Supplementary Cementitious Material
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AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Topic Highlighted What Are Supplementary Cementitious Material (SCM)? Fly Ash Introduction Production Classification Advantages Disadvantages Ground Granulated Furnace Blast Slag Introduction Classification GGBFS Production Growth Advantages Disadvantages Silica Fume Introduction World demand for silica fume Advantages Disadvantages
Natural Pozzolans Introduction Metakaolin Rice Husk Standards Governing The Use Of SCM ACI Documents ASTM Documents British Standards
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
What Are Supplementary Cementitious Material (SCM)?
Concrete is a mixture of cement, sand, coarse aggregate and water. The key to concrete’s success is its versatility and no other sector of the construction industry utilizes this attribute more than the manufactured concrete products industry. Concrete can be designed to withstand the harshest environments while taking on the most inspirational forms. Engineers are continually pushing the limits with the help of innovative chemical admixtures and supplementary cementitious materials. The use of SCMs dates back to the ancient Greeks who incorporated volcanic ash with hydraulic lime to create a cementitious mortar. The Greeks passed this knowledge on to the Romans, who constructed such engineering marvels as the Roman aqueducts and the Coliseum, which still stand today. Early SCMs consisted of natural, readily available materials such as volcanic ash or diatomaceous earth. Nowadays, most concrete mixture contains supplementary cementitious material that forms part of the cementitious component. These materials are majority byproducts from other processes or natural materials. The major benefits of SCM is its ability to replace certain amount of Portland cement and still able to display cementitious property, thus reducing the cost of using Portland cement. More recently, strict air-pollution controls and regulations have produced an abundance of industrial byproducts that can be used as supplementary cementitious materials such as fly ash, silica fume and ground granulated blast furnace slag. The use of such byproducts in concrete construction not only prevents these products from being land-filled but also enhances the properties of concrete in the fresh and hydrated states. The SCMs can be divided into two categories based on their type of reaction: hydraulic or pozzolanic. Hydraulic materials react directly with water to form cementitious compounds, while pozzolanic materials which by themselves do not have any cementitious property but when used with Portland cement, react to form cementitious combination. It chemically reacts with calcium hydroxide (CH),
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
a soluble reaction product, in the presence of moisture to form compounds possessing cementing properties. The word “pozzolan” was actually derived from a large deposit of Mt. Vesuvius volcanic ash located near the town of Pozzuoli, Italy. Example of SCM that will be discussed further in this context are fly ash, ground granulated blast furnace slag, silica fume and natural pozzolans.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Fly Ash Introduction A primary goal is a reduction in the use of Portland cement, which is easily achieved by partially replacing it with various cementitious materials, preferably those that are byproducts of industrial processes. Fly ash is by far the most widely used supplementary cementitious material in the manufactured concrete products industry because of low cost (about half that of cement), availability and property-enhancing characteristics. Fly ash is a byproduct of the combustion of ground coal for use in electric power plants. It’s a fine residue of mineral impurities that melt and recrystallize within the air stream moving through the combustion boiler. The material is then collected from exhaust gases using electrostatic precipitators or filters. Fly ash was first used in large-mass concrete structures such as dams to reduce cost and minimize the heat of hydration. Additional research revealed propertyenhancing benefits of fly ash, including resistance to certain harmful chemicals, sulfate attack and alkali silica reaction (ASR). The oil crisis during the 1970s also led to the construction of additional coal-burning power plants throughout the United States creating an abundance of fly ash. Fly ash is a variable material and its composition is determined by the chemical composition of the coal used by the power plant. As a byproduct of coal combustion, it would be a waste product to be disposed of at great cost, if we don’t make good use of it. By utilizing its cementitious properties, we are adding value to it as we “beneficiate” it as a major aspect of green building construction. Besides that, fly ash has an adverse effect on maintaining a stable air-void system, especially for higher carbon content like Class F fly ashes. The carbon content of fly ash is often given by the Loss on Ignition Value (LOI) shown on the material certification report. This value is obtained by drying the sample of ash and then massing it. The sample is then ignited at 750 oC in a muffle furnace. The loss in weight represents the quantity of unburnt carbon present in the material and is often a good indication of how it will affect the air content of the concrete. Trial batches should always be cast prior to using a new material. The air content of the concrete should be measured regularly when using a fly ash with a LOI value greater than 3 percent. Either type of fly ash can be used as a cement replacement to reduce production costs. Class F fly ashes have also been found to improve sulfate resistance better than Class C fly ashes. However, some ashes with high alumina contents are not as effective in improving sulfate resistance.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Production As shown in graph below, the utilization rates vary greatly from country to country, from as low as 3.5% for India to as high as 93.7% for Hong Kong. The relatively low rate of 13.5% in the US is an indication that there is a lot of room for progress.
Weight (Tonne)
Fly Ash Production and Utilization (1995) 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
91.1
62.0
60.0
57.0
Utilized Produced
13.8 0.4 1.3
China
0.6 0.6
Denmark Hong Kong
2.0
India
2.8
4.7
Japan
8.1 4.3
Russia
USA
Country
The following chart shows the global production and utilization of fly ash based on data from the Minerals Yearbook (MYB). For the year 2002, data were recorded from Coal Ash Association’s 2002 Coal Combustion Product (CCP) FLY ASH GLOBAL PRODUCTION AND UTILIZATION
80,000,000 70,000,000
TONNE
60,000,000 50,000,000 Ultilization Production
40,000,000 30,000,000 20,000,000 10,000,000 0 1997
1998
1999 YEAR
2000
2002
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Classification The four main constituents are silicon dioxide (SiO2), aluminum oxide (Al2O3), iron oxide (Fe2O3) and calcium oxide (CaO). ASTM C 618 classifies fly ash based on the sum of the first three constituents (SiO2, Al2O3, Fe2O3). When this sum exceeds 70 percent the Class F designation is given to the material while their sum must exceed only 50 percent to be classified as a Class C fly ash. Class C fly ashes also contain higher levels of calcium oxide usually exceeding 20 percent. Class F fly ashes are pozzolanic in nature while Class C fly ashes react both pozzolanically and hydraulically. Class F fly ashes have lower calcium contents and are typically derived from higher-ranked coals containing clayey mineral impurities. The principal reaction product of Class F fly ash is suggested to be more gel-like and denser than that from Portland cement hydration. Class F fly ashes react slower than Portland cement, compromising the initial strength gain of the fly ash concrete. Longer set times can be expected as the quantity of fly ash increases, therefore finishing operations may need to be delayed. When using fly ash in the manufactured concrete products industry, accelerated curing methods and extended moisture curing should be used to initiate the pozzolanic reaction and improve initial strength gain.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Advantages o Fly ash is still less expensive than Portland cement. o Fly ash is also widely available, namely wherever coal is being burned. o Fly ash can improve certain properties of concrete, such as durability. o It generates less heat of hydration, well suited for mass concrete applications. o The amount of water can be reduced by 2 to 3 percent for every 10 percent of fly ash used to replace cement. o Depending on how fine the fly ash and the content of unburnt carbon, the water reduction can reach up to 6-10 % with a 25 % cement replacement. o It’s possible to replace 100% of Portland cement by fly ash theoretically, but it requires chemical activator. Optimum replacement level is 30%. o Since fly ash is spherical in shape, it greatly improves the workability of fresh concrete acting like small ball bearings during the mixing and placing process. Disadvantages o There is the relatively slow rate of strength development. But this is irrelevant in applications where high early strength is not required. o More significant is the wide variability of its chemical composition and quality, which is the main reason for the low utilization rates. o Customers generally prefer a consistently uniform color. One concrete manufacturer had to discontinue the use of fly ash, because he could not control the color of his product.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Ground Granulated Blast Furnace Slag (GGBFS) Introduction Ground granulated blast furnace slag (GGBFS) is another excellent cementitious material. It’s a byproduct of industrial process, in this case the steel industry. GGBFS is formed when molten iron blast furnace slag is rapidly chilled by immersion in water. It has limited crystal formation, highly cementitious in nature and, ground to cement fineness, hydrates like Portland cement. Although Portland blast furnace slag cement, which is made by intergrinding the granulated slag with Portland cement clinker (blended cement), has been used for more than 60 years, the use of separately ground slag combined with Portland cement as admixture did not start until late 1970s. For many applications it is now recommended to use a blend of Portland cement, fly ash, and GGBFS. Yet, slag is not as widely available as fly ash. The US steel industry is only a faint image of what it was only a few decades ago, and as a result, the slag marketed in some East Coast states is being imported from Italy. Because of its excellent attributes, the cost of slag is comparable to that of Portland cement, so that there is no advantage in this respect. GGBFS can be substituted for cement on a 1:1 basis. In the absence of mix specific data, the substitution of GGBFS should be limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes which will be exposed to deicing salts. While substitution of GGBFS for up to 70 percent in a mix has been used, there appears to be an optimum substitution percentage which produces the greatest 28 day strength. This is typically 50 percent of the total cementitious material but depends on the GGBFS used. Section 4.2.3.2 of ACI 318-89, "Building Code Requirements for Reinforced Concrete," indicates that substitution rates of up to 50 % may be acceptable for concretes exposed to deicing chemicals. In addition, in mass concreting operations, the heat of hydration may be an overruling factor and substitution rates greater than 50 percent may be deemed suitable. In general, the strength development of concrete incorporating slag is slow at 1- 5 days compared with the control concrete. Between 7 and 28 days, the strength approaches the same with control concrete and beyond this period, the strength of the slag concrete exceeds the control concrete.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Classification ASTM C 989-82 and AASHTO M 302 were developed to cover ground GGBFS for use in concrete and mortar. The three grades are 80, 100, and 120. The grade of a GGBFS is based on its activity index. Activity index is the ratio of the compressive strength of a mortar cube made with a 50 percent GGBFScement blend to that of a mortar cube made with a reference cement. For a given mix, the substitution of grade 120 ground granulated blast furnace slag for up to 50 percent of the cement will generally yield a compressive strength at 7 days and beyond equivalent to or greater than that of the same concrete made without GGBFS. Substitution of grade 100 GGBFS will generally yield an equivalent or greater strength at 28 days. However, concrete made with grade 80 GGBFS will have a lower compressive strength at all ages. It is advisable only grades 100 and 120 GGBFS should be used. However, the use of grade 80 slag may be appropriate in mass concrete where the heat of hydration may be an overriding factor. Therefore, grade 80 GGBFS should be avoided unless in unusual circumstances.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
GGBFS Production Growth Since the late 1800s, ground granulated blast furnace slag has been used as a component of blended hydraulic cement. In Europe, typical applications include traditional structures and those exposed to seawater; in the U.S., its use extends to general construction. As a result of granulated blast furnace slag's high quality and the efforts of all manufacturers to conserve energy, the use of blast furnace slag has grown significantly in the U.S. since the 1970s. According to the Slag Cement Association, slag cement consumption has almost tripled since 1996 and is growing faster than any other product in the cementitious materials industry. Association statistics show that in 2003, 3.1 million metric tons of slag cement was shipped for use in construction projects. The tonnage is a combination of slag cement shipped as a separate product (conforming to ASTM C989) and as a component of blended cement (conforming to ASTM C595). The term slag cement refers to 100% ground granulated blast furnace slag (GGBFS) in North America. Actual ferrous slag production data in the United States do not exist because the iron and steel industry does not routinely measure slag output. Slag outputs in iron and steel production are highly variable and depend, for the most part, on the chemistry of the raw materials and the type of furnace. Typically, for an ore feed with 60% to 66% iron, blast furnace slag production ranges from about 220 to 370 kilograms per metric ton of pig iron produced. Steel slag outputs are approximately 20% by mass of the steel output. According to statistics reported by the International Iron and Steel Institute (IISI), Brussels, the U.S. pig iron production was about 53 million and 55 million metric tons (Mt) in l998 and l997, respectively. Thus, the iron slag production for these years was about 12.5 and 13 Mt in 1998 and 1997, respectively. United States steel production for l998 and l997 was reported to be 108 and 107 Mt, respectively. The expected steel slag production was about 17.2 and 17 Mt in 1998 and 1997, respectively. As with the United States, no data are available on world slag production. The IISI reported the world pig iron output to be about 544 Mt and crude steel production to be 783 Mt in 1998. The estimated figure for ferrous slag production from this output was approximately 200 Mt.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Advantages o Reduce expansion caused by alkali-aggregate reaction. o Like fly ash, GGBFS improve mechanical and durability properties of concrete and generates less heat of hydration. o The rate and quantity of bleeding is less because of the relatively higher fineness of slag. o The higher fineness of slag also increases the air-entraining agent required, compared to conventional concrete. o GGBFS improves workability and decreases the water demand due to the increase in paste volume caused by the lower relative density of slag. o Flexural strength is usually improved by the use of slag cement, which makes it beneficial to concrete paving application. o Slag unlike fly ash does not contain carbon, which cause instability and air loss in concrete. o Helps in the transformation of large pores in the paste into smaller pores, resulting in decreased permeability of the matrix and of the concrete. o Significant reduction in permeability is achieved as the replacement level of the slag increases from 40 to 65% especially by mass. o Because of the reduction in permeability, it requires less depth of cover than conventional concrete requires to protect the reinforcing steel. o Air-entrained slag concrete specimens performed excellently in freezethaw tests, with relative durability factors greater than 91%. o Setting times of concretes increases as the slag content increases. This delay can be beneficial in large pours and in hot weather conditions where it prevents the formation of "cold joints" in successive pours. o Slag is effective in preventing damage due to ASR (alkali silica reaction) due to reduction of total alkalies in the cement-slag blend, the lower permeability of the system, and the tying up of the alkalies in the hydration process.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Disadvantages o Scaling resistance of concretes decreases with GGBFS substitution rates greater than 25 %. o The necessity of proper curing should be emphasized with the use of ground granulated blast furnace slag. o Concretes containing ground granulated blast furnace slag may be more susceptible to cracking caused by drying shrinkage. o At low temperatures, the strengths are substantially reduced up to 14 days, and the percentage of slag used is usually reduced to 25-30%. o Setting times of concretes increases when the slag content increases. Increase of slag content (35 to 65%) can extend the setting time by 60 minutes. o Additionally, the set retardation caused by ground granulated blast furnace slag is temperature sensitive and becomes more pronounced at lower temperatures. o In the absence of special circumstances, the use of ground granulated blast furnace slag as a cement replacement limited to grades 100 and 120 GGBFS. o In the absence of special circumstances or mix specific data, the substitution of GGBFS limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes exposed to deicing salts.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Silica Fume Introduction Perhaps the greatest success story in beneficiating an industrial byproduct is that of condensed silica fume or microsilica, a byproduct of the semiconductor industry. Silica fume is an industrial byproduct of high-purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys. Silica is removed from exhaust gases as it cools and condenses into ultra fine droplets of silica glass. Silica fume has a high content of amorphous silicon dioxide (92 to 94% SiO2). Silica fume is regularly used in high-strength concrete applications or in concrete products that will be subjected to abrasive or corrosive environments such as coastal applications, bridge decks or water conveyance structures. Silica fume is available in a variety of forms. In fact, it is now available not only as a byproduct of the semiconductor industry, but also produced specifically for the concrete industry. Silica fume is spherical in shape and is extremely small, having an average diameter of about one-tenth of a micron (0.1 um). An average silica particle is roughly one one-hundredth the size of a cement grain. Silica fume is usually used as a 5 percent to 10 percent replacement by mass for cement. Research has found that when silica fume is used at a 15 percent replacement level, there are roughly 2 million silica fume particles for each grain of cement present.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Silica fume is densified by putting it into a silo and blowing compressed air from the bottom. The particles tumble and stick together. Densified silica is available in bags or bulk and mixing times may need to be increased to ensure that the particles break down adequately. Slurry silica is a water-based material containing roughly 42 percent to 60 percent by mass of silica fume. Since silica fume is a pozzolan and consumed roughly 50 percent of the calcium hydroxide present in the first 28 days when used at a 10 percent replacement level under normal curing conditions. The pozzolanic reaction is extremely sensitive to temperature and greatly accelerated by steam curing and other accelerated curing methods. World demand for silica fume Worldwide demand for silica fume was projected to rise 4.1% annually through 2006 to 2.1 billion metric tons, although advances were expected to be less robust in more developed areas such as the United States, Japan, and Western Europe.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Advantages o It improves both strength and durability of concrete to such an extent that modern designs rule call for the addition of silica fume. o Silica fume can be used to increase the compressive strength of lightweight concrete as well as conventional concrete o It reduce the wall thickness or other dimensions product and overcome transportation limitations o Silica fume particles pack around the aggregate more efficiently, modifying the paste structure and preventing bleeding. o Silica fume ultimately reduces the porosity of the hydrated cement matrix through improved particle packing. o Silica fume fills the voids between cement particles just as cement fills the voids between sand and coarse aggregate. o It fills up a critical region known as the interfacial transition zone (ITZ) and in conventional concrete is characterized by a massive calcium hydroxide layer laden with voids (weak link between the paste and aggregates) Disadvantages o Silica fume is typically more expensive than cement o The material is difficult to handle because of its extreme fineness. o It’s easy to become airborne, thus raising health issue concerns. o Silica fume concrete is slightly darker in color and has been reported being “sticky” during finishing. o Due to its small size and high surface area (20,000 m2/kg), silica fume minimizes bleeding, which may lead to plastic shrinkage cracking. o Water demand is higher because of its high surface area. o Dispersing agent need to be used to overcome surface forces and ensuring adequate particle dispersion.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Natural Pozzolans Introduction Various naturally occurring materials possess, or can be processed to possess pozzolanic properties. These materials are also covered under the standard specification ASTM C618. Natural pozzolans such as metakaolin and calcined shale or clay are manufactured diatomaceous earth A light soil consisting by controlled calcining (firing) of naturally occurring of siliceous diatom minerals. Metakaolin is produced from relatively pure remains and often kaolinite clay and it is used at 5% to 15% by mass of the used as a filtering cementitious materials. Calcined shale or clay is used at material higher percentage by mass. Other natural pozzolans include volcanic glass, zeolitic trass or tuffs, rice husk ash and diatomaceous earth. The ancient Greeks and Romans used a combination of lime and volcanic ash to make a cementitious mortar to construct many of the impressive monuments which still stand today. The earliest known use of a pozzolan actually dates back to about 4500 BC. It consisted of a mixture of lime and diatomaceous earth from the Persian Gulf. ACI defines natural pozzolans as “either a raw or calcined natural material that has pozzolanic properties.” Calcining is the process of altering the composition or physical state by heating a material below the temperature of fusion. Sources of natural pozzolans that do not require calcining to increase their reactivity are typically located west of the Mississippi River. The price and availability of raw or processed natural pozzolans is dependent on the location of such materials. Metakaolin Research has indicated that most natural pozzolans produce hardened concrete properties similar to industrial byproduct pozzolans. Some investigators have even reported that natural pozzolans are more effective in controlling alkali silica reaction than fly ash. More reactive pozzolans such as metakaolin and rice husk ash are often used in the same manner and proportions as silica fume. Metakaolin is a calcined or “thermally activated” clay and is produced with high purity kaolin-containing clay that is purified by water processing prior to low temperature thermal activation between 600 and 900 oC. The material is then ground to a very high fineness (0.5 to 20 um) and marketed as
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
high-reactivity metakaolin. Research has shown that calcium hydroxide produced during cement hydration will be completely consumed when highreactivity metakaolin is used at a 20 percent replacement level. Metakaolin will increase the concrete’s strength, reduce permeability and improve workability when a water reducing agent is used. Metakaolin is white in color and ideal for use in architectural concrete. Rice Husk Rice husk ash (RHA) is a natural byproduct from the processing of paddy rice. The husks, which are approximately 50 percent cellulose, 30 percent lignin and 20 percent silica, are incinerated by controlled combustion leaving behind an ash that predominantly consists of amorphous silica. Rice husk ash is highly pozzolanic due to its extremely high surface area (50,000 to 100,000 m2/kg). Research has shown that higher compressive strengths, decreased permeability, resistance to sulfate and acid attack, and resistance to chloride penetration can all be expected when a high-quality RHA is used in amounts of 5 percent to 15 percent by mass of cement.
Global production of rice, the majority of which is grown in Asia, is approximately 550 million tonnes/year. The milling of rice generates a waste material - the husk surrounding the rice grain. This is generated at a rate of about 20% of the weight of the product rice, or some 110 million tonnes per year globally. The husk in turn contains between 15 and 20% of mineral matter the majority of which is amorphous silica. There is a growing demand for finely divided amorphous silica in the production of high strength, low permeability
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
concrete, for use in bridges, marine environments, and nuclear power plants. This market is currently filled by silica fume. Limited supply and high demand has pushed the price of silica fume to as much as US$1,000/tonne in some markets. Rice husk has the potential to generate 16.5 to 22 million tonnes of ash containing over 90% amorphous silica that could be used as a substitute for silica fume.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Standards Governing The Use Of SCM ACI Documents ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete” ACI 232.1, “Use of Raw or Processed Natural Pozzolans in Concrete” ACI 232.2, “Use of Fly Ash in Concrete” ACI 234, “Guide for the Use of Silica Fume in Concrete” ASTM Documents ASTM C989-05, “Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars” ASTM C 618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolans for Use in Concrete” ASTM C 1240, “Standard Specification for Silica Fume Used in Cementitious Mixtures” British Standards BS 6610:1996 Specification for Pozzolanic pulverized-fuel ash cement BS 3892-2:1996 Pulverized-fuel ash. Specification for pulverized-fuel ash to be used as a Type I addition BS 3892-1:1997 Pulverized-fuel ash. Specification for pulverized-fuel ash for use with Portland cement BS 3892-3:1997 Pulverized-fuel ash. Specification for pulverized-fuel ash for use in cementitious grouts BS EN 14227-3:2004 Hydraulically bound mixtures. Specifications. Fly ash bound mixtures BS EN 14227-4:2004 Hydraulically bound mixtures. Specifications. Fly ash for hydraulically bound mixtures BS EN 450-1:2005 Fly ash for concrete. Definition, specifications and conformity criteria BS EN 450-2:2005 Fly ash for concrete. Conformity Evaluation
Topic Highlighted What Are Supplementary Cementitious Material (SCM)? Fly Ash Introduction Production Classification Advantages Disadvantages Ground Granulated Furnace Blast Slag Introduction Classification GGBFS Production Growth Advantages Disadvantages Silica Fume Introduction World demand for silica fume Advantages Disadvantages
Natural Pozzolans Introduction Metakaolin Rice Husk Standards Governing The Use Of SCM ACI Documents ASTM Documents British Standards
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
What Are Supplementary Cementitious Material (SCM)?
Concrete is a mixture of cement, sand, coarse aggregate and water. The key to concrete’s success is its versatility and no other sector of the construction industry utilizes this attribute more than the manufactured concrete products industry. Concrete can be designed to withstand the harshest environments while taking on the most inspirational forms. Engineers are continually pushing the limits with the help of innovative chemical admixtures and supplementary cementitious materials. The use of SCMs dates back to the ancient Greeks who incorporated volcanic ash with hydraulic lime to create a cementitious mortar. The Greeks passed this knowledge on to the Romans, who constructed such engineering marvels as the Roman aqueducts and the Coliseum, which still stand today. Early SCMs consisted of natural, readily available materials such as volcanic ash or diatomaceous earth. Nowadays, most concrete mixture contains supplementary cementitious material that forms part of the cementitious component. These materials are majority byproducts from other processes or natural materials. The major benefits of SCM is its ability to replace certain amount of Portland cement and still able to display cementitious property, thus reducing the cost of using Portland cement. More recently, strict air-pollution controls and regulations have produced an abundance of industrial byproducts that can be used as supplementary cementitious materials such as fly ash, silica fume and ground granulated blast furnace slag. The use of such byproducts in concrete construction not only prevents these products from being land-filled but also enhances the properties of concrete in the fresh and hydrated states. The SCMs can be divided into two categories based on their type of reaction: hydraulic or pozzolanic. Hydraulic materials react directly with water to form cementitious compounds, while pozzolanic materials which by themselves do not have any cementitious property but when used with Portland cement, react to form cementitious combination. It chemically reacts with calcium hydroxide (CH),
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
a soluble reaction product, in the presence of moisture to form compounds possessing cementing properties. The word “pozzolan” was actually derived from a large deposit of Mt. Vesuvius volcanic ash located near the town of Pozzuoli, Italy. Example of SCM that will be discussed further in this context are fly ash, ground granulated blast furnace slag, silica fume and natural pozzolans.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Fly Ash Introduction A primary goal is a reduction in the use of Portland cement, which is easily achieved by partially replacing it with various cementitious materials, preferably those that are byproducts of industrial processes. Fly ash is by far the most widely used supplementary cementitious material in the manufactured concrete products industry because of low cost (about half that of cement), availability and property-enhancing characteristics. Fly ash is a byproduct of the combustion of ground coal for use in electric power plants. It’s a fine residue of mineral impurities that melt and recrystallize within the air stream moving through the combustion boiler. The material is then collected from exhaust gases using electrostatic precipitators or filters. Fly ash was first used in large-mass concrete structures such as dams to reduce cost and minimize the heat of hydration. Additional research revealed propertyenhancing benefits of fly ash, including resistance to certain harmful chemicals, sulfate attack and alkali silica reaction (ASR). The oil crisis during the 1970s also led to the construction of additional coal-burning power plants throughout the United States creating an abundance of fly ash. Fly ash is a variable material and its composition is determined by the chemical composition of the coal used by the power plant. As a byproduct of coal combustion, it would be a waste product to be disposed of at great cost, if we don’t make good use of it. By utilizing its cementitious properties, we are adding value to it as we “beneficiate” it as a major aspect of green building construction. Besides that, fly ash has an adverse effect on maintaining a stable air-void system, especially for higher carbon content like Class F fly ashes. The carbon content of fly ash is often given by the Loss on Ignition Value (LOI) shown on the material certification report. This value is obtained by drying the sample of ash and then massing it. The sample is then ignited at 750 oC in a muffle furnace. The loss in weight represents the quantity of unburnt carbon present in the material and is often a good indication of how it will affect the air content of the concrete. Trial batches should always be cast prior to using a new material. The air content of the concrete should be measured regularly when using a fly ash with a LOI value greater than 3 percent. Either type of fly ash can be used as a cement replacement to reduce production costs. Class F fly ashes have also been found to improve sulfate resistance better than Class C fly ashes. However, some ashes with high alumina contents are not as effective in improving sulfate resistance.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Production As shown in graph below, the utilization rates vary greatly from country to country, from as low as 3.5% for India to as high as 93.7% for Hong Kong. The relatively low rate of 13.5% in the US is an indication that there is a lot of room for progress.
Weight (Tonne)
Fly Ash Production and Utilization (1995) 100.0 90.0 80.0 70.0 60.0 50.0 40.0 30.0 20.0 10.0 0.0
91.1
62.0
60.0
57.0
Utilized Produced
13.8 0.4 1.3
China
0.6 0.6
Denmark Hong Kong
2.0
India
2.8
4.7
Japan
8.1 4.3
Russia
USA
Country
The following chart shows the global production and utilization of fly ash based on data from the Minerals Yearbook (MYB). For the year 2002, data were recorded from Coal Ash Association’s 2002 Coal Combustion Product (CCP) FLY ASH GLOBAL PRODUCTION AND UTILIZATION
80,000,000 70,000,000
TONNE
60,000,000 50,000,000 Ultilization Production
40,000,000 30,000,000 20,000,000 10,000,000 0 1997
1998
1999 YEAR
2000
2002
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Classification The four main constituents are silicon dioxide (SiO2), aluminum oxide (Al2O3), iron oxide (Fe2O3) and calcium oxide (CaO). ASTM C 618 classifies fly ash based on the sum of the first three constituents (SiO2, Al2O3, Fe2O3). When this sum exceeds 70 percent the Class F designation is given to the material while their sum must exceed only 50 percent to be classified as a Class C fly ash. Class C fly ashes also contain higher levels of calcium oxide usually exceeding 20 percent. Class F fly ashes are pozzolanic in nature while Class C fly ashes react both pozzolanically and hydraulically. Class F fly ashes have lower calcium contents and are typically derived from higher-ranked coals containing clayey mineral impurities. The principal reaction product of Class F fly ash is suggested to be more gel-like and denser than that from Portland cement hydration. Class F fly ashes react slower than Portland cement, compromising the initial strength gain of the fly ash concrete. Longer set times can be expected as the quantity of fly ash increases, therefore finishing operations may need to be delayed. When using fly ash in the manufactured concrete products industry, accelerated curing methods and extended moisture curing should be used to initiate the pozzolanic reaction and improve initial strength gain.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Advantages o Fly ash is still less expensive than Portland cement. o Fly ash is also widely available, namely wherever coal is being burned. o Fly ash can improve certain properties of concrete, such as durability. o It generates less heat of hydration, well suited for mass concrete applications. o The amount of water can be reduced by 2 to 3 percent for every 10 percent of fly ash used to replace cement. o Depending on how fine the fly ash and the content of unburnt carbon, the water reduction can reach up to 6-10 % with a 25 % cement replacement. o It’s possible to replace 100% of Portland cement by fly ash theoretically, but it requires chemical activator. Optimum replacement level is 30%. o Since fly ash is spherical in shape, it greatly improves the workability of fresh concrete acting like small ball bearings during the mixing and placing process. Disadvantages o There is the relatively slow rate of strength development. But this is irrelevant in applications where high early strength is not required. o More significant is the wide variability of its chemical composition and quality, which is the main reason for the low utilization rates. o Customers generally prefer a consistently uniform color. One concrete manufacturer had to discontinue the use of fly ash, because he could not control the color of his product.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Ground Granulated Blast Furnace Slag (GGBFS) Introduction Ground granulated blast furnace slag (GGBFS) is another excellent cementitious material. It’s a byproduct of industrial process, in this case the steel industry. GGBFS is formed when molten iron blast furnace slag is rapidly chilled by immersion in water. It has limited crystal formation, highly cementitious in nature and, ground to cement fineness, hydrates like Portland cement. Although Portland blast furnace slag cement, which is made by intergrinding the granulated slag with Portland cement clinker (blended cement), has been used for more than 60 years, the use of separately ground slag combined with Portland cement as admixture did not start until late 1970s. For many applications it is now recommended to use a blend of Portland cement, fly ash, and GGBFS. Yet, slag is not as widely available as fly ash. The US steel industry is only a faint image of what it was only a few decades ago, and as a result, the slag marketed in some East Coast states is being imported from Italy. Because of its excellent attributes, the cost of slag is comparable to that of Portland cement, so that there is no advantage in this respect. GGBFS can be substituted for cement on a 1:1 basis. In the absence of mix specific data, the substitution of GGBFS should be limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes which will be exposed to deicing salts. While substitution of GGBFS for up to 70 percent in a mix has been used, there appears to be an optimum substitution percentage which produces the greatest 28 day strength. This is typically 50 percent of the total cementitious material but depends on the GGBFS used. Section 4.2.3.2 of ACI 318-89, "Building Code Requirements for Reinforced Concrete," indicates that substitution rates of up to 50 % may be acceptable for concretes exposed to deicing chemicals. In addition, in mass concreting operations, the heat of hydration may be an overruling factor and substitution rates greater than 50 percent may be deemed suitable. In general, the strength development of concrete incorporating slag is slow at 1- 5 days compared with the control concrete. Between 7 and 28 days, the strength approaches the same with control concrete and beyond this period, the strength of the slag concrete exceeds the control concrete.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Classification ASTM C 989-82 and AASHTO M 302 were developed to cover ground GGBFS for use in concrete and mortar. The three grades are 80, 100, and 120. The grade of a GGBFS is based on its activity index. Activity index is the ratio of the compressive strength of a mortar cube made with a 50 percent GGBFScement blend to that of a mortar cube made with a reference cement. For a given mix, the substitution of grade 120 ground granulated blast furnace slag for up to 50 percent of the cement will generally yield a compressive strength at 7 days and beyond equivalent to or greater than that of the same concrete made without GGBFS. Substitution of grade 100 GGBFS will generally yield an equivalent or greater strength at 28 days. However, concrete made with grade 80 GGBFS will have a lower compressive strength at all ages. It is advisable only grades 100 and 120 GGBFS should be used. However, the use of grade 80 slag may be appropriate in mass concrete where the heat of hydration may be an overriding factor. Therefore, grade 80 GGBFS should be avoided unless in unusual circumstances.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
GGBFS Production Growth Since the late 1800s, ground granulated blast furnace slag has been used as a component of blended hydraulic cement. In Europe, typical applications include traditional structures and those exposed to seawater; in the U.S., its use extends to general construction. As a result of granulated blast furnace slag's high quality and the efforts of all manufacturers to conserve energy, the use of blast furnace slag has grown significantly in the U.S. since the 1970s. According to the Slag Cement Association, slag cement consumption has almost tripled since 1996 and is growing faster than any other product in the cementitious materials industry. Association statistics show that in 2003, 3.1 million metric tons of slag cement was shipped for use in construction projects. The tonnage is a combination of slag cement shipped as a separate product (conforming to ASTM C989) and as a component of blended cement (conforming to ASTM C595). The term slag cement refers to 100% ground granulated blast furnace slag (GGBFS) in North America. Actual ferrous slag production data in the United States do not exist because the iron and steel industry does not routinely measure slag output. Slag outputs in iron and steel production are highly variable and depend, for the most part, on the chemistry of the raw materials and the type of furnace. Typically, for an ore feed with 60% to 66% iron, blast furnace slag production ranges from about 220 to 370 kilograms per metric ton of pig iron produced. Steel slag outputs are approximately 20% by mass of the steel output. According to statistics reported by the International Iron and Steel Institute (IISI), Brussels, the U.S. pig iron production was about 53 million and 55 million metric tons (Mt) in l998 and l997, respectively. Thus, the iron slag production for these years was about 12.5 and 13 Mt in 1998 and 1997, respectively. United States steel production for l998 and l997 was reported to be 108 and 107 Mt, respectively. The expected steel slag production was about 17.2 and 17 Mt in 1998 and 1997, respectively. As with the United States, no data are available on world slag production. The IISI reported the world pig iron output to be about 544 Mt and crude steel production to be 783 Mt in 1998. The estimated figure for ferrous slag production from this output was approximately 200 Mt.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Advantages o Reduce expansion caused by alkali-aggregate reaction. o Like fly ash, GGBFS improve mechanical and durability properties of concrete and generates less heat of hydration. o The rate and quantity of bleeding is less because of the relatively higher fineness of slag. o The higher fineness of slag also increases the air-entraining agent required, compared to conventional concrete. o GGBFS improves workability and decreases the water demand due to the increase in paste volume caused by the lower relative density of slag. o Flexural strength is usually improved by the use of slag cement, which makes it beneficial to concrete paving application. o Slag unlike fly ash does not contain carbon, which cause instability and air loss in concrete. o Helps in the transformation of large pores in the paste into smaller pores, resulting in decreased permeability of the matrix and of the concrete. o Significant reduction in permeability is achieved as the replacement level of the slag increases from 40 to 65% especially by mass. o Because of the reduction in permeability, it requires less depth of cover than conventional concrete requires to protect the reinforcing steel. o Air-entrained slag concrete specimens performed excellently in freezethaw tests, with relative durability factors greater than 91%. o Setting times of concretes increases as the slag content increases. This delay can be beneficial in large pours and in hot weather conditions where it prevents the formation of "cold joints" in successive pours. o Slag is effective in preventing damage due to ASR (alkali silica reaction) due to reduction of total alkalies in the cement-slag blend, the lower permeability of the system, and the tying up of the alkalies in the hydration process.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Disadvantages o Scaling resistance of concretes decreases with GGBFS substitution rates greater than 25 %. o The necessity of proper curing should be emphasized with the use of ground granulated blast furnace slag. o Concretes containing ground granulated blast furnace slag may be more susceptible to cracking caused by drying shrinkage. o At low temperatures, the strengths are substantially reduced up to 14 days, and the percentage of slag used is usually reduced to 25-30%. o Setting times of concretes increases when the slag content increases. Increase of slag content (35 to 65%) can extend the setting time by 60 minutes. o Additionally, the set retardation caused by ground granulated blast furnace slag is temperature sensitive and becomes more pronounced at lower temperatures. o In the absence of special circumstances, the use of ground granulated blast furnace slag as a cement replacement limited to grades 100 and 120 GGBFS. o In the absence of special circumstances or mix specific data, the substitution of GGBFS limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes exposed to deicing salts.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Silica Fume Introduction Perhaps the greatest success story in beneficiating an industrial byproduct is that of condensed silica fume or microsilica, a byproduct of the semiconductor industry. Silica fume is an industrial byproduct of high-purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys. Silica is removed from exhaust gases as it cools and condenses into ultra fine droplets of silica glass. Silica fume has a high content of amorphous silicon dioxide (92 to 94% SiO2). Silica fume is regularly used in high-strength concrete applications or in concrete products that will be subjected to abrasive or corrosive environments such as coastal applications, bridge decks or water conveyance structures. Silica fume is available in a variety of forms. In fact, it is now available not only as a byproduct of the semiconductor industry, but also produced specifically for the concrete industry. Silica fume is spherical in shape and is extremely small, having an average diameter of about one-tenth of a micron (0.1 um). An average silica particle is roughly one one-hundredth the size of a cement grain. Silica fume is usually used as a 5 percent to 10 percent replacement by mass for cement. Research has found that when silica fume is used at a 15 percent replacement level, there are roughly 2 million silica fume particles for each grain of cement present.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Silica fume is densified by putting it into a silo and blowing compressed air from the bottom. The particles tumble and stick together. Densified silica is available in bags or bulk and mixing times may need to be increased to ensure that the particles break down adequately. Slurry silica is a water-based material containing roughly 42 percent to 60 percent by mass of silica fume. Since silica fume is a pozzolan and consumed roughly 50 percent of the calcium hydroxide present in the first 28 days when used at a 10 percent replacement level under normal curing conditions. The pozzolanic reaction is extremely sensitive to temperature and greatly accelerated by steam curing and other accelerated curing methods. World demand for silica fume Worldwide demand for silica fume was projected to rise 4.1% annually through 2006 to 2.1 billion metric tons, although advances were expected to be less robust in more developed areas such as the United States, Japan, and Western Europe.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Advantages o It improves both strength and durability of concrete to such an extent that modern designs rule call for the addition of silica fume. o Silica fume can be used to increase the compressive strength of lightweight concrete as well as conventional concrete o It reduce the wall thickness or other dimensions product and overcome transportation limitations o Silica fume particles pack around the aggregate more efficiently, modifying the paste structure and preventing bleeding. o Silica fume ultimately reduces the porosity of the hydrated cement matrix through improved particle packing. o Silica fume fills the voids between cement particles just as cement fills the voids between sand and coarse aggregate. o It fills up a critical region known as the interfacial transition zone (ITZ) and in conventional concrete is characterized by a massive calcium hydroxide layer laden with voids (weak link between the paste and aggregates) Disadvantages o Silica fume is typically more expensive than cement o The material is difficult to handle because of its extreme fineness. o It’s easy to become airborne, thus raising health issue concerns. o Silica fume concrete is slightly darker in color and has been reported being “sticky” during finishing. o Due to its small size and high surface area (20,000 m2/kg), silica fume minimizes bleeding, which may lead to plastic shrinkage cracking. o Water demand is higher because of its high surface area. o Dispersing agent need to be used to overcome surface forces and ensuring adequate particle dispersion.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Natural Pozzolans Introduction Various naturally occurring materials possess, or can be processed to possess pozzolanic properties. These materials are also covered under the standard specification ASTM C618. Natural pozzolans such as metakaolin and calcined shale or clay are manufactured diatomaceous earth A light soil consisting by controlled calcining (firing) of naturally occurring of siliceous diatom minerals. Metakaolin is produced from relatively pure remains and often kaolinite clay and it is used at 5% to 15% by mass of the used as a filtering cementitious materials. Calcined shale or clay is used at material higher percentage by mass. Other natural pozzolans include volcanic glass, zeolitic trass or tuffs, rice husk ash and diatomaceous earth. The ancient Greeks and Romans used a combination of lime and volcanic ash to make a cementitious mortar to construct many of the impressive monuments which still stand today. The earliest known use of a pozzolan actually dates back to about 4500 BC. It consisted of a mixture of lime and diatomaceous earth from the Persian Gulf. ACI defines natural pozzolans as “either a raw or calcined natural material that has pozzolanic properties.” Calcining is the process of altering the composition or physical state by heating a material below the temperature of fusion. Sources of natural pozzolans that do not require calcining to increase their reactivity are typically located west of the Mississippi River. The price and availability of raw or processed natural pozzolans is dependent on the location of such materials. Metakaolin Research has indicated that most natural pozzolans produce hardened concrete properties similar to industrial byproduct pozzolans. Some investigators have even reported that natural pozzolans are more effective in controlling alkali silica reaction than fly ash. More reactive pozzolans such as metakaolin and rice husk ash are often used in the same manner and proportions as silica fume. Metakaolin is a calcined or “thermally activated” clay and is produced with high purity kaolin-containing clay that is purified by water processing prior to low temperature thermal activation between 600 and 900 oC. The material is then ground to a very high fineness (0.5 to 20 um) and marketed as
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
high-reactivity metakaolin. Research has shown that calcium hydroxide produced during cement hydration will be completely consumed when highreactivity metakaolin is used at a 20 percent replacement level. Metakaolin will increase the concrete’s strength, reduce permeability and improve workability when a water reducing agent is used. Metakaolin is white in color and ideal for use in architectural concrete. Rice Husk Rice husk ash (RHA) is a natural byproduct from the processing of paddy rice. The husks, which are approximately 50 percent cellulose, 30 percent lignin and 20 percent silica, are incinerated by controlled combustion leaving behind an ash that predominantly consists of amorphous silica. Rice husk ash is highly pozzolanic due to its extremely high surface area (50,000 to 100,000 m2/kg). Research has shown that higher compressive strengths, decreased permeability, resistance to sulfate and acid attack, and resistance to chloride penetration can all be expected when a high-quality RHA is used in amounts of 5 percent to 15 percent by mass of cement.
Global production of rice, the majority of which is grown in Asia, is approximately 550 million tonnes/year. The milling of rice generates a waste material - the husk surrounding the rice grain. This is generated at a rate of about 20% of the weight of the product rice, or some 110 million tonnes per year globally. The husk in turn contains between 15 and 20% of mineral matter the majority of which is amorphous silica. There is a growing demand for finely divided amorphous silica in the production of high strength, low permeability
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
concrete, for use in bridges, marine environments, and nuclear power plants. This market is currently filled by silica fume. Limited supply and high demand has pushed the price of silica fume to as much as US$1,000/tonne in some markets. Rice husk has the potential to generate 16.5 to 22 million tonnes of ash containing over 90% amorphous silica that could be used as a substitute for silica fume.
AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com
Standards Governing The Use Of SCM ACI Documents ACI 211.1, “Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete” ACI 232.1, “Use of Raw or Processed Natural Pozzolans in Concrete” ACI 232.2, “Use of Fly Ash in Concrete” ACI 234, “Guide for the Use of Silica Fume in Concrete” ASTM Documents ASTM C989-05, “Standard Specification for Ground Granulated Blast-Furnace Slag for Use in Concrete and Mortars” ASTM C 618, “Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolans for Use in Concrete” ASTM C 1240, “Standard Specification for Silica Fume Used in Cementitious Mixtures” British Standards BS 6610:1996 Specification for Pozzolanic pulverized-fuel ash cement BS 3892-2:1996 Pulverized-fuel ash. Specification for pulverized-fuel ash to be used as a Type I addition BS 3892-1:1997 Pulverized-fuel ash. Specification for pulverized-fuel ash for use with Portland cement BS 3892-3:1997 Pulverized-fuel ash. Specification for pulverized-fuel ash for use in cementitious grouts BS EN 14227-3:2004 Hydraulically bound mixtures. Specifications. Fly ash bound mixtures BS EN 14227-4:2004 Hydraulically bound mixtures. Specifications. Fly ash for hydraulically bound mixtures BS EN 450-1:2005 Fly ash for concrete. Definition, specifications and conformity criteria BS EN 450-2:2005 Fly ash for concrete. Conformity Evaluation
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