Which byproduct of coal combustion is used as a supplementary cementitious material in concrete?

• A. Fly ash.
• B. Slag.
• C. Silica fume.
• D. Gypsum.

Answer: (A)

The key idea is that fly ash is a coal combustion byproduct used as a supplementary cementitious material in concrete. When coal is burned, fine particles are captured and collected as fly ash. Added to concrete, fly ash partly replaces cement and participates in pozzolanic reactions with calcium hydroxide released during hydration. This refines pore structure, reduces heat of hydration, can increase long-term strength, and improves durability and workability.

Other options don’t fit as coal combustion byproducts used as SCMs: slag comes mainly from iron/steel production and, while it can be cementitious in some forms, is not a coal combustion byproduct; silica fume is produced from silicon metal production; gypsum is used as a set regulator in cementitious systems, not as a coal-fired byproduct.