Portland cement and non-sintered Hwangto were used as binders. Crushed stone with a maximum size of 20 mm was used as coarse aggregate, and river gravel with a maximum size of 4.75 mm was used as fine aggregate. To consistently control the workability of the fresh concrete, a polycarboxylic-based superplasticizer was used. The physical properties of the materials used in the experiments are shown in Table 1, and the chemical properties of the Portland cement and non-sintered Hwangto used are shown in Table 2.

The mix proportions utilized in this experiment are shown in Table 3. The water-to-binder ratio (W/B) was 0.41, and the target strength for the plain mix was set at 30 MPa, which was achieved with a measured strength of 34.6 MPa. The test specimens were designated as Plain41, NHTC41-15, and NHTC41-30, with non-sintered Hwangto replacing 15% and 30% of the cement, respectively. The concrete specimens were cast in cylindrical molds with dimensions of ø 100 mm x 200 mm. Initial curing was performed underwater for the first 28 d, followed by curing in a climate room at 20 ± 2 °C and 60 ± 5% humidity until the age of 91 d.

The details of this study are presented in Table 4. The heating curve is shown in Figure 1 [11]. The heating rate was 1 °C/min, and the target temperatures were 20 °C, 100 °C, 200 °C, 300 °C, 500 °C, and 700 °C. After reaching the target temperature, a 1-h holding period was set to ensure a uniform temperature distribution, followed by cooling at room temperature for 24 h before measurements were performed.

To evaluate the energy-absorption capacity of the test specimens, the method used by previous researchers was applied. The area under the stress-strain curve of the concrete specimen until the cracking point was defined as the dissipated energy, as shown in Figure 2 [12].

Photographs of the equipment and test specimens used in the experiment, including a universal testing machine, a load cell for compressive-strength measurement, and strain gauges installed on both sides for stress-strain measurement, are shown in Figure 3. The unit weight, compressive strength, stress-strain behavior, and dissipated energy of the specimens were measured to assess their mechanical properties at 91 d of age. The compressive strength was determined as the average of three measurements based on KS F 2405.

The unit weight of the test specimens is shown in Figure 4. At room temperature, the unit weights were 2,291, 2,226, and 2,212 kg/m³ for Plain41, NHTC41-15, and NHTC41-30, respectively. Owing to the density difference between cement and non-sintered Hwangto, the NHTC specimens showed lower unit weights at room temperature. At 200 °C, the residual unit weights were 0.97, 0.95, and 0.95, respectively, i.e., with Plain indicating a slightly higher value. However, at 500 °C, the residual unit weights were 0.92, 0.93, and 0.93, respectively, and at 700 °C, they were 0.90, 0.91, and 0.91, respectively. In other words, the NHTC specimens exhibited higher residual unit weights at higher target temperatures. This is attributed to the characteristics of Hwangto undergoing chemical decomposition at higher temperatures compared with those of cement [8]. At room temperature, the compressive strength decreased with higher incorporation rates of non-sintered Hwangto. Compared with Plain, NHTC41-15, and NHTC41-30 exhibited lower values of compressive strength by approximately 11.3% and 27.2%, respectively.

The compressive strength of the test specimens is shown in Figure 5. At room temperature, the compressive strengths of Plain, NHTC41-15, and NHTC41-30 were 48.9, 43.4, and 35.6 MPa, respectively, which shows a decreasing trend as the incorporation rates of non-sintered material increases. Compared with Plain, NHTC41-15 and NHTC41-30 exhibited lower values of compressive strength by approximately 11.3% and 27.2%, respectively. As the target temperature increased, the NHTC specimens showed lower residual compressive strengths up to 500 °C. This is attributed to the loss of free water, absorbed water, and structural water, thus causing the shrinkage and structural deformation of Hwangto at temperatures below 400 °C [13,14]. However, at 700 °C, NHTC41-15 and NHTC41-30 exhibited higher values of residual compressive strengths by 0.23 and 0.29 compared to Plain, respectively.

According to M. Karatas, mortars with incorporated kaolin and calcined kaolin exhibited lower compressive strengths than ordinary Portland cement as the incorporation rate increased. However, the residual compressive strength increased with the target temperature. This improvement is attributed to the increased reactivity of kaolin as temperature increases, thus causing the specimens to be degraded less [15].

Additionally, the higher residual compressive strength of the NHTC specimens at higher target temperatures is believed to be caused by the decomposition of some carbonate minerals in Hwangto at temperatures above 600 °C, which subsequently combined with atmospheric moisture during the cooling process to form calcium hydroxide [13].

The stress-strain behavior of the test specimens is shown in Figures 6(a-c). The typical concrete exhibits brittle fracture properties, with strain indicated until the maximum deformation. In the temperature range of 20 °C-300 °C, where strength degradation is relatively minimal, a steep slope was observed. However, at target temperatures of 500 °C and above, a gradual slope was observed due to the rapid decrease in strength and an increase in strain. This is attributed to the increase in porosity within the concrete at temperatures above 500 °C, which is likely due to the evaporation of free water inside the concrete, thus resulting in a more ductile failure compared with that at room temperature [16].

At target temperatures below 300 °C, the difference in the maximum strain between the Plain and NHTC specimens ranged from 2.7% to 7.9%, with the NHTC specimens showing lower values. At 500 °C, NHTC41-15 exhibited a higher strain by approximately 12.9% compared with Plain, whereas at 700°C, NHTC41-30 exhibited a higher strain by approximately 15.3% compared with Plain. Additionally, NHTC41-15 showed higher strains at temperatures below 500 °C compared with the other specimen. However, at 700 °C, NHTC41-30 exhibited the highest strain among the specimens.

The energy dissipated by the test specimens is shown in Figures 7(a-c). Compared with the case at room temperature, higher levels of energy were dissipated at 100 °C-500 °C. This indicates that within this temperature range, the strain increased more significantly relative to the decrease in the compressive strength. Additionally, owing to the higher compressive strength of Plain, its dissipated energy was 5.6% to 25.2% higher than those of NHTC41-15 and NHTC41-30 NHTC in the target temperature range of 100 °C to 300 °C, respectively. At 500 °C and 700 °C, the highest dissipated energies were indicated by NHTC41-and NHTC41-30, respectively. Although NHTC41-30 exhibited relatively lower dissipated energies in absolute terms, its residual dissipated energy at 700 °C was 83.8%, which was higher than those of Plain and NHTC41-15 (58.2% and 57.6%, respectively), compared with the case at room temperature.