Aggregates for concrete — which statements about common rock/brick aggregates are correct? Consider the following statements: (a) Sandstones may be classified as calcareous, siliceous, or ferruginous sandstones. (b) Concrete made with sandstone aggregates tends to crack more due to relatively higher drying shrinkage. (c) Very hard, close-grained crystalline limestones are suitable aggregates but provide low concrete strength. (d) Concrete made with broken bricks can exhibit good fire-resistant qualities. Which option identifies the correct statements?

Introduction / Context:
Aggregate type strongly influences concrete properties such as strength, modulus, shrinkage, durability, and fire performance. Identifying correct generalizations about commonly used aggregates helps in selecting appropriate materials for specific performance goals.

Given Data / Assumptions:

• Normal-weight aggregates from natural rocks or crushed masonry.
• Well-graded mixes with comparable cement contents.
• General tendencies, acknowledging natural variability.

Concept / Approach:

(a) Sandstone classifications by dominant cementing/constituent (calcareous, siliceous, ferruginous) are widely used — correct. (b) Sandstone aggregates often lead to relatively higher drying shrinkage than strong igneous rocks, increasing cracking propensity — broadly correct. (c) Very hard close-grained crystalline limestones are indeed suitable and typically produce good strength; stating they provide “low strength” is misleading — incorrect. (d) Brick aggregate concrete, though lower in strength, exhibits good fire resistance — correct.

Step-by-Step Solution:

Verification / Alternative check:

Materials texts report shrinkage ranking often as: lightweight > sedimentary (incl. many sandstones) > dense igneous; and note brick aggregate's favourable performance in fire tests.

Why Other Options Are Wrong:

• (b),(d) only omits the true statement (a).
• (a),(c) includes an incorrect statement.
• “All” is invalid due to (c).
• “Only (c)” is clearly wrong.

Common Pitfalls:

• Treating all limestones as weak; quality varies with texture and purity.

Final Answer:

Only (a), (b), and (d) are correct.