The behaviour of the mixture of MgO and SiO2 with water was studied and analyzed by DTA-TGA-EGA and XRD after ageing for 1,3, 10,30 and 180 days at 20 degrees C. The samples also were aged at 30 degrees C and 40 degrees C for 1 and 3 days. Additionally, after 180 days the samples were heated in 500 degrees C, 700 degrees C and 900 degrees C for 24 h. The paste was composed of magnesium oxide and rnicrosilica in 1:1 molar ratio and water in addition of 50%wt. of oxides. The DTA-TGA-EGA curves in the temperature range 20-950 degrees C were analyzed. The aim of this work was to study the reaction and phase composition in the MgO-SiO2-H2O phase system. This system has wide applications in refractory materials, especially in basic castables. The compounds of this system may constitute an innovative bonding phase in a new generation basic castables. (C) 2013 Elsevier B.V. All rights reserved.