CMMI-EPSRC RENACEM: Response to CO2 exposure of concrete with natural supplementary cementitious materials

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Concrete can traditionally be thought of a mixture of mineral aggregates, water, and Portland cement. However, modern concrete mixtures are much more complex, also containing chemical admixtures and supplementary cementitious materials (SCMs) to enhance properties in the fluid and hardened states. SCMs are widely used in the US, UK and internationally to replace a portion of the Portland cement, improving concrete long term durability, reducing cost, and reducing CO2 emissions associated with concrete production. The most commonly used SCMs are waste-derived, including coal combustion ashes and blast furnace slags from iron-making. However, changes in industrial processes (e.g. co-firing of coal with biomass, use of renewable energy, structural changes in the global iron industry) and increasing demand for SCMs are leading to a shortage of high quality, conventional SCMs. The use of widely-available natural SCMs, such as calcined clays and volcanic minerals, is rising dramatically, as their chemistry and mineralogy are more homogeneous, aiding in quality control compared to waste-derived SCMs. Standardization is advancing in the US and the UK to enable broader use of these materials, but the fundamental science of their use needs further investigation. Natural SCMs, like waste-derived ones, generally have positive impacts on concrete durability, cost, and environmental footprint. However, one concern with all SCMs is that they often increase the vulnerability of concrete to carbonation. Carbonation occurs when CO2 enters the material, chemically reacting and reducing pH, leading to corrosion of steel reinforcement as well as changes in the integrity of the cementitious matrix. In concretes with natural SCMs, the mechanisms governing carbonation and related degradation are largely unknown. Considering that the field is pushing toward use of increasing volumes of natural SCM use in concrete (so-called LC3 systems), understanding the contribution of natural SCMs toward carbonation degradation is critical for future proof this technology. Furthermore, it is possible that manipulating the composition of the systems to reduce or prevent carbonation is possible, but has not previously been explored. RENACEM is a joint US-UK collaboration between leading infrastructure materials researchers to elucidate the fundamental science explaining the long-term performance of concretes produced with natural SCMs. We will understand the chemical interactions between concretes and atmospheric CO2, and its transport, to identify meaningful methodologies to be used for their assessment. This will underpin the adoption of new methods for testing carbonation of concretes with natural SCMs and prediction models.