Short Answer:

Good aggregates are those that are strong, clean, durable, and properly sized for their intended use. They should not contain clay, silt, organic matter, or other impurities that may weaken the concrete or mortar. Properly graded aggregates help in forming a solid and stable concrete mix.

The desirable properties of good aggregates include hardness, toughness, proper shape, and good bonding with cement paste. These properties ensure that the concrete made from them is strong, long-lasting, and resistant to wear and weather. Using good-quality aggregates is essential for strong and durable construction work.

Detailed Explanation:

Desirable properties of good aggregates

Aggregates are an important part of construction materials used in concrete, mortar, roadwork, and many other civil engineering applications. The performance and strength of concrete depend a lot on the quality of aggregates used. To ensure durability and safety in construction, aggregates must meet several desired properties. These properties affect the strength, durability, workability, and overall quality of the final structure.

Strength and Hardness:
Good aggregates should be strong and hard so they can withstand the load and pressure applied on concrete. They should resist crushing and not break down easily under heavy loads. Hard aggregates help in increasing the compressive strength of concrete.
Toughness and Impact Resistance:
Aggregates should be able to absorb shocks and vibrations without breaking. This is important in structures that face heavy traffic or repeated loads, like roads and pavements. The aggregate should have a good toughness value, measured by the Aggregate Impact Test.
Durability:
A good aggregate must be durable, which means it should resist weathering effects such as moisture, temperature changes, and chemicals. Durable aggregates ensure the long life of concrete in harsh environmental conditions.
Cleanliness and Purity:
Aggregates should be free from dust, clay, silt, organic materials, and other harmful substances. These impurities reduce the bonding between cement and aggregates, leading to weak concrete. Clean aggregates form a strong paste with cement.
Proper Grading:
Grading means having particles of different sizes in correct proportions. Well-graded aggregates fill up the spaces better and reduce the need for excess cement paste. This improves strength, reduces voids, and saves cost.
Shape and Texture:
The shape and surface texture of aggregates affect the workability and strength of concrete. Angular aggregates with rough surfaces form better bonds with cement but may reduce workability. Rounded aggregates are easier to mix but may reduce bonding. A proper balance is needed depending on the work.
Specific Gravity and Water Absorption:
High specific gravity means denser and stronger aggregates. Also, low water absorption is important because aggregates that absorb more water reduce the strength of concrete and affect water-cement ratio.
Resistance to Wear and Tear:
Aggregates should resist wear, especially for roads and floors where surface friction is common. The abrasion value should be low, which is tested using the Los Angeles abrasion test.
Chemical Stability:
Aggregates should not react with cement or water to produce harmful substances. They should be chemically stable to prevent any long-term damage such as cracks or expansion in concrete.
Thermal Stability:
Aggregates must not expand or contract excessively with temperature changes. Stable thermal behavior helps maintain the structure’s shape and prevents cracking.

Conclusion:

The desirable properties of good aggregates are essential for ensuring that the concrete or structure remains strong, durable, and cost-effective. Proper selection based on strength, durability, shape, and cleanliness improves performance and reduces maintenance. Always use tested and certified aggregates for best results in civil construction.