The most useless aggregate is one whose surface texture is:
Correct Answer: C. glassy
📚 Detailed Explanation: Why Glassy Texture is the Most Undesirable
Surface texture of an aggregate determines how well the cement paste can grip it mechanically after hardening. IS 2386 (Part I) classifies aggregate surface texture from glassy (smoothest) to honey-combed/porous (roughest). Each type offers a different degree of bond with the cement matrix.
Why C (glassy) is the most useless: A glassy surface — seen in certain cherts, obsidian fragments, or over-burnt aggregate — is completely smooth at the microscopic level. There are no roughness features for cement paste to grip. This produces the weakest possible aggregate-paste bond. Even smooth rounded aggregate (option A) has some surface irregularities, whereas glassy is completely featureless. Granular (B) and honeycomb/porous (D) surfaces provide excellent bond through their texture and porosity.
Surface Texture Classification (IS 2386 Part I)
| Texture | Bond Quality | Examples |
|---|---|---|
| Glassy | Worst | Chert, flint, obsidian |
| Smooth | Poor | River gravel, polished granite |
| Granular | Moderate | Sandstone, oolitic limestone |
| Rough | Good | Basalt, rough-crushed stone |
| Crystalline | Good | Granite (coarse-grained) |
| Honeycomb / Porous | Excellent | Pumice, clinker, lightweight aggregate |
Key Concepts for Students
- Glassy = worst bond because the atomic-level flatness prevents mechanical interlocking.
- Porous aggregates provide excellent bond because cement paste penetrates the surface pores and keys in — but they also absorb mix water, so moisture correction is essential.
- Surface texture affects tensile and flexural strength more than compressive strength, because crack propagation along the aggregate-paste interface is a tensile failure mode.