Alkali sorption by C-S-H and C-A-S-H gels

In Part I, it was shown that alkali partition between C-S-H gel and an aqueous phase can be represented by a partition function, Rd, the numerical value of which, at constant temperature, is defined by the Ca/Si ratio. This Rd value is constant or nearly so over wide ranges of NaOH and KOH concentrations up to ∼0.3 M. In the present paper, Al has been introduced to form C-A-S-H gels, and the influence of Al on alkali sorption properties was determined: Approximately 6–7% replacement of Si by Al was used. Microprobe evidence is presented to show that the Al is actually in solid solution. Introduction of Al into C-S-H markedly increases Rd, indicating enhancement of alkali binding. The results underpin and quantify the beneficial effects of alkali binding arising from the introduction of aluminous supplementary cementing materials, such as fly ash, into cement pastes.