## 2 Proportioning of Alkali-Activated Composites' Cementitious Materials
### Test Results and Analysis
#### 2.3.1 The Influence of the Slag-to-Fly Ash Ratio on the Strength of Alkali-Activated Composites
As alkalinity increases, the UCS of specimens cured for 3, 7, and 28 days initially rises and then falls; the 28-day UCS increased by 12.8%, 5.3%, and 42.3%, respectively, for each group relative to the previous one. At 10% alkalinity, the highest UCS values for 3, 7, and 28 days were recorded in group B at 43.32 MPa, 45.22 MPa, and 56.33 MPa, respectively. Figure 3: Group B specimens’ unconfined compressive strength at various ages. Figure 2: Group A specimen’s unconfined compressive strength at various ages. At 5% and 12.5% alkalinity, UCS showed an initial decrease followed by an increase over the curing period. At 7.5% and 10.0% alkalinity, UCS consistently increased over time. Specifically, the 7-day UCS of group B specimens varied by \(-\)2.02%, 7.42%, 4.39%, and \(-\)7.20% compared to their 3-day UCS values. After 3 days of curing, the UCS of alkali-activated composites reached at least 75% of their 28-day strength. By 7 days, their strength generally met or exceeded 75% of the 28-day strength, specifically 77.84%, 79.56%, 80.28%, and 88.60%, although the strength increase from 3 to 7 days was minimal. The phenomenon observed can be attributed to the fact that, within a certain range, an appropriate increase in alkalinity is characterized by an increase in the alkali components within the reaction system. It facilitates a more thorough dissolution of silicoaluminate raw materials. Consequently, an optimal level of alkalinity not only accelerates the alkali activation reaction rate between alkali activators, slag, and fly ash, but also serves as a crucial prerequisite for ensuring the cohesive effectiveness of alkali-activated composites. An appropriate concentration of hydroxide ions (OH\({}^{-}\)) accelerates the depolymerization and polymerization of the silicoaluminate components in slag and fly ash, thereby increasing the strength of the alkali-activated composites. However, excessive alkalinity leads to an increase in OH\({}^{-}\) within the alkali-activated composites system, resulting in the premature deposition of silicate products and inhibiting the formation and reaction rate of SiO\({}_{4}\) and AlO\({}_{4}\) tetrahedra. In addition, the presence of unreacted Na\({}^{+}\) bound to the hydrated layers of slag and fly ash prevents the further dissolution and release of reactive SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), inhibiting the activation reaction between slag and fly ash and the formation of alkali-activated composites, thereby reducing their strength. On the other hand, if the alkalinity is too low, the alkali-activated composites' reaction suffers from insufficient alkalinity to break the bonds between the metal and oxygen elements, resulting in a lower dissolution rate of Si and Al elements. This incomplete reaction process within the alkali-activated composite binder results in reduced strength. It can therefore be concluded that an alkalinity of 10.0% is optimal to achieve the best cementing effect with slag and fly ash. 3.3 The Influence of the Alkali Activator Modulus on the UCS Behavior of Alkali-Activated Composites  
Figure 4 shows the effect of various alkali activators on the unconfined compressive strength of alkali-activated composites over different ages. The trend indicates that, as the modulus of the alkali activator increases, the UCS of the alkali-activated composites initially increases and then decreases after 3, 7, and 28 days of curing. The peak strength across all ages groups is achieved when the alkali activator modulus is 1.3, resulting in UCS values of 43.32 MPa, 45.22 MPa, and 56.33 MPa, respectively. As the alkaline activator modulus increases, the 28-day strength of each alkali-activated composite group increases by 4.96% and 9.21%, before decreasing by \(-\)8.60%. It is noteworthy that, at a curing age of 3 days, the UCS reaches over 55% of the 28-day-cured body strength. By 7 days, the strength generally surpasses 75% of the 28-day-cured body strength, specifically 71.95%, 76.10%, 80.28%, and 77.21%.