What to look for when using fly ash in precast products
All precast concrete producers can now use a group of materials called “fly ash” to improve the quality and durability of their products. Fly ash improves concrete’s workability, pumpability, cohesiveness, finish, ultimate strength, and durability as well as solves many problems experienced with concrete today–and all for less cost. Fly ash, however, must be used with care. Without adequate knowledge of its use and taking proper precautions, problems can result in mixing, setting time, strength development, and durability.
What Is Fly Ash?
Fly ash is a group of materials that can vary significantly in composition. It is residue left from burning coal, which is collected on an electrostatic precipitator or in a baghouse. It mixes with flue gases that result when powdered coal is used to produce electric power. Since the oil crisis of the 1970s, the use of coal has increased. In 1992, 460 million metric tons of coal ash were produced worldwide. About 10 percent of this was produced as fly ash in the United States. In 1996, more than 7 million metric tons were used in concrete in the U.S. Economically, it makes sense to use as much of this low-cost ash as possible, especially if it can be used in concrete as a substitute for cement.
Coal is the product of millions of years of decomposing vegetable matter under pressure, and its chemical composition is erratic. In addition, electric companies optimize power production from coal using additives such as flue-gas conditioners, sodium sulfate, oil, and other additives to control corrosion, emissions, and fouling. The resulting fly ash can have a variable composition and contain several additives as well as products from incomplete combustion.
Most fly ash is pozzolanic, which means it’s a siliceous or siliceous-and-aluminous material that reacts with calcium hydroxide to form a cement. When portland cement reacts with water, it produces a hydrated calcium silicate (CSH) and lime. The hydrated silicate develops strength and the lime fills the voids. Properly selected fly ash reacts with the lime to form CSH–the same cementing product as in portland cement. This reaction of fly ash with lime in concrete improves strength. Typically, fly ash is added to structural concrete at 15-35 percent by weight of the cement, but up to 70 percent is added for mass concrete used in dams, roller-compacted concrete pavements, and parking areas. Special care must be taken in selecting fly ash to ensure improved properties in concrete.
There are two classes of fly ash: “F” is made from burning anthracite and/or bituminous coal, and “C” is produced from lignite or subbituminous coal. In Canada, there is a further distinction. When the lime content is 8-20 percent, it is classified Cl, and when it is higher, it is class C.
In the United States and other parts of the world where U.S. standards have been adopted, the chemical part of the specification requires only a combined total of silica, alumina, and iron oxide. It does not specify the amount of silica that reacts with lime to produce added strength. The alumina content could be high in fly ash, which could be detrimental because more sulfate to control its reactivity might be required. Sulfate is added to the cement to control only the setting reactions of the aluminates and ferrites in the cement. However, the amount is limited because expansive reactions are possible after the concrete has set. This amount of sulfate does not take into account the extra aluminates that can be added when fly ash is used. Too much iron oxide will retard the setting time.
Although in ASTM C618, the loss on ignition listed in the table of requirements is less than 6 percent, a footnote actually allows up to 12 percent. Incomplete combustion products such as carbon, which affects air entrainment, water-cement ratio, set, and the concrete’s color, could cause this ignition loss. Fly ash is considered to have met C618’s requirements if the 7- or 28-day strength of a sample with 20 percent fly ash reaches 75 percent of the control strength in an ASTM C109 test.
Both class C fly ash and slag have about 35 percent silica and much lower calcium oxide than portland cement. In most cases, lower calcium oxide means better durability. In some fly ash, alumina and iron oxide can be quite high, leading to lower strength and unusual setting time problems. The carbon content was reported in some to be so high that it was beyond the special footnoted exception in ASTM C618.
The advantages of using fly ash far outweigh the disadvantages. The most important benefit is reduced permeability to water and aggressive chemicals. Properly cured concrete made with fly ash creates a denser product because the size of the pores are reduced. This increases strength and reduces permeability.
Today, there are at least two ways to make fly ash more beneficial: a dry process that involves triboelectric static separation and a wet process based on froth flotation. These procedures generally lower the carbon content and the LOI of fly ash. The cost of an additional storage bin should be easily covered by the reduction in the cost of the concrete and the added benefits to the concrete. Low-carbon fly ash or the use of a better air-entraining agent at a higher-than-usual addition rate can control the problem of freeze-thaw durability.
Advantages in Fresh Concrete
Since fly ash particles are spherical and in the same size range as portland cement, a reduction in the amount of water needed for mixing and placing concrete can be obtained. In precast concrete, this can be translated into better workability, resulting in sharp and distinctive corners and edges with a better surface appearance. This also makes it easier to fill intricate shapes and patterns. Fly ash also benefits precast concrete by reducing permeability, which is the leading cause of premature failure. The use of fly ash can result in better workability, pumpability, cohesiveness, finish, ultimate strength, and durability. The fine particles in fly ash help to reduce bleeding and segregation and improve pumpability and finishing, especially in lean mixes.
Advantages in Hardened Concrete
Strength in concrete depends on many factors, the most important of which is the ratio of water to cement. Good quality fly ash generally improves workability or at least produces the same workability with less water. The reduction in water leads to improved strength. Because some fly ash contains larger or less reactive particles than portland cement, significant hydration can continue for six months or longer, leading to much higher ultimate strength than concrete without fly ash.
There have been several cases in which the early strength of concrete was low, particularly where a significant portion–30 percent or more–of the portland cement was replaced with fly ash. This need not be a serious problem today, since set time is also controlled by many other factors that can be altered to compensate for added fly ash, if necessary.
The observed slow set and low early strength obtained with fly ash has caused a reduction in the amount of this mineral admixture used in concrete. Although some fly ash materials will reduce early strength and slow the setting time it does not have to be the case today. Some fly ash actually accelerates set. The addition of accelerators, plasticizers and/or a small amount of additional CSF, as well as the proper beneficiated fly ash, can mitigate this problem.
Properly proportioned concrete containing fly ash should create a lower cost. Because of the reduced permeability and reduced calcium oxide in properly selected fly ash, it should be less susceptible to the alkali-aggregate reaction. Sulfate and other chemical attacks are reduced when fly ash is added. Fly ash, which has little effect on creep, has been suspected of contributing to corrosion because it reacts with the calcium hydroxide. Fly ash, in fact, does not materially reduce alkalinity, and the reduced permeability helps to protect the concrete from chloride penetration, the cause of rebar corrosion (see Rosenberg’s article on corrosion in the Fall 1999 issue of MC Magazine). A superplasticizer combined with fly ash can be used to make high-performance and high-strength concrete. Concrete containing fly ash generally performs better than plain concrete in drying shrinkage tests.
The quality of fly ash is important–but it can vary. Poor-quality fly ash can have a negative effect on concrete. The principle advantage of fly ash is reduced permeability at a low cost, but fly ash of poor quality can actually increase permeability. Some fly ash, such as that produced in a power plant, is compatible with concrete. Other types of fly ash must be beneficiated, and some types cannot be improved sufficiently for use in concrete.
Some concrete will set slowly when fly ash is used. Though this might be perceived as a disadvantage, it can actually be a benefit by reducing thermal stress. When cement sets, it produces 100 calories per gram so that the temperature of a structure may rise 135 degrees. Certain fly ash can be used to keep the temperature from rising too high (less than 45 degrees). However, concrete with fly ash can set up normally or even rapidly, since many other factors control the set and strength development.
Freeze-thaw durability may not be acceptable with the use of fly ash in concrete. The amount of air entrained in the concrete controls the freeze-thaw durability, and the high carbon content in certain fly ash products absorbs some air entraining agents, reducing the amount of air produced in the concrete, making the concrete susceptible to frost damage. High-carbon fly ash materials tend to use more water and darken the concrete as well. It is not recommended to use a high-carbon (greater than 5 percent) content fly ash, but if it must be used, the proper air content can be reached by increasing the dosage of an air-entraining agent.
Slow set and low early strength need not be consequences of using fly ash. Most of the time, high- fineness and low-carbon fly ash will result in high early strength. Sometimes, additional lime, an accelerator or a superplasticizer will be needed. Fly ash also can be mixed with a small amount of condensed silica fume (CSF) to improve set or early-strength properties. Certainly, careful attention to the mix design and water content is always necessary to obtain proper set and early strength development.
Precasters should try to obtain fly ash with as high a silica content as possible. Silica reacts with lime from cement to produce strength and reduce permeability (class F fly ash should have 50 percent silica content; class C should have 35 percent silica content).
Ask that the water requirement be less than the control, that the color, density and fineness have a minimum variation (<5 percent) and that the strength activity index at 3, 7 and 28 days be 90 percent of the control. If protection from the alkali aggregate reaction is needed, then the fly ash should be tested in ASTM C 441 with 25 percent of the cement replaced with the fly ash. Some class C fly ash will not protect against the alkali-aggregate reaction. Lastly, it is important for the precast concrete producer to test the mix design continually, because fly ash is a group of materials that comes from burning coal.
Dr. Arnie Rosenberg is a former research director at Grace Construction Products and now a guest researcher at the National Institute for Standards and Testing, working on the characterization of fly ash.