Concrete is the most widely used man-made material globally, making it a subject of extensive analysis regarding its environmental footprint. Its production releases greenhouse gases into the atmosphere, primarily from the manufacture of cement, the binder in concrete. Less commonly understood is the material’s ability to naturally absorb atmospheric carbon dioxide (CO2). This continuous, passive process occurs over the entire lifespan of a concrete structure and must be understood for an accurate assessment of the material’s full environmental life cycle.
The Chemical Process of CO2 Absorption
Yes, concrete naturally absorbs atmospheric carbon dioxide through a process known as carbonation. This chemical reaction is the reverse of the high-temperature calcination process used to manufacture cement. During cement production, limestone is heated, releasing CO2 and forming calcium oxide, which later forms calcium hydroxide in the concrete mix. The absorption begins when atmospheric CO2 penetrates the pores of the hardened concrete, dissolving in the pore water to form carbonic acid. This acid then reacts with the calcium hydroxide, a primary component of the hydrated cement paste.
The reaction converts the calcium hydroxide into calcium carbonate, a stable mineral, permanently locking the CO2 within the solid matrix of the concrete structure. Carbonation begins at the exposed surface and slowly progresses inward over years or decades. This conversion causes a reduction in the material’s pH level. In structural concrete containing steel reinforcement, engineers limit the speed of carbonation to prevent rebar corrosion. The mechanism of absorption remains the same across all concrete types, but the rate is highly dependent on the surrounding environment and the physical properties of the material.
Key Factors Affecting Carbonation Rate
The rate of carbonation is significantly influenced by the physical characteristics of the concrete and the conditions of its exposure. The material’s porosity and permeability dictate how easily CO2 can diffuse into the cement paste. A higher water-to-cement ratio in the initial mix leads to greater porosity, allowing for faster and deeper carbonation penetration.
Environmental moisture content is a significant modifier, as the process requires a specific range of relative humidity to proceed efficiently. Carbonation is most rapid when relative humidity is between 50% and 70%, providing sufficient pore water for the CO2 to dissolve. If the concrete is too dry, the reaction cannot occur; if it is too saturated, water-filled pores block the diffusion of CO2 gas.
The dimensions of the concrete structure and its exposure also influence the overall rate. In large, dense, reinforced concrete structures, carbonation is deliberately slow and limited to shallow surface layers throughout its service life. Conversely, products like unreinforced concrete blocks or thin mortar layers, which have a greater surface area relative to their volume, carbonate much more quickly.
The most dramatic acceleration of the carbonation rate occurs after the concrete’s service life has ended. When a structure is demolished and the concrete is crushed for recycling, the massive increase in exposed surface area allows CO2 to penetrate non-carbonated interior material. This crushing phase facilitates a significant amount of CO2 reabsorption in a comparatively short time.
Measuring Lifetime CO2 Reabsorption
Quantifying the total amount of CO2 reabsorbed by concrete is necessary for a complete life cycle assessment of the material. Studies account for the slow, continuous absorption during the structure’s use phase and the accelerated absorption that follows demolition and crushing. The total amount of CO2 that concrete can reabsorb is theoretically equivalent to the amount released during the calcination of the limestone used to make the cement.
Global analyses indicate that this ongoing carbonation process can offset a meaningful portion of the initial emissions. Research suggests that over a 50- to 100-year lifespan, including the post-demolition phase, concrete structures reabsorb approximately 20% to 40% of the CO2 emitted during cement calcination. This reabsorbed amount is a permanent sequestration, as the carbon is chemically bound in the form of calcium carbonate.
The post-demolition phase, where concrete is crushed and recycled as aggregate, is particularly important to the overall absorption total. By exposing the inner, non-carbonated material, the crushing process allows the majority of the lifetime CO2 uptake to occur rapidly, often within a few years. While concrete remains a net emitter of greenhouse gases, this natural reabsorption significantly reduces the material’s overall environmental impact over its full life cycle.