Lightweight aggregates: These are primarily obtained from which geological source for use in lightweight concrete?

Introduction / Context:
Lightweight concrete relies on aggregates with low density. Natural lightweight aggregates commonly originate from specific geological processes that produce vesicular textures.

Given Data / Assumptions:

• Natural aggregates considered (not manufactured like expanded clay or sintered fly ash).
• We are to identify the most typical natural source.

Concept / Approach:
Volcanic activity can produce rapidly cooled lavas with trapped gas bubbles, yielding vesicular rocks such as pumice and scoria. These materials have low unit weights and are widely used as lightweight aggregates. While these are igneous, the key feature is the volcanic vesicular origin, distinguishing them from dense intrusive igneous or non-igneous rocks.

Step-by-Step Solution:
Recall lightweight natural aggregates: pumice, scoria, volcanic cinders.Associate them with volcanic processes producing vesicular texture.Select “Volcanic source”.

Verification / Alternative check:
Specifications cite pumice/scoria as standard natural lightweight aggregates; many other rock types are too dense.

Why Other Options Are Wrong:
Sedimentary/metamorphic rocks are generally denser; “igneous in general” is too broad and includes dense forms like granite.

Common Pitfalls:
Confusing general igneous category with the specific vesicular volcanic subtypes that provide low density.

Final Answer:
Volcanic source (e.g., pumice, scoria).