# An Investigation on the Synthesis of Alkali Activated Materials from Thermally Modified Clays
## 3 Results and Discussion
### Precursor Characterization
Figure 1: Diffraction analysis of the starting raw materials used to synthesize AAM binders: (**a**) comparison between the local red clay before and after calcination; (**b**) comparison between the commercial metakaolin (MK), local common clay calcined at 800 \({}^{\circ}\)C (CRC), and fumed silica (SF). \begin{table}
\begin{tabular}{l c c c c c c c c c} \hline  & **LOI \({}^{*}\)** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **CaO** & **Fe\({}_{2}\)O\({}_{3}\)** & **MgO** & **TiO\({}_{2}\)** & **K\({}_{2}\)O** & **Na\({}_{2}\)O** & **Others** \\ \hline
**MK** & 1.9 & 57.0 & 34.5 & 0.2 & 1.2 & 0.2 & 1.8 & 0.4 & nd \({}^{*}\) & 2.9 \\ \hline
**CRC** & 1.2 & 56.0 & 33.9 & 0.2 & 4.2 & 0.5 & 0.4 & 3.4 & 0.1 & 0.3 \\ \hline
**SF** & 1.3 & 97.4 & 0.5 & 0.2 & nd \({}^{*}\) & 0.1 & nd \({}^{*}\) & 0.2 & 0.2 & 0.2 \\ \hline \end{tabular} \({}^{*}\) LOI: loss on ignition, nd: less than quantification limit.  
\end{table} 
Table 3: The chemical composition of the raw materials (% by mass). The particle size distribution of the precursors is provided in Figure 2. It can be seen that SF showed a coarser particle size distribution when compared with the remnant precursors. The calcined clays presented finer grain size distributions, with a smaller d10, d50, and d90 being found in CRC (Table 4). Among other factors, the precursors' particle size distribution determines the surface exposed to the alkaline medium, and thus influences the precursors' reactivity, the reaction kinetics, and the products formed. The finer particles of CRC precursors can possibly compensate to some extent for the lower degree of structural disorder and amorphous phases present in CRC when compared to MK (Figure 1b), thus resulting in precursors with potential akin to alkali activation technology. This mechanism seems to be corroborated by the strength and porosity results discussed in Section 3.2.6. In Figure 2 and Table 4, it can also be seen that the RC and CRC data points largely overlap with no significant differences being observed in the particle size distribution. Such similarity between the CRC and RC samples shows that the calcination process occurred without significant agglomeration of the clay particles, and thus without showing signs of sintering. It should be mentioned here that no grinding process was conducted after calcination to avoid mechanically imposed changes in the particle size distribution and reactivity, which would be intermingled in the interpretation of results. Overall, and despite the chemical, mineralogical, and physical differences regarding the benchmark precursor (MK), the results showed auspicious characteristics for the alkali activation of the examined local clays after calcification. (CS) of MK-binders produced with varying solid-to-liquid ratios (S/L ratio) while keeping other synthesis parameters constant (Table 1). After 7 days of curing, the SL0.90 samples recorded the highest CS values (9.8 \(\pm\) 0.7 MPa) whereas SL1.00 showed the lowest CS (7.1 \(\pm\) 1.2 MPa). The SL1.10 binders showed intermediate CS values of 9.2 \(\pm\) 2.0 MPa but considerable standard deviations were recorded in all binders, which can be attributed to the specimens' early stage of maturation and ongoing polymerization reactions [17]. Later strength gains were observed in all binders, with SL1.00 being the most performant in terms of CS at 28 days. AP showed the opposite trend, with significant reductions being observed as curing progressed. Such reductions in AD can be mostly related to free water egress, as suggested by the linear relationship between solution load and AD at 28 days. Conversely, CS cannot be described as a linear function of the S/L ratio (nor of AD), as an inflection point appears to exist around a S/L ratio of 1.0. At a higher S/L ratio, AAM pastes presented a less workable consistency and increases in viscosity that could result in air entrapment within the structure in the samples.