Design improvements
There is growing interest in reducing concrete-related carbon dioxide emissions, both in academia and industry, especially with the possibility of a future introduction of a carbon tax. Various approaches have been suggested to reduce emissions.
One reason why cement manufacturing produces so many emissions is because the cement precursor mixture must be heated to very high temperatures for clinker to form. Alite (CaSiO), a mineral present in concrete that cures—that is, it reaches a high degree of hardness—has a great responsibility in this; see crosslinking "Crosslinking (chemistry)") and hydration reaction, we are not talking about curing a disease—in a matter of hours after it is poured (pouring the concrete into a formwork) and is therefore responsible for much of its initial strength. However, the alite must also be heated to 1500 °C in the clinker formation process.
Some research suggests that alite can be replaced by a different mineral, such as belite (CaSiO). Belite is also a mineral already used in concrete. It has a roasting temperature of 1,200 °C, significantly lower than that of wing. Additionally, belite is stronger once the concrete cures. However, belite takes days or months to fully set, causing the concrete to weaken for a longer time. Current research is focused on finding possible additives, such as magnesium, that could speed up the curing process. It should also be considered that belite requires more energy to grind, which can make the impact of its entire life cycle similar or even greater than that of alite.[20].
Another approach has been the partial replacement of conventional clinker with alternatives such as fly ash, solid ash and slag, all by-products of other industries that would otherwise end up in landfills ("Landfill (garbage)"). Fly and solid ash come from coal-fired thermoelectric plants, while slag is a residue from the blast furnaces of the steel industry. These materials are slowly gaining popularity as additives, especially because they can potentially increase the strength, decrease the density, and prolong the durability of concrete.[21].
The main obstacle to mixing more fly ash and slag into concrete may be the risk of building with new technology that has not been exposed to prolonged field testing. Until a carbon tax is introduced, companies are unwilling to take the risk of using new concrete mix recipes, even if they reduce emissions. However, there are some examples of “green” concrete and its implementation. One example is a concrete company called Ceratech that has begun manufacturing concrete with 95% fly ash and 5% liquid additives.[20] Another is the I-35W Saint Anthony Falls Bridge, which was built with a novel concrete mix that included different compositions of Portland cement, fly ash, and slag depending on the bridge part and material property requirements.[22]
Several startups are developing and testing alternative cement production methods. For example, Sublime of Somerville, Massachusetts, uses a kilnless electrochemical process, and Fortera captures carbon dioxide from conventional plants to make a new type of cement. Blue Planet, of Los Gatos, California, captures the 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, permanently injecting and storing carbon dioxide into the concrete as it is mixed.[24].
Furthermore, concrete production requires large quantities of water, with global production accounting for almost a tenth of global industrial water use.[25] This is equivalent to 1.7% of total global water withdrawals. A study published in Nature Sustainability in 2018 predicts that concrete production will increase future pressure on water resources in regions susceptible to drought, writing: "In 2050, 75% of water demand for concrete production will likely occur in regions expected to experience water stress."[26].
Carbonation is the formation of carbonates. In the case of cement and concrete, calcium carbonate (CaCO) is formed through a chemical reaction (carbonation) which, if used in concrete, can sequester carbon dioxide.[27] The rate of carbonation depends mainly on the porosity of the concrete and its water content. Carbonation in the pores of concrete occurs only when the air humidity (relative humidity, RH) is between 40 and 90%: when the RH is above 90%, carbon dioxide cannot enter the pores of the concrete, and when it is below 40%, it cannot dissolve in water.[28].
There are 2 main methods for carbonating concrete: weathering carbonation and early carbonation.[29].
Weathering carbonation occurs in concrete when calcium compounds react with carbon dioxide () in the atmosphere and water () in the pores of the concrete. The reaction is the following. First, through weathering, CO reacts with water in the pores of concrete to form carbonic acid:
The carbonic acid then reacts with calcium hydroxide to give calcium carbonate and water:.
After calcium hydroxide (Ca(OH)) has been sufficiently carbonated, the main component of cement, calcium silicate hydrate (CSH, which is not a chemical formula), can be decalcified, that is, caused to release calcium oxide (), which in turn can be carbonated:.
Early carbonation occurs when CO is introduced in the early stages of fresh, ready-mixed concrete or during initial curing, which can occur naturally by exposing the concrete to air, or be artificially accelerated by increasing CO uptake by the concrete.[29].
Gaseous carbon dioxide is converted to solid carbonates and can be permanently captured in concrete. The reactions of CO and hydrated calcium silicate (CSH) in cement were described in 1974 in cement chemical notation (CCN) as:[30].
A Canadian company patented and commercialized a novel technology that uses early carbonation to sequester CO. This is achieved by directly injecting liquid carbon dioxide from large emitters (e.g. thermal power plants) into the wet mix concrete stage. CO is mineralized, thus retaining this greenhouse compound in concrete infrastructure for long periods of time.
In a study published in the Journal of Cleaner Production, the authors created a model that shows that the CO thus sequestered improves the compressive strength of concrete while reducing emissions, allowing a reduction in the cement load in concrete and, at the same time, a "4.6% reduction in the carbon footprint."[31].
Another proposed method to capture greenhouse gas emissions is to absorb during the curing process through the use of an additive — specifically an 𝛾-phase dicalcium silicate. The use of fly ash or another suitable substitute could theoretically reduce emissions to below zero (negative emissions), compared to emissions from Portland cement concrete (400 kg/ ). The most effective method to produce this emissions-negative concrete would be to use exhaust gases from a power plant, where an insulated chamber could control temperature and humidity.[32].
In August 2019, a reduction cement was announced that "reduces the overall carbon footprint of precast concrete by 70%."[33] The base of this cement is mainly wollastonite () and rankinite") (), unlike traditional Portland cement, based on alite ().
The patented process of manufacturing reduced emissions concrete begins with the bonding of particles through liquid phase sintering, also called "reactive hydrothermal liquid phase densification" (rHLPD).[34] A solution of water and CO penetrates the particles, reacting under ambient conditions to form a bond that creates non-hydraulic calcium silicate cement, with reduced lime (CSC). The difference between traditional concrete with Portland cement and these carbonated calcium silicate concretes (CSC-C) lies in the reaction of the final curing process between a water-CO solution and a family of calcium silicates. According to a study of a reduced emissions cement, called Solidia, "curing of CSC-C is a slightly exothermic reaction in which the low-lime calcium silicates of CSC react with in the presence of water to produce calcite (CaCO) and silica (Silicon(IV) oxide") () as follows:.
Early carbonation methods have gained recognition for their significant carbon sequestration capabilities. However, some authors have argued that the effect of early carbonation curing may subsequently succumb to weathering carbonation. For example, a 2020 article states: "Experimental results suggest that early carbonated concretes with high water/cement ratios (>0.65) are more likely to be affected by weathering carbonation."[35] The article warns that this can weaken the strength of the concrete over its useful life.
Another aspect to consider in carbon concrete is surface peeling due to cold weather conditions and exposure to antifreeze salts and freeze-thaw cycles (frost weathering). Concrete produced by carbonation curing also shows superior performance when subjected to physical degradations, for example freeze-thaw damage, particularly due to a pore densification effect enabled by the precipitation of carbonation products.[36].
The Italian company Italcementi designed a type of cement that supposedly reduces air pollution by decomposing the pollutants that come into contact with the concrete produced with this cement, using titanium dioxide "Titanium(IV) Oxide"), which absorbs ultraviolet light and thereby promotes reactions that decompose pollutants. However, some environmental experts remain skeptical and question whether this special material can "eat" enough pollutants to be economically viable. The Jubilee Church in Rome is built with this type of concrete.[37].
Titanium dioxide "Titanium(IV) oxide") (TiO), a semiconductor material that has been shown to exhibit photocatalytic behavior, has been used to remove nitrogen oxides (called NO and also NO) from the atmosphere. There are 6 nitrogen oxides, each with various names. For example, NO is called dinitrogen monoxide, nitrogen (I) oxide, nitrous oxide, and hyponitrous anhydride. Nitric oxide "Nitrogen(II) oxide") and nitrogen dioxide are gases that, if released into the atmosphere, contribute to the formation of acid rain and smog (Spanish translation of English smog admitted[38] by the RAE). Since NO formation only occurs at high temperatures, nitrogen oxides are typically produced as a byproduct of the combustion of hydrocarbons, for example in vehicle engines.
In addition to contributing to urban pollution, NO has been shown to harm health and the environment in multiple ways, including triggering respiratory distress (dyspnea), 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 NO concentration of 40 μg/m (micrograms per meter). cubic of air).[40] One of the ways that have been proposed to reduce NO concentrations, especially in urban environments, is to use photocatalytic TiO mixed with concrete to oxidize NO and NO and form nitrate. In the presence of light, TiO generates electrons and holes that allow NO to be oxidized to NO, and then NO to HNO (nitric acid) through a hydroxyl radical attack. detailed below:
The generation of holes and electrons through the activation of TiO is schematized as follows:
Electron/hole trapping:.
Hydroxyl radical attack:.
Recombination of electrons and holes:.
Another route for nitrogen oxidation uses UV radiation to form NO.[41].
The use of dye-sensitized solar cells embedded in concrete has been proposed as a method to reduce the carbon footprint of buildings. The use of these solar cells allows the generation of energy by the exterior surface of the building itself, which if combined with batteries, would provide constant energy day and night (depending on the number of solar cells, the degree of sunshine and energy consumption). The top layer of the concrete would be a thin layer of solar cells sensitized with dye. These cells are particularly attractive due to their ease of mass production, either by roll printing or painting, and reasonably high efficiency in transforming 10%[42] of the solar energy that falls on them into electricity. An example of the commercialization of this concept is the German company Discrete, which produces concrete with these cells. Their process uses a spray coating method to apply electricity-generating organic dyes onto concrete.[43].
Energy storage has become essential for many renewable energy generation methods such as solar or wind energy, which are intermittent energy producers (they generate electricity when the sun shines or the wind blows) that require storage to be used constantly.
Currently, 96% of the world's energy storage comes from reversible hydroelectric power plants (also called "pumped power plants"), which, when there is "leftover" renewable electricity, use it to pump water up an elevated dam and then, when it is lacking, let the water fall to drive hydraulic turbines. However, pumping stations require specific geographies that can be difficult to find, especially in flat countries. A similar concept that uses cement instead of water has been implemented by Energy Vault, a Swiss startup. They created a facility that uses an electric crane surrounded by stacks of 35-ton concrete blocks, which can be produced from waste products, to store energy. When there is "leftover" electricity, it is used to lift the blocks, and when it is missing, the blocks are allowed to fall slowly, spinning a dynamo (electric generator), which injects energy into the electrical grid. The facility would have a storage capacity of between 25 and 80 megawatt hours MWh.[44].
Other improvements with environmental impact not directly related to emissions have been proposed for concrete. There has recently been much research into “smart” concretes, which use electrical and mechanical signals to respond to changes in loading conditions. One variety uses a carbon fiber reinforcement that provides an electrical response, which can be used to measure the stress to which the construction is subjected. This allows the structural integrity of the concrete to be monitored without installing sensors.[45].
The road construction and maintenance industry consumes tons of high emission intensity concrete every day for the maintenance of roads and urban infrastructure. As populations grow, this infrastructure becomes increasingly vulnerable to vehicle impact, creating an ever-increasing cycle of damage, debris from traffic-damaged infrastructure, and ever-increasing consumption of concrete for repairs. A major advance in the infrastructure industry involves the use of recycled oil waste to protect concrete from damage and allow infrastructure to become dynamic, able to be easily maintained and upgraded without disturbing existing foundations. This innovation theoretically preserves the foundation throughout the useful life of a building.
Another area of concrete research is so-called "waterless" concrete for use in extraplanetary colonization. These concretes most commonly use sulfur as a non-reactive binder, allowing the construction of concrete structures in environments with little or no water. These concretes are, in many ways, indistinguishable from normal hydraulic concrete: they have similar densities, can be reinforced (reinforced concrete), and, in fact, gain strength more quickly than normal concrete.[46] Its application is yet to be explored on Earth, but with concrete production accounting for up to 2-thirds of the total energy consumption of some developing countries,[15] any improvements are worth considering.