More and more biomass fly ash is produced because of the increasing usage of renewable energy sources. Therefore, novel applications in which biomass fly ashes can be utilized efficiently are needed. One interesting option is to use biomass fly ashes in hardening applications such as earth construction. In this study, the self-hardening of fly ashes from a bubbling fluidized bed combustion of peat, sludges from the papermaking industry, forest industry residuals, and other wood wastes was investigated. The selfhardening compressive strength of fly ashes after 28 days and one year of hardening was measured for four different fly ash samples. The compressive strength for self-hardening samples was dependent on reactive calcium, aluminum, silicate, and sulfate content, but calcium (Ca) and aluminum (Al) seemed to be the most important components. According to the X-ray diffraction measurements, the main reaction products are ettringite, hydrocalumite, monocarboaluminate, and various types of Ca-Al phases. (C) 2015 Elsevier Ltd. All rights reserved.