Blast furnace slag — composition and common uses Which statement best describes typical blast furnace slag and its practical applications after granulation or air cooling?

Introduction / Context:
Blast furnace slag is the non-metallic by-product formed when fluxes combine with gangue during ironmaking. Far from being waste, properly processed slag is a valuable construction material with diverse end uses.

Given Data / Assumptions:

• Slag considered is from conventional iron blast furnaces.
• Post-processing may include granulation (for cement) or crushing/screening (for aggregates).
• Composition is dominated by silicates and aluminosilicates of calcium with other oxides.

Concept / Approach:
Chemically, blast furnace slag comprises calcium silicates with alumina and magnesium, often described generally as calcium, aluminium, and ferrous/magnesium silicates. Air-cooled slag makes good railroad ballast and aggregate due to its angularity and durability. Slag can be mixed with tar/asphalt for road bases and wearing courses. Granulated slag, when ground, serves as a supplementary cementitious material, improving durability and reducing clinker demand.

Step-by-Step Solution:

Verification / Alternative check:
Civil engineering specifications list air-cooled slag for aggregates and asphalt mixes; ground granulated blast furnace slag (GGBFS) is standardized in cement.

Why Other Options Are Wrong:

• Each single-use statement is correct but incomplete; the comprehensive choice is “all of the above.”
• “Mainly pure lime” is incorrect; slag is not pure CaO and is quite suitable for various construction uses.

Common Pitfalls:
Assuming slag is waste; ignoring performance benefits of GGBFS in concrete (sulfate resistance, lower heat of hydration).

Final Answer: