Ambient pressure carbonation curing of reinforced concrete for CO2 utilization and corrosion resistance

CO2 utilization for concrete production: Commercial deployment and pathways to net-zero emissions

Approximately 10 % of global anthropogenic CO2 emissions arise from the cement and concrete industry driven by urban expansion and a constant need for infrastructure renewal. Reusing waste CO2 to make new construction materials produces circular carbon flows and constitutes a key step toward a carbon-negative economy. To establish a holistic view of the field, this paper examines upscaled technologies with industrial deployments for utilizing CO2 in manufacturing cement-based materials and analyzes their interplay for attaining net-zero emissions (NZE) in the concrete sector. By scrutinizing the status quo, it suggests that NZE agendas should be diversified catering to the wide-ranging built products. Small-sized precast elements and lightweight components lead the way in carbon-neutral manufacturing, while the market-dominating ready-mix concrete is by far difficult to decarbonize and relies on the incorporation of pre‑carbonated ingredients, preferably sourced from alkaline wastes, to leverage large-scale CO2 utilization. To expedite the race to NZE, it is necessary to combine the development of CO2 utilization and low-CO2 cement to create decarbonization strategies tailoring for individual products. In this regard, the paper reveals credible pathways and research needs to facilitate their implementation in sustainable construction.

Experimental Study on Effects of CO2 Curing Conditions on Mechanical Properties of Cement Paste Containing CO2 Reactive Hardening Calcium Silicate Cement

Human survival is threatened by the rapid climate change due to global warming caused by the increase in CO2 emissions since the Second Industrial Revolution. This study developed a secondary cement product production technology by replacing cement, a conventional binder, with calcium silicate cement (CSC), i.e., CO2 reactive hardening cement, to reduce CO2 emissions and utilize CO2 from the cement industry, which emits CO2 in large quantities. Results showed that the carbonation depth, compressive strength increase rate, and CO2 sequestration rate increased as the CSC content increased, suggesting that CSC can be applied as a secondary cement product.

Influence of Citric Acid on the Fundamental Properties of CO2 Cured Magnesium Oxysulfate Paste

Magnesium oxysulfate (MOS), mainly composed of magnesium oxide and magnesium sulfate, is a kind of gas-hardening cementing material with low energy consumption and CO₂ emissions. In order to develop environment-friendly cement-based materials, MOS needs to be studied systematically. The paper mainly investigates the influence of citric acid (a retarder) on the working and mechanical properties of MOS paste. In this study, the setting time of fresh MOS paste is determined. The flexural and compressive strengths of hardened specimens exposed to the environment of water dry-wet (D-W) alternations, freeze-thaw (F-T) cycles, and sulfate D-W alternations are investigated. Furthermore, the drying shrinkage (D-S) rate of MOS paste is tested for 3 days and 28 days. The specimens are cured in standard or CO₂ curing environments. A scanning electron microscope energy spectrum (SEM-EDS) is obtained to analyze the morphology of hydration products. Results show that citric acid can increase the setting time of MOS paste. The citric acid and CO₂ curing have a positive effect on the mechanical strengths and the resistance to erosion by water, F-T cycles, and sulfate D-W alternations. The D-S rate decreased in relation to the increasing dosages of citric acid and increased with CO₂ curing. MOS with 0.3% of the total binder material mass shows the best erosion resistance. As observed in the results of SEM-EDS, the CO₂ curing and the citric acid can make the hydration products denser.

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.

Improvement of CO2-Cured Sludge Ceramsite on the Mechanical Performances and Corrosion Resistance of Cement Concrete

The application of CO₂ curing on sludge ceramsite may improve its mechanical properties, and then increase the corresponding corrosion resistance. In this study, the influence of CO₂-cured sludge ceramsite on the strength and long-term properties of cement concrete is investigated. CO₂ curing time ranges from 0 h to 2 d. The cylinder compressive strength and water absorption rate of CO₂-cured sludge ceramsite are first determined. Additionally, the flexural and compressive strengths, the chloride permeability and the freeze-thaw damage, as well as the corresponding thermal conductivity of cement concrete, are tested. Furthermore, the corrosion resistance of reinforcement inner-sludge-ceramsite cement concrete is measured. Finally, the scanning electron microscope photos of sludge ceramsite are obtained. Results show that the cylinder compressive strength of CO₂-cured sludge ceramsite is 15.1, ~34.2% higher than that of sludge ceramsite. Meanwhile, the water absorption rate of CO₂-cured sludge ceramsite is 39.6, ~82.4% higher than that of sludge ceramsite. The compressive strength and the flexural strength of cement concrete with CO₂-cured sludge ceramsite are 11.4 and 18.7, ~21.6% and ~31.5% higher than the cement concrete with sludge ceramsite, respectively. The resistance of NaCl freeze-thaw cycles, determined by comparing the mass loss rate and the loss rates of mechanical strengths, is effectively improved by CO₂ curing, while the thermal conductivity of cement concrete is decreased by CO₂ curing. The corrosion resistance of inner reinforcement is improved by the application of CO₂ curing on sludge ceramsite.

Influence of Carbon Dioxide Curing on the Corrosion Resistance of Reinforced Cement Mortar under the External Erosion of NaCl Freeze–Thaw Cycle

Carbon dioxide (CO2)-cured concrete is a novel material that can effectively reduce CO2 emissions in the atmosphere. However, limited research has been found to investigate the corrosion behavior of CO2-cured reinforced concrete. In this paper, the corrosion resistance of reinforced cement mortar is investigated. The mortars were cured in CO2 for 1 day~28 days. Water–cement ratios (w/c) of 0.3, 0.4 and 0.5 were designed. The corrosion resistance of inner steel bars was researched by the methods of ultrasonic velocity, electrical parameters (AC electrical resistance, Tafel curve method and AC impedance spectroscopy). Moreover, scanning electron microscope was selected for observing the micro-morphology of CO2-curing mortar. X-ray diffraction spectrum was used to characterize components of steel bars’ passive films. The results show that CO2 can effectively increase electrical resistivity and ultrasonic velocity, thus improving the corrosion resistance of reinforced cement mortar. The enhancement of carbon dioxide curing increases with the increasing w/c. The mass-loss rate, the electrical resistivity and the decreasing rate of ultrasonic velocity increase with the increasing sodium chloride freeze–thaw cycles, indicating the continuous increase in the corrosion degree of reinforcement. The corrosion deterioration degree of steel bars decreases with the increasing CO2-curing time. Specimens with w/c of 0.3 and 0.4 show the highest and lowest corrosion deterioration resistances after sodium chloride freeze–thaw cycles. Microscopic characterization found that CO2 curing could increase the corrosion resistance of the inner steel bars by improving the compactness of the cement matrix. Moreover, the iron oxides on the surface of the passivation film decreased after CO2 curing.

Carbonation and Corrosion Problems in Reinforced Concrete Structures

Reinforced concrete (RC) has been commonly used as a construction material for decades due to its high compressive strength and moderate tensile strength. However, these two properties of RC are frequently hampered by degradation. The main degradation processes in RC structures are carbonation and the corrosion of rebars. The scientific community is divided regarding the process by which carbonation causes structural damage. Some researchers suggest that carbonation weakens a structure and makes it prone to rebar corrosion, while others suggest that carbonation does not damage structures enough to cause rebar corrosion. This paper is a review of the research work carried out by different researchers on the carbonation and corrosion of RC structures. The process of carbonation and the factors that contribute to this process will be discussed, alongside recommendations for improving structures to decrease the carbonation process. The corrosion of rebars, damage to passive layers, volume expansion due to steel oxidation, and crack growth will also be discussed. Available protection methods for reducing carbonation, such as rebar structure coating, cathodic protection, and modifier implementation, will also be reviewed. The paper concludes by describing the most significant types of damage caused by carbonation, testing protocols, and mitigation against corrosion damage.

Progress in Sensors for Monitoring Reinforcement Corrosion in Reinforced Concrete Structures—A Review

Non-destructive monitoring methods and continuous monitoring systems based on them are crucial elements of modern systems for the management and maintenance of assets which include reinforced concrete structures. The purpose of our study was to summarise the data on the most common sensors and systems for the non-destructive monitoring of reinforced concrete structures developed over the past 20 years. We considered systems based on electrochemical (potentiometry, methods related to polarisation) and physical (electromagnetic and ultrasonic waves, piezoelectric effect, thermography) examination methods. Special focus is devoted to the existing sensors and the results obtained using these sensors, as well as the advantages and disadvantages of their setups or other equipment used. The review considers earlier approaches and available commercial products, as well as relatively new sensors which are currently being tested.