An increasing amount of biomass fly ash is produced because of the increased usage of renewable energy sources. Therefore, novel applications in which biomass fly ashes can be utilized efficiently are required. One interesting option is to use biomass fly ashes in hardening applications, such as earth construction. In this study, the self hardening of ground fly ash from a Finnish power plant with a bubbling fluidized bed boiler that burns forest industry residuals and peat was investigated. Fly ash was ground with three different mills, namely, tumbling ball mill, impact mill, and jet mill, to different particle size distributions. Results showed that the amount of reactive components for ground fly ash samples was more or less the same with the original sample, but the particle size distribution was different. Moreover, the water requirement for self-hardening samples was completely different depending on the grinding mechanism. When particle size distribution was wider (after ball and impact milling), the water requirement was much lower due to the better packing of the particles. A smaller water requirement also led to greater compressive strength even the fly ash samples were chemically and mineralogically the same. After ball milling, obtaining four times higher self-hardening compressive strength, from 5 MPa to 20 MPa, was possible. For jet milled fly ash, the particle size distribution was narrow due to the classifier in the mill, and the water requirement was high. Therefore, the packing and self-hardening compressive strength of jet milled fly ash was lower than that of impact and ball milled samples. (C) 2016 Elsevier B.V. All rights reserved.