Which of the following statements about aggregates is correct?

The correct statement regarding aggregates is that coarse aggregate can include natural stone. Coarse aggregates are typically larger particles, which may include materials such as gravel, crushed stone, or natural stones like granite, limestone, or basalt. The inclusion of natural stone in the classification of coarse aggregates is essential, as they are widely used in concrete and asphalt production due to their durability and strength.

Coarse aggregates can be obtained from both natural sources and quarried or crushed materials, making them versatile in construction applications. This characteristic is important because the physical properties of the stones can contribute significantly to the overall performance of concrete mixes.

In contrast, the other statements do not accurately reflect the standards and practices regarding aggregates. For instance, the idea that coarse aggregates must always be crushed is not correct, as they can also be naturally occurring. The notion that fine aggregate must not exceed 20% of total aggregate is misleading; the proportion of fine to coarse aggregate can vary depending on the specific concrete mix design requirements. Lastly, while recycled materials can be used as aggregates, they are not the sole source, as both natural and manufactured aggregates are also common in construction.