Mechanisms of enhancement in early hydration by sodium sulfate in a slag-cement blend – Insights from pore solution chemistry

The action of added sodium sulfate (Na2SO4) leading to increased reactivity and early strength in slag: cement binders remains unclear. In this study, early hydration reactions and resultant compressive strength in a 50:50 slag:cement binder in the presence of Na2SO4 were investigated. Early strength increases in the presence of Na2SO4 were shown to be due to a combination of increased alite hydration and increased slag dissolution. Increased alite hydration was due to neither reduced dissolved Al concentration nor increased alite under-saturation but related to increased ionic strength. Increased slag dissolution was associated with both increased pH and decreased Ca activity with the two being connected through the portlandite solubility limit. Na2SO4 was shown to substantially enhance slag dissolution at fixed pH 13 with this action attributed to greater under-saturation of slag as a result of ettringite formation. Na2SO4 was shown to be superior to alternate activators in a slag:cement binder.