Microstructural investigation of calcium aluminate cement-based ultra-high performance concrete (UHPC) exposed to high temperatures

This study investigated the microstructural and chemical changes of calcium aluminate cement (CAC)-based UHPC exposed to high temperatures. Upon exposure to 100 degrees C, C(3)AH(6) was formed by the dehydration of CAH(10). A further increase in the exposure temperature to 450 degrees C resulted in the formation of a new phase C(12)A(7), which is attributed to the dehydration of C(3)AH(6) and AH(3). The compressive strength of UA50 and UA70 increased significantly due to the formation of C-A-(S)-H gel resulting from the further hydration of anhydrous CAC and silica fume upon exposure to 450 degrees C. The hydration reaction of CAC in UHPC led to a significant increase in the micro pores (< 100 nm), thereby releasing the vapor pressure. After exposure to 800 degrees C the formation of calcium aluminosilicate (C-A-S) gel was observed as a consequence of the sintering of anhydrous CAC and dehydrated C-A-(S)-H with adjacent micro silica, contributing to the residual strength.