In building materials and construction engineering, identify the source from which good-quality sand is never obtained. Choose the best single answer based on salt content, grain shape, and cleanliness standards used at site inspections.

Introduction / Context:
Sand quality strongly influences mortar and concrete performance. This question tests your understanding of permissible sources of fine aggregates for construction and why some sources are rejected in practice.

Given Data / Assumptions:

• We are evaluating typical field sources: river, nala (seasonal stream), sea, and crushed gravel fines.
• Standards require sand to be clean, hard, chemically inert, and with limited harmful salts and organic matter.

Concept / Approach:

Sea sand contains soluble salts (notably chlorides and sulfates) that cause corrosion of embedded steel and disrupt cement hydration. Unless it undergoes rigorous desalination and grading (rare at small sites), sea sand is rejected for structural work. River or well-graded stream sands are usually acceptable after washing and sieving. Manufactured fines (from gravel) can be used if they meet grading, shape, and dust limits.

Step-by-Step Solution:

Verification / Alternative check:

Field tests (salt taste, silver nitrate check) and lab tests (chloride content limits) confirm sea sand’s unacceptability without treatment.

Why Other Options Are Wrong:

River: commonly acceptable after washing. Nala: similar to river source if clean. Gravel powder: acceptable as manufactured sand when it meets grading and dust limits.

Common Pitfalls:

Assuming sun-drying removes salts—it does not. Confusing “clean appearance” with chemical suitability.

Final Answer:

sea