C3S hydration in diluted and stirred suspenSIons: (III) NMR study of C-S-H precipitated during the two kinetic steps

29Si NMR experiments on C-S-H formed during the two kinetic steps of C3S hydration reveal the presence of Q1 and Q2 signals. For a very low percentage of the reaction, a Q0 signal is also observed. However, this signal is not believed to be induced by C-S-H and has been attributed to C3S,h (C3S superficially hydroxylated). Two discontinuities occur when the Q1/Q2 ratio is plotted against the lime concentration in solution. These discontinuities are representative of two invariant points involving three different forms of C-S-H: C-S-H(SO), C-S-H(SI) and C-S-H(SII). C-S-H(SO) and C-S-H(SI) are in equilibrium at the first invariant point and C-S-H(SI) and C-S-H(SII) at the second one. C-S-H(SI) and C-S-H(SII) are precipitated during the first and second kinetic steps respectively, hence the C-S-H precipitated during each kinetic step is different. C-S-H(SO) is not likely to be formed during the hydration of C3S in ‘pure’ water at room temperature.