Read Part One of CO2 Reducing Cement here.
Carbon dioxide curing
Cements are typically classified as being hydraulic or non-hydraulic in nature. Portland cement falls into the hydraulic category; that is, it sets and hardens by hydration, a chemical reaction between the cement powder and water. When Portland cement comes into contact with water, the lime-rich alite and belite phases are converted into amorphous calcium silicate hydrate gel and calcium hydroxide. The hydration process proceeds relatively slowly. Portland cement-based concretes can take up to 28 days to reach the target hardness.
Solidia cement is a non-hydraulic cement. The low lime wollastonite/pseudowollastonite and rankinite phases react minimally with water. However, in the presence of liquid water and CO2, the cement will react with CO2 according to the general formulae:
- CaO·SiO2 + CO<2 + H2O --> CaCO3 + SiO2 + H2O.
- 3CaO·2SiO2 + 3CO2 + H2O --> 3CaCO3+ 2SiO2 + H2O. (Note that no water is consumed in this reaction).
The calcite and silica phases are thermodynamically stable to temperatures in excess of 500 °C, thereby offering an effective way to safely and permanently sequester CO2. One tonne of Solidia cement, used as a bonding agent to set and harden concrete, can sequester up to 300 kg of CO2 during the curing process. Unlike Portland cement-based concretes, concrete products that are hardened with CO2-cured cement do not consume water.
The reaction products and microstructure of CO2-cured cement are able to effectively bond discrete aggregate particles, such as sand and crushed stone, into strong, durable concrete products. Concrete mixtures consisting of 50% aggregate, 33% sand and 17% Solidia cement can reach ASTM C39 compressive strengths in excess of 10 000 psi (69 MPa) and ASTM C78 flexural strengths in excess of 1100 psi (7.5 MPa). These strengths can be achieved in relatively short curing times compared to those of Portland cement-based concretes. For example, thin concrete products such as roof tiles (thickness ~10 mm) can reach target hardness within 10 hours. Thicker concrete products such as railroad sleepers/ties (thickness ~250 mm) can reach target hardness within 24 hours. Fast curing times offer the concrete manufacturer greater flexibility in equipment utilisation, inventory management and production planning.
The introduction of Solidia cement as a non-hydraulic, cementitious binder used to produce concrete products offers the potential to significantly reduce the CO2 footprint of the cement and concrete industries. By manufacturing this new type of cement, the industry will be able to reduce its CO2 emissions by consuming less limestone and fossil fuels. By setting and hardening their products with this cement, the concrete industry will be able to produce strong and durable products and reduce its manufacturing cycle time while permanently sequestering CO2 in its products. The cement and concrete industries can realise these benefits while preserving both their raw materials supply chains and their capital investment in plants and equipment.
Solidia cement provides the cement and concrete industries with an alternative to traditional Portland cement that offers important sustainability and performance benefits:
- Cement production with 30% reduction in CO2 emissions.
- A concrete curing process that safely and permanently sequesters CO2 in quantities equal to 30% of the mass of cement used.
- Production of concrete products that will effectively reduce the CO2 footprint associated with cement manufacturing and use by up to 70%.
- CO2-cured concrete products that equal or exceed the performance of conventional water-cured Portland cement-based concretes.
- Accomplishing all of the above in a manner that is compatible with the raw materials supply chains, manufacturing equipment and unit processes of the cement and concrete industries.
Written by Nick DeCristofaro and Sada Sahu, Solidia Technologies®. This is an abridged version of the full article, which appeared in the January 2014 issue of World Cement. Subscribers can view the full article by logging in.
Read the article online at: https://www.worldcement.com/the-americas/09012014/co2_reducing_cement_part_two_carbon_dioxide_curing_572/