Mechanical and Microstructural Changes of Alkali-Activated Binder Due to the Leaching Process

Abstract

This paper investigates the resistance of alkali-activated binder to leaching. This type of binder was obtained by alkali-activation of blast-furnace slag. The slag was activated by aqueous solution of sodium silicate. The resistance of alkali-activated slag to leaching was studied on mortar prisms, with the use of an accelerated test with concentrated solution (50%) of ammonium nitrate at 20 ºC. Strength of mortar was determined after 30, 60, and 90 days of exposure to ammonium nitrate solution. Portland-slag cement (CEM II/А-Ѕ 42.5R) was used as the reference material. After 30 days of exposure to ammonium nitrate solution the strength of alkali-activated slag, with respect to control samples, was slightly reduced (about 5%). At the same time the reference Portland-slag cement sample showed drastic strength reduction (over 50%). After 90 days of exposure the reduction of Portland-slag cement strength was over 80%. In the case of the reference cement sample, significant loss of portlandite, i.e. calcium, was established by X-ray and scanning electron microscopy (SEM/EDS) analysis. The loss of calcium lead to a progressive decalcification of calcium–silicate–hydrates (C–S–H), drastic reduction of strength and destruction of the samples studied. In comparison with the reference cement sample decalcification of alkali-activated slag was a very slow process, whereby a minor change in the structure was observed. Based on the results of investigation it can be concluded that alkali-activated slag has showed high resistance to leaching