DTA-TGA in combination with XRD and wet chemical techniques was employed to explain the effect of chemical composition of the starting raw mix, maintaining the Al2O3 content constant, on the formations of hydrogarnet and Al-substituted 11 Angstrom tobermorite. Three mixes with bulk Ca/(Al + Si) atom ratios of 1.0, 0.8, and 0.6 in the industrially important CaO-Al2O3-SiO2-H2O system were investigated following hydrothermal treatment at 180 degrees C for up to 23 h. The stability of hydrogarnet decreased with increasing silica content in the bulk composition and was extended in the presence of portlandite and lime-rich calcium silicate hydrate. The formation of Al-substituted 11 Angstrom tobermorite accelerated with decreasing bulk Ca/(Al + Si) atom ratio. Changes in the exotherm profiles, due to the crystallization of beta-wollastonite, with reaction time provided evidence for increasing incorporation of Al into the 11 Angstrom tobermorite crystal lattice. The concurrent decrease in the hydrogarnet amount provided the proof that this phase supplied most of the Al for the formation of Al-substituted 11 a tobermorite.