## 4 Mechanical Test Results and Discussion  
### Compressive Strength  
Figure 9 shows the compressive strength test results of the 7 d and 28 d cured specimens. Specim A7, containing only OPC as a binder, exhibited a compressive strength of 93 MPa. This value varied based on the type and dosage of the mineral additives and the curing time of the specimen. Generally, the substitution of OPC with other reactive or non-reactive mineral additives lowered the compressive strength at the age of 7 d. The silica flour notably decreased the compressive strength at the early curing age to 72 and 79 MPa for specimens B7 and C7, respectively. Specim D7, with 50% GGBS, also had a lower compressive strength of 85 MPa. Specim E7, with 100% GGBS, however, had a higher compressive strength of 107 MPa. Specimens F7 and G7, with SF, also had lower compressive strengths of 85 MPa and 92 MPa, respectively. Lastly, specimens H7 and I7, with 15% and 30% dosages of CKD, exhibited significantly higher compressive strengths of 102 MPa and 112 MPa, respectively.  
Figure 9 shows that the additional curing at 40 \({}^{\circ}\)C from 7 d to 28 d enhanced the compressive strength, and the degree of increase varied according to the types and dosages of the mineral additives used. Specim A28 had a compressive strength of 103 MPa, 10.3% higher than specimen A7. Silica flour decreased the compressive strength of the 7 d cured specimens B7 and C7, as mentioned previously. However, specimens B28 and C28 showed compressive strengths of 91 MPa and 104 MPa, respectively. The rate of increase in specimens B28 and C28, compared to B7 and C7, were 26.3% and 31.2%, respectively. GGBS enhanced the compressive strength of the cementitious composites after 28 d of curing at 40 \({}^{\circ}\)C. Specim D7 had a lower compressive strength than specimen A7, whereas specimens D28 and E28 had higher compressive strengths than specimen A28, measuring at 108 MPa and 116 MPa, respectively. Their rates of increase from specimens D7 and E7 were 28.0% and 7.8%, respectively. Specimens F28 and G28, with SF, also increased by 26.5% and 9.6% from those of specimens F7 and G7 to 108 MPa and 100 MPa, respectively. Lastly, the compressive strengths of specimens H28 and I28 with CKD increased by 18.8% and 6.4%, measuring 121 MPa and 119 MPa from specimens H7 and I7, respectively.  
After 28 d of curing, the reactive GGBS, SF, and CKD improved the compressive strength of the specimens in comparison to that of specimen A28, which only employed OPC, except for specimen G28, which constituted 30% of SF. On the other hand, in the case of specimens B28 and C28, containing silica flour, the compressive strength was not enhanced in comparison to that of specimen A28, which employed OPC.  
As mentioned earlier, the mineral additives, which secondarily react in the hydrating cementitious composite, require more time than OPC. Even though the mineral additives contain smaller sizes than those present in OPC and fill the voids, the rate of compressive strength development resulting from the bond between various components generally becomes slower. It was observed from the SEM images shown in Figures 4-8 that the particles in the mineral additives could effectively densify the matrix in the cementitious matrix but still dissolved in the matrix of the 7 d cured specimens. However, in the case of CKD, the specimens exhibited direct reactivity and a higher specific surface area than OPC. This led to the rapid growth of hydrates of various sizes and properties that could bind the matrix. However, as reported by [PERSON] [10], the SF substituting more than 20% of OPC decreases the rate of compressive strength development at an early stage due to a limit on the concentration of the CH. Figure 7 also shows that the microscopic SF particles could fill the voids in the cementitious matrix but continued to dissolve in the matrix of specimen F7. Therefore, the 7 d cured specimens, including SF, had lower compressive strength as compared to specimen A7.