Alkali binding in cement pastes: Part I. The C-S-H phase

Abstract

The binding of sodium and potassium into cement paste influences the performance of concrete: for example, alkali balances between solid and paste constituents and pore fluid affect the potential for reaction with alkali-susceptible aggregates. However, quantification of the binding potential into paste solids has proven to be difficult, although much empirical data are available from pore fluid analyses. In this study, single-phase homogeneous C-S-H phases have been prepared at Ca:Si molar ratios of 1.8, 1.5, 1.2, and 0.85 and reacted with six alkali hydroxide concentrations, both NaOH and KOH, between 1 and 300 mM, giving a grid of 48 alkali concentrations and Ca:Si ratios. A steady-state alkali partition is attained in less than 48 h. A distribution coefficient, R d, was calculated to express the partition of alkali between solid and aqueous phases at 20°C. The numerical value of R d is independent of alkali hydroxide concentration and depends only on Ca:Si ratio. Approximate reversibility is demonstrated, so the R d values are constants of a C-S-H over wide ranges of alkali concentration. The trend of R d values indicates that alkali binding into the solid improves as its Ca:Si ratio decreases.