Study on the influences of silica and sodium in the alkali-activation of ground granulated blast furnace slag

Slag is activated with alkali-silicate solutions made with nano-silica and waterglass. The reaction kinetics and the reaction products formed are evaluated for different silica and sodium contents. Isothermal calorimetric measurements with in-situ mixing reveal increased early reactivity with added silica, while the main hydration reaction is delayed. The presence of Na with dissolved silica in the activating solution does not influence the initial dissolution and early reactivity of slag. At the same time, it has an accelerating effect on the main hydration peak of slag. Dissolution studies of slag in alkaline solution with added silica indicate the immediate formation of calcium silicate hydrates after dissolution contributes to the early reactivity. The enhanced early reaction product formation in the presence of dissolved silica leads to the very rapid development of the internal structure. Rheological measurements indicate an early loss of workability due to increased early reactivity. The increase in time for the main hydration reaction of slag in the presence of added silica, however, delays the setting behavior. The primary reaction product in alkali-activated slag is calcium silicate hydrate with aluminum uptake, C-(A)-S-H. The addition of silica in the activating solution leads to its larger incorporation in the C-(A)-S-H and higher compressive strength. The sodium content in the activating solution does not contribute to the ultimate strength. The sodium in the alkali-activated slag is present in the form of a water-soluble amorphous product