# Strength Development of Metakaolin-Based Alkalial-Activated Cement
## 3 Results and Discussion
### Strength Development with MKF as Co-Binder  
Strength development is perhaps the most important property of cementitious materials, both for well cementing applications and within civil engineering. For geopolymer cements, the strength development of the aluminosilicate gel binder is dependent upon the Si/Al ratio in the mixture [35, 36, 37], where a Si/Al ratio of about 2 is optimal with respect to the obtained compressive strength. As noted above, all samples were based upon MKB as the main precursor, with MKF as a co-binder. The intention of using the more reactive MKF as the co-binder was that it would dissolve quickly and react with the activator to form an initial K-A-S-H gel in the slurry, thereby ensuring setting and sufficient early strength. Furthermore, this initially precipitated K-A-S-H gel should act as nucleation sites for subsequent K-A-S-H gel formation when the less reactive MKB precursor also dissolves. It should be noted that it was not verified experimentally in this study that the precipitated  
Figure 6: Setting time of MKB-based cement samples with varying amounts of MKF as co-binder. Figure 7: Rheology flow curves for MKB-based slurries with varying amounts of MKF as co-binder. binder phase was indeed K-A-S-H, but due to the low Ca content of the MK precursors this is a credible assumption. Figure 8 shows the obtained compressive strength after 1, 7, and 28 days curing time for the metakaolin-based cement samples with 10% MKF (B90F10) and varying Si/Al ratios in the mix design (the KOH concentration was 6.5 M). It can be seen from the figure that although the early strength (1 day) was relatively low, there was a significant increase in strength from 1 day to 7 days, and also from 7 days to 28 days for most of the samples. The early strength is probably caused by K-A-S-H gel precipitation from the co-binder MKF, whereas the delayed strength increase strongly indicates that MKB also reacted and participated in the strength development. For example, for the sample with a Si/Al ratio of 1.4, there was an observed strength increase from about 8 MPa at 7 days to about 17 MPa at 28 days, which is more than a doubling of compressive strength. Such a significant increase strongly indicates that the low reactive MKB has dissolved and been successfully alkali-activated, and that the initial K-A-S-H gel from the MKF acts as nucleation sites for the subsequent geopolymerization of the less reactive MKB precursor. Moreover, the measured compressive strengths were significantly dependent upon the Si/Al ratio in the slurry. The highest strengths are obtained at Si/Al ratios of 1.4 and above, which is considerably lower than the predicted optimum of 2. However, it should be noted that the calculated Si/Al ratios of the mixtures were based upon the assumption that only the kaolinite parts of the metakaolin samples dissolved, and that the quartz parts remained inert. However, the Si solubility results for MKB (Figure 5) indicate that this assumption is at least partly incorrect. It is likely that the high quartz content in MKB dissolves at least partially over time, thereby increasing the actual Si/Al ratio in the slurry above the simplified, calculated ratios. The actual Si/Al ratio of the samples may therefore be closer to 2 than their denoted values, which would potentially explain the high compressive strengths found for the samples with Si/Al ratios below 2. In addition to the Si/Al ratio, the activator concentration is also important in influencing the strength development of aluminosilicate cements [6,34]. Figure 9 shows the obtained compressive strength after 1, 7, and 28 days curing time for B90F10 cement samples with varying KOH concentrations and Si/Al ratios of both 1.2 and 1.6. The resulting compressive strength was heavily dependent upon the KOH concentration of the activator, where concentrations above 5-6 M led to lower compressive strengths, possibly due to an increased solubility of the K-A-S-H gel.