This paper describes the effect of particle size of fly ash and solid to liquid ratio on the microstructure and flexural strength of fly ash geopolymer for coating applications. Sodium hydroxide concentration of 12 M was used for the preparation of geopolymers. The mechanically activated fly ash was divided into two parts based on the particle size (MFA A (11.81 μm) & MFA B (8.59 μm)). The decrease of particle size of fly ash and increase of S:L ratio produced dense and compact geopolymer which is due to the increase of reactivity of fly ash with the decrease of particle size and increase of water content. The decrease of particle size increased the flexural strength of geopolymer from 13.39 MPa (OFA-1) to 23.84 MPa (MFA B-III). The decrease of S:L ratio showed irregular trend with optimum flexural strengths of 13.39 MPa, 16.74 MPa, and 23.84 MPa obtained for OFA-1, MFA A-II, and MFA B-III respectively. The optimum flexural strength of 23.84 MPa was obtained. The mechanical activation is a useful technique to increase the reactivity of fly ash and produce dense and compact geopolymer with higher flexural strength. © 2023, Association of American Publishers. All rights reserved.