Which of the following materials is commonly used for refractory in boiler construction?

Fireclay is commonly used for refractory in boiler construction due to its high resistance to heat and thermal shock. Refractory materials are designed to withstand the intense temperatures and corrosive environments found in boilers and other high-heat applications. Fireclay, which is a type of clay that can withstand high temperatures, is specifically formulated to maintain its strength and integrity under these conditions.

Its ability to provide insulation and resist chemical attacks makes it ideal for linings in furnaces, kilns, and boilers. In contrast, concrete is not typically used in these high-temperature applications as it can crack and degrade. Steel, while strong and durable, cannot handle extreme temperatures without losing structural integrity, and fiberglass is generally not suitable for high-heat applications due to its lower melting point and susceptibility to heat damage. Thus, fireclay stands out as the appropriate choice for refractory material in boiler construction.