Microstructural Analysis of Ternary Hybrid Alkaline Binders Containing Slag, Fly Ash and Portland Cement

Although the use of supplementary cementitious materials (SCMs) is a well-known practice with over a century of experience, the mechanism of reaction of SCMs remains not fully understood, and further research is necessary. In addition, the incorporation of alkaline activators into cementitious systems with a low content of Portland cement (PC) is gaining popularity because it resolves issues such as long setting times and slow strength development. These challenges are typically identified when high volumes of SCMs are used. Scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS) analysis can provide insights into the phase assemblage of ternary hybrid alkaline binders. Information concerning the microstructure, as well as the presence of different gels identified as the main binding products in the systems are key for understanding the complex formation of secondary hydration products. In many cases, such secondary hydration products influence the durability of the cementitious matrix when exposed to the environment. This study evaluates ternary cements containing ground granulated blast furnace slag (GGBFS), a low-calcium fly ash (FA), PC, and sodium sulfate as an activator. The chemistry and morphology of remnant GGBFS and FA particles provided important information in terms of the ongoing reactions. Calcium silicate hydrates with partial substitution by aluminium from the SCMs were identified as the main reaction products forming in these systems.