Investigation of Reactivity of Fly Ash and Its Effect on Mixture Properties

The reactivity of fly ash can vary considerably when used as a supplementary cementitious material (SCM). This paper demonstrates a framework for a standard test method to quantify the maximum reactivity of fly ash. The test is based on mixing magent-garde calcium hydraxide (CH) and fly ash in a 3:1 mass ratio and exposing the mixture to 0.5 M potassium hydroxide (KOH) solution. Isothermal calorimetry and thermogravimetric analysis (TGA) am used to measure heat release and CH consumption, respectively, which am clone in conjunction with thermodynamic calculations, as a basis to characterize the maximum reactivity of the fly ash. Fifteen fly ashes were tested using the method, which revealed that the reactivities ranged from 33 to 75%. Thermodynamic modeling was used to demonstrate the effect of fly ash reactivity on the properties of blended OPC-fly ash mixtures with different fly ash replacement levels (0 to 80%) and at various reactivities (0 to 100%). It was shown that the reactivity of fly ash is a critical factor determining durability-related parameters of mixtures such as CH content, C-S-H type and content, and the pH of the pore solution.