What material is most frequently employed to absorb humidity in dry laying up?

Activated alumina is the correct choice for absorbing humidity in dry laying up due to its unique properties. This material is a highly porous form of aluminum oxide that has a vast surface area and is structurally designed to attract and hold moisture effectively. It is commonly used in various applications, including as a desiccant in gas and liquid storage systems, to maintain dry conditions.

Its ability to absorb water vapor makes activated alumina particularly effective in environments where moisture control is critical. This is essential for preventing spoilage or degradation of sensitive materials or chemicals that may be stored in moisture-sensitive conditions.

In contrast, other materials listed do not possess the same efficiency or suitability for humidity absorption. Calcium silicate, while it does absorb moisture, is less common in dry laying up applications compared to activated alumina. Silicon dioxide is often used for the same purpose but primarily in the form of gels and not as frequently in dry laying applications. Iron oxide is typically not used for moisture absorption and serves different functions in industry. Thus, activated alumina stands out as the preferred material for this specific application.