December 29, 2016
by Christopher Bennett, CSI, Assoc. AIA, and Rae Taylor, PhD
Carbon dioxide (CO2) is a naturally occurring gas byproduct of every organism that breathes oxygen; it is found deep in the oceans and high in the atmosphere. It is also vitally important—without CO2, photosynthesis would not be possible, and life would not exist. However, having too much of this greenhouse gas (GHG) can also bring harmful consequences directly affecting climate and humankind. Concrete, unfortunately, plays a role in this.
Carbon dioxide is created in various ways, from the decomposition of organic material to the natural rhythms of the oceans to seismic activity. Human-centered sources of CO2 include deforestation, the burning of fossil fuels, and—to a significant extent—the production of concrete. This may be unsurprising given the material’s scope—concrete is everywhere. As suggested by Bill Dubois in his October 2016 article for The Construction Specifier, “Healthy Concrete Systems,” the building product could be considered part of the muscular skeletal structure supporting the entire built environment. (See The Construction Specifier article by Bill DuBois and Christopher Bennett, “Healthy Concrete Systems: Defending Design Intent,” which appeared in the October 2016 issue. Visit www.constructionspecifier.com/healthy-concrete-systems-defending-design-intent.) Concrete is in our homes and in our schools; it spans waterways in the forms of bridges and connects destinations with endless miles of road.
Concrete is the most widely used manmade building material in the world. It is relatively inexpensive to manufacture, and basic concrete construction is easy to teach. The material’s convenience propelled it to its top position as a building product, while its strength and versatility has kept it there despite obvious contributions to total CO2 emissions. There is no likely contender for a substitute to concrete on the horizon—replacing concrete as a building technology simply is not an option. That said, there are ways to reduce its environmental impacts.
The manufacturing process of concrete, with emphasis on cement, has been under pressure to be greener for decades. Currently, fuel consumption in kiln-fired production of Portland cement, along with CO2 emission during calcination of carbonaceous rock and use of cement, uses a significant amount of energy and produces large amounts of GHG emissions. In 2010, 3.2 billion tons of CO2 were emitted to the atmosphere from the production of 3.9 billion tons of cement. Going forward, cement production is expected to grow each year by about 2.5 percent, potentially reaching 4.4 billion tons by 2050. Without any significant changes to how the global construction industry approaches concrete, the planet will continue to be burdened with additional CO2.
Many countermeasures, such as improving the thermal efficiency of kilns and cooler systems, or using alternative fuels rather than carbon-based ones, have already been put in place, but with limited effect. While it is possible to make further advancements, the industry seems reluctant for numerous reasons, including:
The solution seeming to have the most direct promise of a positive effect would be replacing ordinary Portland cement (i.e. the main culprit of CO2 release) with another material still providing strength. This suggestion is not a new one—as early as the 1960s, the potential of ground granulated blast furnace slag (GGBS) as a cement replacement material was discovered. (See H.F.W. Taylor’s Cement Chemistry [2nd edition, vol. 2], published in 1997 by Thomas Telford, for more on Locher’s 1966 work.) Earlier than this, fly ash was known to have similar properties (“Properties of cements and concretes containing fly ash,” by R.E. Davis et al, was published by the American Concrete Institute [ACI] in 1937.), even though the reason was not fully understood until more recently. (See R. Taylor et al’s “Composition and Microstructure of 20-year-old Ordinary Portland Cement-ground Granulated Blast-furnace Slag Blends Containing 0 to 100% Slag,” published in Cement and Concrete Research [40(7)] in 2010.)
Before exploring alternatives to Portland cement, it is important to understand this ingredient’s role in concrete. When mixed with water, cement undergoes an incredibly complex reaction; it produces a variety of compounds depending on its original composition. Of these, the most important is calcium silicate hydrate (C-S-H)—the ‘glue’ holding the concrete together. Without C-S-H, one would simply have a loose collection of sand and stone—certainly nothing resembling the Three Gorges Dam. Construction workers used some 16 million m3 (21 million cy) of concrete in this structure.
Fly ash, GGBS, and SCMs
Use of fly ash as a cement replacement has become increasingly common due to the material’s resistance to sulphate attack, chloride-ion penetration, and de-icing salt-scaling. These benefits are the result of fly ash’s pozzolanic reaction, consuming the calcium hydroxide formed from the hydration reaction
of ordinary Portland cement and water, which produces supplementary C-S-H. With the introduction of additional C-S-H comes an increase in density and strength. (A pozzolan is a silicate-based material that reacts with the calcium hydroxide generated by hydrating cement to create materials with cementitious characteristics See R. Helmut’s “Fly Ash in Cement and Concrete” published by the Portland Cement Association in 1987.) The strength of fly-ash blends has been reported to be initially low, but improved over the long term. (See the fourth edition of Lea’s Chemistry of Cement [Elsevier Ltd, 1998] for Massazza’s work.) Other benefits include increased workability and less water demand, lower heat of hydration, and reduced drying shrinkage.
ASTM standards define Class F fly ash as a pozzolan, whereas Class C has some cementitious properties. In the British standard BS 3892-1997, Pulverized-Fule Ash, Class F fly ash was used in the mixing of samples nine years earlier. This means fly ash does not have cementitious properties, but when finely divided and introduced to water in the presence of calcium hydroxide, it chemically reacts to form a compound that does.
GGBS, on the other hand, comes from the production of iron in a blast furnace to which flux is added to the charge of iron ore. At 1400 to 1550 C (2552 to 2822 F), it produces slag, which is a liquid containing all the impurities from the iron ore and coke. The principal oxide components are lime (added as flux), silica, and alumina. If this liquid is rapidly cooled by converting the liquid into small droplets (granulation), crystallisation is prevented. This allows it to form a glassy structure with latent hydraulicity, which, after grinding, resembles sand.
The oxide composition of GGBS varies between plants due to different ores being used. However, composition remains roughly the same for one plant’s output. As GGBS hydraulicity is latent, hydration reactions are very slow, so an activator is required. There is a range of activators that can be used, such as alkali, lime, and gypsum. However, the most common is Portland cement.
Due to the extensive body of research on cement and cementitious materials, we now understand more about C-S-H, the original minerals required, the ideal conditions vital to its correct formation, and possible replacement materials for Portland cement (i.e. supplementary cementitious materials [SCMs]). This knowledge reveals that mix design is of the utmost importance.
Natural materials and those taken as waste from other industries have varied mineral composition. This gives rise to the main disadvantage of SCMs—quality control of the material. Concrete suppliers rely almost exclusively on the SCM supplier to ensure the quality of the product, which can vary significantly and have a profound impact on the hardened concrete. Control of fly ash is greater than with GGBS, as it is more variable, and can affect setting, air content, and overall strength.
Another disadvantage to SCMs involves the delayed setting time and low early strength. Too often, this results in subsidence and plastic shrinkage, along with thermal and volume stresses. These cause unintentional cracking, structural problems, and overall negative impacts on architectural quality.
There is another option—one involving a look back at history to find what the modern construction industry’s predecessors did right to make constructions such as the Colosseum, aqueducts, the Pantheon, or the Appian Way.
Preparing for the future by returning to the past
The Ancient Romans made durable and resilient concrete, creating monuments and buildings that withstood centuries while maintaining mechanical and aesthetic performance. Shown at the beginning of the article, Trajan’s Market is the world’s oldest shopping mall—a large complex of Roman ruins opposite the Colosseum. Designed by Apollodorus of Damascus, and built in 100−110 AD, it is an example of the durability of Roman concrete. Fully understanding how they did this would be critical in producing a modern, durable, and environmentally responsible concrete material.
Much research has built on earlier studies in the characterization of constructs such as the seawater harbors of Ancient Rome. Now, this knowledge must be applied to the development and optimization of modern concrete. How can we maintain strength with a variety of natural resources? Can mimicking the Roman process fulfill current demands, including the modern construction schedule? Al-tobermorite research by Paulo Monteiro’s team at the University of California, Berkeley alongside the U.S. Department of Energy’s (DOE’s) Lawrence Berkeley National Laboratory (LBNL) found the Roman concrete recipe not only used less lime when manufacturing concrete, but the limestone was also baked at a lower temperature than modern Portland cement, reducing fuel demands and thus CO2 emissions. (See M.D. Jackson et al’s “Unlocking the Secrets of Al-tobermorite in Roman Seawater Concrete,” in American Mineralogist (vol. 98) in 2013 or “Material and Elastic Porperties of Al-Tobermorite in Ancient Roman Seawater Concrete” in the June 2013 edition of the Journal of the American Ceramic Society.)
Monteiro and his team proved the Romans were not only making concrete at two-thirds or less the temperature of the Portland cement process (creating less CO2), but the concrete was also more resilient, surviving, wind, water, and chemical attacks for 2000 years. If modern design/construction professionals know far more about cement than third-century Romans, why is newly designed infrastructure struggling to stand the tests of time?
In April 2016, the United States signed the Paris Agreement written by the United Nations Framework Convention on Climate Change (UNFCCC), agreeing to fight the increase in global average temperature. (For more on this agreement, see The Construction Specifier article, “How Paris COP21 Drives Low-carbon Building Energy Efficiency” by Paul Bertram, FCSI, CDT, CSC, LEED AP, GGP. Visit www.constructionspecifier.com/how-paris-cop21-drives-low-carbon-building-energy-efficiency.) As such, this country, along with the 191 others who signed the treaty, will be moving more toward sustainable and renewable energy. This means concrete will become more invaluable to support the new technologies and the longevity of construction materials. With knowledge and clear, correct specifications, it is possible to design just as strong, just as true, yet more resilient concrete to help cut carbon dioxide emissions.
We must start thinking more about reducing CO2 emissions from concrete, and one of the best ways to do this effectively is to look to the past—Roman maritime concrete can provide fresh perspective. This legacy highlights not only that it is possible to make concrete with fewer carbon emissions, but also that the material can be made better and more resilient.
Concrete microstructure will determine the macrostructure and, therefore, the durability of the home, road, bridge, airport, or nuclear waste storage facility where the concrete is used. This means the characterization of cement is vital to the built environment and to ensuring creation of a resilient infrastructure.
This highlights the necessity for ongoing research—not just to reduce the cement content of concrete, but also to increase knowledge of cement. Such knowledge will further the optimization of cements and concretes in order to better exploit alternative energy sources, such as those contributing to the goals of the Paris Agreement.
Chris Bennett, CSI, is a concrete consultant for commercial projects in North America. He specializes in document creation, contractor training, and technology testing for MasterFormat Divisions 03, 07, and 09. He can be reached via e-mail at firstname.lastname@example.org.
Rae Taylor, PhD, holds a doctorate in civil engineering and materials science from the University of Leeds, and a post-graduate certificate in technology management from the Open University. Her principal research interests lie in the field of materials science and improving the environmental impact of construction materials, with a focus on the effect of cement replacement materials and additives on cement microstructure. Taylor has published on the topic of cement in numerous academic journals and conferences, such as the Journal of the American Ceramic Society, American Mineralogist, and Cement and Concrete Research. She can be reached at email@example.com.
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