CO2 AS A PERFORMANCE-ENHANCING ADMIXTURE IN READYMIX CONCRETE PART 2

An online presentation by Sean Monkman on the American Concrete Institute (ACI) platform.

The ACI recently hosted a presentation on the innovative topic of utilising CO₂ as a performance-enhancing admixture in ready-mix concrete. This is the Part 2 of a four-part article.

…continued from Part 1.

Motivation for CO₂ Utilisation

As Monkman continued his presentation, he addressed the urgency of exploring alternative uses for CO₂, recognising that burying it underground is not the only solution. He highlighted the ambitious goal set by the industry, aiming for one cement plant capturing CO₂ every week, starting from 2030 onwards. This goal emphasises the increasing focus on carbon capture to meet sustainability targets. Monkman underscored the need to derive value from CO₂ within the cement and concrete value chain.

Introduction to CO₂ as an admixture

The core of Carbon Cure Technologies’ approach involves utilising CO₂ as an admixture in the concrete production process. Monkman provided an overview of the system, showcasing a tank of cryogenic CO₂ connected to a control box. This control box facilitates the injection of CO₂ in a controlled dose, much like a conventional admixture, whether in a truck-mixed or central batch situation. This integration seamlessly incorporates CO₂ into the batching control system, treating it as another essential ingredient in the concrete mixture.

Comparing CO₂ and admixtures

Drawing parallels between CO₂ and traditional admixtures, Monkman illustrated how the system mirrors existing practices in concrete production. He emphasised the ease with which the CO₂ injection system can be retrofitted into existing concrete plants, similar to the implementation of traditional admixtures. The presentation showcased a supply of cryogenic CO₂ in liquid form, which transforms into a mixture of solid and gas upon injection into the concrete at atmospheric temperature and pressure.

Chemical reactions and impact on concrete

Monkman explained the chemical reactions that occur when CO₂ is used as an admixture. As cryogenic CO₂ is injected and flash converts, it adheres to the wet, mixed concrete, initiating a chemical reaction. In simplified terms, the reaction involves the formation of calcium carbonate, a new mineral within the cement paste. This process catalyses the formation of CSHL (calcium silicate hydrate gel), contributing to the concrete’s strength.

Concrete strength enhancement

To support the efficacy of CO₂ as an admixture, Monkman shared findings from practical applications. In one case, CO₂ was injected into a truck at a pre-industrial stage, showcasing varied doses and their corresponding impact on concrete strength. The results demonstrated an optimum dosage, with lower and higher doses showing diminishing returns. The strength gains were not only observed at early ages but were sustained over an extended period, from 7 days to 91 days, highlighting the long-term benefits of CO₂ utilisation in concrete production.

Industrial application and performance

Monkman concluded his presentation with a case study from 2014, where the CO₂ injection technology was integrated into the batching cycle of a concrete plant. The results showed an 8% increase in strength at seven days and a 7% increase at 28 days, with no adverse effects on air content or slump. The successful integration of CO₂ as an admixture led to an enhanced efficiency of the cement in the mix, showcasing its potential to contribute to the industry’s sustainability goals.

In summary, Sean Monkman’s presentation at the American Concrete Institute provided valuable insights into the innovative use of CO2 as a performance-enhancing admixture in ready-mix concrete. The integration of this technology not only aligns with industry goals for carbon capture but also demonstrates tangible benefits in terms of concrete strength and sustainability. As the concrete industry continues to evolve, such advancements play a crucial role in shaping a more environmentally friendly and efficient future.

Continued in Part 3…