Microstructures and mechanical properties of sodium-silicate-activated slag/co-fired fly ash cementless composites

Abstract

Our objective in this study was to promote the recycling of industrial by-products by combining ground granulated blast-furnace slag (G) and circulating fluidized bed co-fired fly ash (FA). We then added sodium silicate as an alkali activator (A) to a combination of G and FA (GFA) to create a novel cement-free cementitious material (AGFA). One of our main research aims was to explore the influence of the activator on GFA in terms of working and mechanical properties and drying shrinkage behavior. Hydration products were analyzed using XRD and SEM-EDS. The setting time of GFA was longer than that of AGFA, and the water absorption and fluidity were both higher. Adding sodium silicate to the GFA was shown to greatly improve mechanical properties through the formation of hydration products containing C–S–H, C-A-S-H, and C–N-A-S-H gels, which intertwined to form a dense microstructure. Compared to AGFA, the GFA was less affected by dry shrinkage, due to the expansive formation of ettringite (from anhydride) during the hydration process. Note that the reaction of the alkaline agent in AGFA with the anhydride in FA reduced the amount of ettringite that formed. The alkaline agent also promoted the depolymerization of vitreous G and FA particles, resulting in the formation of large quantities of gel, the process of which consumed much of the free water, resulting in further shrinkage. Further, it provides a good feasibility and application prospect in environmentally friendly building material of GFA and AGFA cementless composites and is expected to replace traditional cement-based composites.