Alkalis in Portland cement

The predominant source of alkalis in concrete is the Portland cement. As noted in Chapter 2, Portland cement contains relatively minor amounts of both sodium (Na) and potassium (K) and these are usually expressed as oxides, Na2O and K2O, in a typical chemical analysis. Since these alkalis tend to behave fairly similarly in Portland cement concrete, it is common practice to convert the potassium oxide component of a cement to an equivalent amount of sodium oxide and report the total equivalent sodium oxide content, which is calculated as follows:

Na2Oe = ˆ Na2O + ‡ 0.658 x K2O

where Na2Oe is the equivalent sodium content, Na2O is the sodium oxide content, K2O is the potassium oxide content and the 0.658 is a conversion factor based on the relative molecular weights of the oxides (Na2O/K2O). A typical chemical analysis for a Portland cement is shown in Table 7.2. The equivalent alkali content of this particular cement is 0.63% Na2Oe (ˆ 0.15% ‡ 0.658 x  0.73%) and the equivalent alkali content of Portland cements generally ranges from about 0.20 to 1.20% Na2Oe.

Although the alkalis represent a small component of Portland cement, they dominate the pore solution chemistry of concrete and, as discussed in detail in Chapter 2, after the first day of hydration at normal temperatures, the pore solution has become essentially a mixed solution of NaOH and KOH with low levels of other dissolved ionic species. The concentration of alkali metal hydroxides in solution depends on a number of factors, particularly the alkali content of the cement, the water/cement ratio (w/c) and the degree of hydration, and typically ranges from about 0.15 to 0.85 mol/l, corresponding to pH values ranging from approximately 13.2 to 13.9.

Helmuth et al. (1993) developed an empirical equation for predicting the hydroxyl ion concentration of the pore solution of mature Portland cement pastes as follows:

[OH-] = 0.339(Na2Oe)/(w/c) + 0.022 -+  0.06 mol/l

For example, equation 7.2 predicts a hydroxyl ion concentration of 0.43 mol/l in the pore solution of a hydrated cement paste produced with a cement with an alkali content of 0.60% Na2Oe and w/c ˆ =0.50.

Alkalis may be contributed from other concrete-making materials (e.g., supplementary cementing materials, aggregates or admixtures) or external sources (e.g., deicing salts or seawater) and this is discussed in Section 7.7.2.

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