State whether the following is true or false.
Bogue calculations give the exact phase composition of a clinker.
Answer: A
They assume equilibrium and fixed phase compositions, neither of which holds exactly in a real clinker. Quantitative X-ray diffraction gives a more accurate answer where one is needed. Bogue remains useful because it costs nothing beyond an oxide analysis.
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Assertion (A): Quantitative X-ray diffraction is sometimes preferred to the Bogue calculation.
Reason (R): Bogue assumes equilibrium and fixed phase compositions that a real clinker does not achieve.
Answer: B
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. Bogue computes the phases from the oxide analysis on the assumption of equilibrium and pure phase compositions, neither of which a real clinker attains - the phases contain substituted ions and the cooling was not slow enough for equilibrium. Diffraction measures what is actually present, and the two figures can differ by several points.
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Consider the following statements about clinker assessment.
1. Free lime is measured chemically as an operating control.
2. Microscopy reveals crystal size and cooling history.
3. The Bogue calculation measures phases directly.
Answer: C
The first two are correct. The third confuses calculation with measurement, since Bogue estimates the phases from the oxide analysis using assumptions that a real clinker only approximates. Calculation and measurement answer the same question by different routes.
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Alkali sulphates in clinker are found chiefly
Answer: D
Correct answer: in the interstitial material and on crystal surfaces
Alkalis and sulphur volatilise in the burning zone and condense further back, so they end up concentrated in the last material to solidify and on the surfaces of the crystals. Being at the surface makes them readily soluble, which is why they influence early setting and the alkali content of the pore solution out of proportion to their quantity.
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If a clinker contains 1.2 per cent alkali expressed as sodium oxide equivalent, how much is that in a tonne of clinker?
Answer: A
One and a fifth per cent of 1,000 kilograms is 12. Low-alkali cement is usually specified below 0.6 per cent, so this clinker would be well outside that limit. Low-alkali cement is specified where reactive aggregates are to be used.
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The sulphate to alkali ratio in a kiln is monitored because
Answer: B
Correct answer: an imbalance promotes build-ups and affects setting
Sulphur in excess of the alkalis available to combine with it recirculates in the kiln and condenses as sticky build-ups in the preheater, which eventually block it. Alkali in excess of the sulphur leaves free alkali in the cement, which accelerates setting and contributes to alkali-silica reaction. The ratio is held near unity for both reasons.
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State whether the following is true or false.
Clinker chemistry ends at the kiln outlet and cannot change afterwards.
Answer: C
Correct answer: False
Prehydration in storage, the addition of gypsum at the mill, and the reaction of alkalis with atmospheric carbon dioxide all alter the material after it leaves the kiln. Cement chemistry continues right up to the point at which the cement is used, which is why storage conditions and cement age are recorded.
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Assertion (A): The clinker cooler is regarded as part of the clinkering process rather than merely a handling stage.
Reason (R): The rate of cooling determines which phases survive and in what form.
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. Alite is unstable below about 1,250 degrees and decomposes to belite and free lime if cooled slowly, and the aluminate crystallises coarsely and reacts too fast if given time. Rapid cooling freezes the phases in their high-temperature form, so the cooler determines the product as much as the burning zone does.
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Clinker leaving the cooler is typically at a temperature of about
Answer: A
Correct answer: 100 degrees Celsius or a little above
A well-performing grate cooler brings the clinker down from about 1,400 degrees to a little over 100, recovering most of the heat into the secondary and tertiary air. Clinker much hotter than that heats the cement mill and can dehydrate the gypsum, so cooler performance is watched for that reason as well as for the heat recovery.
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State whether the following is true or false.
Hot clinker entering the cement mill is a problem as well as a lost opportunity.
Answer: B
Excessive mill temperature dehydrates the gypsum and can cause false set in the finished cement. Cooler performance therefore affects the grinding plant as well as the heat balance of the kiln. A fault at one end of the plant shows up at the other.
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