Recycled Cement Offers a Low-Carbon Alternative Without Compromising Strength

An engineering revolution in cement material recycling could potentially reduce hugely the carbon output from cement making. Scientists from the University of São Paulo and Princeton University discovered how to recycle cement from crushed-down buildings into a sustainable, equally functional counterpart to traditional Portland cement. This revolution can now be used to mitigate the environmental problem of excessive cement-making emissions as well as generate new avenues of revenue in the demolition and building waste materials industries.

Converting Building Rubbish to High-Grade Cement
Building material concrete, which is everywhere, produces enormous amounts of rubbish when buildings are demolished. United States construction and demolition rubbish alone exceeded twice that of domestic waste in 2018. Although some is salvaged, most of it finds its way into low-grade applications like road base or fill. The new study shows that the cement obtained from waste concrete can be refined into a high-performance binder, thus limiting the amount of new cement being produced.

It starts with crushing old concrete into a dust. Scientists believe that of the five gigatons of waste concrete produced per year, one gigaton is readily recyclable. It is also calcined at 500°C, at a high enough temperature to recover its binding properties without liberating free oxygen. The "thermoactivated" cement can then be mixed with traces of finely ground Portland cement or limestone to enhance strength and workability. The product has the strength of conventional cement but reduces emissions by much.

Reducing Cement Industry Emissions
Portland cement is the cause of approximately 8% of carbon emissions worldwide, largely because of the huge energy input required to produce it. The new recycled cement produces 198 to 320 kilograms of CO₂ per metric ton, up to 40% less than current low-carbon alternatives like limestone calcined clay cement (LC3). If deployed in full along with other cement-replacement technologies, industry emissions would fall by up to 61%.

The secret to this diminution is recycling of material otherwise lost. In contrast to conventional cement manufacturing, where limestone is over-heated beyond 1400°C and carbon dioxide is emitted in the process, the recycling process does not over-heat and chemically decompose. By reducing the heat necessary for processing, the process dramatically reduces energy consumption and emissions.

Challenges in Scaling the Technology
While recycled cement presents a very promising solution, there are several problems that need to be resolved before its large-scale application. One such critical one is improved sorting and processing of demolition rubble. Unlike the production of new cement, where processes are standardized, recycling needs effective gathering, separation, and preparation of material from buildings demolished.

Apart from that, most national building codes currently specify the ratio of cement instead of its performance. "Recipe-based" standards are hard to stray from new materials even if they possess similar or greater strength and durability. Some Latin American and some European nations are starting to adopt performance-based codes that may lead to more utilization of recycled cement and other low-carbon options.

A Step Toward Circular Construction
The principle of circularity—where waste from buildings being demolished is reused in new development—is picking up speed as cities increasingly look for more sustainable development trends. Recycling construction waste using the new cement technology can, in theory, make the building industry a more circular economy. Rather than landfilling demolished material, cities can simply integrate them into new development projects, eliminating waste and emissions.

The study is the product of collaboration between Princeton scientists and the University of São Paulo. It illustrates how international collaboration can propel innovation towards sustainability. The researchers are still carrying out research and analyzing the long-term stability of recycled cement and means to further optimize the process.

Conclusion
Manufacturing high-performance recycled cement is a giant leap towards minimizing the environmental footprint of the building sector. Using waste already available, reducing emissions, and being as strong as regular cement, this technology has the potential to transform building construction. With adequate policy and infrastructure, recycled cement can be an integral part of a shift towards green building.

Source & Credits:
Original content by Princeton University and University of São Paulo. Adaptation from the Andlinger Center for Energy and the Environment.