Which of the following sentences is INCORRECT in the case of mortar?

Discussion - Mortar Properties MCQ

A. Even a small percentage of mica, if present, considerably lowers the tensile strength and adversely affects the compressive strength.

B. There is a gain of compressive strength by the replacement of less than 25% of cement by hydrated lime.

C. Cement lime mortars are helpful in autogenously healing of cracks.

D. It requires about twice as much cement to produce mortar of a given strength when fine sand is used as it does with coarse sand.

Correct Answer (Incorrect Statement): B. There is a gain of compressive strength by the replacement of less than 25% of cement by hydrated lime.

🔬 Understanding Mortar Properties

Mortar is the workable paste used to bind building blocks like bricks and stones together, and to plaster surfaces. Its properties are determined by the quality and proportion of its ingredients: binder (cement/lime), fine aggregate (sand), and water. This question tests detailed knowledge of how different factors affect mortar performance.

⚖️ Detailed Analysis of the Statements

Let's evaluate each statement to find the one that is false.

(a) Effect of Mica in Sand

TRUE. Mica is a flaky, plate-like mineral. If present in sand, these flakes create planes of weakness within the mortar. They prevent a strong bond from forming between the sand and cement paste. This significantly reduces both the tensile and compressive strength of the mortar, even in small quantities.

(b) Replacement of Cement with Lime

FALSE. This statement claims a *gain* in compressive strength, which is incorrect. Hydrated lime is a much weaker binding material than Portland cement. While adding lime to a cement mortar (creating a "composite mortar") greatly improves its workability, water retention, and flexibility, it does so at the expense of ultimate strength. Replacing any portion of cement with lime will result in a reduction, not a gain, in the final compressive strength compared to a pure cement mortar of the same binder-to-sand ratio.

(c) Autogenous Healing in Lime Mortars

TRUE. Cement-lime mortars possess a remarkable property called "autogenous healing" or self-healing. They contain free lime (calcium hydroxide) that can dissolve in any moisture that enters a fine crack. This lime-rich water then reacts with carbon dioxide from the air to precipitate calcium carbonate (limestone), which seals the crack over time.

(d) Effect of Sand Fineness

TRUE. The strength of mortar comes from the cement paste completely coating the sand particles. Fine sand has a much larger total surface area per unit volume than coarse sand. To achieve the same strength, every particle needs to be coated with a sufficient layer of cement paste. Therefore, to coat the vastly larger surface area of fine sand, you need significantly more cement paste (and thus more cement) than you would for the same volume of coarse sand.

💡 Study Tips for Mortar

Lime is for Workability, NOT Strength: Remember this trade-off. Adding lime makes mortar easier to work with but reduces its final strength.
Mica is a Flaky Menace: Mica's flaky shape is bad for mortar strength.
Fine Sand is "Needy": Fine sand has a huge surface area and is "needy" for more cement paste to achieve good strength.
Lime Heals: Associate the presence of lime with the special property of self-healing cracks.