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Building products that have a portion of their constituent materials from recycled products reduce the need for virgin materials in new construction. Using recycled materials reduces the need to landfill these materials. It also reduces the environmental impacts from extracting and processing virgin materials.
 
Post-consumer recycled material is defined as waste generated by households or by commercial, industrial and institutional facilities as the end-users of a product. These are products that are sold and used for a specific purpose and then need to be disposed, such as newspaper, containers, computers, and batteries. Post-consumer materials include crushed concrete and masonry from demolished buildings that are reused as aggregate for concrete in new buildings.
 
Post-Industrial material is defined as material from the waste stream of a manufacturing process. These include materials such as fly ash and slag cement which are not manufactured to be sold, but are the result of a manufacturing process for another product.

 

Concrete incorporates three major types of recycled materials:
  1. Fly ash, slag cement, and silica fume are industrial by-products that are used as a partial replacement for portland cement in concrete. These supplementary cementitious materials (SCMs) are pre-consumer materials.
  2. Recycled material or recycled concrete can be used as aggregates in concrete.
  3. Spent solvents, used oils, tires, and medical waste are used as fuel in many cement plants. Industrial byproducts are used as ingredients for manufacturing portland cement.

Fly Ash, Slag Cement, and Silica Fume (SCMs)
Fly ash, slag cement, and silica fume are industrial by-products; their use as a partial replacement for portland cement does not contribute to the energy and CO2 impacts of cement in concrete. If not used in concrete, these materials would use valuable landfill space. Fly ash is a by-product of the combustion of pulverized coal in electric power generating plants. Slag cement, also called ground granulated blast furnace slag, is made from iron blast-furnace slag. Silica fume is a by-product from the electric arc furnace used in the production of silicon or ferrosilicon alloy. These types of industrial by-products are considered post-industrial or pre-consumer recycled materials.


Fly ash, Slag Cement and Silica Fume
Fly ash, Slag Cement and Silica Fume
Fly ash, Slag Cement and Silica Fume

From top: Fly ash, Slag Cement and Silica Fume

These SCMs are used as a partial replacement for the portland cement in concrete. Fly ash is commonly used at replacement levels up to 25%; slag cement up to 60%; and silica fume up to 5% to 7%. When slag cement replaces 50% of the portland cement in a 7500 psi concrete, greenhouse gas emissions per cu yd of concrete are reduced by 45%. Because the cementitious content of concrete is about 7 to 15%, these SCMs typically account for only 2% to 8% of the overall concrete material in buildings.

Testing determines the optimum amounts of SCMs used with portland or blended cement, the relative cost and availability of the materials, and the specified properties of the concrete. When SCMs are used, the proportioned mixture (using the project materials) should be tested to demonstrate that it meets the required concrete properties for the project. SCM frequently enhance the durability of concrete. Some SCMs increase curing times and can affect the construction schedule.

The durability of products with recycled content materials should be carefully researched during the design process to ensure comparable life cycle performance.
 
Recycled Aggregates
The environmental attributes of concrete can be further improved by using aggregates derived from industrial waste or using recycled concrete as aggregates. Blast furnace slag is a lightweight aggregate with a long history of use in the concrete industry.

Recycled concrete can be used as aggregate in new concrete, particularly the coarse portion. The Federal Highway Administration (FHWA) reports that eleven states use recycled concrete aggregate in new concrete. These states report that concrete with recycled aggregate performs equal to concrete with natural aggregates. When using the recycled concrete as aggregate, the following should be taken into consideration:

  • Recycled concrete as aggregate will typically have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates.
  • Too many recycled fines can also produce a harsh and unworkable mixture. Many transportation departments have found that using 100% coarse recycled aggregate, but only about 10% to 20% recycled fines, works well.The remaining percentage of fines is natural sand.
  • In crushing the concrete, it is difficult to control particle size distribution, meaning that the “aggregate” may fail to meet grading requirements of ASTM C33 – “Standard Specification for Concrete Aggregates”.
  • The chloride content of recycled aggregates is of concern if the material will be used in reinforced concrete. This is particularly an issue if the recycled concrete is from pavements in northern climates where road salt is freely spread in the winter. The alkali content and type of aggregate in the system is probably also unknown, and therefore if mixed with unsuitable materials, a risk of alkali-silica reaction is possible.
Recycled Materials in the Cement Plant
Cement is made by heating common minerals – typically crushed limestone, clay, iron ore, and sand – to temperatures of about 2700°F (the temperature of molten iron). Achieving these high temperatures requires large quantities of fuel, mainly coal, petroleum coke, and natural gas. Waste materials often have energy value and can be used as fuel. Common wastes such as spent solvents, printing inks, paint residues, and cleaning fluids often are designated as hazardous because they are flammable and have high fuel value. These and other high-energy wastes, such as used motor oil and scrap tires, cannot be safely disposed of in landfills. However, they can be safely burned to destruction as fuel in a cement kiln while reducing the need to use fossil fuels. The chemistry of cement production makes the kiln ideal for waste destruction. Not only is the energy recovered, but many wastes also contain materials essential for cement making.

The cement industry has steadily increased its use of waste materials to fuel cement kilns, and currently relies on the burning of waste materials to satisfy 10% of these energy needs. Cement plants also burn many industrial wastes, including sludge from the petroleum industry, and agricultural wastes such as almond shells.

Cement producers also include non-hazardous byproducts from other industries in the raw ingredients used to make new cement. Common industrial byproducts used include fly ash resulting from the production of electricity, mill scale resulting from steel making, and foundry sand resulting from metal castings. This practice not only recycles wastes, but also reduces the amount of raw materials taken from quarries. Cement manufacturers even reuse cement kiln dust (CKD), the primary byproduct of cement manufacture, by recycling it back to the kiln as an ingredient for new cement. CKD is also sold to the agriculture industry for use as fertilizer.

Recycled Steel
Virtually all reinforcing steel used in concrete is made from recycled steel. Steel forms are recycled when they become worn or obsolete.

 

BOOKMARK
Resources
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Located at BookstoreConcrete: Sustainability and Life Cycle (2007)
Portland Cement Association. Item Code: SN3011
Available for download for free This report presents the results of the LCI of three concrete products: ready mixed concrete, concrete masonry, and precast concrete.
Located at BookstoreDesign and Control of Concrete Mixtures, 14th Edition (2002)
S.H. Kosmatka, B. Kerkhoff, and W.C. Panarese, Portland Cement Association, Item Code EB001, 372 pages
Available for $80 Definitive reference on concrete technology covers fundamentals and detailed information on freshly mixed and hardened concrete. Extensively updated and expanded, this new edition discusses materials for concrete, such as portland cements, supplementary cementing materials, aggregates, admixtures and fibers; air entrainment; procedures for mix proportioning, batching, mixing, transporting, handling, placing, consolidating, finishing, and curing concrete; precautions necessary during hot- and cold-weather concreting; causes and methods of controlling volume changes; commonly used control tests for quality concrete; special types of concrete, such as high-performance, lightweight, heavyweight, no-slump, roller-compacted, shotcrete, mass concrete and many more. Applicable ASTM, AASHTO, and ACI standards are referred to extensively.
Located at BookstoreOptimizing the Use of Fly Ash in Concrete (2007)
Portland Cement Association, Item ID: IS548
The optimum amount of fly ash varies not only with the application, but also with composition and proportions of all the materials in the concrete mixture (especially the fly ash), the conditions during placing (especially temperature), construction practices (for example, finishing and curing) and the exposure conditions. This document discusses issues related to using low to very high levels of fly ash in concrete and provides guidance for the use of fly ash without compromising the construction process or the quality of the finished product. Available for a fee.
Located at BookstoreSupplementary Cementing Materials for Use in Blended Cements (1996)
Portland Cement Association. Item Code: RD112
Available for $40. Provides information on using fly ash, slag, silica fume and natural pozzolans in the manufacturing of blended cements and the effects of these materials on cement and concrete. This report is also found on CD019 and DVD019.
Located at BookstoreSupplementary Cementing Materials For Use in Concrete (2002)
Michael Thomas and Michelle L. Wilson. Portland Cement Association. Item Code: CD038
Available for $35. The first of a series of interactive distance learning programs specifically designed for training individuals on cement and concrete technology. This fully-narrated CD provides an intense self-contained course on supplementary cementing materials (SCMs) and their impact on the durability, workability, economy, and sustainability of concrete.
Download DocumentAchieving LEED® Credits with Segmental Concrete Pavements—Part 2 (2006)
Rob Burak, P.Eng.-ICPA Director of Engineering, Interlocking Concrete Pavement Magazine, August, 2006
This 4 page article continues from the May issue on how LEED® credits can be earned under the five principal categories. It details how points Sustainable Sites (SS) can be earned through heat island effect both in non roof and roof, material and resources, by reducing construction waste, resource reuse, by using recycled materials, using materials manufactured within the region as well as incorporating innovative improvements in building materials and design and durable materials. Applications: Heat Island Effect, Materials and Resources, Construction Waste Management, Resource Reuse, Recycled Content, Regional Materials, Innovation and Design Process, Durable Materials.
Download DocumentCoal Combustion Product Partnership Fact Sheet
2 page fact sheet
C2P2 is part of EPA’s Resource Conservation Challenge
Download DocumentReclaimed Industrial By-Products Key to Concrete of the Future (1998)
Environmental Council of Concrete Organizations, #EV18, 4 pages
Available for free. This bulletin points out the usefulness and environmental benefits of recycling industrial byproducts such as fly ash and silica fume for use in high-performing concrete.
Download DocumentRecycling Concrete and Masonry (1999)
Environmental Council of Concrete Organizations, #EV22, 11 pages
Available for free. This bulletin contains valuable information on how the recycling process impacts the environment, the advantages of recycling concrete and masonry as an aggregate in new concrete and applications for recycled concrete and masonry.
Download DocumentSlag Cement and the Environment (2003)
Slag Cement Association, SCIC #22, 4 pages
Available for free. From the "Slag Cement in Concrete" series, describing environmental benefits of slag. Includes life cycle inventory data on material, energy and emissions reductions, as well as LEED, reflectivity data and other info.
Download DocumentSlag Cement LEED NC 2.1 Guide (2005)
Slag Cement Association
Available for free. This 17-page publication discusses how slag cement can help contribute to achieving 9 different points toward for LEED™-NC certification.
Download DocumentSteel Reinforcing Bars: Recycled (2002)
Concrete Reinforcing Steel Institute, 2 pages
Available for free. This document describes the recycling benefits for steel reinforcing bars used in concrete construction. This document is available as a free download from Concrete Reinforcing Steel Institute.
Download DocumentSustainable Manufacturing Fact Sheet: Iron and Steel By-Products (2005)
Portland Cement Association, #IS326, 4 pages
Available for free. Color brochure describes the utilization of steel making by-products in the cement manufacturing process, saving virgin materials and reducing waste.
Download DocumentSustainable Manufacturing Fact Sheet: Power Plant By-Products (2005)
Portland Cement Association, #IS331, 4 pages
Available for free. Color brochure describes the utilization of power plant by-products in the cement manufacturing process, saving virgin materials and reducing waste.
Download DocumentSustainable Manufacturing Fact Sheet: Tire Derived Fuel (2005)
Portland Cement Association. Item Code: IS325
Available for free. By utilizing a cement kiln's controlled combustion environment, scrap tires can be an environmentally-sound source of energy in the manufacture of cement. This fact sheet shows how the popularity of tire-derived fuel has increased over the past two decades and summarizes its environmental benefits.
Located at External Web SiteAmerican Coal Ash Association (2006)
A website dealing with the use of coal ash in concrete products.
Located at External Web SiteBuilding Even Better Concrete (2007)
Originally printed in the December 2007 of Architectural Record, this article by Joann Gonchar, AIA of McGraw-Hill looks at the current trends in cement and concrete construction that improve performance and reduce environmental impact. One hour of AIA Continuing Education Credit is available on-line through McGraw-Hill by reading the article and completing a brief test. http://construction.com/CE/articles/0712edit-1.asp#
Located at External Web SiteChanges In Store (2006)
Wal-Mart showcases green concrete technologies at its store in Texas.
This 4 page article was originally featured in the May 2006 edition of Concrete Producer Magazine, by Hanley Wood. Wal-Mart testing a range of green strategies at this prototype store in McKinney, TX. Along with other green strategies, concrete was used as interior finish flooring, reducing VOC's and maintenance, and pervious pavement in the parking area to improve ground water quality and quantity.
Located at External Web SiteConcrete Reinforcing Steel Institute (2006)
An industry resource website
Located at External Web SiteConcrete's Contrubition to Sustainable Development
Concrete is the most widely used building material on earth. It has a 2, 000 year track record ofhelping build the Roman Empire to building today's modern societies. As a result ofits versatility, beauty, strength,·and durability, concrete is used in most types ofconstruction, including homes, buildings, roads, bridges, airports, subways, and water resource structures. And with today's heightened awareness and demandfor sustainable construction, concrete performs well when compared to other building materials. Concrete is a sustainable building material due to its many eco{riendly features. The production ofconcrete is resource efficient and the ingredients require little processing. Most materials for concrete are acquired and manufactured locally which minimizes transportation energy. Concrete building systems combine insulation with high thermal mass and low air infiltration to make homes and buildings more energy efficient. Concrete has a long service life for buildings and transportation infrastructure, thereby increasing the period between reconstruction, repair, and maintenance and the associated environmental impact. Concrete, when used as pavement or exterior cladding, helps minimize the urban heat island effect, thus reducing the energy required to heat and cool our homes and buildings. Concrete incorporates recycled industrial byproducts such as fly ash, slag, and silica fume that helps reduce embodied energy, carbon footprint, and waste.
Located at External Web SiteConcrete, Slag Cement and the Environment
Slag Cement Association
Contact the Slag Cement Association for a copy of this PowerPoint presentation designed for Architects/Engineers interested in concrete and slag cement environmental benefits.
Located at External Web SiteConstruction Waste Managment Database
Whole Building Design Guide
Enter your geographic information to determine where you can take concrete to be recycled.
Located at External Web SiteICF Points to LEED (2008)
Insulating Concrete Form Systems contribute to LEED credits
This two page .pdf summarizes the credits available to designers and building owners when using high performing insulating concrete forms in wall construction. Documents available for download to ICFA members.
Located at External Web SiteReclaimed By-Products Boost Concrete Performance (2004)
Tom Kuennen, James Informational Media, Inc., Better Roads Magazine
Reclaimed, recyclable industrial byproducts now being used in high-performance concrete are providing the durability and strength that portland cement concrete bridges and pavements need to stand up to the traffic loads and maintenance practices of decades to come. View this article from the January, 2004 edition of Better Roads Magazine published by James Informational Media, Inc.
Located at External Web SiteSilica Fume Association (2006)
An industry association website.