A new type of carbon-neutral concrete has been commercially used for the first time, in a high-profile skyscraper being built in Manhattan. The binder utilized in the concrete is made of granite instead of traditional greenhouse-gas-emitting limestone cement.
Concrete consists of three main ingredients: cement, an aggregate (such as sand and/or gravel) and water. The cement undergoes a chemical reaction when mixed with the water, ultimately forming a solid that binds the mixture together.
Traditional Portland-style cement is made by grinding up limestone and other raw materials, then heating the resulting powder to temperatures of up to 1,450 ºC (2,642 ºF). Unfortunately, the processes by which that heat is generated produce a lot of carbon dioxide.
What's more, as the heated limestone forms into cement via a process known as calcination, it releases trapped carbon dioxide as a byproduct. The combination of that CO2 and the CO2 produced in the heat-generating process is estimated to be responsible for 5% to 8% of all human-caused greenhouse gas emissions.
Various groups have set out to reduce cement's carbon footprint by replacing the limestone with feedstocks such as discarded clay, fly ash, and magnesium silicate. Last year we also heard about C-Crete, a Portland-cement-free type of concrete made by California-based C-Crete Technologies. At that time, a version of the material that used slag as a binder was being used to retrofit a 120-year-old Seattle building's foundation.
This July, a newer granite-binder version was poured to form a 12-cubic-yard (9.2-cubic-meter) topping slab in the upper lobby of 270 Park Avenue, a 1,388 ft (423 m)-tall skyscraper being constructed in New York City. A topping slab is a layer of concrete placed over another concrete surface to enhance its appearance and functionality.
C-Crete president Rouzbeh Savary tells us that the new cement-alternative is made by grinding up raw granite, followed by a number of proprietary steps that cause the powdered granite to be reactive with water – thus allowing it to act as a binder. No heat is required at any point in the process.
Additionally, because granite is by nature a non-carbonate rock, it neither contains nor releases any CO2. It is one of the planet's most abundant rocks, however, so availability shouldn't be an issue. Savary says that granite-based C-Crete should cost the same as traditional concrete.
As far as performance goes, the new form of C-Crete is claimed to "demonstrate pumpability, workability, setting time and surface finish akin to conventional concrete, while meeting ASTM international standards for mechanical and durability properties, a fact verified by independent third parties."
Source: C-Crete Technologies
