Study of fly ash based zeolite by using slow positrons

•Zeolites like materials were synthesized from fly ash by hydrothermal method.•Positron annihilation measurements show that increasing of NaOH increases porosity.•The increasing of synthesis temperature diminishes porosity. After coal fly ash activation with alkaline NaOH solution by the hydrothermal method, it was possible to synthesize the zeolites type Cancrinite and Analcime. Slow positron beam experiments have been performed on two set of samples. One in which the NaOH concentration was varied and another one in which the synthesis temperature range was varied. The mineralogical composition was determined by X ray diffraction, the morphology of one of the zeolite samples as compared with the coal fly ash was observed by 3D X-ray imaging. Positron results suggest that increasing the NaOH concentration produces an increase in open volume and/or defects while increasing the temperature in the sample preparation diminishes it.