Environmental impact of concrete
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.[1] Many depend on circumstances. A major component is cement, which has its own environmental and social impacts and contributes largely to those of concrete.
The
Concrete dust
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Building demolition and natural disasters such as earthquakes often release a large amount of concrete dust into the local atmosphere. Concrete dust was concluded to be the major source of dangerous air pollution following the Great Hanshin earthquake.[5]
Toxic and radioactive contamination
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The presence of some substances in concrete, including useful and unwanted additives, can cause health concerns. Natural
Carbon dioxide emissions and climate change
The cement industry is one of the two largest producers of carbon dioxide (CO2), creating up to 5% of worldwide man-made emissions of this gas, of which 50% is from the chemical process and 40% from burning fuel.
One area of the concrete life cycle worth noting is its very low
A 2022 report from the
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
One reason why the carbon emissions are so high is because cement has to be heated to very high temperatures in order for
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.[21]
The main 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.
Furthermore, the production of concrete requires large amounts of water, and global production accounts for almost a tenth of worldwide industrial water use.[25] 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".[26]
Carbon concrete
Concrete can be carbonated by two main methods: weathering carbonation and early age carbonation.[29]
Weathering carbonation occurs in concrete when calcium compounds react with carbon dioxide () from the atmosphere and water () in the concrete pores. The reaction is as follows. First, through
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 () can carbonate:
Early age carbonation is when CO2 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 CO2.[29] 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:[30]
In a study published in the Journal of Cleaner Production, the authors created a model showing that sequestered CO2 improved the compressive strength of concrete while reducing CO2 emissions, thus allowing for a cement loading reduction while also having a "4.6% reduction in the carbon footprint".[31]
Another proposed method of capturing emissions is to absorb CO2 in the
In August 2019, reduced CO2 cement was announced which "reduces the overall carbon footprint in precast concrete by 70%".[33] The base of the cement is primarily wollastonite () and rankinite (), in contrast to traditional Portland cement, based on alite () and belite (). The patented process of reduced-emissions concrete manufacture begins with the bonding of particles through liquid phase
"[35]
However, as early age carbonation methods have gained recognition due to their substantial carbon sequestration proficiencies, some authors have argued that the effect of early-age carbonation curing may succumb to weathering carbonation later on. For example, a 2020 article writes, "Experimental results suggest that early-age carbonated concretes with high w/c ratios (>0.65) are more likely to be affected by weathering carbonation".[36] The article cautions that this may weaken its strength abilities in the corrosion stages during life service.
Italian company
Another aspect to consider in carbon concrete is surface scaling due to cold climatic conditions and exposure to de-icing salt and freeze-thaw cycles (frost weathering). Concrete produced by carbonation curing also shows superior performance when subject to physical degradations, e.g., freeze-thaw damage, particularly due to a pore densification effect enabled by the precipitation of carbonation products[38]
The vast majority of CO2 emissions from concrete come from cement manufacturing. Therefore, methods to reduce cement materials in each concrete mix are the only known methods to reduce the emissions.[citation needed]
Photocatalysis to reduce smog
Titanium dioxide (TiO2), a semiconductor material shown to exhibit photocatalytic behavior, has been used to remove nitrogen oxides (denoted NOx) from the atmosphere. NOx species, i.e., nitric oxide and nitrogen dioxide, are atmospheric gases that contribute to acid rain and smog formation, both of which are the result of urban pollution. Since NOx formation only occurs at high temperatures, nitrogen oxides are typically produced as a byproduct of hydrocarbon combustion. In addition to contributing to urban pollution events, NOx has been demonstrated to cause a wide variety of adverse health and environmental effects, including triggering respiratory distress, reacting with other atmospheric chemicals to form harmful products such as ozone, nitroarenes, and nitrate radicals, and contributing to the greenhouse effect. The World Health Organization (WHO) has recommended a maximum NOx concentration of 40 μg/m3.[39] One proposed route of decreasing NOx concentrations, especially in urban settings, is to use photocatalytic TiO2 mixed into concrete to oxidize NO and NO2 to form nitrate. In the presence of light, TiO2 generates electrons and holes that allow for NO to oxidize into NO2 and NO2 to then form HNO3 (nitric acid) via a hydroxyl radical attack. The molecule adsorption reactions are given below:
- O2 + ⬚ → Oads
- H2O + ⬚ → H2Oads
- NO + ⬚ → NOads
- NO2 + ⬚ → NO2ads
Generation of holes and electrons via TiO2 activation is described below:
- TiO2 + hν → e− + h+
Trapping of electron/hole:
- h+ + H2Oads → OH· + H+
- e− + O2ads → O2−
Hydroxyl radical attack:
- NOads + OH· → HNO2
- HNO2 + OH· → NO2ads + H2O
- NO2ads + OH· → NO3− + H+
Electron and hole recombination:
- e− + h+ → heat
Another pathway for the oxidation of nitrogen uses UV irradiation to form NO3.[40]
Embedded solar cells
Energy storage
Energy storage has become an important consideration for many renewable energy generation methods, especially for popular methods such as solar or wind energy, both of which are intermittent energy producers that require storage for constant use. Currently, 96% of the world’s energy storage comes from pumped hydro, which uses excess generated electricity to pump water up a dam and then allowed to fall and turn turbines that produce electricity when the demand exceeds generation. The problem with pumped hydro, however, is that the setup requires specific geographies that can be difficult to find. A similar concept that uses cement instead of water has been realized by Energy Vault, a Swiss startup. They created a setup that uses an electric crane surrounded by stacks of 35-ton concrete blocks, which can be produced using waste products, to store energy by using excess energy generation to power the crane to lift and stack the concrete blocks. When energy is needed, the blocks are allowed to fall and the rotated motor would send energy back to the grid. The setup would have a storage capacity of 25-80 MWh.[43]
Other improvements
There are many other improvements to concrete that do not deal directly with emissions. Recently, much research has gone into "smart" concretes: concretes that use electrical and mechanical signals to respond to changes in loading conditions. One variety uses carbon fiber reinforcement which provides an electrical response that can be used to measure strain. This allows for monitoring the structural integrity of the concrete without installing sensors.[44]
The
Another area of concrete research involves the creation of certain “waterless” concretes for use in extraplanetary colonization. Most commonly, these concretes use sulfur to act as a non-reactive binder, allowing for construction of concrete structures in environments with no or very little water. These concretes are in many ways indistinguishable from normal hydraulic concrete: they have similar densities, can be used with currently existing metal reinforcement, and they actually gain strength faster than normal concrete[45] This application has yet to be explored on Earth, but with concrete production representing as much as two-thirds of the total energy usage of some developing countries,[18] any improvement is worth considering.
Changes in use
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Concrete is one of the world's oldest man-made building materials. Over the years, significant environmental limitations have been placed on the creation and use of concrete due to its carbon footprint. Manufacturers responded to these limitations by altering concrete's production processes, and recycling old concrete rubble to use as aggregate in new concrete mixtures to reduce these emissions. Concrete has immersed from natural resources into man-made processes; evidence of the use of concrete dates back over 8,000 years ago. Today, many construction companies and concrete manufacturers have cut the use of Portland cement in their mixtures due to its production process emitting significant amounts of greenhouse gases into the atmosphere.
Alternatives to concrete
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There are in fact many alternatives to concrete. One being Green concrete that is produced by recycled waste materials from various industries , another being Ashcrete, a material made from a mix of lime and water that acts similar to cement. Black furnace slag is also a strong alternative made from molten iron slag into water, along with Micro Silica, Papercrete, composite cement, and post-consumer glass.[46]
Depending on the amounts required or used overall and the amounts needed, in combination with other materials, for structural stability per building, many other materials also have a substantial negative impact on the environment. For instance, while research and development to reduce these emissions are ongoing, steel accounted for ~8 % of the world's total greenhouse gas emissions as of 2021.[47][48]
Clay
Mixtures of clay are an alternative construction material to concrete that have a lower environmental footprint. In 2021, the first prototype 3D printed house, Tecla, printed from locally-sourced soil and water as well as fibers from rice husks and a binder was completed.[49][50][51] Such buildings could be very inexpensive, well-insulated, stable and weatherproof, climate-adaptable, customizable, get produced rapidly, require only very little easily-learnable manual labor, require less energy, produce very little waste and reduce carbon emissions from concrete.[citation needed]
Surface runoff
In an attempt to counteract the negative effects of impervious concrete, many new paving projects have begun to use pervious concrete, which provides a level of automatic stormwater management. Pervious concrete is created by careful laying of concrete with specifically designed aggregate proportions, which allows for surface runoff to seep through and return to the groundwater. This both prevents flooding and contributes to groundwater replenishment.[56] If designed and layered properly, pervious concrete and other discreetly paved areas can also function as an automatic water filter by preventing certain harmful substances like oils and other chemicals from passing through.[57] Unfortunately there are still downsides to large scale applications of pervious concrete: its reduced strength relative to conventional concrete limits use to low-load areas, and it must be laid properly to reduce susceptibility to freeze-thaw damage and sediment buildup.[56]
Urban heat
Both concrete and asphalt are the primary contributors to what is known as the urban heat island effect.[25] According to the United Nations Department of Economic and Social Affairs 55% of the world’s population reside in urban areas and 68% of the world’s population is projected to be urban by 2050; also, "the world is projected to add 230 billion m2 (2.5 trillion ft2) of buildings by 2060, or an area equal to the entire current global building stock. This is the equivalent of adding an entire New York City to the planet every 34 days for the next 40 years".[58] As a result, paved surfaces represent a major concern because of the additional energy consumption and air pollution they cause.[59]
The potential of energy saving within an area is also high. With lower temperatures, the demand for air conditioning theoretically decreases, saving energy. However, research into the interaction between reflective pavements and buildings has found that, unless the nearby buildings are fitted with reflective glass, solar radiation reflected off pavements can increase building temperatures, increasing air conditioning demands.[60]
Moreover, heat transfer from pavements, which cover about one-third of a typical U.S. city,[3] can also influence local temperatures and air quality. Hot surfaces warm the city air through convection, so using materials that absorb less solar energy, such as high-albedo pavements, can reduce the flow of heat into the urban environment and moderate the UHIE.[61] Albedos range from about 0.05 to about 0.35 for currently used pavement material surfaces. Over a typical life service, pavement materials that begin with high albedo tend to lose reflectance, while those with low initial albedo may gain reflectance[62]
The Design Trust for Public Space found that by slightly raising the albedo value in New York City, beneficial effects such as energy savings could be achieved.,[63] by replacement of black asphalt with light-colored concrete. However, in winter this may be a disadvantage as ice will form more easily and remain longer on light colored surfaces as they will be colder due to less energy absorbed from the reduced amount of sunlight in winter.[64]
Another aspect to consider is thermal comfort effect, as well as the need for more mitigation strategies, which don’t threat the health and wellbeing of pedestrians particularly during heat waves.[65] A study that appeared in Building and Environment in 2019 performed experiments to project the impact of heat waves and high albedo materials interactions in the northern Italian city of Milan. By calculating the "Mediterranean Outdoor Comfort Index" (MOCI) in presence of a heat wave, where high albedo materials was used in all surfaces. The study identified a deterioration of the microclimate where high amounts of high albedo materials were located. The use of the high albedo materials was found to "lead to the establishment of multiple inter-reflections and a consequent increase in micrometeorological variables such as average radiant temperatures and air temperatures. To be more detailed, these changes lead to an increase in the MOCI that in the afternoon hours can even reach 0.45 units".[66]
Overall urban configurations should remain of concern when making decisions as people are exposed to weather and thermal confort conditions. The use of high albedo materials within an urban environment can be of positive effect with proper combination of other technologies and strategies such as: vegetation, reflective materials, etc. Urban heat mitigation measures could minimize impacts on microclimate as well as human and wildlife habitats.[67]
Handling precautions
Handling of wet concrete must always be done with proper protective equipment. Contact with wet concrete can cause skin
Concrete recycling
Concrete recycling is an increasingly common method of disposing of concrete structures. Concrete debris was once routinely shipped to landfills for disposal, but recycling is increasing due to improved environmental awareness, governmental laws and economic benefits.
Concrete, which must be free of trash, wood, paper and other such materials, is collected from demolition sites and put through a crushing machine, often along with asphalt, bricks and rocks.
Reinforced concrete contains rebar and other metallic reinforcements, which are removed with magnets and recycled elsewhere. The remaining aggregate chunks are sorted by size. Larger chunks may go through the crusher again. Smaller pieces of concrete are used as gravel for new construction projects. Aggregate base gravel is laid down as the lowest layer in a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete can sometimes be used as the dry aggregate for brand new concrete if it is free of contaminants, though the use of recycled concrete limits strength and is not allowed in many jurisdictions. On 3 March 1983, a government-funded research team (the VIRL research.codep)[citation needed] approximated that almost 17% of worldwide landfill was by-products of concrete-based waste.
See also
- Longship, a CCS project storing CO2 emissions from a cement factory
- Greenhouse gas emissions § Buildings and construction
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