It could be that the answer is...
neither of them.
"Many scientists currently think at least 5 percent of humanity's carbon footprint comes from the concrete industry, both from energy use and the carbon dioxide (CO2) byproduct from the production of cement, one of concrete's principal components."
has been a much echoed news feed emanating from West Coast's, Washington State Univ and NSF National Science Foundation.
The accompanying news story and two news related posts are also quoted. All three are concerned with reducing concrete's (industry) carbon footprint.
When all is said and done my favourite most informative website turned out to be the last but not least "Concrete Thinking for a sustainable world." The site of the Portland Cement Association presents an extremely well documented "Technical Brief", which explains, in a simple and clear fashion all the main aspects of cement and concrete, their difference, their CO2-GHG emission and absorption, the importance of Life Cycle Assessement and Balance through time and much more. The site introduces their panel of experts called appropriately "Concrete Thinkers for Green Buildings" and the site includes a series of no less than 38 videos entitled "Recycled Inside and Out".
cf. Sources and References at the end of my post(s)
Since no lowly focused effort can make any significant impact on the CO2-GHG emissions climate change global warming issue (cf for example, DJ MacKay Cambridge Univ-ebook Without hot air" and Socolow-Pacala of Princeton Univ. Wedge approaches_many references upon request) I have added at a link to an interesting wide angled view of Carbon Cycle Science at the end of my now lengthy post.
1. West Coast's case:
Yet several studies have shown that small quantities of CO2 later reabsorb into concrete, even decades after it is layed, when elements of the material combine with CO2 to form calcite.[as most high school chemistry students and of course, all concrete materials scientists and civil engineers know full well]
A study appearing in the June 2009 Journal of Environmental Engineering suggests that the re-absorption may extend to products beyond calcite, increasing the total CO2 removed from the atmosphere and lowering concrete's overall carbon footprint.[can't wait for this, nor can the planet - brings back fond memories of my A-Level High School Chemistry Class writing all the possible chemical equations imaginable, later consolidated by a good university metallurgical grounding in chemical thermodynamics ].
While preliminary, the research by civil and environmental engineering professor Liv Haselbach of Washington State University re-emphasizes findings first observed nearly half a century ago--that carbon-based chemical compounds may form in concrete in addition to the mineral calcite-now in the light of current efforts to stem global warming.
"Even though these chemical species may equate to only five percent of the CO2 byproduct from cement production, when summed globally they become significant," said Haselbach. "Concrete is the most-used building material in the world."
Researchers have known for decades that concrete absorbs CO2 to form calcite (calcium carbonate, CaCO3) during its lifetime, and even longer if the concrete is recycled into new construction--and because concrete is somewhat permeable, the effect extends beyond exposed surfaces. [cf. video link Recycled Inside and Out in the Source and reference links below]
While such changes can be a structural concern for concrete containing rebar, where the change in acidity can damage the metal over many decades, the CaCO3 is actually denser than some of the materials it replaces and can add strength. [some may recall bad memories of 9/11 and the materials and engineering studies of why the twin towers failed in such a disastrous fashion - and may wish to confront this news feed and the related ones with these materials analysis?]
Haselbach's careful analysis of concrete samples appears to show that other compounds, in addition to calcite, may be forming. Although the compounds remain unidentified, she is optimistic about their potential.
"Understanding the complex chemistry of carbon dioxide absorption in concrete may[had better?] help us develop processes to accelerate the process in such materials as recycled concrete or pavement. ["Perhaps"?] this could help us achieve a nearly net-zero carbon footprint, for the chemical reactions at least, over the life-cycle of such products."
That is the thrust of Haselbach's current NSF-funded work, where she is now looking at evaluating the life-cycle carbon footprint of many traditional and novel concrete applications, and looking for ways to improve them.
"This work is part of the portfolio of studies that NSF is funding in this vital area," added Bruce Hamilton, director of NSF's environmental sustainability program and a supporter of Haselbach's work. "Research relating to climate change is a priority."
[Whatever the criticism sometimes harsh, I hope they are constructive and do contribute to "our shared aim of a net-zero materials carbon footprint
Widely spread news feed, here is quoted from Physorg.com
The concrete industry is a contributor to the global carbon cycle particularly with respect to the contribution of carbon dioxide in the manufacturing of cement (calcination). The reverse reaction of carbonation is known to occur in concrete, but is usually limited to exterior surfaces exposed to carbon dioxide and humidity in the air. As alternate concrete uses expand which have more surface area, such as crushed concrete for recycling, it is important to understand surface adsorption of carbon dioxide and the positive impacts it might have on the carbon cycle.X-ray photoelectron spectroscopy (XPS)[wikipedia html] is used in this study to evaluate carbon species on hydrated cement mortar surfaces. Initial estimates for carbon absorption in concrete using other techniques predict the potential for carbonate species to be a fraction of the calcination stoichiometric equivalent.
The XPS results indicate that there is a rapid and substantial uptake of carbon dioxide on the surfaces of these mortars, sometimes exceeding the calcination stoichiometric equivalents, indicative of carbon dioxide surface complexation species. On pure calcite, the excess is on the order of 30%. This accelerated carbon dioxide surface adsorption phenomenon may be important for determining novel and effective carbon sequestration processes using recycled concrete.
LINK to ACS Abstract
2. East Coast punch line:
"While government leaders argue about the practicality of reducing world emissions of carbon dioxide, scientists and engineers are seeking ways to make it happen."
One group of engineers at MIT decided to focus its work on the nanostructure of concrete, the world's most widely used material. The production of cement, the primary component of concrete, accounts for 5 to 10 percent of the world's total carbon dioxide emissions; the process is an important contributor to global warming.
In the January 2007 issue of the Journal of the Mechanics and Physics of Solids, the team reports that the source of concrete's strength and durability lies in the organization of its nanoparticles. The discovery could one day lead to a major reduction in carbon dioxide emissions during manufacturing.
"If everything depends on the organizational structure of the nanoparticles that make up concrete, rather than on the material itself, we can conceivably replace it with a material that has concrete's other characteristics-strength, durability, mass availability and low cost-but does not release so much CO2 into the atmosphere during manufacture," said Franz-Josef Ulm, the Esther and Harold E. Edgerton Professor of Civil and Environmental Engineering.
The work also shows that the study of very common materials at the nanoscale has great potential for improving materials in ways we might not have conceived. Ulm refers to this work as the "identification of the geogenomic code of materials, the blueprint of a material's nanomechanical behaviour."
Cement is manufactured at the rate of 2.35 billion tons per year, enough to produce 1 cubic meter of concrete for every person in the world. If engineers can reduce carbon dioxide emissions in the world's cement manufacturing by even 10 percent, that would accomplish one-fifth of the Kyoto Protocol goal of a 5.2 percent reduction in total carbon dioxide emissions.
Ulm considers this a very real possibility.
He and Georgios Constantinides, a postdoctoral researcher in materials science and engineering, studied the behavior of the nanostructure of cement. They found that at the nano level, cement particles organize naturally into the most densely packed structure possible for spherical objects, which is similar to a pyramid-shaped pile of oranges.
The nanogranular nature of C–S–H
Journal of the Mechanics and Physics of Solids, Volume 55, Issue 1, January 2007, Pages 64-90
Georgios Constantinides, Franz-Josef Ulm
3. Or again
"Working on the railroad? Using concrete could help environment"
In the study, May 7th, 2009 ,Robert Crawford points out that there have been long-standing concerns about environmental consequences of manufacturing railway sleepers because it involves harvesting large amounts of timber. Reinforced concrete sleepers are an alternative that offer greater strength, durability and long-term cost savings, he said. Critics of using concrete sleepers have charged that their manufacture increases greenhouse gas emissions as it involves higher consumption of fuel when compared to production of wood sleepers.
Crawford studied the greenhouse gas emissions of wooden and reinforced concrete sleepers based on one kilometer (0.62 miles) length of track over a 100-year life cycle. He found that emissions from reinforced concrete sleepers can be from two to six times lower than those from timber. “The results suggest strongly that reinforced concrete sleepers result in lower life cycle greenhouse emissions than timber sleepers,” the report states. [This begs questions on what are today's advanced rail-track practices and at the high end of the adventure of the French high speed train rail system the TGV_Train à Grande Vitesse etc. does it not? ]
ref. “Greenhouse Gas Emissions Embodied in Reinforced Concrete and Timber Railway Sleepers”, Environmental Science & Technology
Read more on Carbon Cycle Science...
Sources and Links:
1. a. Physorg.com b. LINK to ACS Abstract
2. ref.The nanogranular nature of C–S–H, Journal of the Mechanics and Physics of Solids, Volume 55, Issue 1, January 2007, Pages 64-90,Georgios Constantinides, Franz-Josef Ulm
3. Link Physorg.com
4. Concrete Thinking for a sustainable world Technical Brief > Green in Practice 102 - Concrete, Cement, and CO2
5. Read more on Carbon Cycle Science...