Utilization of Recycled Materials in Illinois Highway Construction
- Microsilica is a by-product of the production of silicon metal or ferro-silicon alloys. It is supplied in densified bulk trucks, large bags, or provided as a water-based slurry. It is most commonly furnished bagged (6).
- Physical Properties:
- Microsilica (SiO2), also known as silica fume, is a gray powdery material largely consisting of amorphous silicon dioxide, and has a mean particle size between 0.1 and 0.2 mm-100 times finer than portland cement (6).
- Engineering Value:
- Microsilica's high silica content is also high in purity and pozzolanic properties. Reacting with calcium hydroxide (products of cement's pozzolanic reaction), microsilica will produce calcium silicates that will result in denser concrete with higher strengths-increasing compressive strengths up to 100 MPa (14,500 psi) or more-lower permeability, and improved durability. In the specific application of bridge deck overlays, the decrease in permeability slows the rate of corrosion on reinforcing members by impeding chloride or sulfate intrusion (3, 6).
- Present Applications:
- The Department's primary use for microsilica is in bridge deck overlays. Since the late 1980s, over 150 concrete deck overlays have incorporated microsilica. Small amounts of microsilica are also used in high performance shotcrete for structural repairs. Outside of the Department, microsilica is utilized in multi-story building construction (3).
- Quantity Used:
- 115 tons (2001 MISTIC estimate).
- Economic Impact:
- Even though the price of microsilica is substantially higher than that of portland cement, the Department has contributed approximately $50,600 toward its recycling-eliminating disposal costs. Overall, this material has the potential to extend the life of a structure 25 to 30 years, thus lowering its life cycle cost.