Which of the following aggregates is used in Group I moderate lightweight concrete?

Expanded aggregates such as perlite are indeed used in Group I moderate lightweight concrete. The use of lightweight aggregates like perlite helps to achieve a reduced density in concrete while maintaining adequate strength and durability. These aggregates are typically characterized by their lower specific gravity compared to normal aggregates, allowing for significant weight reduction in the final concrete product.

In the context of moderate lightweight concrete, perlite provides enhanced insulation properties and can improve the overall workability of the concrete mix. This makes it suitable for applications where lower weight and better thermal performance are desired.

The other options do not align with the characteristics and purposes of Group I moderate lightweight concrete. For instance, natural river sand and fine crushed stone would contribute to a higher density concrete mix rather than a lightweight one. Cement and gravel mixtures, while used in standard concrete applications, do not fall under lightweight aggregates and would generally pertain to conventional concrete formulations.