Converting industrial waste into a value-added cement material through ambient pressure carbonation.

Converting industrial wastes into value-added building products in an environmental management strategy is a challenging yet vital component of the industrial process. Steel slag (SS), an industrial waste by-product from the steel-making process, is typically disposed of in landfill which consumes land resources and pollutes the environment. This paper explores the possibility of a closed-loop system to convert steel slag into a cement material through carbonation activation, thereby significantly reducing the amount of steel slag waste sent to landfills across Canada. The production of this cementing material can occur next to the steel mill, utilizing steel slag and carbon dioxide collected on-site to fabricate carbon-negative products. To save energy and allow production to be feasible on an industrial scale, ambient pressure (AP) carbonation is developed to reduce carbon emissions while improving their performance. High pressure (HP) carbonation curing and normal hydration (NH) references were also implemented at the same time to justify the application of AP carbonation in reducing CO2 emission. The results of this study found AP carbonation-activated SS compacts have comparable CO2 uptake (about 7.5 tons CO2/100 tons slag) and mechanically compressive strength values as those subjected to HP carbonation, suggesting that AP could be used to replace HP in carbonation curing to ensure a lower energy input. Additionally, AP seemed to possess as effective carbonation as HP. The studies investigated by multiple techniques including X-ray diffractometer (XRD), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopic analysis, and scanning electron microscopy (SEM) aim to identify the microstructure development of carbonated SS paste to assess carbonation results. Developed with life cycle assessment (LCA), environmental impact evaluation shows that AP presents a smaller global warming potential (GWP) value than HP. The comparable CO2 sequestration, satisfactory engineering properties, enhanced microstructure and lesser environmental impact in AP carbonation confirm the feasibility of replacing high pressure with extremely low pressure to cure concrete products. The use of AP carbonation for cement material created using steel slag reduces carbon emissions, energy usage, and natural resource consumption.

Dimensional stability of cement paste and concrete subject to early-age carbonation curing.

Early-age carbonation curing of concrete is receiving more interest in terms of performance improvement and emission reduction. However, the volume change of cement-based products subject to carbonation curing may become a concern because of the potential carbonation shrinkage and its related shrinkage cracking. The purpose of this study was to investigate the dimensional stability of cement paste and concrete subject to the early-age carbonation curing. It was found that the carbonation curing introduced first an initial shrinkage due to water evaporation upon gas injection and then generated an expansion due to CO2 uptake and carbonate precipitation. As carbonation proceeded, the deformation was switched to a secondary shrinkage after expansion. The residual deformation due to carbonation curing was shrinkage in cement paste samples and expansion in concrete samples. This was because the relative expansion due to carbonate precipitation in paste was not large enough to compensate for the shrinkage caused by water loss. However, for concrete samples, the introduction of aggregates reduced the pore spaces in concrete, leading to an expansion owing to the limited precipitation. The results of carbon dioxide uptake, XRD, and SEM analysis confirmed that calcium carbonate formation played a critical role in the relative expansion. The study also showed that cement-based products were more resistant to weathering carbonation after the early-age carbonation curing. After 61-day weathering carbonation exposure, both paste and concrete samples exhibited carbonation shrinkage as a result of carbonation of hydration products. However, the magnitude of shrinkage was much smaller in carbonation curing than in weathering carbonation because of the short period of exposure. Both carbonations did not significantly affect the compressive strength of carbonated products. Carbonation curing likely makes concrete products more dimensionally stable in the long-term service.