This post follows naturally from my two previous posts on CO2 reduction in Concrete manufacturing and use throughout it's life-span. The figure opposite sums-up much of what has be written previously and is taken from a full well presented article in the NYT of 11 April 2009 entitled Concrete Is Remixed With Environment in Mind.As often quoted in my pages (Napoleon) "A good drawing is worth more than a long discourse" the reader is cordially invited to consult the figure-click to enlarge.
The article is an excellent paper for public understanding of cement and concrete. All sort of careful chemistry is going on: Some add silica fume industrial waste which improves impermeability and gives reinforced steel bars corrosion protection from road salts. Some add titanium oxide to accelerate oxidation which breaks down organic airborne pollutants so producing a permanently attractive white surface.
NYT reporter Henry Fountain, goes much further than the scientists reported in my earlier post and introduces NYT readers to the much more heartening and ambitious aim of "reversing the manufacturing CO2 emissions equation" to achieve a negative carbon emissions, or overall absorption during the "concrete life-cycle", by both reducing the emissions during production and absorption of CO2 during it's useful life.
"Some researchers want to eventually eliminate Portland cement entirely and replace it with other cements to produce zero-carbon, or even carbon-negative, concrete."
Dr. Brent R. Constantz, company founder, of Calera does not describe Calera as a cement company:
“We’re primarily driven by the need to capture large amounts of CO2 and sequester it,”
NB. The high standard fall-out from Dr. Constantz, background in cements, having made specialty products for use in orthopedic surgery. But he
Back-ground from from NYT- cement manufacture basic process:
"Portland cement is at the heart of concrete’s environmental problems. About a ton of CO2 is emitted for every ton of cement produced. The basic manufacturing process involves burning limestone and other minerals at about 2,700 degrees Fahrenheit(about 1480°C) to create an intermediate product called clinker.
“Essentially, we’re trying to make the same minerals that they did in 1825,” said Mr. Stehly, who is head of a committee addressing sustainability issues at the American Concrete Institute.
The cement industry, particularly in the United States and Europe, has reduced CO2 emissions through the use of more efficient kilns and processes, and is now allowed to add some ground unburned limestone to the clinker, reducing the actual cement in the mix. But about half of the CO2 from cement cannot be eliminated — it is produced in the reaction, called calcination, that occurs as the limestone (which consists of calcium carbonate) is being burned."NYT points to two innovative companies, strongly engaged in this adventure:
1. Calera Corporation, is developing a process to bubble gas-fired electric power plant flue gases through seawater or other brackish water, using the CO2 in the gases to precipitate carbonate minerals for use as cement or aggregates in concrete. The process mimics, to some extent, what corals and other calcifying marine organisms do.
2. Carbon Science associated with Novacem, a British start-up, is developing a cement that does not use carbonates and can make concrete that absorbs carbon dioxide.
At a site adjacent to a gas-fired electricity generation plant in Moss Landing, Calif., the Calera Corporation is developing a process to bubble power plant flue gases through seawater or other brackish water, using the CO2 in the gases to precipitate carbonate minerals for use as cement or aggregates in concrete. The process mimics, to some extent, what corals and other calcifying marine organisms do.
Calera calculates that producing a ton of these minerals consumes half a ton of CO2, so the resulting concrete could potentially be carbon negative — sequestering carbon dioxide permanently.
Brent R. Constantz, the company’s founder, has a background in cements, having made specialty products for use in orthopedic surgery. But he does not describe Calera as a cement company. “We’re primarily driven by the need to capture large amounts of CO2 and sequester it,” he said.
The company probably will begin by making aggregate, because the barriers to making a commercially acceptable product are lower than with cement. Even with aggregate, any new product must meet standards and must be accepted by the concrete industry, which can be conservative. “Any time you introduce anything new,” Dr. Constantz said, “it’s a challenge.”More about Calera in Scientific American[pdf].
2. Carbon Science associated with Novacem, a British start-up, is developing a cement that does not use carbonates and can make concrete that absorbs carbon dioxide.
"To reduce concrete’s carbon footprint to near zero or less, different approaches are needed. Novacem, a British start-up, is developing a cement that does not use carbonates and can make concrete that absorbs carbon dioxide. Carbon Sense Solutions, in Halifax, Nova Scotia, wants to bubble CO2 through wet cement, sequestering the gas through carbonation (a process that occurs naturally, though very slowly, under normal conditions)."
My professional house journal, Materials World, almost a year earlier (7 months ago) in their news report entitled Concrete carbonation,MW 01 Oct. 2008 described the above second highly innovative company(2) in a balance way.
The pros (a) and cons(b)
a)The pros:
Combustion flue gases will be redirected to the curing process. The resulting effluent is scrubbed of CO2 in under an hour. The gas is stored in the concrete as calcite with no further reactions occurring.
‘Calcite, otherwise known as limestone, is the process feedstock for cement. We are simply reverting it back to its natural and most stable state. You can call this cradle-to-cradle engineering,’ says Robert President of Carbon Sense Solutions. The material is said to store up to half the weight of cement as CO2.
Niven is guarded about revealing more about the process, but says, compared to previous efforts at concrete carbonation, this work involves ‘a new reactor design that achieves complete carbonation, faster processing and improved material properties [faster early strength development, lower permeability, reduced shrinkage cracking and efflorescence resistance]’.
b) The cons:
However, concrete and cement science expert Dr Charles Fentiman of Fentiman Consulting in Southwater, UK, is sceptical about the ability to achieve complete carbonation during curing. He reserves judgement until the work is taken out of the laboratory and shown to overcome the practical problems that have impeded academics and industry for decades.
He says, ‘This seems to be an idea of making concrete elements and giving a warm cure in CO2. [But] in my experience, as soon as cement hydration starts, the CO2 coats everything and blocks further hydration. It does accelerate hardening, but then ongoing strength development is low and the concrete remains porous because hydration is blocked’.
Fentiman explains that academics have previously tried to overcome this through super-critical carbonation after the concrete has cured and the cement hydrated. However, ‘this would greatly slow the manufacturing process and the extra cost would need to be covered by the end user’.
Prepare for the worst but hope for the Best.
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