PURPOSE: To improve the strength, impact resistance and heat resistance of the reinforced inorganic hardened product and prevent the generation of cracks by adding flied ash wool as a fiber reinforcing material to an inorganic hardening material.

CONSTITUTION: The flied ash wool is obtained by melt-spinning flied ash at a high temperature of a thousand and several hundred degree into fibers, the flied ash being produced on the combustion of coal. In order to constantly maintain the quality of the fibers, it is desirable that the components of the flied ash as the raw material are stable, and the flied ash having a low silicon dioxide content and a high alumina content is preferable because it is considered that the flied ash has a small viscosity at a high temperature and can be melted at a low temperature. The surface of the reinforced inorganic hardened product is smooth and glassy, and has an average particle diameter of approximately 3μm. The lengths of the fibers are 20-300mm. The content of the flied ash wool in the inorganic hardened product (e.g. cement, gypsum, lime, or pozzolan hardened product) is usually 5-80wt.%, preferably 15-40wt.%, on the strength of the hardened product. When lightweight sand is especially used as an aggregate, the use of the flied ash wool as the reinforcing material is effective.