Binders and cements — the initial setting time of lime–pozzolana binder (used in mortars and low-heat concretes) is approximately:

Introduction / Context:
Lime–pozzolana binders (using hydrated lime with reactive siliceous/aluminous pozzolana) develop strength through pozzolanic reactions forming C-S-H and related hydrates. Knowing their initial setting time is important for mixing, transport, and placing schedules, especially in heritage conservation and low-heat applications where Portland cement is not preferred.

Given Data / Assumptions:

• Standard laboratory conditions are assumed (normal temperature and humidity).
• Initial setting time refers to the onset of rigidity measured by needle penetration methods.
• Typical formulations of lime–pozzolana binder are considered (not rapid-hardening blends).

Concept / Approach:

Compared with ordinary Portland cement (OPC), which must not set in less than 30 minutes per standards, lime–pozzolana systems tend to set more slowly because the reaction depends on the availability of dissolved silica/alumina and lime. A commonly cited benchmark for initial set is about 60 minutes, allowing adequate working time while ensuring early stability for finishing operations.

Step-by-Step Solution:

Verification / Alternative check:

Conservation and traditional construction guidelines often cite ∼1 hour as a practical initial set for lime–pozzolana mixes, though exact values vary with fineness, temperature, and mix proportions.

Why Other Options Are Wrong:

30 minutes — more typical of OPC minimum; too short for many lime–pozzolana mixes.
90 or 120 minutes — possible in cool conditions but generally longer than the common benchmark used in exam contexts.

Common Pitfalls:

Assuming all binders follow OPC times; ignoring environmental effects that can extend or shorten set times.

Final Answer:

60 minutes