State whether the following is true or false.
Bagged cement has a shorter effective shelf life than cement kept in a sealed silo.
Answer: C
Paper bags admit moisture over time, whereas a silo is comparatively airtight. Warehouses therefore rotate bagged stock and standards recommend using it within a few months of manufacture. Lumps that cannot be crushed by hand indicate real deterioration.
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The most useful single laboratory check on a cement mill's output is
Answer: D
Fineness can be measured within minutes and correlates well with early strength, so it serves as the control variable. Strength itself takes days or weeks and is confirmation rather than control. The two roles are quite distinct in a quality system.
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Assertion (A): Cement mills are controlled on fineness rather than on strength.
Reason (R): Strength results take days to obtain whereas fineness is available within minutes.
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. Strength is what the customer buys, but a 28-day result arrives four weeks after the cement was made and is useless for controlling the mill. Blaine is available in minutes and correlates well enough with strength to serve as the control variable, with strength testing confirming the relationship.
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Consider the following statements about finish grinding.
1. Gypsum is interground with the clinker.
2. Mill temperature must be kept below the dehydration point of gypsum.
3. Fineness has no effect on the water demand of the cement.
Answer: B
The first two are correct. The third is wrong, since a greater surface area requires more water to wet it, and water demand rises with fineness alongside early strength. Both effects follow from the same increase in surface area.
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A cement described as blended contains
Answer: C
Correct answer: clinker together with one or more other main constituents
A blended cement contains a main constituent besides clinker - slag, fly ash, limestone, pozzolana or a combination - in a proportion the standard defines. Gypsum does not count, being a set regulator present in every Portland cement rather than a main constituent.
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State whether the following is true or false.
Ordinary Portland cement is a blended cement because gypsum is added to it.
Answer: D
Correct answer: False
Gypsum is a set regulator added in a small quantity to every Portland cement, not a main constituent, so its presence does not make a cement blended. Ordinary Portland cement is clinker and gypsum by definition, and a blended cement is what results when a further main constituent is added.
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The residue and Blaine figures for a cement give different information because
Answer: A
A sieve tells you only what is larger than its aperture, whereas the permeability test responds to the total surface including the fines. Reporting both gives a fuller picture than either alone does. Specifications increasingly call for both figures to be reported.
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State whether the following is true or false.
Sieve residue alone is a sufficient description of cement fineness.
Answer: B
It says nothing at all about the material finer than the aperture, which is most of the cement and most of the reactive surface. A Blaine figure or a full distribution is needed to describe the powder properly. Most of the reactive surface lies below any practical sieve aperture.
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A cement mill produces 120 tonnes an hour with a separator return of 240 tonnes an hour. What is the circulating load as a percentage?
Answer: C
Correct answer: 200 per cent
The circulating load is the returned material as a percentage of the finished product: \[ \frac{240}{120} \times 100 = 200\ \mathrm{per\ cent}. \] The mill itself is therefore handling 360 tonnes an hour to deliver 120, which is normal for a closed circuit and is what allows the separator to keep the product uniform.
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Water spray inside a cement mill is used to
Answer: D
A carefully controlled quantity evaporates and removes heat without wetting the cement. Too much would cause pre-hydration, so the spray is metered against the measured mill outlet temperature. Control of the dose is what makes the technique safe.
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