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Cement Substitutes

By-products from other manufacturing or electric generating processes can be substituted for cement

Producing cement uses a great deal of energy, so finding a waste product that can substitute for cement makes good environmental sense. According to Environmental Building News (EBN), as much greenhouse gas is created producing the portland cement used in the U.S. as operating 22 million compact cars. In addition, the U.S. imports about 20% of the 100 million metric tons (tonnes) of cement it uses annually, adding to its cost and wasting more energy. Burning coal to make electric power creates a great deal of waste "fly ash," and a smaller amount of slag is created when producing iron in blast furnaces. Coal fly ash, blast furnace slag and other mineral admixtures can substitute for cement in concrete mixes for buildings, saving energy, disposing of a waste product, improving the quality of the concrete, and reducing cost. Cement substitutes should be distinguished from concrete additives, such as plasticizers and air entrainment agents; and from aggregate substitutes, such as ground glass or ground scrap rubber.

Types of Cement Substitutes

Fly ash is one of the byproducts of burning coal to create electric power. Two-thirds of the 55 million tonnes of fly ash produced in the U.S. in 1999 were sent to waste piles, with only 9 million tonnes used to make concrete. The carbon content of fly ash is a major concern. Class C fly ash, most of which is produced in the west from lignite coal, contains little carbon. However, Class F fly ash, produced primarily from anthracite and bituminous coal, contains significant amounts of carbon. Class C and Class F material also differ from each other and from source to source with regard to strength, rate of strength gain, color and weatherability. Insuring a consistent supply is a concern among concrete suppliers.

Slag is a by-product from production of both iron and steel, and ground iron slag from blast furnaces can be used for making concrete. About 12.4 million tonnes of blast furnace slag was used in the U.S. in 1999, of which 2 million tonnes were used in concrete. In addition, another 1.1 million tonnes were imported for use by the construction industry. Because the demand for the product is rising while the supply is falling, new grinding plants are coming on line to process imported slag. The added energy used to ship and grind the slag makes it somewhat less energy-saving than fly ash, but far better than portland cement.

Silica fume was once a cheap waste product; but high demand has made it a high-cost admixture, used primarily for bridges and other structures where top weathering performance and high strength are needed. Concrete made from silica fume is expensive, however, not only because of the material cost, but because the powdery fineness of the fume makes it hard to handle. It is often turned into a slurry before use.

Rice hull ash, as long as quality is controlled, is another material that can be used to replace cement. So far, its use remains in the laboratory stage, although a consistent-quality ash needed for concrete is available from AgroSilica, Inc. in Lake Charles, LA.

Slow Strength Gain

To varying degrees, cement substitutes work in two ways. First, they hydrate and cure like portland cement. Second they are "pozzolans," providing silica that reacts with hydrated lime, an unwanted byproduct of concrete curing. Blast-furnace slag is most like portland cement and least like a pozzolan. Class F fly ash is most like a pozzolan, with Class C fly ash between . While stronger and more durable in the end, it takes more time for pozzolans to gain strength than it does portland cement. For most construction purposes, high early strength is very desirable because it allows quicker finishing of slabs and earlier removal of forms. Reducing the amount of water, in part, can compensate for slow strength gain. Researchers have made concrete in the lab from high-percentages of cement substitute by drastically reducing the water content and adding "superplasticizers" to maintain the required slump, but such mixes are not yet common and may be costly. Mixes with 15% to 25% fly ash, and somewhat higher percentages of slag, can be used in home-building with only modest slowing of strength gain. Higher percentages can be used in footings, where high early strength is typically not important. Precasters and concrete masonry unit (CMU) producers can maintain precise control of the mix, and use more admixture. However, they require high early strength for fast re-use of forms, so precast concrete seldom has high percentages of cement substitutes.

Air Entrainment and Carbon Content

Some fly ash, notably most of the Class F fly ash used in the east, contains high levels of carbon (unburned coal particles resulting from the lower-temperature "low-NOx" burning that improves air quality). Carbon particles absorb the soapy air-entraining chemicals used to improve cold weather performance, and in this way make the air content unpredictable. This problem has led some northern suppliers to substitute slag admixture for fly ash, since slag contains no carbon. The fly ash industry is addressing this problem by processing high-carbon fly ash to remove most of the carbon. Unrelated to carbon, concrete with mineral admixtures may require more air-entraining chemicals to ensure freeze-thaw protection, because the small particles of these minerals can fill voids in the concrete that would otherwise be air bubbles.

Strength

Strength is improved by the substitution of some mineral admixtures for portland cement. Class C fly ash and slag improve strength more than Class F flyash. In applications where high strength is critical (such as high-rise buildings) silica fume is the cement substitute of choice, resulting in compressive strengths of 15,000 psi and higher.

Color

Class C fly ash results in a buff-colored concrete; Class F is a darker grey. Slag concrete is lighter in color with high reflectivity. During curing, slag concrete may show a blue-green mottling, called "greening." However, the color is usually gone from the surface in a week. Its disappearance depends on oxidation, so slag cement is not recommended for swimming pools.

Weatherability

There are three weatherability conditions that cement substitutes help alleviate:

Permeability and Chloride-Induced Corrosion: De-icing salts can migrate through pores in the concrete, break down the passive protective layer around the resteel, and cause corrosion that leads to spalling. The pozzolanic action of cement substitutes removes the calcium hydroxide that makes the concrete permeable, and therefore is highly desirable in roadways. A high percentage of fly ash is not recommended for slabs and paving exposed to the weather because of dusting and scaling of the surface.

Alkali-Silica Reaction (ASR): High-silica aggregates and high-alkali cement (which is becoming more common) can create ASR, which causes internal expansion and crazing of concrete. Cement substitutes, especially slag, remove the alkalinity through pozzolanic action. Class C fly ash varies in this ability, while Class F fly ash is very effective.

Sulfate Attack: Concrete made with 60% or more slag is very effective in mitigating attack by sulfates, found in some arid soils, seawater and wastewater. The pozzolanic action of fly ash also contributes to sulfate resistance.

Although the Federal government and the heavy construction industry have used cement substitutes for decades, and they rely on them for special situations, residential contractors are less familiar with their use. As the fly ash industry develops processes to remove carbon, variations in the composition of fly ash will become less important and will encourage its use. The U.S. blast furnace slag supply is declining and the demand growing, so future growth in its use depends on imports. Silica fume remains costly and difficult to handle, and Rice hull ash and other potential substitutes are not yet being marketed.


Environmental Performance

All cement substitutes have the dual benefit of replacing energy-intensive portland cement, and of using material that would otherwise be landfilled.

Quality and Durability

Strength is improved by the substitution of some mineral admixtures for portland cement. Class C fly ash and slag improve strength more than Class F flyash. In applications where high strength is critical (such as high-rise buildings) silica fume is the cement substitute of choice, resulting in compressive strengths of 15,000 psi and higher.


Easy

Most concrete suppliers stock either fly ash or slag or both, typically in separate silos to allow mixing with portland cement in various percentages. Also available are pre-mixed cements with a specific percentage of portland cement and cement substitute. Pre-mixed cements have the advantage of controlled quality, but limit flexibility in mix design. Rice hull ash is not commercially available as a cement substitute.


Concrete made with 100% silica fume can cost 2-3 times the normal price. Ground transportation also raises the price of substitutes, so they are typically used within 50 miles of a source.


Not Applicable


There are no regulatory code barriers to the use of concrete substitutes that provide equal or better performance than portland cement. The primary prescriptive ASTM Standards for cement substitutes are:

ASTM C618-01 "Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use as a Mineral Admixture in Concrete" is the specification that covers coal fly ash and raw or calcined natural pozzolan for use as a mineral admixture in concrete.

ASTM C595-02a "Standard Specification for Blended Hydraulic Cements" is the specification covering five classes of blended hydraulic cements for both general and special applications, using slag or pozzolan, or both, with portland cement, or portland cement clinker or slag with lime.

The primary performance-based ASTM Standard for cement is:

ASTM C1157-02 "Standard Performance Specification for Hydraulic Cement" is a performance specification covering hydraulic cements for both general and special applications. There are no restrictions on the composition of the cement or its constituents. The specification classifies cements by type based on specific requirements for general use, high early strength, resistance to attack by sulfates, heat of hydration, and low reactivity with alkali-reactive aggregates.


Not Applicable


Cement substitutes should be added to the concrete along with portland cement, rather than added to the raw materials (to avoid the energy loss in reheating the substitute during cement production). Most substitutes are stored in separate silos at the concrete plant and added to batches as required. Concrete made from a mixture of portland cement and 15% to 25% cement substitutes has improved workability. Higher percentages with lower water content reduce workability and require the addition of superplasticizers to maintain a workable slump, raising the cost. Cement substitutes are in common use in precast concrete, including concrete masonry units (cmu) and aerated autoclaved concrete products.


Not Applicable


All cement substitutes have the dual benefit of replacing energy-intensive portland cement, and of using material that would otherwise be landfilled. In the case of blast-furnace slag, some waste product is imported, somewhat reducing its positive energy impact. Small percentages of fly ash or slag will reduce concrete cost by replacing higher-cost portland cement. As the percentage of substitutes rises and water content falls to control strength gain, superplasticizer additives and more precise control begin to raise the cost.

Disclaimer: The information on the system, product or material presented herein is provided for informational purposes only. The technical descriptions, details, requirements, and limitations expressed do not constitute an endorsement, approval, or acceptance of the subject matter by the NAHB Research Center. There are no warranties, either expressed or implied, regarding the accuracy or completeness of this information. Full reproduction, without modification, is permissible.