The kinetic of the reaction of sand with aqueous NaOH corresponding to the ratio SiO2/Na2O = 2 was studied in a pressure vessel at 220 degrees C and 2.7 MPa. Since the kinetic curves could not be obtained directly from the experimental data, a new method is proposed to plot the entire kinetic graph from experimental data. An analytical expression of the type alpha = A [1-exp(-Bt)] describes the system perfectly. The constants A and B were calculated for a silica sample having a narrow granulometric distribution (range i.e. 300-315 mu m). The value of A is found to be almost constant, between 0.95 to 0.99 and B ranges from 0.03 to 0.14 when [OH-] increases from 0.5 to 12.5 mol/L. The kinetic order with respect to OH- is equal to 0.470 +/- 0.013 and the kinetic constant at 220 degrees C is 3.933 x 10(-6) g/m(2).s.