is apparent that its scaling resistance is more sensitive to mixture proportioning, method of placement, finishing and curing than Portland cement concrete.
Optimizing fly ash content in concrete
The properties of fresh concrete and the mechanical properties and durability of hardened concrete are strongly influenced by the incorporation of the fly ash into the mixture. The extent to which fly ash affects these properties is dependent not only on the level and the composition of the fly ash, but also on other parameters including the composition and proportions of the other ingredients in the concrete mixture, the type and size of the concrete component, the exposure conditions during and after placement, and construction practices. Clearly there is no one replacement level best suited for all applications. When using higher quantity levels in reinforced concrete, consideration is to be given to whether the combination of the concrete quality (w/cm ratio), degree of moist curing, depth of cover, and exposure condition pose a risk of carbonation-induced corrosion.
Application of Fly ash in brick production
A considerable amount of work has been done to find useful applications of fly ash in the brick production. A brick has been introduced using Class C fly ash, water and air entraining agent. When casting the brick, it was compressed at 27 MPa and used steam bath for curing for 24 hours at 66 deg C. This brick has improved freezing and thawing properties, as well as achieved greater durability with lower cost than the traditional clay bricks. For the brick production, 40 % of fly ash with 8 % to 10 % of lime and definite proportion of water is desirable. An advantage in using fly ash is that heat flow resistance can be 15 % to 40 % higher than in quartz sand. Therefore fly ash increases thermal insulation of the final product as well as reduces the heat of hydration in concrete.
High performance fly ash bricks have been produced using fly ash, water and chemical additives such as plasticizer, Carboxymethyl Cellulose (CMC) and calcium chloride (CaCl2) in small quantities. Addition of CaCl2 as an admixture helps to accelerate the curing process considerably. It increases corrosion of any metallic object which can be in contact with these bricks. Raw bricks are cured for three days before firing. These fly ash bricks are around 28 % lighter than clay bricks and possess compressive strength higher than 40 MPa.
Class F Fly ash, with super plasticizers, lime, silica fume, sludge treatment agent and Portland cement is desirable to be used with a water/cement ratio of 0.58 and fine aggregate / cement ratio of 2.75 in making cement based fly ash blocks. Silica fume and sludge treatment agent has been used to improve the strength. Based on 90 day strength, the fly ash blocks can be used as secondary structural members for marine engineering since the leaching of the block’s soluble components has been very low, and hence the solid blocks are considered as environmentally acceptable in the seawater.
A study on the light weight fired bricks from Class F fly ash has shown that the compressive strength of the brick is similar to that of common clay bricks. The study has concluded the optimum composition as a combination of 70/30 for fly ash/common sand with 15 % sodium silicate and 5 % lime to produce best performing brick in terms of strength, mouldability, and water absorption