Phase Analysis of Alkali-Activated Slag Hybridized with Low-Calcium and High-Calcium Fly Ash

Tao Jiang and Ying Jin
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Tao Jiang: Test Station for Mechanics and Structural Engineering, National Center for Materials Service Safety, University of Science and Technology Beijing, Kunlun Road 12, Beijing 102206, China
Ying Jin: Test Station for Atmospheric Environment Corrosion, National Center for Materials Service Safety, University of Science and Technology Beijing, Kunlun Road 12, Beijing 102206, China

Sustainability, 2022, vol. 14, issue 7, 1-13

Abstract: This paper investigates the hydrated phase assemblage, microstructure, and gel composition of sodium hydroxide (NaOH)-activated fly ash–slag blends with either low-calcium or high-calcium fly ash. The results show that the nature of precipitated calcium–aluminosilicate–hydrate (C-A-S-H) and alkali aluminosilicate-hydrate (N-A-S-H) depends on the fly ash composition and slag-to-fly ash ratio. However, regardless of fly ash composition and slag-to-fly ash ratio, a universal linear compositional relationship exists between Al/Ca ratio and Si/Ca ratio in precipitated gels. This indicates that there exists a structural limitation on the incorporation of Al 3+ for Si 4+ in the tetrahedral silicate of C-A-S-H, N-A-S-H, or metastable N-C-A-S-H gels. In a hybrid slag–fly ash system, the framework structure of precipitated gels is an assemblage of aluminosilicate units with heterogeneous Ca 2+ and Na + distribution. The amount and reactivity of calcium and alkalis seem to play a critical role in determining the structure and properties of precipitated gels in hybrid systems. The low cementitious capability in alkali-activated high-calcium fly ash may be attributed to the unstable N-C-A-S-H gel structure with concurrent high Na and Ca contents.

Keywords: alkali-activated binder; geopolymer; alternative cementitious materials; fly ash; blast furnace slag (search for similar items in EconPapers)
JEL-codes: O13 Q Q0 Q2 Q3 Q5 Q56 (search for similar items in EconPapers)
Date: 2022
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