ONE-EIGHTH STAGE 2018
Prempeh College: 62 points
St. Francis Xavier: 29 points
Saviour SHS: 18 points
ROUND 1
QUESTION
Name the main source of the calcium in clinker, and describe the process(es) that raw material goes through during the formation of clinker.
ANSWER: Limestone (calcium carbonate, CaCO$_3$)
NOTE: The process part of the question: limestone is crushed, mixed with clay and heated in a rotary kiln; it decomposes to calcium oxide and carbon dioxide, and the calcium oxide combines with the silica and alumina of the clay to form calcium silicates and aluminates, which fuse into clinker.
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PRACTICE QUESTIONS
1. QUESTION: Name the main raw material that supplies the silica and alumina in cement clinker.
ANSWER: Clay (or shale)
SOLUTION:
Clay is an aluminosilicate; in the kiln it combines with calcium oxide to form calcium silicates and aluminates.
2. QUESTION: Name the gas released when limestone decomposes in a cement kiln.
ANSWER: Carbon dioxide
SOLUTION:
$CaCO_3\rightarrow CaO+CO_2$
3. QUESTION: Name the substance added to clinker when it is ground to make cement, and state its purpose.
ANSWER: Gypsum (calcium sulfate); it slows down the setting of the cement.
SOLUTION:
Without gypsum, the cement would set too quickly to be worked.
4. QUESTION: Name the type of kiln in which cement clinker is made.
ANSWER: A rotary kiln
SOLUTION:
The slowly turning, sloping kiln moves the raw mix towards the hottest zone.
5. QUESTION: What happens when cement is mixed with water and left to stand?
ANSWER: It reacts with the water (hydrates) and sets into a hard mass.
SOLUTION:
The calcium silicates form hydrated crystals that bind together.
6. QUESTION: What name is given to a mixture of cement, sand, stones and water that sets hard?
ANSWER: Concrete
SOLUTION:
Steel rods set inside it give reinforced concrete.