A number of new materials are presently being introduced for application in high temperature environments. Though some of these are old materials fabricated with new methods or processes, many are actually new. This paper focuses on the use of high strength concretes or mortars for application to high temperature environments, such as furnaces or boilers for coal gasification and similar processes requiring high pressure and temperature. This paper reports the results of an investigation on the high.temperature performance of mortar of different strengths incorporating 0%, 8% and 16% of condensed silica fume (CSF). The experiments described have been carried out to compare the performance of different strength level mortars with different CSF contents. Generally speaking, it was found that low strength mortar with CSF is less susceptible to strength loss after high temperature exposure than mortar without CSF. On the other hand, high strength mortar with CSF seems to be more sensitive to high temperature than high strength mortar without CSF since it retains only 65% of its room temperature strength after heating to 3200C as compared to regular high strength mortar which retains 95% of its room temperature strength.

Though there may be a possible practical problem in using high strength CSF concrete, this shown loss of strength in high strength CSF concrete upon heating may prove very important in giving us insight into the fundamental mechanisms of strengthening concrete with CSF. An explanation is hypothesized on the observed behavior of high strength CSF concrete, which will produce a theory for one of the ways in which CSF strengthens portland cement paste. It is theorized that the CSF touohens the portlandite in high strength paste, but after heating to 320 C, the CSF particles which have been converted to C-S-H have lost their ability to toughen the portlandite.