Alkali activated waste fly ash as sustainable composite: Influence of curing and pozzolanic admixtures on the early-age physico-mechanical properties and residual strength after exposure at elevated temperature

TLDR: In this paper, three mineral admixtures, namely blast furnace slag, silica fume and metakaolin, were used to synthesize binary alkali activated binders whose major volumetric fraction was waste fly ash.

Abstract: Waste fly ash, referred in literature also as “weathered”, represents a major management issue for coal-fired power plants. The low qualification of this relevant fraction of produced fly ash is not adequate for recycling in structural concrete. Non-structural applications may represent an economically feasible solution to maximize the recovery of coal fly ash from power plants. A potentially valuable recycling track may be represented by alkaline activation for the production of sustainable precast elements based on geopolymeric binders. In this work, three mineral admixtures, namely blast furnace slag, silica fume and metakaolin, were used to synthesize binary alkali activated binders whose major volumetric fraction was waste fly ash. Three curing temperatures (20, 40, 60 °C) were also considered. Early age kinetics and properties development were assessed by means of strength development and ultrasonic pulse velocity. Residual compressive strength after exposure at high temperature was also assessed in order to highlight typical geopolymeric thermal resistance which is generally observed for higher value raw materials. Furthermore, microstructural analysis was carried out by means of scanning electron microscopy. Silica fume exhibited an increasing detrimental effect (formation of agglomerates/partial foaming) with early age curing temperature, while blast furnace slag and metakaolin revealed to be effective admixtures without strict need of higher curing temperatures.