## 1 Introduction  
An important durability aspect of concrete structures is their resistance to the alkali-silica reaction (ASR) [1]. This is a deleterious process produced by several chemical reactions between amorphous silica present in some reactive aggregates and sodium, potassium, and hydroxyl ions, among others present in the concrete pore solution. Such reactions produce a gel that expands with the absorbed water. This reaction evolves over several steps [2]; the first one consists of the scission of siloxane bonds (Si-O-Si) in aggregate by hydroxyl ions to generate alkali silicate gel and silicic acid (silanol bonds), followed by the alkali-silica gel expansion due to the water absorption. Three conditions at least have to be satisfied for the concrete alkali-silica reaction (ASR) to take place. First, a high alkali amount must be in the concrete pore solution, second, an amorphous silica aggregate must be present in the concrete, and third, a high humidity level in the environment much be maintained over time, in order to ensure more than 80% relative humidity in the concrete pores [3].  
The alkalis concentration in the pore solution of concrete is also a key parameter, because it is one of the major factors for the likelihood of the immobilization of contaminants with Portland cement, and on the reactivity enhancing of coal fly ash and ground granulated blast-furnace slag when are used as Supplementary Cemertitious Materials (SCM). Also, the hydroxyl concentration in the pore solution, in equilibrium with the alkali concentration, was important, due to its influence on the passive layer formation around steel reinforcement, to prevent its corrosion. Thus, the partition of alkali ions in the cement paste hydrates and in the pore solution is of great importance.  
Usually, atomic absorption spectrophotometry (AAS), flame emission spectrophotometry (FES) or flame photometry is the standard method for the determination of sodium and potassium in cement and concrete components [4; 5; 6; 7].  
[PERSON] [8] proposed an alkali partition theory, and a method for determining the alkali binding capacities of hydrated Portland cement paste. Therefore, part of the alkali ions are bound in the C-S-H gel and monosulphate phases (AF\({}_{\text{m}}\)) in hydrated cement pastes and the others remain in the pore solution. Following this method, the moles of alkali ions released by the Portland cement hydration which become available for the pore solution could be predicted.  
In this work, we develop a simple estimation procedure for the determination of the available sodium and potassium in the pore solution of concrete, based on the [PERSON]'s empirical theory [8]. The proposed procedure will allow predicting the development of alkali ion concentrations and in turn, the hydroxyl concentration in the pore solution.  
This method considers the chemical composition of the Portland cement and the soluble alkali metal content. Then, the fixed alkalis can be calculated. The developed estimation procedure may be applied for the analysis of common Portland cement, CEM I, so it would be suitable for use in the procedure for routine classification of these materials with regard to their usage in engineered barriers for waste confinement design.