Effect of fire on compressive strength of early cured mortars with Ground Granulated Blast Furnace Slag (GGBFS)

This study investigated the effect of high temperature on early cured mortar with Ground Granulated Blast Furnace Slag (GGBFS). The mortar (W/B=0.4, 0.5) was cured in air for 3, 7, 14, 28, 56 days, and then exposed to high temperatures (300 degrees C, 450 degrees C, 600 degrees C, 850 degrees C, Standard Temperature Curve (STC)). The TGA tests were conducted to understand the material properties of the cement mortars with different W/B ratios and curing ages. After exposed to the high temperatures, residual compressive strength tests (RCS) and ultrasonic pulse velocity tests (UPV) were conducted on the next day, 35th day after casting, and 21-day recurring. The results revealed that the UPV decreased for all specimens after exposed to high temperature, and the reduction was more significant for specimens with higher temperature. Internal structural damage caused by the high temperature lowered their densities. In addition, the RCS of mortars with W/B=0.4 significantly decreased after the mortars exposed to 300 degrees C, while the RCS did not change much with the temperatures of 450 degrees C and 600 degrees C. Between the temperatures of 300 and 450 degrees C, C-S-H gel generated and enhanced the RSC. The RCS decreased significantly after the mortars exposed to 850 degrees C. Furthermore, an increase in RCS at 450 degrees C was found for the specimens of W/B=0.5. The increase may be caused by enhanced hydration reaction between unhydrated cement and crystal water due to heating. Additionally, insignificant recovery of the UPV and RCS after post-fire-curing existed. Moisture supply with air-curing to continue and expedite the rehydration reactions was not effective. (C) 2013 International Association for Fire Safety Science. Published by Elsevier Ltd. Open access under CC BY-NC-ND license.