CO2 AS A PERFORMANCE-ENHANCING ADMIXTURE IN READYMIX CONCRETE PART 3

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 3 of a four-part article.

…continued from Part 2.

Customising technology for varied conditions

As Carbon Cure Technologies extended the application of their technology to different concrete producers, Monkman emphasised the adaptability of the system to diverse conditions. Local producers, he noted, often have unique preferences regarding cement type, mix designs, and strength classes. Acknowledging these variations, the technology has demonstrated its performance across a range of conditions, showcasing its versatility within the concrete industry. Monkman shared an example of seven mixes sorted by strength class, each with different water-cement ratios, cement contents, and SCM (supplementary cementitious materials) contents.

Addressing challenges with limestone cements

Monkman acknowledged the challenges associated with the introduction of limestone cements in the context of CO₂ utilisation technology. Traditionally, the recommended dosage for the technology ranged between 0.05% and 0.3%. However, when applied to limestone cements, initial attempts did not yield the expected results. In response, the dosage was doubled to 0.4%, leading to a significant strength benefit of 7%. This adjustment underscored the need to tailor the technology to accommodate variations in cement types, reinforcing its adaptability.

Unveiling strength gain mechanisms

Delving into the fundamental aspects of the technology, Monkman presented findings from fundamental work on paste activated with CO₂. Using backscattered electron microscopy and elemental mapping, the researchers identified the presence of calcium carbonate. Additionally, analysis of the low-density calcium silicate hydrate (CSH) and high-density CSH revealed a lower calcium-to-silica ratio in the CO₂-activated system.

Understanding the significance of calcium silicate hydrate

Monkman drew connections between the observed lower calcium-to-silica ratio in the CO₂-activated system and existing literature on calcium silicate hydrate. The research indicated that a lower calcium-to-silica ratio in the gel corresponds to a stronger gel. The findings were further substantiated by nanoindentation work, demonstrating that the gel in the CO₂-activated system exhibited greater strength. This evidence contributes to a deeper understanding of the mechanisms behind the strength gains observed in concrete treated with CO₂ as an admixture.

Looking ahead

As Monkman delivered this segment of the presentation, he left the audience with a glimpse into the ongoing research and the promising potential of CO₂ utilisation technology. The ability to customise the technology for different concrete producers and adapt it to emerging challenges, such as those posed by limestone cements, positions CO₂ as a versatile and impactful admixture in the quest for sustainable and high-performance concrete. The fundamental understanding of the gel’s strength and the ongoing research efforts pave the way for continued innovation in the realm of concrete production and sustainability.

Efficiency gains and material optimisation

Monkman highlighted the practical implications of the strength benefits observed with CO₂ utilisation technology. Beyond providing concrete producers with the opportunity to offer higher-strength products to their customers, it also enables them to use materials more efficiently. Using a three-way comparison, Monkman demonstrated how, with CO₂ addition, a concrete mix with reduced cement content (5.5% reduction) could achieve comparable or improved strength efficiency compared to the reference mix. This efficiency gain translates to enhanced sustainability by reducing the environmental impact associated with cement usage.

Cement efficiency and carbon savings

Examining the metric of cement intensity, Monkman showed how the technology contributed to improved cement efficiency over extended periods. By reducing the cement content in concrete mixes and maintaining or enhancing performance, concrete producers can achieve a significant reduction in carbon-intensive cement usage. Monkman illustrated this point with a case study where a 5.5% reduction in cement resulted in approximately 130kg of CO₂ saved per load of concrete, emphasising the environmental benefits of adopting CO₂ utilisation technology.

Durability considerations

Addressing concerns about the durability of concrete treated with CO₂, Monkman emphasized that the technology does not lead to carbonation-related durability issues. He clarified that CO₂ utilisation involves a chemical activation of cement hydration, not the atmospheric carbonation that occurs over a long period. The small amount of CO₂ used during the hydration process has negligible effects on pH and does not compromise the durability of the concrete. Monkman referred to recently published work in the same materials journal, providing further evidence of the technology’s durability.

Understanding pH effects

Monkman further clarified the misconception regarding pH changes in concrete due to CO₂ utilisation. While acknowledging a temporary pH drop in the first 5-10 minutes of hydration, he emphasised that this does not have a significant impact on the overall chemistry of the hydration process. In service, the pH of the concrete remains unaffected, and any short-term pH reduction during initial hydration is not detrimental to the long-term performance of the concrete.

Continued in Part 4…