One factor that is seldom considered in striving for a high degree of fire or heat resistance in a concrete structure is the design of the concrete mixture itself. And yet, to name just one example, a study has shown that an appropriate change in the type of aggregate used can sometimes double the fire endurance of concrete. Crushed brick, charmottes, corundum and other special aggregates can be used for concrete to be exposed to fire or high temperatures. We are concerned here with aggregates commonly used in general concrete construction. Carbonate aggregates undergo loss of carbon dioxide at temperatures mainly in the ranges of 1320 to 1794 degrees F for calcium carbonate and 1365 to 1540 degrees F for magnesium carbonate. This chemical change absorbs heat without corresponding temperature rise. At the same time it builds up a film of carbon dioxide at the concrete surface to produce an insulating layer, especially when the gas is being generated in considerable volume. Siliceous aggregates do not have this built in safeguard. In the past they have also been faulted because of the expansion accompanying the silica inversion which occurs at a temperatures of about 1,600 degrees F. The effect of the silica inversion does not become important in most fires, however, and it is likely that the main differences in the fire endurance between siliceous and carbonate aggregates are ascribable to the benefits obtained by driving off carbon dioxide from the carbonate aggregates. To sum up, if designers of a concrete mix wishes to achieve a high degree of fire endurance in the concrete itself without going to the use of special refractory materials, they should heed the following advice: of the aggregate available, use the one that exhibits the greatest stability at high temperatures; use a low aggregate-cement ratio; if available, employ pozzolanic or blast furnace slag cement; design the mix for a strength in excess of what is structurally needed, of what is needed to endure the greatest temperature anticipated; cure the concrete long enough to achieve the needed strength; and thoroughly dry the concrete before exposing it to fire or to really great heat.