The role of sodium during the hydration of alkali-activated slag

This paper presents the results of an investigation into the role of sodium cations during the hydration of alkali-activated slag (AAS). This subject was studied by a combination of powder X-ray diffraction, high-resolution solid-state nuclear magnetic resonance spectroscopy and atomic absorption spectrum analysis. It was found that: there is no evidence that sodium cations take part in cation interchange or play a catalytic role in the early stages of hydration; no sodium cations of a detectable quantity are chemically incorporated into the structure of C-S-H solid solution; and no hydrates of zeolitic type or other crystalline phases containing sodium are formed in the AAS pastes studied. It has been concluded that most sodium in AAS pastes is present in pore solution and may be partly sorbed by C-S-H, and that sodium cations may only have the effect of balancing and maintaining OH⁻ anions during the AAS hydration. The structural incorporation of sodium cations in C-S-H is deemed very limited.