Study finds concrete reabsorbs under 10% of cement's CO2

What happened

Cement is the binder that holds concrete together. According to the UCLA Newsroom, making it is responsible for about 10 percent of global carbon dioxide emissions, mostly from heating limestone to make its key ingredient. Some people have argued that concrete pays part of this back, because hardened concrete slowly absorbs carbon dioxide from the air. A team led by Professor Gaurav Sant at the University of California, Los Angeles (UCLA), tested that claim.

The study was published in Communications Sustainability, a journal in the Nature Portfolio group. The paper's authors come from UCLA, Missouri University of Science and Technology, Arizona State University and the University of California, Davis. They estimate that by 2030, concrete in use around the world will absorb about 0.23 billion tonnes of carbon dioxide each year. That is less than 10 percent of the roughly 3 billion tonnes the cement industry is expected to emit that year.

The paper notes that earlier studies suggested this natural process could offset between 10 and 50 percent of the emissions from making clinker, the main ingredient of cement. UCLA says some estimates went as high as 57 percent. The new study finds the effect is real but small and very slow. The authors say the industry should instead use less cement per structure, replace part of the cement with lower-carbon materials, use alternative fuels and capture carbon at the plant.

The engineering behind it

Fresh cement paste is strongly alkaline. The paper explains that carbon dioxide from the air enters the concrete's pores and reacts with compounds such as calcium hydroxide and calcium silicate hydrate. These reactions form calcium carbonate, the same mineral found in limestone. This process is called carbonation. It locks the carbon away permanently, but it also lowers the pH of the concrete as it moves inward from the surface. The paper adds that about one tonne of carbon dioxide is released for each tonne of clinker produced.

The speed of carbonation is limited by diffusion, the slow movement of gas through the tiny pores of the concrete. The researchers combined chemical models with diffusion models and ran many random combinations of inputs, a method called Monte Carlo simulation. The inputs included the mix design, the amount of cement, the porosity of the concrete, and the ratio of exposed surface area to volume. They assumed a service life of 50 years.

Shape matters a great deal. The paper finds that an uncracked beam, slab or pavement, fully exposed to air, would need up to 1,000 years to carbonate halfway. A concrete masonry block, which is thin and porous, would take about 14 years. Crushed recycled concrete in small pieces could carbonate fully in about one year. In its first year of service, a typical concrete element absorbs less than 2 percent of the emissions from making its cement.

After 50 years, the model estimates a typical carbonation depth of about 14 millimetres, with a range from about 4 to 38 millimetres. Demolition can speed up carbonation by breaking concrete into pieces. But the researchers point out that crushed concrete is usually buried in landfill, stockpiled or used as road base, where little air reaches it.

Why carbonation is a problem for engineers

For structural engineers, carbonation has always been a durability threat rather than a benefit. The paper notes that it is well known to damage reinforced concrete. Steel bars inside concrete are protected from rust by the high alkalinity around them. When the carbonation front reaches the steel and the pH falls, that protection is lost and the steel can start to corrode. Rusting steel expands and cracks the concrete around it.

This is why design codes, in general, set a minimum cover, the thickness of concrete between the steel and the surface, and limits on the water-to-cement ratio. A denser concrete with lower porosity slows carbonation. The new study uses the same physics, but turns the question around: instead of asking how long the steel stays safe, it asks how much carbon the concrete takes back. The answer is that the same slow diffusion that protects steel also limits carbon uptake.

What it means in Nepal

The sources do not discuss Nepal, so this section is about skills. The study is a useful example of how to test a climate claim with engineering numbers. A statement such as concrete absorbs its own emissions sounds reasonable. Checking it needs chemistry, diffusion physics, real data on carbonation depth, and honest ranges instead of single numbers. Civil engineers who can do this kind of checking can judge green building claims for themselves.

The practical message for designers is the one the authors give. The largest cuts come from how cement is made and how much of it is used. Using concrete efficiently, choosing blended cements with lower-carbon substitutes where standards allow, and specifying enough cover and a dense mix for durability are all choices a site or design engineer makes. A structure that lasts longer also spreads its emissions over more years of use.