by Nick Gromicko, CMI®
Cement substitutes are materials that may be substituted, to some degree, for cement in order to improve different properties, such as strength and longevity. The use of cement substitutes is generally encouraged because of the environmental advantages gained from their diversion from the waste stream, the reduction of the energy required in their re-purposing (as compared to the manufacture of cement), and the conservation of raw materials, such as silica, alumina and iron oxide. In fact, as much greenhouse gas is created during the production of cement used in the U.S. as the operation of 22 million cars. In addition, the U.S. imports about 20% of its cement, which adds to its cost and wastes a great deal of energy, according to Environmental Building News.
Inspectors are more likely to encounter cement substitutes in heavy construction as opposed to residential construction, where contractors are less familiar with their use. Cement substitutes are distinguished from aggregate substitutes, such as ground scrap rubber and ground glass, and concrete additives, such as air-entrapment agents and plasticizers. Inspectors will not be able to visually identify concrete that has had substitutes for concrete incorporated into the mix.
The most common cement substitutes include the following:
Silica fume, also known as microsilica, is a byproduct of the combustion of quartz, coal and wood chips during the production of silicon metals. silica improves compressive strength, bond strength, and the abrasion resistance of concrete. Prior to the 1970s, its release into the atmosphere was permitted, but environmental concerns eventually forced its collection and deposition into landfills. It then became economical for silica fume to be used in various applications, chiefly in high-performance concrete. Consisting of fine silicon dioxide particles that are approximately one-hundredth the size of the average cement particle, silica fume is the cement substitute of choice where high strength is critical, such as in high-rise buildings. Cement that contains silica fume looks darker than ordinary cement. Although a respirator should be worn while handling pure silica fume, a cement-silica fume mix is not considered dangerous to humans.
Fly ash is a fine, light, glassy residue generated during ground- or powdered-coal combustion. Contractors find that fly ash enables cement to flow better in pump hoses and makes it more workable under hand-finishing. It includes substantial amounts of silicon dioxide and calcium oxide, both of which are natural ingredients in coal-bearing rock. Mixed with cement during the construction of the Hoover Dam during the 1930s, it wasn’t until the 1980s when its use in construction became commonplace. There are two types of fly ash:
Class C fly ash is produced from the burning of younger lignite or sub-bituminous coal, and it contains considerably more lime and is stronger than its alternative, Class F fly ash. It is preferable for green building projects and is the standard type of fly ash found in residential applications. Inspectors can identify this type of cement substitute by its buff, tan, or occasionally orange coloring.
Class F fly ash results from the burning of harder, older anthracite and bituminous coal. Excessive carbon, which may be indicated by a dark colored cement mix, means that the coal was not burned thoroughly, which may reduce the concrete’s freeze-thaw resistance.
Fly ash contains a number of hazardous minerals, such as mercury, cadmium, arsenic, lead and selenium. There is little evidence that these substances can leach out of the concrete, although disposal and re-use of cement containing fly ash has raised health concerns.
Slag is a byproduct of the production of iron and steel in blast furnaces. The benefits of the partial substitution of slag for cement are improved durability, reduction of life-cycle costs, lower maintenance costs, and greater concrete sustainability. The molten slag is cooled in water and then ground into a fine powder. Slag is used in very high concentrations, often occupying more than half of the total composition of cement. The energy required to grind and ship slag makes it less energy-efficient than fly ash but better than Portland cement. Slag concrete is reflective and lighter in color than fly ash and silica fume, and it may initially have a blue-green coloring that typically disappears within a week. Known as “greening,” this discoloration will not disappear if the slag is used in swimming pools or other wet locations.
Cement substitutes can alleviate the following types of concrete weathering:
alkali-silica reaction, in which crazing and the expansion of concrete results from the interaction between high-alkali cement and high-silica aggregates. Much of the alkalinity can be removed through the action of slag, while Class F fly ash is also effective;
corrosion, in which de-icing salts migrate through pores in the concrete to corrode the reinforcement steel and rebar. Cement substitutes mitigate this corrosion by removing the calcium hydroxide that makes the concrete permeable; and
sulfate attack, in which concrete is attacked by sulfates that are found in some arid soils, seawater and wastewater. Concrete that incorporates fly ash or are composed of more than 60% slag are effective in limiting attack by sulfates.
In summary, cement substitutes are used to enhance certain qualities of cement and reduce the environmental and financial costs of cement creation.