The environmental impact of concrete, its manufacture, and its applications, are complex, driven in part by direct impacts of construction and infrastructure, as well as by CO2 emissions; between 4-8% of total global CO2 emissions come from concrete. Many depend on circumstances. A major component is cement, which has its own environmental and social impacts and contributes largely to those of concrete. In comparison with other construction materials (aluminium, steel, even brick), concrete is one of the least energy-intensive building materials.
The cement industry is one of the main producers of carbon dioxide, a greenhouse gas.
Concrete is used to create hard surfaces which contribute to surface runoff that may cause soil erosion, water pollution and flooding. Conversely, concrete is one of the most powerful tools for flood control, by means of damming, diversion, and deflection of flood waters, mud flows, and the like. Light-colored concrete can reduce the urban heat island effect, due to its higher albedo. However, original vegetation results in even greater benefit. Concrete dust released by building demolition and natural disasters can be a major source of dangerous air pollution. The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns due to toxicity and (usually naturally occurring) radioactivity. Wet concrete is highly alkaline and should always be handled with proper protective equipment. Concrete recycling is increasing in response to improved environmental awareness, legislation, and economic considerations. Conversely, the use of concrete mitigates the use of alternative building materials such as wood, which is a natural form of carbon sequestering.
Toxic and radioactive contamination
The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns. Natural radioactive elements (K, U, Th, and Rn) can be present in various concentration in concrete dwellings, depending on the source of the raw materials used. For example, some stones naturally emit Radon, and Uranium was once common in mine refuse. Toxic substances may also be unintentionally used as the result of contamination from a nuclear accident. Dust from rubble or broken concrete upon demolition or crumbling may cause serious health concerns depending also on what had been incorporated in the concrete. However, embedding harmful materials in concrete is not always dangerous and may in fact be beneficial. In some cases, incorporating certain compounds such as metals in the hydration process of cement immobilizes them in a harmless state and prevents them from being released freely elsewhere.P.K. Mehta: Concrete technology for sustainable development – overview of essential elements, O.E. Gjorv, K. Sakai (Eds.), Concrete technology for a sustainable development in the 21st century, E&FN Spon, London (2000), pp. 83–94
Carbon dioxide emissions and climate change The cement industry is one of the two largest producers of carbon dioxide (CO2), creating up to 8% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel.The Cement Sustainability Initiative: Our agenda for action, World Business Council for Sustainable Development, page 20, published 1 June 2002 The produced for the manufacture of structural concrete (using ~14% cement) is estimated at 410 kg/m3 (~180 kg/tonne @ density of 2.3 g/cm3) (reduced to 290 kg/m3 with 30% fly ash replacement of cement). The CO2 emission from the concrete production is directly proportional to the cement content used in the concrete mix; 900 kg of CO2 are emitted for the fabrication of every ton of cement, accounting for 88% of the emissions associated with the average concrete mix. Cement manufacture contributes greenhouse gases both directly through the production of carbon dioxide when calcium carbonate is thermally decomposed, producing lime and carbon dioxide,EIA – Emissions of Greenhouse Gases in the U.S. 2006-Carbon Dioxide Emissions and also through the use of energy, particularly from the combustion of fossil fuels.
One area of the concrete life cycle worth noting is its very low embodied energy per unit mass. This is primarily because the materials used in concrete construction, such as aggregates, pozzolans, and water, are relatively plentiful and can often be drawn from local sources.Building Green. (1993). Cement and Concrete: Environmental Considerations. Retrieved 2 November 2015.http://www.wbcsdcement.org/pdf/tf2/cementconc.pdf This means that transportation only accounts for 7% of the embodied energy of concrete, while cement production accounts for 70%. Concrete has a total embodied energy of 1.69 GJ/tonne, lower per unit mass than most common building materials besides wood. However, concrete structures often have high masses, so this comparison is not always directly relevant to decision making. Additionally, this value is based only on mix proportions of up to 20% fly ash. It is estimated that a 1% replacement of cement with fly ash represents a 0.7% reduction in energy consumption. With some proposed mixes containing as much as 80% fly ash, this could represent a considerable energy saving.
A 2022 report from the Boston Consulting Group found that investments in greener forms of cement would lead to greater greenhouse gas reductions, per dollar, than investments in many other green technologies—though investments in plant-based meat alternatives would reap considerably greater reductions than even this.
Mitigation Design improvements There is a growing interest in reducing carbon emissions related to concrete from both the academic and industrial sectors, especially with the possibility of future carbon tax implementation. Several approaches to reducing emissions have been suggested.
Cement production and use Carbon emissions from cement manufacture are high because of the high temperatures required to produce clinker. A major component of clinker is alite (Ca3SiO5). Alite is produced at 1,500 °C in the clinker-forming process. Some research suggests that alite can be replaced by a different mineral, such as belite (Ca2SiO4). Belite is also a mineral already used in concrete. It has a roasting temperature of 1,200 °C. Furthermore, belite actually cures to give stronger concrete. However, belite takes on the order of days or months to set completely, which leaves concrete weak for longer. Belite takes more energy to grind, which may make its full life of impact similar to or even higher than alite.
Another approach has been the partial replacement of conventional clinker with such alternatives as fly ash, bottom ash, and slag, all of which are by-products of other industries that would otherwise end up in landfills. Fly ash and bottom ash come from thermoelectric power plants, while slag is a waste from blast furnaces in the ironworks industry. These materials are slowly gaining popularity as additives, especially since they can potentially increase strength, decrease density, and prolong durability of concrete.
One obstacle to wider implementation of fly ash and slag may be largely due to the risk of construction with new technology that has not been exposed to long field testing. Until a carbon tax is implemented, companies are unwilling to take the chance with new concrete mix recipes even if this reduces carbon emissions. However, there are some examples of "green" concrete and its implementation. One instance is a concrete company called Ceratech that has started manufacturing concrete with 95% fly ash and 5% liquid additives. Another is the I-35W Saint Anthony Falls Bridge, which was constructed with a novel mixture of concrete that included different compositions of Portland cement, fly ash, and slag depending on the portion of the bridge and its material properties requirements. Several startup companies are developing and testing alternative cement production methods. Sublime of Somerville, Massachusetts uses a no-kiln electrochemical process, and Fortera captures carbon dioxide from conventional plants to make a new kind of cement. Blue Planet of Los Gatos, California captures emitted carbon dioxide to produce synthetic concrete. CarbonCure Technologies of Halifax, Nova Scotia has retrofitted its carbon mineralization systems at hundreds of concrete plants around the world, injecting and permanently storing carbon dioxide in concrete as it is being mixed.
Furthermore, the production of concrete requires large amounts of water, and global production accounts for almost a tenth of worldwide industrial water use. This amounts to 1.7 percent of total global water withdrawal. A study that appeared in Nature Sustainability in 2018 predicts that concrete production will in the future increase pressure on water resources in regions susceptible to drought conditions, writing, "In 2050, 75% of the water demand for concrete production will likely occur in regions that are expected to experience water stress".
Carbon concrete Carbonatation, sometimes called carbonation, is the formation of calcium carbonate (CaCO3) by chemical reaction, which, if used in concrete, can sequester carbon dioxide. The speed of carbonation depends primarily on the porosity of the concrete and its moisture content. Carbonation in concrete pores happens only at a relative humidity (RH) of 40-90%—when RH is higher than 90%, carbon dioxide cannot enter the concrete pores, and when RH is lower than 40%, cannot be dissolved in water.
Concrete can be carbonated by two main methods: weathering carbonation and early age carbonation.
Weathering carbonation occurs in concrete when calcium compounds react with carbon dioxide (CO2) from the atmosphere and water (H2O) in the concrete pores. The reaction is as follows. First, through chemical weathering, reacts with water in the concrete pores to form carbonic acid:
Carbonic acid then reacts with calcium hydroxide to form calcium carbonate and water:
Once the calcium hydroxide (Ca(OH)2) has sufficiently carbonated, the main component of cement, calcium silicate hydrate gel (C-S-H), can be decalcified, i.e., liberated calcium oxide (CaO>) can carbonate:
Early age carbonation is when is introduced to the early stages of fresh premix concrete or upon initial curing, which can occur both naturally through exposure or be artificially accelerated by augmenting a direct intake of . Gaseous carbon dioxide is converted to solid carbonates and can be permanently stored in concrete. The reactions of CO2 and calcium silicate hydrate (C-S-H) in cement was described in 1974 in cement chemist notation (CCN) as:
A Canadian company patented and commercialized a novel technology that uses early age carbonation to sequester . This is achieved by directly injecting recycled liquid carbon dioxide from third-party industrial emitters into the concrete wet-mix stage during the manufacturing process. is then chemically mineralized, sequestering the greenhouse gas pollutant in concrete infrastructure, buildings, roads, etc. for long periods of time.
In a study published in the Journal of Cleaner Production, the authors created a model showing that sequestered improved the compressive strength of concrete while reducing emissions, thus allowing for a cement loading reduction while also having a "4.6% reduction in the carbon footprint".
Another proposed method of capturing emissions is to absorb CO2 in the curing process through the use of an admixturespecifically, a dicalcium silicate in phaseas the concrete cures. The use of fly ash or another suitable substitute could theoretically bring CO2 emissions below 0 kg/m3, compared to Portland cement concrete emissions of 400 kg/m3. The most effective method of production of this concrete would use the exhaust gas of a power plant, where an isolated chamber could control temperature and humidity.