Concrete is one of the world’s necessary evils. It is versatile, strong, relatively inexpensive, easy to apply, easy to make, sets quickly and potentially lasts for thousands of years. However, in its most widely-used form, it is a non-sustainable, one-time use, high-energy product with a large carbon footprint.
But with its discovery made in ancient times, it is “… a surprisingly simple material” as the Guardian newspaper reported in March 2016, “usually made from approximately 10% Portland cement, 3% supplementary cementitious inclusion (for example fly ash or ground granulated blast furnace slag), 80% aggregates (such as gravel and sand), and 7% water.” And yet as the report explains, it has a high impact on the planet, as, “…nearly every aspect of its production – from mining and transporting the raw materials, to heating them to over 1,400°C (often using fossil-fuel-based energy) in a kiln, and the subsequent chemical process of turning limestone into small rocks of cement called clinker – releases huge amounts of carbon dioxide.”
The addition of waste by-products such as fly ash from coal combustion or blast furnace slag (from iron manufacturing) already plays a part in lowering concrete’s environmental impact. According to the specialist website, greenconcreteinfo, “Use of such by-products in concrete prevents 15 million metric tons a year of these waste materials from entering landfills. [Plus] Utilizing these ‘supplemental cementitious materials’ as a replacement for cement improves the strength and durability of concrete and also further reduces the CO2 embodied in concrete by as much as 70%, with typical values ranging from 15% to 40%.”
But now chemical engineers are looking at more radical ways to both strengthen concrete (so that less is needed), whilst using more sustainable feedstock. For example, the 2015 winner of the ‘Manufacturing, Construction and Innovation’ prize of the ‘Australian Innovation Challenge’ was Shi Yin, a PhD student at James Cook University, who under the supervision of Dr. Rabin Tuladhar, has developed an improved concrete made from recycled industrial plastic waste. As Dr Tuladhar explained to the university website, “We’ve produced recycled polypropylene fibres from industrial plastic wastes. With our improved melt spinning and hot drawing process we now have plastic fibres strong enough to replace steel mesh in concrete footpaths.” He said, “Using recycled plastic, we were able to get more than a 90 per cent saving on CO2 emissions and fossil fuel usage compared to using the traditional steel mesh reinforcing. The recycled plastic also has obvious environmental advantages over using virgin plastic fibres.”
Use of recycled plastic fibres in concrete eliminates the need for steel mesh and saves significant amounts of CO2 associated with steel production. Comprehensive life cycle assessment shows the production of recycled plastic fibre produces 90% less CO2 and eutrophication (contamination of water bodies with nutrients) compared to the equivalent steel.
But there are other techniques underway to lower concrete’s environmental impact, as listed in the Cement Technology Roadmap produced by the World Business Council for Sustainable Development and the International Energy Agency. A roadmap that outlines the following concrete improvements;
“Novacem is based on magnesium silicates (MgO) rather than limestone (calcium carbonate) as is used in Ordinary Portland Cement. Global reserves of magnesium silicates are estimated to be large, but these are not uniformly distributed and processing would be required before use. The company’s technology converts magnesium silicates into magnesium oxide using a low-carbon, low temperature process, and then adds mineral additives that accelerate strength development and CO2 absorption. This offers the prospect of carbon-negative cement.
Calera is a mixture of calcium and magnesium carbonates, and calcium and magnesium hydroxides. Its production process involves bringing sea-water, brackish water or brine into contact with the waste heat in power station flue gas, where CO2 is absorbed, precipitating the carbonate minerals.
Calix’s cement is produced in a reactor by rapid calcination of dolomitic rock in superheated steam. The CO2 emissions can be captured using a separate CO2 scrubbing system.”
Whilst these developments are to be applauded, researchers at MiT are taking the study of concrete to the next level. Indeed, given the impact that concrete has on our planet, both good and bad, MiT has set up the MIT Concrete Sustainability Hub (CSHub) to be better placed to understand what this amazing material does.
One part of this analysis involves examining the material at its smallest, molecular level, using the MIT-CNRS laboratory called MultiScale Material Science for Energy and Environment, hosted at MIT by the MIT Energy Initiative (MITEI).
The reason behind this is simple, as Prof Franz-Josef Ulm explained to the university website, “[Previous researchers] didn’t go to the very small scale to see what holds it [concrete] together — and without that knowledge, you can’t modify it.”
As the MiT website reports, “An MIT-led team has defined the nanoscale forces that control how particles pack together during the formation of cement ‘paste’ the material that holds together concrete and causes that ubiquitous construction material to be a major source of greenhouse gas emissions. By controlling those forces, the researchers will now be able to modify the microstructure of the hardened cement paste, reducing pores and other sources of weakness to make concrete stronger, stiffer, more fracture-resistant, and longer-lasting.”
With such nanoscale analysis, the study, “… defined the forces that control how particles space out relative to one another as cement hydrate forms. The result is an algorithm that mimics the precipitation process, particle by particle. By constantly tracking the forces among the particles already present, the algorithm calculates the most likely position for each new one — a position that will move the system toward equilibrium. It thus adds more and more particles of varying sizes until the space is filled and the precipitation process stops.” 
The diagrams show the structure of cement hydrate as determined by the researchers’ model, which calculates the positions of particles based on particle-to-particle forces. Each simulation box is about 600 nanometers wide. The packing fraction (the fraction of the box occupied by particles) is assumed to be 0.35 in the left diagram and 0.52 in the right one.
With an understanding of what makes concrete work, the team has begun to work on techniques that will make for cheaper, greener concrete. For example, the simplest way to lower its environmental impact is to recycle it. But today’s methods of recycling concrete generally involve cutting it up and using it in place of gravel in new concrete; an approach that doesn’t reduce the need to manufacture more cement. So the researchers’ idea is to reproduce the cohesive forces they’ve identified in cement hydrate. “If the microtexture is just a consequence of the physical forces between nanometer-sized particles, then we should be able to grind old concrete into fine particles and compress them so that the same force field develops,” says Ulm, “We can make new binder without needing any new cement — a true recycling concept for concrete!”
Another method being researched is how to improve concrete used in making roads that will lower vehicle fuel consumption. As the study explains, “the fuel efficiency of vehicles is significantly affected by the interaction between tires and pavement. Simulations and experiments in the lab … suggest that making concrete surfaces stiffer could reduce vehicle fuel consumption by as much as 3% nationwide, saving energy and reducing emissions.”
The team is also studying the ingredients for concrete to see if its 2,000 year old recipe can be improved. “For example, a promising beginning-of-life approach may be to add another ingredient — perhaps a polymer — to alter the particle-particle interactions and serve as filler for the pore spaces that now form in cement hydrate. The result would be a stronger, more durable concrete for construction and also a high-density, low-porosity cement that would perform well in a variety of applications. For instance, at today’s oil and natural gas wells, cement sheaths are generally placed around drilling pipes to keep gas from escaping.”
As one of the lead researchers, Roland Pellenq, senior research scientist in the MIT Department of Civil and Environmental Engineering (CEE) and research director at France’s National Center for Scientific Research (CNRS) explains, “A molecule of methane is 500 times smaller than the pores in today’s cement; so filling those voids would help seal the gas in.”
Environmentalists claim that concrete has too big an impact on our planet, and that replacement materials should be found; and for the most part they are correct. Concrete has a negative effect on our environment, as the RSC reports, “The material is used so widely that world cement production now contributes 5% of annual anthropogenic global CO2 production.”
And yet we continue to use vast amounts of concrete, as the MiT report makes clear, “Each year, the world produces 2.3 cubic yards of concrete for every person on earth, in the process generating more than 10% of all industrial CO2 emissions. New construction and repairs to existing infrastructure currently require vast amounts of concrete, and consumption is expected to escalate dramatically in the future. “
Consumption is expanding because we need to house our growing, urbanising population. As Pellenq says, “To shelter all the people moving into cities in the next 30 years, we’ll have to build the equivalent of several hundred New York cities. There’s no material up to that task but concrete.”
The challenge that is left for the chemical industry is finding a sustainable version that is still versatile, strong, relatively inexpensive, easy to apply, easy to make, set quickly and potentially lasts for thousands of years that the construction industry demands.
The development and research is coming close to an answer that will satisfy all these requirements, but with the RSC reporting that, “by 2050, concrete use is predicted to reach four times the 1990 level” will it come soon enough?