Refractory concrete is used in two general applications in the fireplace industry. Producers manufacturing zero clearance pre-fabricated fireplaces use refractory concrete to cast brick-like facades for use in the hearth. Gas log manufacturers also use refractory concrete to mold logs with unprecedented detail.
The use of cements by this industry is usually limited to the four lower temperature grades as hearth temperatures rarely exceed 2,000°F. Many times the choice of cement is made more by color or rheology requirements than refractory performance.
Formulations for these concretes are typically simple in nature utilizing locally available lightweight aggregates in conventional proportions. Expanded shale, slate and clay are quite prevalent as the aggregate system (http://www.escsi.org) as they have been thermally processed and are therefore capable of long term durability in a fireplace. Careful consideration must be made with these aggregates, however, as their open surface porosity can cause variable moisture contents which in turn can affect the rheology and long term strength of the concrete. Free silica in the form of quartz can also cause cracking in these concretes.
Ciment Fondu®: The world's first commercially manufactured calcium aluminate cement patented by J. Bied of Lafarge in 1908. Originally designed for sulfate resistance, the refractory properties of this cement are well known. Its composition of 40% alumina and 15% total iron oxide is suitable for a fireplace environment however the high iron content can cause a red color to appear over time in non-painted logs.
Secar® 41: Increasing the purity of the raw materials used for Fondu® increases the alumina content to 48% and lowers the total iron oxide content to 7% to form a more pure variation of cement. This cement is widely used in the manufacture of gas logs and is suitable for fireplace panel applications.
Secar® 51: Using higher quality raw materials produces another increase in refractoriness. At 52% alumina and <3% total iron, this light gray cement has become the industry standard color for panels used in prefabricated fireplaces.