3.5.2. Compressive Strength Test of Mortar and Concrete

The compressive strength of mortar was measured by 30 t class universal testing machine, and the average value was used by measuring the compressive strength in a triplicate set of specimens at 2, 4, 8, 16, 32 and 64 times for the final setting time [21]. In addition, the compressive strength of the concrete was measured 200 t class universal testing machine to measure the compressive strength in a triplicate set of specimens at ages 3, 7, 14, and 28 days, and the average value was considered as a result.

#### **4. Results and Discussion**

#### *4.1. Setting Time of Mortars*

The initial and final setting time of mortar with GGBFS was measured by a penetration resistance test. Figure 2 shows the effect of GGBFS and temperature on the setting time of mortar. As expected, mortars of the same GGBFS replacement ratio show a decrease in setting time at higher curing temperatures. In addition, compared with OPC mortar, as the GGBFS replacement ratio increases, the setting time increases at the same curing temperature. In case of mortar with 50% of GGBFS compared to OPC mortar at the curing temperature of 5 ◦C, the final setting time was delayed by 15.7 h. When mortar with 50% GGBFS was cured at 5◦ C and 35 ◦C, the final setting time was delayed by 16.9 h. The largest difference in the final setting time was 23.7 h between 50% GGBFS at 5 ◦C and OPC at 35 ◦C. It is considered that impermeable film has formed when GGBFS particle reacts with water. Due to the formation of the impermeable film results in the delay of hydration reaction, which restraining the penetration of water and ion [44–47]. Thus, this result suggests that the setting quickly occur owing to the fast reaction of hydration caused in early age at high temperature. Therefore, the setting time is affected by both curing temperature and GGBFS replacement ratio.

**Figure 2.** Setting time of mortar, according to curing temperature and replacement ratio of GGBFS.
