The rocks which have very high strength are

Detailed Explanation: Strength of Rock Types

The strength of the aggregate directly affects the potential strength of concrete, especially in high-strength concrete where the aggregate may govern the failure mode. Different rock types formed through different geological processes have widely varying strength values.

Why D (granite) is correct: Granite is a coarse-grained igneous rock composed of quartz, feldspar, and mica, formed by slow cooling of magma deep in the earth's crust. This slow crystallisation produces an interlocked crystalline structure with very high strength (150–280 MPa compressive strength) and excellent abrasion resistance. Comparisons: Limestone (A) is a sedimentary rock with variable strength (30–150 MPa) — soft limestones are unsuitable for concrete; Sandstone (B) is a sedimentary rock with even lower strength (20–100 MPa) due to loose cementation of sand grains; Basalt (C) is also a strong igneous rock (100–300 MPa) but is typically harder and less available than granite in India. Granite is the universally cited example of a very high-strength rock for aggregate.

Compressive Strength of Common Rocks

Rock Type	Formation	Typical UCS (MPa)	Suitability as Aggregate
Sandstone	Sedimentary	20–100	Variable; weak varieties rejected
Limestone	Sedimentary	30–150	Good if hard; soft varieties rejected
Basalt	Igneous (extrusive)	100–300	Excellent; dense, strong
Granite	Igneous (intrusive)	150–280	Excellent; high strength aggregate

Key Concepts for Students

• Granite = highest strength among common building rocks — the standard exam answer for this type of question.
• Rock strength must exceed the design concrete strength; for M60+ high-strength concrete, granite or basalt aggregates are specified.
• Aggregate should not be the weak link: IS 456 and IS 10262 require that aggregate Crushing Value (ACV) and Impact Value (AIV) meet minimum standards for the grade of concrete.