Physical properties and pore solution analysis of alkali activated fly ash-slag pastes

The technology of alkaline activated materials (AAMs) has become a sustainable alternative to cement production with respect to CO2 emission and energy consumption. This technology is based on by-products usage from industrial energy and manufacturing processes, such as fly ash or blast furnace slag. The long-term performance of AAMs is decisive for their structural application. Generally, physical properties, such as porosity, pore size distribution, internal pore surface and connectivity determine material long-term behavior. Characterization of these properties in activated fly ash, slag and their blends was performed by Mercury intrusion porosimetry (MIP), Nitrogen adsorption and ESEM. Since the microstructure is greatly influenced by dissolution rate of fly ash and slag in selected activator, the pore solution was characterized by inductively coupled optical emission spectrometry (ICP-OES). Mixtures with the following fly ash and slag content (100:0, 70:30, 50:50, 30:70, 0:100 wt.%) have been prepared using sodium hydroxide/sodium silicate solutions. The increase of slag amount produces highly dense and refined microstructure, where only gel pores were identified and measured by Nitrogen adsorption. ICP-OES analysis has shown high concentrations of sodium and sulfur in solutions of slag rich mixtures, while Al, Si, Ca were almost completely absent from solution suggesting their total incorporation into reaction products of pastes.