What should coarse aggregates be free from to ensure a strong concrete mix?

Coarse aggregates must be free from organic matter to ensure a strong concrete mix because organic materials can adversely affect the setting and hardening process of concrete. Organic matter, such as leaves, roots, or decayed plant material, can interfere with the hydration of cement and may lead to problems with the overall durability and strength of the concrete. When organic impurities are present, they can also create voids within the mix that weaken the bond between the aggregates and the cement matrix.

While water is necessary for the concrete mix to hydrate the cement and achieve strength, it doesn't need to be excluded. Coloring agents are typically not harmful in small quantities, as they are often added deliberately for aesthetic reasons, and air pockets can be managed through proper mixing and vibration techniques. Therefore, the presence of organic matter is particularly detrimental and should be avoided in coarse aggregates to maintain the integrity of the concrete structure.