MCQs on Admixtures and Their Uses
Questions 1–9 — Tap an option to check your answer.
Q1: Which of the following factors influence the durability of concrete?
Difficulty: Medium
Correct Answer: D. All options are correctSolution:Concrete durability is influenced by all three factors: the cover depth to steel (governs chloride/carbonation attack on rebar), the cement content and w/c ratio (govern permeability and strength of the paste), and the shape/size of the member (affects surface-to-volume ratio, drying, thermal gradients, and stress concentrations). IS 456:2000 addresses all three in its durability design requirements.
Q2: Shrinkage of concrete is directly proportional to:
Difficulty: Medium
Correct Answer: A. Cement contentSolution:Drying shrinkage is a property of the cement paste, not the aggregate. More cement means more paste volume — and paste is what shrinks on drying. Aggregate particles are dimensionally stable and actually restrain paste shrinkage. Higher w/c ratio also increases shrinkage, but the direct proportionality stated here refers specifically to cement content.
Q3: Which of the following accelerating admixtures is harmful for reinforced concrete and pre-stressed concrete?
Difficulty: Medium
Correct Answer: B. Calcium chlorideSolution:Calcium chloride (CaCl₂) is the most effective accelerator but is harmful to reinforced and pre-stressed concrete because the chloride ions (Cl⁻) disrupt the passive oxide layer that protects steel reinforcement from corrosion. Above 2% by weight of cement, CaCl₂ is detrimental; above 4%, it severely attacks steel. In pre-stressed concrete, even small amounts are prohibited due to the high-tensile wires being susceptible to stress-corrosion cracking.
Q4: Which of the following is NOT a retarding admixture?
Difficulty: Medium
Correct Answer: B. Calcium chlorideSolution:Retarding admixtures delay the hydration of cement and extend the setting time. Starch, sugar (saccharose), calcium sulphate (gypsum), calcium lignosulphonate, and sodium bicarbonate are all retarders. Calcium chloride is an ACCELERATOR — it increases the rate of hydration and reduces initial setting time. It is therefore NOT a retarding admixture.
Q5: The addition of a small portion of granite polishing waste in the design mix results in:
Difficulty: Medium
Correct Answer: D. Improved packing densitySolution:Granite polishing waste (GPW) is a very fine, hard material produced during the polishing of granite slabs. When used as a partial replacement for sand in concrete, GPW fills the micro-voids between sand particles, reduces porosity and void ratio, and improves packing density. This leads to denser concrete with better strength and lower permeability.
Q6: The addition of a portion of fly-ash in the mix design results in reduction in which of the following?
Difficulty: Hard
Correct Answer: D. Only (i), (ii) and (iii)Solution:Fly ash reduces permeability (by secondary pozzolanic reaction producing more C-S-H gel that fills capillary pores), reduces chemical attack susceptibility (denser matrix; lower Ca(OH)₂ content means less material for acid/sulfate attack), and reduces heat of hydration by 50–60% when 30% fly ash is used (slower pozzolanic reaction vs. rapid C₃S hydration). However, fly ash INCREASES (not decreases) the initial setting time due to its slower reaction rate, so statement (iv) is wrong.
Q7: To overcome the accelerating effect of high temperature on initial setting time, which admixture is added?
Difficulty: Medium
Correct Answer: B. RetardersSolution:At high temperatures, the rate of cement hydration accelerates rapidly, shortening the setting time and leaving less time for mixing, transporting, and placing concrete. Retarders are added to the fresh mix to counteract this effect — they slow down the hydration reaction, extending the setting time and providing adequate workability windows for concrete operations.
Q8: Fly-ash is a by-product of:
Difficulty: Easy
Correct Answer: B. CoalSolution:Fly ash is a fine, powdery residue collected from the flue gases produced by burning pulverised coal in thermal power plant boilers. When coal is combusted at 1200–1700°C, the ash particles are carried out of the boiler with the flue gas and collected by electrostatic precipitators or baghouse filters. Fly ash is widely used as a supplementary cementitious material (SCM) in concrete.
Q9: As per IS 3812 (Part 1): 2013, which ash is prepared from fly ash or bottom ash conveyed in dry form and deposited dry?
Difficulty: Hard
Correct Answer: A. Mound ashSolution:IS 3812 (Part 1): 2013 defines Mound ash as ash prepared from fly ash, bottom ash, or a combination of both, in any proportion, that is transported and deposited strictly in the dry state. This distinguishes it from Pond ash, which is transported in slurry form and deposited wet. Mound ash is typically used for engineered fill or land reclamation.