## 3 Results
### Relationship between the Water-Soluble Alkalis and the Major Clinker Phases
In the case of the sodium ions, the trend was clearly linear (r\({}^{2}\) = 0.98); whereas in the case of potassium, this was less evident. From the analysis water-solubility data for alkalis, it can be concluded that sodium was significantly better bonded into the hydrated cement paste than potassium, as reported in the literature [8]. However, the large size of the solvated sodium ion hinders its uptake by interlayer sites of the C-S-H gel [26]. Alkali cations compete with calcium cations to compensate for the surface negative charge of the C-S-H gel. Bivalent cations present a higher electrostatic interaction; therefore, high amounts of calcium cations can hinder alkali uptake [28]. On the contrary, the pH increases at high alkali concentrations, and this enhances silanol site de-protonation, leading to a more negatively charged C-S-H gel surface. In addition, the high pH induces a lower calcium concentration in equilibrium with the C-S-H gel. Both occurrences cause an increase in alkali metal binding [28]. Figure 4: Effects of different factors on the soluble alkali concentrations in the pore solutions of hydrated Portland cement pastes: (**a**) C\({}_{3}\)S; (**b**) C\({}_{2}\)S; (**c**) C\({}_{3}\)S + C\({}_{2}\)S; (**d**) CaO/SiO\({}_{2}\) contents in clinker. Some authors found an increase in alkali metal uptake in the presence of C-A-S-H gels, which was attributed to SiO\({}_{2}\) substitution by AlO\({}_{2}\)\({}^{-}\), and the need for the uptake of more positive cations, to maintain charge equilibrium [22]. On the contrary, some studies observed less alkali metal uptake in the C-A-S-H gels than in the C-S-H gels [29], while other studies found no significant difference in the systems, with or without aluminum [26]. [PERSON] and [PERSON] [30] investigated the alkali binding capacities of hydrated Portland cement pastes following the same method proposed by [PERSON] [8], but they used a computer model to predict the amount of alkali ions released by the cement hydration. They also proposed that the C-S-H gel is the only hydration product that is able to bind alkalis, instead of the relative fraction of C-S-H and AF\({}_{\text{m}}\) phases that are formed in the paste. Currently, it is considered that the alkali metal uptake by AF\({}_{\text{m}}\) is much lower than that of the C-S-H gel. Given that, most researchers have focused on alkali uptake by the C-S-H gel. They also suggest that the constant values of alkali distribution ratios imply that the alkali-binding capacity of C-S-H gel is linearly proportional to the alkali concentrations in the pore solution; this was verified for Na\({}^{+}\), but not for K\({}^{+}\)[31]. Therefore, [PERSON] and [PERSON] [31] found a linear relationship between the molar concentration of Na\({}^{+}\) in the C-S-H gel, and its concentration in the pore solution, with a proportionality constant of 0.45. The distribution ratio in cm\({}^{3}\)/g found by other authors was 0.39 [8,21]. In the present work, such a constant is 1.6. On the contrary, they did not find a similar relationship in the case of K\({}^{+}\), and then, they proposed a non-linear relationship that is similar to a Freundlich isotherm, between the molar concentration of K\({}^{+}\) in the C-S-H gel, and the molar concentration of K\({}^{+}\) in the pore solution. In the present work, neither a linear relationship was found. Nevertheless, a good relationship was found between the total K\({}_{2}\)O and K\({}_{2}\)O/SiO\({}_{2}\) ratios (Figure 6). SiO\({}_{2}\) is determined by X-ray fluorescence (XRF) in the Portland cement. It was found that the total K\({}_{2}\)O of the paste correlated well with the K\({}_{2}\)O/SiO\({}_{2}\) ratio; increasing the total K\({}_{2}\)O in the clinker appeared to increase the overall K\({}_{2}\)O/SiO\({}_{2}\) ratio. Figure 5: Relationship between the water-soluble alkalis (Na\({}_{2}\)O\({}_{\text{ps}}\) or K\({}_{2}\)O\({}_{\text{ps}}\) in the pore solution (ps)) of the anhydrous cement and the water-soluble alkalis of the hydrated cement paste after 28 days.