Eco-Cements
Eco-Cement is a new more environmentally sustainable type of cement developed by John Harrison of TecEco which incorporates reactive magnesia[1] and wastes that is more environmentally sustainable. Eco-Cement used to make porous concretes absorbs CO2 from the atmosphere to set and harden and in this way mimic nature (See geomimicry). Eco-Cement can also be recycled. Wastes such as fly and bottom ash, slags etc. can also be included for their physical properties as well as chemical composition without problems from delayed reactions.
The Global Sustainability Alliance plan to make the magnesia that is used in Eco-Cements using solar energy in a new kiln that combines heating and grinding and captures CO2 known as the Tec-Kiln. Given this production scenario Eco-Cement concretes have the capacity to become a huge carbon sink and they will be used in the Gaia Engineering tececology to bind wastes and the carbonate building components made using the N-Mg technology..
Net Sequestration Typical Eco-Cement (Assuming Capture during Manufacture)
As stated by Fred Pearce in the article on Eco-Cements published in the New Scientist magazine (Pearce 2002[2] ) “There is a way to make our city streets as green as the Amazon Forest. Almost every aspect of the built environment from bridges to factories to tower blocks, and from roads to sea walls, could be turned into structures that soak up carbon dioxide – the main greenhouse gas behind global warming. All we need to do it is the change the way we make cement.”
TecEco Sustainable Cities
Making the built environment a repository for recyclable resources (referred to as waste) as well as a huge carbon sink is an alternative that is both politically and economically viable.
John Harrison got the idea of using carbon and wastes in building materials from his observations of nature. During earth's geological history, large tonnages of carbon were put away as limestone and coal by the activity of plants and animals. Shellfish build shells from it and trees turn it into their wood. These same plants and animals wasted nothing, the waste from one was the food or home of another. John concluded that the answer to the problems of greenhouse gas and waste was to use them both in building materials. (See geomimicry)
Eco-Cements are made by blending reactive magnesium oxide with conventional hydraulic cements like Portland cement. They are environmentally friendly because in porous concretes the magnesium oxide will first hydrate using mix water and then carbonate forming significant amounts of strength giving minerals in a low alkali matrix. Many different wastes can be used as aggregates and fillers without reaction problems. The reactive magnesium oxide used in Eco-Cements is currently made from magnesite (a carbonate compound of magnesium) found in abundance and that in the Gaia Engineering tececology will be produced from seawater.
When added to concrete reactive magnesia[1] hydrates to magnesium hydroxide, but only in porous materials like bricks, blocks, pavers and porous pavements will it absorb CO2 and carbonate. The greater proportion of the elongated minerals that form is water and carbon dioxide. These minerals bond aggregates such as sand and gravel and wastes such as saw dust, slags, bottom ash etc.
Eco-Cement can include more waste than other hydraulic cements like Portland cement because it is much less alkaline, reducing the incidence of delayed reactions that would reduce the strength of the concrete. Portland cement concretes on the other hand can’t include large amounts of waste because the alkaline lime that forms causes delayed and disruptive reactions.
Eco-Cement Carbonation
The more magnesium oxide in an Eco-Cement and the more porous it is, the more CO2 that is absorbed. The rate of absorption of CO2 varies with the degree of porosity. Carbonation occurs quickly at first and more slowly towards completion.
Carbonation in a Typical Block compared to an Eco-Cement Block
A typical Eco-Cement concrete block would be expected to fully carbonate within a year. Eco-Cement also has the ability to be almost fully recycled back into cement, should the concrete structure become obsolete.
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[1] Reactive magnesia (rMgO) is also variously known as caustic calcined magnesia, caustic magnesia or CCM. The temperature of firing has a greater influence on reactivity than grind size as excess energy goes into lattice energy.
[2] Pearce, F. (2002). "Green Foundations." New Scientist 175(2351): 39-40.