To effectively compact clay soil, what material should be added?

When compacting clay soil, adding lime is effective because it alters the physical and chemical structure of the clay particles. Lime serves as a soil stabilization agent, which helps improve the soil's workability and compaction properties. When lime is mixed with clay, a chemical reaction occurs that enhances the soil’s cohesion and reduces plasticity. This results in a more stable and solid material, making it easier to compact and suitable for construction purposes.

Using lime also promotes better drainage and reduces shrink-swell potential, which can be critical in preventing future issues with settling or expansion. In contrast, sand can improve drainage but may not effectively address the cohesive properties of clay. Compost increases organic material but may not provide the necessary stability for compacting clay. Gypsum can improve the soil's structure but is typically not as effective as lime in enhancing the compaction of clay soils.