Evaluation reactivity of synthetic fly ash in pozzolanic process from glassy fly ash lechability

Abstract

Fly ash from different sources possesses different reactivity, owing to variations in chemical composition. This variable quality of fly ash is a major problem that hinders fly ash utilization. The current fly ash classification, ASTM-618, classifies fly ash into only two types by the sum of the oxides: SiO2, AI2O3 and Fe2 O3 - It is not enough to classify fly ash, which has a broad chemistry range. Thus this study is aimed to investigate the influence of the chemical constituents that may control the ash reactivity. These major constituents of concern are: SiO2, AI2O3, Fe2O3, MgO, CaO, Naz9, and K2O. However, in practice, it is difficult to separate fly ash into different constituents. Thus, synthetic fly ashes were prepared as glasses with specified Ca0 /Na2O +K2O ratios. They were immersed in simulated concrete pore solutions in a single pass flow through (SPFT) leaching unit. The SPFT technique was used instead of a closed system to maintain a constant degree of undersaturation and the desired pH. The concentrations of oxides leached from the synthetic ashes were measured by inductive coupled plasma method (ICP). It was found that the leachability of glass rapidly increased when the pH increased. This rate was also used for assessing the effect of Ca0/Na2O) + K2O ratio on reactivity of synthetic glass. Glass with high Ca0 /Na2 O +K2O ratios has less reactivity. The correlations between pozzolanic reactivity and glass composition was shown as a contour line on the ternary plot with the axes, A1203+Si02+Fe203, CaO and Na2O+K2O, which are known as glass network formers, intermediates, and network modifiers. This might be primary criteria for assessing the reactivity of real fly ash.