Lime mortars: Why is pozzolana (surkhi) blended with lime in traditional construction?

Difficulty: Easy

Correct Answer: To impart hydraulicity (ability to set in presence of water)

Explanation:


Introduction / Context:
Traditional lime mortars can be improved by adding pozzolanic materials such as surkhi (finely ground burnt clay). This blend creates hydraulic lime mortars that set even under damp conditions—valuable for foundations, water structures, and humid climates.


Given Data / Assumptions:

  • Lime alone (air lime) hardens mainly by carbonation and is slow-setting.
  • Pozzolana reacts with lime in presence of moisture.
  • Desired performance: early strength and setting in wet environments.


Concept / Approach:
Pozzolans contain reactive silica and alumina. In the presence of calcium hydroxide and water, they form cementitious compounds (C-S-H type gels), granting hydraulicity. This reaction reduces free lime, improves durability, and allows setting even when air access is limited.


Step-by-Step Solution:
Identify limitation: pure fat lime sets slowly by carbonation and is unsuitable for persistently wet zones.Add pozzolana: reactive components combine with Ca(OH)2 to form water-stable hydrates.Result: mortar gains hydraulic properties and improved early strength in damp environments.


Verification / Alternative check:
Historic structures using surkhi-lime mortars exhibit long-term durability in water-exposed situations, confirming imparted hydraulicity.


Why Other Options Are Wrong:
Shrinkage reduction and cost benefits may occur, but they are secondary to imparting hydraulicity.Setting time may decrease somewhat, but the core reason is hydraulic setting capability, not merely quick set.


Common Pitfalls:
Assuming pozzolana is just an extender; it is a reactive additive that chemically strengthens the matrix.


Final Answer:
To impart hydraulicity (ability to set in presence of water)

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