Testing of Cement Questions and Answers

Answer: C

The mortar test uses a standard sand and a fixed water content, which no real concrete matches. It classifies the cement for comparison rather than predicting what a particular mix will give. Mix design uses the class as an input rather than as an answer.

Answer: D

The mean load is 200 kilonewtons, or 200,000 newtons, and dividing by 5,000 square millimetres gives 40 newtons per square millimetre. A newton per square millimetre is the same thing as a megapascal. The equivalence is worth remembering, since standards use both units.

Answer: A

Correct answer: remove aggregate variability from the result

The test is measuring the cement, so everything else in the mortar must be held constant. Standard sand is specified by source, grading and shape precisely so that a difference between two results can be attributed to the cement and not to the sand. It is not chosen to give the strongest mortar - only the most reproducible one.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the logic of the whole test. Mortar strength depends on the aggregate as well as the binder, so an unspecified sand would introduce a variable indistinguishable from the one being measured. Specifying the sand as tightly as the cement is what makes the result comparable between laboratories and between years.

Answer: C

Correct answer: the time for air to pass through a compacted bed

Air is drawn through a bed of cement compacted to a known porosity, and the time taken is recorded. Finer cement offers more surface for the air to drag against, so it takes longer, and the Kozeny–Carman relation converts that time into a specific surface figure in square metres per kilogram.

Answer: D

Correct answer: False

The Blaine apparatus measures a flow time and nothing else. The specific surface is calculated from it through a permeability relation that assumes a particular pore geometry, so the figure is inferred rather than observed - and a different method, such as nitrogen adsorption, gives a different number for the same cement. Blaine figures are therefore comparable with each other and not with values from other techniques.

Answer: A

Correct answer: dissolving hydrated and unhydrated samples in acid and comparing

This is the heat of solution method. The heat released when unhydrated cement dissolves in acid, less the heat released when a hydrated sample of the same cement dissolves, is the heat the hydration itself gave out. It measures the total heat rather than the rate; isothermal conduction calorimetry is the alternative where the rate over the first days is what matters.

Answer: B

Heating drives off combined water and carbon dioxide taken up from the air, so a high figure suggests the cement has been stored badly. Standards set a maximum for exactly this reason. Fresh cement from the mill shows a very low figure indeed.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. Loss on ignition heats the sample to about 950 degrees and weighs what is driven off - chiefly the water and carbon dioxide absorbed as the cement pre-hydrated and carbonated in damp storage. A high figure therefore indicates a cement that has already begun to react in the bag, and it will be correspondingly weaker.

Answer: D

The loss of 0.028 g on a starting mass of 1.000 g is 2.8 per cent. Most standards permit up to about four or five per cent, so this sample would pass comfortably. Rising values in successive samples point to a storage problem.