## 3 Experimental Results
Figure 12: Relationship between mortar compressive strength at different curing temperatures (curing age 7 d). Thus, concrete specimens were prepared using the mortar mix. Among the VG20 mixes that achieved a mortar compressive strength of 123 MPa at maximum curing temperatures of 70 and 80 \({}^{\circ}\)C, the VG20 mix cured at 70 \({}^{\circ}\)C (VG20-70) was selected for further comparison. This selection aimed to reduce energy consumption during curing. Additionally, another set of VG20 concrete specimens was prepared at a maximum curing temperature of 20 \({}^{\circ}\)C (VG20-20). For further comparison, concrete cured at a maximum curing temperature of 70 \({}^{\circ}\)C was also prepared using the SF10 mix (SF10-70), as it exhibited the highest strength among the SF mixes. Figures 14 and 15 illustrate the relationship between the activity index and the maturity index for mixtures containing VG and SF, respectively. The activity index represents the ratio of the compressive strength of the blended mortar with admixtures to that of OPC mortar, which is set as 100. This value is specified in the Test Method for Activity Index Using Mortar in JJS A 6207. An activity index greater than 100 indicates a high reactivity of the admixture. The maturity index accounts for the cumulative effect of temperature and time on concrete exposure. It was calculated based on the Standard Method for Estimating Concrete Strength Using the Maturity Method (ASTM C1074). Specifically, the reference temperature was set at 0 \({}^{\circ}\)C, and the maturity index was determined by calculating the area under the steam-curing program curve, as shown in Figure 9.  
Figure 14: Relation between the activity index and maturity index in VG. Figure 13: Pores on the mortar specimens. (**a**) VG20, curing temperature of 80 \({}^{\circ}\)C; (**b**) SF20, curing temperature of 80 \({}^{\circ}\)C.  
For concretes mixed with VG and SF (except for SF20), an increase in the maturity index correlated with a rise in the activity index. For VG, the activity index of VG10 was higher when cured at 20 \({}^{\circ}\)C, whereas VG20 exhibited a higher activity index than VG10 under steam curing. This phenomenon is attributed to VG's high glass content and the cement's high alkalinity. During mixing and heat curing, these conditions create a high-temperature environment that promotes the frequent dissolution of the glass content, resulting in the formation of robust hydration products. Figure 16 illustrates the relationship between the rate of compressive strength increase and the maturity index from 1 to 7 d of age. The rate of increase in compressive strength is expressed as a percentage, calculated by dividing the compressive strength at 7 d by that at 1 d. It is generally understood that while steam curing enhances early compressive strength, the subsequent increase over time is lower compared to constant-temperature curing [39]. This study further highlights a distinct difference in the rate of compressive strength increase depending on whether steam curing was applied. The results suggest that the rate of increase was higher when moisture was supplied after steam curing. However, since the specimens in this experiment underwent air curing, this effect was observed. Regardless of the maximum cur  
Figure 16: Relationship between compressive strength increase rate and maturity index. Figure 15: Relation between the activity index and maturity index in SF. of increase in compressive strength remained approximately 105% with steam curing, emphasizing the critical role of 1 d compressive strength in steam-cured concrete. Figure 17 illustrates the relationship between the age of the concrete specimens and their compressive strength. The compressive strength values represent the average of three specimens, with standard deviation (SD) indicated by error bars (average value \(\pm\) SD). The compressive strength of the concrete specimens was lower than that of the mortar specimens, recording 129 MPa at 1 d (a 14% decrease) and 135 MPa at 7 d (a 14.5% decrease).