Semi-wet carbonation: Transforming construction waste into sustainable resources

Hong Kong’s skyline generates vast amounts of construction waste through daily demolition. While some of this waste is used for land reclamation, much still ends up in landfills. Prof. C.S. Poon, Michael Anson Professor in Civil Engineering and Distinguished Research Professor of the Department of Civil and Environmental Engineering at The Hong Kong Polytechnic University, is dedicated to advancing construction waste recycling and had developed a new semi-wet carbonation technique, transforming waste into valuable building aggregates and promoting more sustainable construction practices in the city. 

Recycling concrete not only diverts substantial material from landfills but also presents a significant opportunity for reducing carbon emission. Central to this recycling effort is the use of recycled concrete aggregate (RCA), which can serve as a substitute for natural aggregates that typically comprises 60%–80% of concrete’s volume in new construction. However, RCA’s higher porosity and water absorption compared to natural aggregates pose challenges for its widespread adoption, as these properties can compromise the mechanical strength and durability of new concrete.

To address these limitations, PolyU researchers have turned to carbonation—a process that reacts CO₂ with calcium-bearing phases in RCA, forming calcium carbonate and thereby improving the properties of an aggregate. Beyond enhancing RCA’s performance, carbonation also offers a route for permanent CO₂ sequestration, aligning with global efforts to mitigate climate change. 

Traditionally, two main carbonation techniques have been explored: semi-dry carbonation and wet carbonation. Semi-dry carbonation employs water vapour as the reaction medium, typically under high humidity conditions (50–100% relative humidity). This method is relatively simple but suffers from slow reaction rates, with degrees of carbonation ranging from 10% to 20% after several days. The limited water availability in semi-dry carbonation restricts the diffusion of CO₂ and calcium ions, thereby impeding the formation of calcium carbonate.

In contrast, wet carbonation immerses RCA in liquid water, facilitating faster and more complete reactions. Here, the water-to-solid ratio is a critical parameter, often ranging from 5 to 100, to ensure sufficient moisture for efficient carbonation. Wet carbonation can achieve degrees of carbonation between 10% and 20% within a few hours. However, the process is not without drawbacks: it demands significant water input and involves energy-intensive pre- and post-treatment steps such as drying, filtration and washing. These requirements complicate large-scale implementation and raise concerns about water consumption and wastewater management.

Recognising the need for a more practical and sustainable approach, Prof. Poon and his research team have introduced a novel semi-wet carbonation method in Cement and Concrete Research. This technique bridges the gap between semi-dry and wet carbonation by employing a fine water mist at the solid-liquid interface of RCA. 

The semi-wet environment creates a thin, spatially confined water film on the RCA surface, which proves highly effective for carbonation reactions. Remarkably, the process achieves a carbonation degree of 10.6% within just 30 minutes—comparable to, or even surpassing, the rates observed in wet carbonation under similar conditions. 
 
The semi-wet method also brings about a 3.6% reduction in water absorption and a 20% decrease in porosity, both of which are critical for improving the quality and durability of RCA in construction applications.
 
Another key innovation in this approach is the use of sodium bicarbonate as an accelerator. The addition of sodium bicarbonate creates a weakly alkaline environment that lowers the free energy barrier for CO₂ speciation, as confirmed by molecular dynamics simulations. This environment favours the rapid conversion of CO₂ into carbonate ions, thereby accelerating the overall carbonation process. 

Comparative analysis between semi-wet and wet carbonation reveals several important distinctions. While both methods achieve similar degrees of carbonation in the initial stages, the semi-wet process is markedly more efficient in terms of water usage and energy consumption. Furthermore, the semi-wet process influences the evolution of the silicate phase in RCA. It suggests enhanced reactivity of the treated RCA and potentially better bonding of RCA within new concrete. 

In summary, the semi-wet carbonation technique represents a significant advancement in the sustainable utilisation of recycled concrete aggregate. By combining high carbonation efficiency with minimal water consumption and simplified processing, this method addresses the key limitations of existing carbonation strategies. The use of sodium bicarbonate as an accelerator further enhances the process, offering a practical route for industrial CO₂ capture and utilisation. As the construction industry seeks to reduce its environmental footprint, the adoption of semi-wet carbonation could play a pivotal role in converting concrete waste from a liability into a valuable resource for both material recovery and climate mitigation.
 
Source: Innovation Digest