Concrete made with recycled steel slag is 17 percent stronger than traditional concrete.

Disposing of slag is a millennia-old problem, but the "waste material" is highly sought in construction.

Recycled slag performed 8 percent better in concrete than "raw" slag.

Australian scientists are trying to close the loop on steel waste and sewage wastewater. When they made new concrete using these waste materials, the results were 17 percent stronger than concrete made with traditional materials.

Let’s talk about slag. Steel is first made in extremely high-temperature blast furnaces, where natural iron ore or recycled scrap is purified by fire, and then combined with the right elements to make different kinds of alloys. There are strict definitions for these types, and each has narrow windows for which elements are allowed (and in which quantities). During this process, impurities are sloughed off continuously. These leftovers are called slag.

Slag is both plentiful and potentially valuable. For some alloys, even the so-called byproduct of the furnace process is just a second saleable product. And regular slag is highly prized as a strong, durable aggregate material for concrete.

“The global steel making industry produces over 130 million tons of steel slag every year," water engineer Biplob Pramanik, of Melbourne’s RMIT University, said in a statement. And while that slag has typically gone directly from furnace to contractor, the RMIT scientists saw an opportunity to add a useful step.

The chemical composition of slag means it naturally filters and holds metals contained in waste water, at least until it’s saturated to a certain extent.

This is pretty well understood, but the RMIT study is the first of its kind to examine whether this pollution-doped slag ends up performing better as concrete aggregate. They found the post-sewage concrete was 17 percent stronger than concrete made with traditional aggregate material, and they believe this is because what’s undesirable in waste water can be desirable in construction. The slag is acting as an informal “recapture” system for valuable metals.

In the study, researchers focused on phosphorus, which is both one of the world’s vital fertilizers and a “pollutant of concern,” they say. Above ground, phosphorus allows crops to grow and is a critical part of the life cycle. Below ground, it leaves disruptive deposits in water systems and distorts the ecosystems of bodies of water where it’s eventually dumped by, well, over-fertilizing ocean plants and algae.

By designing the right kind of waste water treatment system, scientists can arrange what these researchers refer to as a “circular economy,” meaning the same resource serves multiple purposes by reapplying its exhausted or leftover materials to a new task. The idea of reprocessing or repurposing isn’t new to any mining or smelting industry, where continuous improvements mean companies can even run their own waste through newer equipment and reactions to extract a new round of useful ore or material.

“Use of slag as a replacement of conventional aggregates show much better cement paste to aggregate bond performance and provides a monolithic effect,” the researchers conclude .

Slag that’s prepared in a continuous wash of phosphorus-rich wastewater ends up making a cleaner, tighter bond when combined with cement to make concrete. It’s this “monolithic” bond that makes the concrete stronger, because force pressing down on the surface is countered by the support of a fully solid material instead of a micro Swiss cheese of looser materials.

For more on monolithic versus aggregated materials, check out a 2011 Mythbusters project about making a Newton’s cradle from a series of wrecking balls. In order for the mechanism to work, energy has to travel in a straight line through all five wrecking balls with as little loss as possible.

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