# Strength Development of Metakaolin-Based Alkalial-Activated Cement
## 3 Results and Discussion
### Slurry Properties and Setting Time  
During the initial experiments, MKB was found to have a low reactivity towards alkali activation, i.e., the samples did not set within a reasonable time at room temperature when activators were added to MKB as the sole precursor. This low reactivity could be due to the relatively low Al solubility (Figure 5), as sufficient amounts of dissolved Al and Si are required for geopolymerization reactions to occur [5; 34]. It has been found for other low-reactive aluminosilicate precursors, such as mine tailings, that the reactivity can be improved by the addition of minor amounts of another, more reactive aluminosilicate precursor that acts as a \"co-binder\" in the slurry [27; 28]. In other words, the co-binder helps to start the geopolymerization reaction, thereby enabling the low-reactive precursor to react as well. Consequently, another metakaolin sample (MKF) was included in the mixture as a co-binder to ensure the proper setting of the cement slurry. The MKF sample was expected to be considerably more reactive than MKB, due to its lower particle size (Figure 4) and higher Al solubility (Figure 5).  
Figure 6 shows the setting times of the metakaolin-based cement samples with varying amounts of MKF as a co-binder, and where the KOH concentration was 6.5 M. While it was observed that neat MKB samples set after about a week, the setting time decreased significantly with increasing amounts of MKF as a co-binder. Indeed, the addition of at least 6% MKF was needed to ensure a setting time below 24 h, which is required in order to ensure proper cement placement [7]. However, although the addition of MKF led to a shorter setting time, the fineness of the MKF particles (Figure 4) also generated a considerable increase in slurry viscosity. Figure 7 shows the rheology flow curves (shear rate vs. shear stress) for MKB-based slurries with varying amounts of MKF, and where the KOH concentration was 6.5 M. (Regarding the shear stress values in Figure 7, it should be noted that the viscometer had an upper, inherent limitation of 170 Pa, and therefore, all data points with actual shear stress values above this limit were discarded).  
It is beyond the scope of the present paper to perform a thorough rheological analysis of these slurries; however, a simplified, qualitative assessment can be performed. From the shapes of the flow curves, it can be assumed that the slurries follow a Bingham rheology model, where the flow curve is a straight line and the intersection between the flow curve and the y-axis determines the yield stress of the slurry [7]. Consequently, the viscosity of the slurry is determined by slope of the flow curve, where steeper slopes mean higher viscosity. Therefore, it can be seen from Figure 7 that increasing amounts of MKF in the slurry led to steeper curves and thereby a higher viscosity, which may result in slurry pumpability problems. Nonetheless, at 10% MKF and below, the vis  
Figure 5: Solubility of Si and Al for metakaolin samples MKB and MKF after 18 h dissolution in 5 M KOH.  
for this mix design. Therefore, combined with its sufficiently low setting time (Figure 6), 10% MKF was added as a co-binder in the subsequent strength development tests.