What would you do if you couldn’t confirm the compressive strength of your mix? You’d be worried, and that’s just the beginning. Compressive strength is the first method used to determine the strength of your mix - and its performance, ability to withstand use, and its safety. For a producer, knowing compressive strength is essential.
Compressive strength is the ability of concrete to carry compression loads on its surface without showing any crack or deflection. It is commonly assessed using cubes or cylindrical specimens at a defined loading rate, and it is probably the most important parameter from the structural design perspective.
Concrete compressive strength is affected by several factors, including quality of raw materials, mix design, conditioning, concrete age, and porosity [1]. The latter also directly influences concrete durability.
You might have heard that CarbonCure Technologies (CCT) increases compressive strength in mix designs - and you’re probably wondering how we know that. Read on to learn more.
How does CO2 injection improve concrete strength?
When the cement portion of the concrete reacts with water, a chemical reaction takes place where two main calcium-based hydration products are formed: calcium-silicate hydrate (CSH) and calcium hydroxide (CH). CSH is the preferred hydration product as it is dense in nature and fills the voids that water leaves behind as it is consumed during the cement reaction. This leads to a porosity reduction in concrete; thereby, increasing strength.
When CO2 is injected into the fresh concrete, an additional chemical reaction occurs between the CO2 and the calcium phases present in the cement. This reaction and the regular cement hydration reaction occur simultaneously.
The CO2 mineralizes forming nano-sized calcium carbonate (calcite) particles that further reduce concrete porosity and promote the formation of more CSH by providing nucleation sites, both contributing to an increase in the concrete strength. These effects are synthesized into the term increased cement hydration efficiency of concrete (strength-to-cement content ratio).
CO2 injected ready-mix concrete typically exhibits strength improvements just by increasing the cement efficiency of concrete (Figure 1). This allows for a potential cement reduction of about 3-6% and further increase in the cement efficiency.
Okay, you might say. I get the nanoparticles. But they’re pretty small.
How do we know CO2 is mineralized? And how do we know that nanoparticles increase strength?
CCT researchers have detected the nanoparticles in simplified cement model systems using scanning electron microscope techniques (Figure 2). In those studies, it was demonstrated that in normal cement hydration during the first 10 minutes, there would be some very early CSH and CH formation. When CO2 is added, there is the formation of some nanoparticles of calcite (about 55 nm in average size in the image example below) that increase the cement hydration efficiency, as discussed in the previous section.
How is the nano-calcite created using CarbonCure different from the nanoparticles added ex-situ to concrete?
The formation of calcite nanoparticles remove some of the calcium in the pore solution, causing the formation of CSH with a lower calcium-to-silica ratio, which has been proven to be mechanically stronger [2, 3]. Additionally, these particles have been observed to form in a better dispersed state than in systems where nanoparticles of calcite are added ex-situ to concrete (i.e., as a filler additive) [2]. Hence, the CCT nanoparticles are utilized to their maximum efficiency, contributing positively to the mechanical properties of concrete.
Figure 2. Rhombohedral-shaped calcite nanoparticles observed in a pure alite system specimen formed through the mineralization of CO2 (right) compared to a reference specimen without CO2 (left)
As concrete experts, we’re the first to be able to understand how essential it is for producers and engineers to feel confident with our data. If you’ve got questions, or you want to learn more, reach out to us anytime at sales@carboncure.com.
References
1. Lian, C., Zhuge, Y., & Beecham, S. (2011). The relationship between porosity and strength for porous concrete. Construction and Building Materials, 25(11), 4294-4298.
2. Monkman, S., Sargam, Y., & Raki, L. (2022). Comparing the effects of in-situ nano-calcite development and ex-situ nano-calcite addition on cement hydration. Construction and Building Materials, 321, 126369.
3. Monkman, S., Lee, B. E., Grandfield, K., MacDonald, M., & Raki, L. (2020). The impacts of in-situ carbonate seeding on the early hydration of tricalcium silicate. Cement and Concrete Research, 136, 106179.
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