Which substance is used to retard the setting action of cement?

The correct answer is Option 2: Gamma-rays. In the context of specialized civil engineering and nuclear construction, high-energy electromagnetic radiation is utilized to modify the hydration kinetics of cement.

The reasoning is as follows:

• Retardation Mechanism: Exposure to Gamma-rays induces structural changes and radiolytic effects in the cement paste. This process interferes with the initial crystallization of calcium silicate hydrate (C-S-H) gels, effectively delaying the setting time.
• Gamma-rays vs. Others: Unlike Ultraviolet, X-rays, or Infra-red rays, Gamma-rays possess the highest penetration power and energy. This allows them to penetrate deeply into dense concrete mixtures, ensuring a uniform retardation effect throughout the mass, which is critical for large-scale or specialized pours.
• Conclusion: While gypsum is the traditional chemical retarder used during manufacturing, Gamma-irradiation serves as a physical method to control setting action in advanced applications.

Therefore, Option 2 is the scientifically accurate choice for retarding cement setting via radiation.