Producers and Contractors are Drawing Criticism Over the Carbon Footprint of Concrete!

Cement, the binding agent in concrete, is the second most frequently used substance throughout the World – behind only water.  Given that water is mandatory to the survival of all living things – says mountains about the human need for cement.  Much like living things cannot exist without water – traditional concrete would not exist without cement.  And concrete, using cement, is used in the construction of almost everything worldwide, ranging from dams, roads, bridges, foundations, buildings, sidewalks, culverts, wastewater treatment plants, pipes, tunnels, etc.  The need for cement is endless. The World is a concrete jungle!  [But see, “Could “Living Building Materials” a/k/a LBM’s Change the Face of Concrete Construction”, posted 6/7/23].

The only problem with cement is it has a large carbon footprint producing carbon dioxide or CO2 . The worldwide cement industry accounts for 8% of gross CO2 production, falling between aviation at ±2.5% and agriculture at ±12%.  According to the BBC, when the carbon footprint of cement manufacturing and concrete production are combined,  the union becomes the third largest worldwide emitter of CO2  behind only the US (13.6% in 2022) and China (30.7% in 2022).  That says mountains about the combined carbon footprint of cement and concrete.  Numbers do not lie. 

The production of concrete differs from many other building materials in that it requires a two-step process.  First, cement must be manufactured.  The active ingredient in manufacturing cement is calcium.  The most commonly used source of calcium is limestone.  In order to extract calcium from limestone, the material must be superheated to ±2,700 degrees in kilns.  Superheating relies entirely upon fossil fuels which release CO2 to the atmosphere.  The process separates limestone into CO2, and the end product of calcium.  During the process more CO2 is released to the atmosphere. Calcium remains in the kiln in the form of rock like “clinkers” which are then ground to a fine powder to make cement.  Approximately 1,370 pounds of CO2 are released throughout the process for every 1,000 pounds of cement manufactured. 

Next, concrete must be produced.  The principal ingredients of concrete are cement, sand, and stone.  All must be transported to the concrete plant by either truck or train.  More CO2  is released to the atmosphere.   The ingredients are then mixed to form a matrix based upon the desired design properties.  Water is then added to the matrix either at the plant or at the construction site.  The drum on the truck mixes the matrix and water into a flowable mass.  Meanwhile the addition of water to the matrix causes the calcium within the cement to mineralize through a process called “heat of hydration”.  This process releases additional CO2 to the atmosphere.  Every country and company in the World using concrete releases CO2 to the atmosphere in this manner.  China currently produces twice as much concrete as all other countries combined and is the biggest producer of CO2.

Many countries, including the US, are beginning to address the CO2  released by the production of concrete.  California and New York recently passed laws requiring the reduction of CO2 released from the production of concrete.  California requires concrete producers to cut carbon emissions by 40% below the 2019 levels no later than 2035.  New York requires concrete producers to reduce CO2 emissions below future emissions standards (yet to be defined) if they seek to furnish concrete for public works projects.  In May of 2021, 50 major corporations - including Microsoft, Google, and Salesforce – pledged to incorporate  clean technologies in future construction, relating to steel, aluminum, and concrete.

Concrete companies are also working to resolve the CO2  issue using either carbon capture techniques or the use of calcium sources, other than limestone.  Carbon capture techniques include preventing the release of CO2  to the atmosphere, causing the CO2  to interact with other compounds to create a liquid, or entraining the CO2 back into the concrete.  None of these carbon capture techniques have proven successful thus far.  Calcium based limestone replacement techniques include the use of bacteria or algae in lieu of calcium which has no carbon footprint, or the use calcium silicate rocks in place of limestone which has a smaller carbon footprint.  While these techniques show some promise, both are in the development stages.  Whatever the solution might be or from where it may come, one thing is for certain – the concrete industry will be forced to reduce its carbon footprint.