Abstract: The phase composition of the pressed stone of lime-fly ash mixture after autoclaving for 8.50 and 100 h at 0.8 MPa isotherm with no addition or with 1 and 2% Na2SO4 is represented by Na2SO4 gel-like C-A-S-H, aluminum substituted tobermorite, hydrogarnet katoite. Adding of 1 and 2% Na SO makes Na2SO4 no qualitative change in phase composition but significantly increases the rate of phases formation. Thus, 2% Na SO contributes to a substantial increase in the synthesis of Al-tobermorite but reduces the C-A-S-H phase Na2SO4 formation. Katoite hydrogarnet content remains unchanged. Strengthof the stone is always directly proportional to the content of a gel C-A-S-H phase. Its content is proportional to the number of Al-tobermorite in non-additional mixtures but does not correspond to the content of tobermorite in compositions with addition of 2% Na2SO4 .