Thermal degradation of potassium-activated ternary slag-fly ash-silica fume binders

Abstract

Alkaline activation of ternary precursors composed of ground granulated blast-furnace slag (GGBS), fly ash (FA), and silica fume (SF), offers more versatile mixture design of alkali-activated binders with improved performance and optimum resource utilization. In this work, the influence of relative proportions of GGBS-FA-SF precursors on the ambient- and high-temperature properties of alkali-activated ternary binders (AATB) with potassium activators is investigated. Nine AATB mixtures are formulated using the simplex centroid design method, and their fresh and hardened properties, as well as microstructure information, are mapped on ternary contour plots. The results show that AATB with above 30% FA fraction gains strengths upon exposure to 200 °C, likely ascribed to heat-induced densification and strengthening of crosslinked potassium-aluminosilicate-hydrate (K-A-S-H). Nevertheless, regardless of the relative proportions of GGBS-FA-SF precursors, AATB exhibits consecutive strength decline as temperature further rising to 600 °C, resulting from extensive dehydration cracking and deleterious phase transformation. In particular, the formation of akermanite and anorthite is triggered, respectively, in GGBS- and in FA-enriched AATB systems at temperatures above 600 °C. An increased FA inclusion strengthens the volumetric stability of AATB upon heat exposure, while AATB with a higher SF fraction suffers more aggravated mechanical degradation. Besides, considerable softening and deformation occur in AATB containing above 30% FA at 800 °C, indicating a lower sintering temperature of K-A-S-H than calcium-aluminosilicate hydrate (C-A-S-H).