Investigation on the roles of solution-based alkali and silica in activated low-calcium fly ash and slag blends

Abstract

An evaluation of strength-gain in binders with equal mass proportions of low-calcium fly ash and slag activated with alkaline solutions containing dissolved silica is conducted. The roles of Si and Na in the activating solution on compressive strength, reaction kinetics, and content and phase composition of reaction products are evaluated. The early reactions within the activated binder are due to the dissolution and hydration of slag. In the presence of silica, there is early precipitation of calcium silicate hydrate (CSH) before the hydration of slag. Silica in the activating solution delays the primary hydration of slag. The reactivity of silica depends on the SiO2/Na2O ratio in the activating solution. A less polymerized form of silica with higher reactivity is produced at a higher Na content in the activating solution. More reactive silica form produces a more rapid early precipitation of CSH following the initial dissolution of slag. Fly ash in the binder provides additional Si and Al to the calcium aluminosilicate hydrate (CASH) formed from the slag reaction. The Na identified in the reaction products is in a water-soluble form and not chemically bound to the CASH. The Na does not contribute directly to the compressive strength. In the long-term, the content and the silica incorporation in the CASH scales with the silica content in the activating solution and it does not depend on the form of silica. Increasing the activating solution silica content results in its larger incorporation in CASH and produces compressive strength enhancement. The form of silica has a negligible effect on the dissolution of fly ash in the binder and ultimate strength achieved.