# An Investigation on the Synthesis of Alkali Activated Materials from Thermally Modified Clays
## 3 Results and Discussion
### Precursor Characterization
#### 3.2.6 Influence of CRC Incorporation  
In Sections 3.2.1-3.2.5, the baseline MK system was investigated and the optimal synthesis conditions were selected. A binder synthesized with the S/L = 1.0, C/S = 3.0, SH/SS = 1.0, NaOH molarity = 12 M, and cured in C3 conditions (Table 1) was defined as the reference binder (R0; 0.0 wt.% CRC) to investigate the effects of MK replacement by CRC. Figure 8 presents the representative photographs of the alkali activated binders produced with the progressive replacement of MK by CRC (0, 25, 50, 75, and 100 wt.%).  
Figure 7: The apparent density and compressive strength of the alkali activated binders at 7 and 28 days of curing as a function of the curing conditions used as per Table 2.  
Figure 9 shows the AD and CS of the binders with varying CRC dosages after 7 and 28 days of curing. Residual increases in AD could be seen as the CRC content rose (AD\({}_{28\mathrm{dmin}}\), R0 = 1.37 \(\pm\) 0.02 g/cm\({}^{3}\); AD\({}_{28\mathrm{dmax}}\), R100 = 1.45 \(\pm\) 0.01 g/cm\({}^{3}\)), which can be attributed to the presence of iron in the CRC composition. CRC incorporation levels up to 75 wt.% resulted in slight increases in the yearly age CS. An optimal replacement level seemed to exist around 50 wt.% CRC, where the R50 samples recorded the highest CS values after 7 days of curing (12.5 \(\pm\) 0.7 MPa), representing a 32.6% increase in comparison to the pure MK-binders (Figure 9). MK total replacement by CRC seemed to have no deleterious effects on early CS, but considerably increased the mechanical performance after 28 days of curing. Curiously, as the CRC content rises, the theoretical SiO\({}_{2}\)/(Al\({}_{2}\)O\({}_{3}\) + Fe\({}_{2}\)O\({}_{3}\)) molar ratio of the mixtures decreases, thus less polymerized structures would be expected. Differences in the reaction rates arising from the CRC particle size distribution and composition (as discussed in Section 3.1) can explain to some extent the late strength developments in the R100 binders, but do not provide a general definition for the trends observed in the CS nor the WA and OP results.  
Therefore, the mineralogical differences were investigated by means of XRD and the diffraction analysis of the binders prepared with the progressive replacement of MK by CRC (0, 50 and 100%) is shown in Figure 10. Amorphous material (broad convexity located at 2\(\theta\)-22-35\({}^{\circ}\)) could be appreciated in all cases, which confirmed polymerization, even when totally replacing MK with CRC. The differences in the intensity of the quartz peaks found in the starting materials remained after alkali activation, but they appeared as much less remarkable. The dissolution of quartz from CRC is consistent with the increase in the relative intensity between the main peaks of microcline (PDF#77-0135) and quartz (PDF#87-0743). However, some crystalline quartz persisted, showing that some Si units did  
Figure 8: Representative photographs of the alkali activated binders produced with the progressive replacement of MK by CRC. From left to right: (i) 0.0%, (ii) 25.0%, (iii) 50.0%, (iv) 75.0%, and (v) 100.0% MK replacement.  
Figure 9: The apparent density and compressive strength of the alkali activated binders at 7 and 28 days of curing as a function of metakaolin replacement by calcined common clay.  
not partake in the reactions. Muscovite (PDF#80-0743) was detected in both the MK and CRC-containing binders, and anhydrous aluminum silicate phases (Al0.5 Si0.75D2.25, PDF#37-1480) identified in CRC precursors could not be detected, suggesting their dissolution and participation in polymerization reactions. The contribution of these phases can explain the porosity values and CS increases to some extent, but dedicated studies are still needed to better understand the composition and morphology of the reaction products that formed to provide a discretionary explanation of the reaction mechanisms involved.