MCQs on Admixtures and Their Uses
Questions 10–18 — Tap an option to check your answer.
Q10: What is the effect of fly ash in concrete?
Difficulty: Easy
Correct Answer: D. Reduces the heat of hydrationSolution:Fly ash reduces the heat of hydration because it reacts pozzolanically — a slow secondary reaction — rather than through the rapid primary hydration of C₃S and C₃A that generates most heat. With 30% fly ash replacement, heat of hydration reduces by 50–60%. Fly ash also improves workability (spherical particles), reduces permeability, and increases ultimate long-term strength.
Q11: Retarders are used for:
Difficulty: Medium
Correct Answer: D. Grouting deep oil wellsSolution:Retarders extend the setting time of concrete/grout. This is essential when grouting deep oil wells, where the grout mixture must remain fluid for a long time while being pumped down the borehole to the required depth. Without retarders, the grout would set before reaching the target depth. Retarders are also used in hot weather concreting and long-distance transport, but the classic exam answer here is deep oil wells.
Q12: Which of the following is commonly used as retarder in cement?
Difficulty: Easy
Correct Answer: B. GypsumSolution:Gypsum (CaSO₄·2H₂O) is the standard retarder added to all Portland cements during the grinding of clinker. Without gypsum, C₃A (the most reactive cement compound) would react instantly with water causing a flash set — the cement would become unworkable within minutes of mixing. Gypsum reacts with C₃A to form ettringite, creating a protective coating that slows the C₃A hydration and gives adequate working time.
Q13: How many types of chemical admixture are there?
Difficulty: Medium
Correct Answer: D. 5Solution:There are 5 major types of chemical admixtures: (1) Accelerators (e.g. CaCl₂ at 2–4%); (2) Retarders (e.g. gypsum, sugar); (3) Air-entraining agents (e.g. soaps, oils at 0.005–1%); (4) Water-repelling agents (e.g. sodium silicate, zinc sulphate); and (5) Gas-forming agents (e.g. aluminium or zinc powder at 0.02% of cement weight). These form the five standard chemical admixture categories.
Q14: Pick up the incorrect statement from the following about admixtures:
Difficulty: Medium
Correct Answer: D. Admixtures give high strength (INCORRECT statement)Solution:Options A, B, and C are all correct: accelerating admixtures do speed up hydration; waterproofing admixtures reduce permeability making concrete effectively waterproof; and acid-resistant admixtures can make concrete resistant to certain acids. Option D is incorrect because admixtures alone do NOT directly confer high ultimate compressive strength — strength is primarily governed by the water-cement ratio, compaction, and cement quality, not by admixtures.
Q15: Pick up the correct statement: A) Calcium chloride acts as retarder, B) Gypsum acts as accelerator, C) Gypsum acts as retarder
Difficulty: Easy
Correct Answer: C. Gypsum (calcium sulphate) acts as a retarderSolution:Gypsum (calcium sulphate dihydrate) is the standard retarder added to all Portland cement at 2–3% during grinding to prevent flash set. Calcium chloride is an accelerator (reduces initial setting time). Therefore A is wrong, B is wrong, and C is correct.
Q16: Addition of sugar in concrete results in:
Difficulty: Medium
Correct Answer: B. Increase in setting time by about 4 hoursSolution:Sugar (sucrose) is a powerful retarder in concrete. When added at approximately 0.2% by weight of cement, it can increase the total setting time by approximately 4 hours. Sugar molecules adsorb onto cement particle surfaces, forming a barrier that prevents water from initiating hydration. Very small amounts of sugar are highly effective — even 0.05% can significantly delay setting.
Q17: Which of the following properties of plastic concrete is modified by air-entraining agents?
Difficulty: Medium
Correct Answer: D. All options are correctSolution:Air-entraining agents introduce millions of tiny, stable, discrete air bubbles (0.05–1.25 mm diameter) into fresh concrete. These bubbles: (A) improve workability by acting as micro-ball-bearings reducing inter-particle friction; (B) reduce segregation by increasing cohesion of the mix; (C) reduce bleeding by interrupting the upward capillary paths for water migration. All three properties of plastic concrete are modified.
Q18: Concrete made by packing dry coarse aggregate and injecting cement-sand mortar under pressure is known as:
Difficulty: Hard
Correct Answer: A. Pre-packed concreteSolution:Pre-packed concrete (also called pre-placed aggregate concrete or intrusion concrete) is made by first placing coarse aggregate in the formwork (packed to minimise voids) and then injecting a cement-sand mortar under pressure using a pump. This method produces very dense concrete with low shrinkage, excellent contact with existing structures, and no segregation — ideal for underwater concreting, repairs, and massive foundations.