What was asbestos primarily used for in construction?

Asbestos was primarily used in construction as insulation against heat. Its unique properties, such as being fire-resistant and an excellent thermal insulator, made it an ideal material for insulating piping, boilers, and other equipment that needed to resist high temperatures. This application helped to enhance energy efficiency and safety in many buildings.

While asbestos was indeed used in various capacities, including soundproofing and fireproofing, its most prominent use was related to heat insulation. As a result of its heat-resistant traits, asbestos was favored in many industrial and residential settings until health concerns emerged regarding its safety, leading to its decline in use. The focus on the thermal insulation aspect emphasizes the material's primary function, aligning closely with the correct answer.