## 3 Results and Discussions
### Fresh State and Rheological Characteristics
#### 3.1.3 Yield Stress of Mortars  
Figure 7a,b present the shear stress-strain relationship for binary and ternary mortar mixes. The starting point of the flat portion of the curve indicating no change in shear stress represents the material's yield stress. The yield stress for binary and ternary mortars varied from 62 to 161 Pa and 51 to 77 Pa, respectively. The control mortar (FPcM) obtained the lowest yield stress of 6.6 Pa due to the formation of shorter C-S-H linkages compared to long chains of geopolymerization/alkali activation consisting of Si-Al with shared oxygen atoms. The shear modulus is the ratio of shear stress to shear strain for pure shear within the elastic/proportional limit. The mortars with reagent one (CSM1, CSM1N, CFSM1 and CFSM1N) exhibited higher shear modulus (164 to 373 Pa/rad), resulting in more significant yield stresses in comparison to the mixes with reagent two (CSM2, CSM2N, CFSM2 and CFSM2N) and the control mix (FPcM) presented in Table 4. This can be attributed to the higher \({}^{\text{Si}}\text{O}_{2}/\text{Al}_{2}\text{O}_{3^{\prime}}\) ratio in the mixes incorporating reagent one, leading to comparatively more and the faster formation of Si-Al linkages and colloidal Si-O-H-Na complexes due to the high alkalinity (pH = 14) of sodium metasilicate. A similar formation of such aggregate complexes of few nanometers in dimension was seen in previous studies utilizing alkaline silicate solution-based reagents [33; 36]. Additionally, the higher L/S ratio in mixes with reagent two compared to their counterparts with reagent one reduced the interparticle frictional forces and resulted in lower yield stresses. This behavior is consistent with the observations reported in a previous study where the yield stress of fly ash-Na/K-silicate suspensions was found to be influenced by interparticle forces [35]. However, the mix 'CSM2N' had the highest shear modulus owing to the comparatively 5% higher GGBFS content than its CSM2 counterparts. This is because the characteristic angular particle shape and high Blaine fineness of GGBFS provided a higher surface area for alkali activation and quicker reaction products comprising both C-S-H and C-A-S-H. This led to the faster densification of the reaction products resulting in higher yield stresses.  
The ternary mortars exhibited lower yield stress varying from 51.2 to 77.4 Pa and correspondingly lower shear modulus (Table 4) than their binary counterparts. The presence of fly ash class F particles with a characteristic round shape in the mix composition enables quicker shear deformation of the material at lower stress values by inducing a ball-bearing-like effect (causing lower inter-particle friction) in the system. Additionally, high fly ash class F content of 55% by mass in the control mix led to lower yield stress for the same reasons mentioned above. Similar observations were reported in a previous study on fly ash-metakaolin-based two-part geopolymer grout, where an increase in the fly ash content decreased plastic viscosity and yield stress. This was attributed to the dense particle distribution and, thus, higher packing density of fly ash. Therefore, the substitution of metakaolin with fly ash improved the packing density and, eventually, enhanced the workability [37].  
A decrease in yield stress was noted with the increased slump flow spread for the mortar mixes as per the equation (Slump flow = \(-1.6796\)\(\times\) yield stress + 397.02) presented in Figure 8, indicating a linear correlation with a coefficient of 0.70. Similar behavior was reported in a previous study on Portland cement-based mortar, where the slump flow was influenced by the yield stress determined using theoretical simulations based on the Bingham model [22].