Alkali Content in Cement and Pore Solution

Cement contains small amormts of alkali oxides (Na20 and K2O), deriving from the minerals from which it is produced. Following hydration these pass into solution as hydroxides  

The alkali content in cement is generally expressed as an equivalent percentage of Na20 by mass of cement. Since the molecular weights of Na20 and K2O are respectively 62 and 94, the equivalent percentage of Na20 is calculated with the formula  

Besides the cement, the other components of concrete (mixing water, aggregates, admixtures and additives) can contain alkalis, although generally in more modest levels. Of these alkalis, only those dissolved in the pore solution can react with aggregates. 

The alkali content in concrete (expressed as kg/m ) is determined by multiplying the percentage of Na20eq of each component in the mixture by its relative proportion (in kg/m ) and dividing by 100. Concrete that contains more than 3 1 /m of alkali can be considered to have a high alkali content [23]. 

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