What Stockpile Managers Need to Know About Aggregates Recycling

As populations, city centers and demand for raw materials expands, recycling has become big business in every industry that deals with tangible goods — including rock quarries, concrete companies, and other businesses that provide aggregate and related materials.

And when commentators like Vince Beiser of the New York Times note that “we are starting to run out” of sand, the pressure to repurpose, recycle, and innovate rises even higher.

What happens to leftover inventory at quarries, or to concrete and other materials after demolition? How are these reused, repurposed or recycled? Who is leading the charge in recycling existing materials and in finding innovative ways to apply the results?

Here, we take a look at the mysteries of aggregates recycling.


Why Recycle Aggregates?

While visionaries like Bea Johnson advocate methods for reducing waste on the individual level, waste from construction and demolition accounts for far more of the contents of landfills — providing a much higher potential to reduce that waste.

Aggregate waste resulting from construction and demolition accounted for 11.6 million tons of solid waste in California in 2008 alone, according to the state’s Statewide Waste Characterization Study. About 977,000 tons of waste consisted solely of asphalt and concrete.

Construction and demolition waste also accounted for about 29 percent of all the waste generated in California in 2008. That’s a lot of landfill space!

Cement and concrete creation are also heavily implicated in the production of greenhouse gases. While many individuals focus on reducing their own greenhouse gas output, the average individual produces only four tons of carbon dioxide per year, according to an Earth on the Edge report. The average resident of the United States produces about eighteen tons: a higher number, but still nowhere near the production from the cement industry, which produces about one ton of greenhouse gases per ton of cement made, according to an 2012 article by Madeleine Rubenstein.

In other words, while individual efforts can help protect the planet and its resources for future generations, industries must lead the way in creating a more sustainable future for everyone. Aggregates recycling is one way that heavy industries can take the lead and realize a clear, stronger future with a continued market for their products and services.


Old Buildings, New Applications: Post-Use Aggregates Recycling

To reduce the amount of waste involved and to save both money and energy, many companies turn to recycled aggregate.

A 2004 study by the Federal Highway Administration estimates that 38 states used recycled concrete as an aggregate base, while 11 states had programs to recycle used concrete into new portland cement concrete (PCC). Recycled aggregate may be used for general bulk fills, bank protection, drainage base or fill, road construction, or in noise barriers or embankments.

The benefits of using hardened recycled concrete include its similarity to natural rock in most applications in which conventional aggregates are used, according to the Portland Cement Association. Some types even offer higher absorption than “virgin” aggregates, due to the lower specific gravity and increased porousness of recycled concrete when compared to other aggregate types.


What’s in the Works? Innovations in Aggregates Recycling

For many years, attempts to reduce environmental impacts through recycling in quarries, cement and concrete production, and other building materials and applications have been going strong.

A 2001 paper by C. Meyer of Columbia University, N. Egosi of RRT Design and Construction, and C. Andela of Andela Products Ltd. details how post-consumer glass can be used in concrete and asphalt, and ARI’s Guy Woodford reports on a quarry that’s saving nearly $1 million per year by recycling water.

Until recently, however, few innovations have focused on cement or concrete themselves — and even fewer have focused on their significant atmospheric impacts. According to Concrete Construction’s Kendra Kozen, concrete creates significant quantities of greenhouse gases each year: about one ton of gas emissions per one ton of cement.

In an effort to improve energy efficiency of concrete production, as well as to recycle and reuse where possible, a number of ongoing projects are changing the way we approach concrete.

Roland Pellenq at MIT actually explored changes to the molecular structure of concrete to make it not only more energy efficient, but also stronger and more resilient. The analysis suggested reducing the calcium-to-silicate ratio, an option that could be pursued in recycled concrete as well as in new materials.

By partnering with Louisiana Tech University, Alchemy Geopolymer Solutions’ Dr. Erez Allouche and other researchers used fly ash instead of portland cement in concrete creation, creating stronger and more fire-resistant concrete with lower greenhouse gas emissions.

Big things are happening at Switzerland’s Ecole Polytechnique Federale de Lausanne (EPFL), where Construction Materials Laboratory head Karen Scrivener recently announced the development of a new form of cement that may help to further reduce the environmental impact of concrete, along with reusing old concrete materials. The new material is less porous and stronger than traditional cement.

Civil and environmental engineer Marwa Al-Ansary has also begun to push the envelope on the development of materials like sulfur concrete, which uses sulfur rather than cement, to create high-strength concrete that does not require the same curing period as conventional materials. Sulfur concrete is strong and durable, and has the added bonus of reusing sulfur produced as a waste material in gas and oil production.

Finally, Stanford professor Brent Constantz is exploring cements derived from seawater and power plant flue gases. If made according to Constantz’s vision, this cement might not only be carbon-neutral but actually carbon-negative, trapping power plant gases instead of allowing them to vent into the atmosphere.


Recycling Sand?

Concrete and asphalt are two popular candidates for recycling post-use: when a building is torn down or a roadway is torn up, for instance. However, leftover inventory of sand and gravel at rock quarries may also be recycled or reused — and they often are in order to meet consistently growing demands for sand, a major component in the creation of concrete.

Rock quarries are one of the world’s top producers of sand and gravel, after the operations that mine these materials specifically. While the sand and gravel produced at rock quarries is often secondary to the quarries’ production goal, it can be used in productive ways:

  • A 2014 paper by Liudmila Alexandrova explored the multiple uses of leftover aggregates from soapstone quarries in the North Savo region of Finland (and how to transport those stockpiles more cost-effectively).
  • A 2012 paper in the Journal of Mining Science by M. Lintukangas, A. Suihkonen, P. Salomaki, and O. Selonen looked into potential uses of abandoned quarries once aggregates and other materials have been removed.

As early as 1941, researchers were seeking replacements for natural sand to use in concrete and other building applications, as a report to the Michigan State Highway Department by E.A. Finney indicates. Today, the race to replace sand, or to use the world’s existing sand more efficiently, is on.

As David Owen notes at the New Yorker, bit by bit the world is running out of sand with the appropriate qualities for concrete and construction uses. Not all sand is appropriate for creating concrete — and for other purposes, ranging from building custom islands off the shores of Dubai to providing an appropriate base for beach volleyball courts.

For construction purposes, sand such as that produced in the Sahara Desert is inappropriate, notes John R. Gillis: The right kind of sand for building is more commonly found in lake beds or quarries. And getting it can have devastating environmental impacts.

As a result, attempts to recycle, reuse and repurpose appropriate sand deposits have focused heavily on the “waste” materials from rock quarries, as well as on other options, as Denis Delestrac noted in a recent TED Talk. New methods for using “unusuable” sand are in the works, as well. Architecture student Magnus Larsson focuses his research on how to use desert sand — often unsuitable for concrete applications — as a building material, creating rigid structures from sand for use as buildings, walls and more.

While much of the talk on recycling and sustainability focuses on topics like food production, clean water, outdoor green spaces, and ethical consumption, a focus on aggregates recycling and efficiency offers one of the most efficient and effective ways to apply materials, cut greenhouse gas emissions, and produce a future that is stronger and more sustainable.

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