Deleterious substances in aggregate are undesirable beacause they may:
Correct Answer: D. All option are correct
📚 Detailed Explanation: Deleterious Substances in Aggregates
IS 383 and IS 456 both list deleterious substances that must be limited in aggregates used for concrete. These substances are harmful because they attack the concrete mix from multiple angles: they can weaken the aggregate itself, interfere with cement chemistry, or worsen the properties of fresh and hardened concrete.
Why D (All option are correct) is correct: Each option identifies a distinct mechanism by which deleterious materials cause harm. Silt and clay films coat aggregate surfaces and reduce bond; organic matter (humic acids) retards or disrupts cement hydration; soft fragments (shale, chalk) have low intrinsic strength; all these ultimately degrade strength, workability, and durability together.
Common Deleterious Substances and Their Effects
| Substance | Mechanism of Harm |
|---|---|
| Silt / clay lumps | Coat aggregate, weakening aggregate-paste bond; increase water demand |
| Organic matter | Humic acids interfere with C3A hydration; delay setting |
| Soft / friable fragments | Intrinsic low strength — fracture under load |
| Coal / lignite | Expand on wetting; cause pop-outs and staining |
| Chlorides / sulphates | Cause steel corrosion and sulphate attack |
Key Concepts for Students
- IS 383 limits: for example, clay lumps ≤1% by weight in coarse aggregate; organic impurities tested by colour test per IS 2386 (Part II).
- The colour test for organic matter compares the colour of the aggregate-NaOH solution to a reference: darker than light yellow = presence of harmful organics.
- Sea sand is deleterious because of chloride content, even though it is not “soft” — this shows deleterious effects span all three option categories.