page_title: Environmental impact of concrete
section_title: Carbon Concrete
hierarchical_section_title: Environmental impact of concrete / Design improvements / Carbon Concrete
caption_attribution_description: English: Pore structures in fresh concrete and air entrained concrete. Fresh concrete
context_page_description: The environmental impact of concrete, its manufacture and applications, are complex. Some effects are harmful; others welcome. Many depend on circumstances. A major component of concrete is cement, which has its own environmental and social impacts and contributes largely to those of concrete.
  The cement industry is one of the main producers of carbon dioxide, a potent greenhouse gas. Concrete causes damage to the most fertile layer of the earth, the topsoil.
  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 proper 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 radioactivity.
context_section_description: Carbonatation in concrete is the formation of Calcium Carbonate (CaCO3) by chemical reaction . The Speed of carbonation depends primarily on the porosity of the concrete and the 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 also when RH is lower than 40% CO
  2 cannot be dissolved in water 
  Concrete can be subjected for the most part by two types of carbonation: weathering carbonation and Early age carbonation. 
  Weathering carbonation, occurs in concrete when calcium compounds react with carbon dioxide CO
  2 from the atmosphere and water  in the concrete pores. The reaction is as follows: 
  First as in chemical weathering CO
  2 reacts with water in the concrete pores to form Carbonic acid:
  carbon dioxide + water → carbonic acid
  Second Carbonic acid then reacts with calcium carbonate:
  Ca(OH)₂ + H₂CO₃ → CaCO₃ + 2H₂O
  carbonic acid + calcium carbonate → calcium bicarbonate
  Third Once the calcium hydroxide (Ca(OH)2) has carbonated, the main component of Cement calcium silicate hydrate gel (also shown as C-S-H) can be decalcified, allowing the liberated CaO to carbonate:
  H₂CO₃ + CaO → CaCO₃ + H₂O
  Early age carbonation is when we introduce CO
  2 to the early stages of fresh premix concrete or upon initial curing, it can be both natural through exposure or accelerated by augmenting a direct intake of CO
  2 . Gaseous carbon dioxide is converted to solid carbonates and can be permanently stored in concrete for emission reduction, the general reaction of CO2 and calcium silicate hydrate in cement was described in 1974  as: 
  C3S + 3 CO2 + H2O → C-S-H + 3CaCO3 + 347 kJ/mol 
  C2S + 2 CO2 + H2O → C-S-H + 2CaCO3 + 184 KJ/mol
  A Canadian company patented and commercialized a novel technology that uses the early age type of carbonation to sequester CO
  2. 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. Whereby a chemical reaction CO
  2 becomes a mineral, sequestering the greenhouse gas pollutant in concrete infrastructures, buildings, roads, etc for long periods of time. Moreover in a study published in the journal of Cleaner Production authors preformed a model where they proved that CO
  2 improved the compressive strength of the concrete while reducing CO
  2 emissions as a result, in so allowing for a cement loading reduction while at the same time having a "4.6% reduction in the carbon footprint" 
  Another proposed method of capturing emissions is to absorb CO₂ in the curing process, by the use of an admixture (a dicalcium silicate y phase) as the concrete cures. The use of coal ash or another suitable substitute, could theoretically have CO₂ emissions below 0 kg/m³, compared to portland cement concrete at 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. 
  In August 2019, reduced CO₂ cement was announced which "reduces the overall carbon footprint in precast concrete by 70%.". The base of these cement being primarily of wollastonite (CaSiO₃) and rankinite (3CaO·2SiO₂) in contrast to traditional portland cement alite (3CaO·SiO₂) belite (2 CaO · SiO₂).
  The patented process of concrete making begins with the bonding of particles through liquid phase sintering also referred to as hydrothermal liquid phase densification (rHLPD). A solution mixed of H
  ₂O and CO
  2 penetrates the particles, in reaction with ambient conditions to create a bond which creates the reduced-lime, non-hydraulic calcium silicate cement (CSC). Moreover the difference between traditional portland concrete and these carbonated calcium silicate concrete (CSC-C) lies in the final curing process reaction between a water-CO
  2 solution and a family of calcium-silicate: 
  "CSC-C curing is a mildly exothermic reaction in which the low-lime calcium silicates in the CSC react with carbon dioxide in the presence of 
caption: Pore structures in fresh concrete and air entrained concrete
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