Concrete is by far the most widely used construction material because of its low cost, availability of raw materials, strength, durability and, most importantly, versatility. Worldwide, more than a ton of concrete is produced every year for each person on the planet, or roughly 2.5 tons per person in North America alone.
The key to concrete’s success has been its versatility; however, continued success will be dictated on another important attribute: sustainability.
The use of SCMs can help contribute significantly to both.
Background on SCMs
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 would go on to construct such engineering marvels as the Roman aqueducts and Coliseum, which still stand today. Early SCMs consisted of natural, readily available materials such as volcanic ash or diatomaceous earth.
More recently, strict air-pollution controls and regulations have produced an abundance of industrial byproducts that can be used as SCMs. Industries are continually looking for innovative ways to use and dispose of industrial byproducts such as fly ash, silica fume and blast furnace slag. The use of such byproducts in concrete construction not only prevents them from being land-filled but also enhances the properties of concrete in the fresh and hydrated states.
Why designers love precast concrete with SCMs
When designers choose materials for a project, they often look to integrate the three R’s: reduce, reuse and recycle. Precast concrete is an excellent choice to achieve all three goals. The use of SCMs in manufacturing concrete can contribute to the first two R’s:
Reduce. Reduce in this context means minimizing waste through reduction of resource use (virgin, reused and recycled) and energy during construction. The use of blended cement or the replacement of portland cement with industrial byproducts such as SCMs reduces the amount of clinker required per cubic yard of concrete, and thus reduces the amount of energy needed and resultant CO2 emissions.
Reuse. Reuse in this context means using again in basically the same form for the same purpose or for a new purpose. When industrial byproducts are used as SCMs or cement replacement, they not only provide a sustainable option through reuse, but also improve concrete properties while reducing cost.
SCMs also have many beneficial effects on concrete, such as lowering permeability, increasing workability and sulfate resistance. The most common cementitious mineral admixtures are silica fume, fly ash and slag cement.
Fly ash. Fly ash is by far the most widely used SCM in the manufactured concrete products industry because of its 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 is 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 bag house filters. According to the American Coal Ash Association, roughly 9.8 million tons of fly ash were used in the production of concrete in the United States during 2010.
Silica fume. Silica fume is an industrial byproduct from the reduction of high-purity quartz during the production of silicon metal or ferrosilicon alloys. Silica fume can be used as an addition to cement, but is usually used as a 5-10% replacement by mass for it. Silica fume is typically more expensive than cement and is considered a property-enhancing material. It 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.
Slag cement. Slag, also referred to as ground granulated blast furnace slag (GGBFS), is a byproduct of smelting iron ore and is primarily comprised of silica, calcium, aluminum, magnesium and oxygen. It is used as a partial cement replacement in concrete either by blending with cement or batching as a separate ingredient.
Slag has been used in proportions of 25 to 70% (by mass of cementitious material), with ranges of 40 to 50% typically optimum for strength (ACI Committee 233 2003). Slag cement can increase workability and delay set time (depending on slag and cement amounts). Hardened property changes include a reduced rate of strength gain with equal or higher compressive strengths at later ages, a higher modulus of rupture at later ages, a reduction in temperature rise during hydration in mass concrete, lower permeability and improved sulfate resistance.
That’s a sustainable argument in which precast products compare favorably with petroleum-based products such as plastics. It’s a strong start, and the cement industry has a solid plan in place to become more sustainable year after year.