Finish grinding refers to the milling of
Answer: A
This is the last processing step before the cement is stored and dispatched, and it fixes the fineness and therefore much of the performance. Raw grinding, coal grinding and crushing all happen earlier and serve different purposes.
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The fineness of finished cement is most commonly reported as
Answer: B
Correct answer: Blaine specific surface in square metres per kilogram
The Blaine air permeability test measures how readily air passes through a compacted bed of the cement and reports the result as surface area per unit mass. It correlates well with strength development and water demand, and it is quick enough to control a mill on - a result in minutes rather than the days a strength test needs.
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An ordinary Portland cement has a Blaine fineness of 320 square metres per kilogram. How much surface area is present in a 50 kg bag?
Answer: C
Correct answer: 16,000 square metres
\[ 320\ \mathrm{m^2/kg} \times 50\ \mathrm{kg} = 16{,}000\ \mathrm{m^2}. \] That is about two hectares of surface in one bag, which is worth pausing over: it is the reason cement hydrates as quickly as it does, and the reason its water demand is what it is.
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Assertion (A): Increasing the fineness of a cement raises both its early strength and its water demand.
Reason (R): A greater surface area reacts faster but also needs more water to wet it.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Hydration begins at the surface, so more surface means faster reaction and higher early strength. But every particle must be wetted before it can react, and more surface needs more water to wet it - which raises the water demand and, at a fixed workability, works against the strength that the fineness was meant to gain.
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State whether the following is true or false.
Grinding cement finer improves every property of the finished product.
Answer: A
Correct answer: False
Fineness is a trade rather than an improvement. Finer cement gives higher early strength but also higher water demand, greater heat of hydration, a shorter shelf life through prehydration, more shrinkage, and a sharply higher grinding cost. The optimum is a balance, and it differs for each cement type.
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Particle size distribution matters as well as overall fineness because it affects
Answer: B
Two cements of identical Blaine figure can behave quite differently if one has a wide spread of sizes and the other a narrow one. A well-graded distribution packs more densely and can reduce the water needed for a given workability.
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State whether the following is true or false.
Two cements with the same Blaine fineness necessarily perform identically.
Answer: C
Correct answer: False
Blaine reports a single number for the whole distribution, and two cements can reach it in quite different ways - one with a narrow spread of particle sizes, another with a mixture of very fine and quite coarse. Their strength development, water demand and workability will differ, which is why the full particle size distribution is examined where performance matters.
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Gypsum content in finished cement is optimised because
Answer: D
The optimum depends on the aluminate content and the fineness, so it is determined by trial rather than assumed. Standards set a maximum on total sulphate, which the optimum must respect. The optimum is found by trial on the actual clinker and mill.
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Assertion (A): Two clinkers of different aluminate content require different quantities of gypsum.
Reason (R): Gypsum retards the aluminate, so more of that phase demands more retarder.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. It is the tricalcium aluminate that reacts violently with water and would flash-set, and gypsum controls it by forming ettringite on the aluminate surfaces. More aluminate therefore needs more gypsum, which is why the optimum sulphate content is determined for each clinker rather than fixed by the standard.
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False set in a cement is caused chiefly by
Answer: B
Correct answer: dehydration of the gypsum in a hot mill
A mill running hot drives water out of the gypsum, converting it to hemihydrate or anhydrite. On mixing, that rehydrates rapidly and forms secondary gypsum crystals which stiffen the paste within minutes - a stiffening from crystal growth rather than from cement hydration, which is why it is a false set. Cooling the mill or injecting water controls it.
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