How Aluminum Hydroxide Works in The Subway
Subway tunnels are critical infrastructures requiring durable and robust construction materials. The application of spraying cement in these tunnels is common to enhance their structural integrity and longevity. Adding aluminum hydroxide (Al(OH)?) to spraying cement offers significant benefits, making the tunnels safer and more durable.
Spraying Cement
Spraying cement, also known as shotcrete, is a mixture of cement, aggregates, and water projected at high velocity onto surfaces. This method is widely used in subway construction for lining tunnels due to its rapid application and strong adhesion. The key features of spraying cement include:
Quick setting and hardening: Allows for fast construction progress.
High strength: Provides excellent support and stability to the tunnel walls.
Good adhesion: Ensures a strong bond with the substrate, reducing the risk of detachment.
Aluminum Hydroxide
Aluminum hydroxide is a fine, white powder used in various industries. It is valued for its chemical properties, including:
Flame retardancy: Decomposes at high temperatures, releasing water vapor and reducing fire risk.
Chemical reactivity: Reacts with other compounds to form stable products.
Non-toxic and environmentally friendly: Safe for use in construction and other applications.
Role of Aluminum Hydroxide in Spraying Cement
Adding aluminum hydroxide to spraying cement improves several properties essential for subway tunnel construction:
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Aluminum hydroxide decomposes at high temperatures, absorbing heat and releasing water vapor. This process delays the spread of fire, providing critical time for evacuation and firefighting efforts.
Subway tunnels are often exposed to corrosive chemicals from groundwater and other sources. Aluminum hydroxide reacts with these chemicals, forming stable compounds that protect the cement matrix from degradation.
The fine particles of aluminum hydroxide fill the microvoids in the cement, increasing its density and overall strength. This results in a more durable tunnel lining capable of withstanding heavy loads and external pressures.
Aluminum hydroxide enhances the formation of a denser cement matrix, reducing porosity and improving the material's resistance to water infiltration. This is crucial for preventing water damage in subway tunnels.
Case Study: Beijing Subway Line Construction
During the construction of a new Beijing subway line, engineers faced challenges related to fire safety and chemical corrosion. To address these issues, they incorporated aluminum hydroxide into the spraying cement used for tunnel lining.
Implementation
Material Preparation: The spraying cement mixture was prepared by adding 8% aluminum hydroxide by weight of the cement. The components were thoroughly mixed to ensure uniform distribution.
Spraying Process: The modified spraying cement was applied to the tunnel walls using standard shotcrete equipment. The application was carried out in layers, allowing each layer to set before applying the next.
Curing and Inspection: After application, the sprayed layers were cured under controlled conditions to achieve optimal strength and durability. Regular inspections ensured that the material performed as expected.