Pathways to Decarbonize Society's Most Popular Material

It is widely accepted that increasing emissions of greenhouse gasses (GHG) in general, and carbon dioxide (CO2) in particular, are impacting global climate with detrimental consequences for humans and ecosystems. Since the largest contributor to emissions is use of fossil fuels (contributing an estimated 75% of GHG emissions and 90% of CO2 emissions), the clearest path to lowering emissions is reducing use of coal, oil, and gas. However, one ubiquitous material, concrete, produces over 7% of anthropogenic CO2 (IEA 2018), with less than half of these emissions attributable to fuel (Chen et al. 2022) and, as such, requires other decarbonization strategies. 

The bulk of concrete is made up of aggregates (e.g., sand and crushed rocks), which are commonly sourced locally. Cement is mixed with water to “glue” the aggregates together and form rock-like materials — including concrete, mortar, and other materials, but referred to herein as “concrete” for simplicity — that can be cast into any shape, gain strength as hydration reactions take place, and are both inexpensive and remarkably durable. This class of material products supports our ability to build roadways, water and wastewater systems, and foundations and structures, as well as other applications. Due to our societal need for such systems, concrete is one of the most consumed materials on Earth. Unfortunately, the production of cement requires a chemical reaction called calcination that emits CO2. The production of cement alone accounts for 70% to 90% of the emissions from concrete, with the calcination reaction being responsible for what is estimated as over 60% of these emissions (Chen et al. 2022).

Concrete is not per se an environmentally unfavorable material; rather, we are consuming so much of this one material that it is causing substantial impacts. Building materials are our most-consumed resources after water. We currently produce almost 2 billion metric tons (Gt) of steel each year (Kelly et al. 2014), with some of that steel being used in appliances, automobiles, and packaging, and the remaining approximately 55% used in construction applications (Cullen et al. 2012). Notably, while steel is used in several forms in construction, it is also often embedded in concrete as reinforcement (although, we note, this form of steel tends to have lower emissions from production than some other steels). Beyond steel, about 1500 billion baked-clay bricks are produced annually (ILO et al. 2017). And we require myriad other materials including lumber, plastics for piping and flooring, asphalt pavements, and glass, to name a few. Yet, by mass, production of each of these materials is dwarfed by the production of concrete. Annual current production of cement is approximately 4.36 Gt (Kelly et al. 2014), and annual production of concrete is about 30 Gt. The widespread use of concrete worldwide is largely due to its remarkably low cost, which results from the abundance of materials needed to produce it. This affordability, however, makes it challenging to introduce more expensive innovations unless they offer significant benefits, such as improved performance, that can justify the higher price in the market. 

GHG emissions from concrete production are driven by the production of clinker in cement. Our modern cement is composed of clinker, a material produced from limestone and clays that are kilned (at approximately 1450 degrees Celsius) and then quenched (rapidly cooled), with other mineral additives. While the mineral additives are commonly directly quarried or are byproducts of other industries, the production of clinker relies on: (1) the calcination process, an energy-driven chemical reaction in which limestone is broken down to provide reactive calcium compounds, directly off-gassing CO2; and (2) energy-derived emissions from both the calcination and kilning stages, where kilning is used to form desired reactive calcium silicates. Currently, the energy resources commonly used in cement production are fossil fuels (e.g., coal), so GHG emissions can potentially be lowered up to about 25% through energy transitions. Use of alternative fuels, such as biomass (e.g., agricultural waste, wood chips), or other mechanisms, such as electrification, in the kilns can support a reduction in reliance on high GHG emissions fuels. Improving kiln efficiency and/or the efficiency of other production processes can also reduce energy demands, thus offering lower energy-related emissions.

Read the full article at  NAE Perspectives