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RESEARCH PRODUCT

The Structure, Stoichiometry and Properties of C-S-H Prepared by C3S Hydration Under Controlled Condition

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Hydrations of tricalcium silicate undertaken by keeping constant lime concentration in solution in an open system and in diluted suspensions for different values of [CaO] ranging between 6.5 and 30 mmol/1 show that the lime concentration in solution is the parameter which determines the main characteristics of the reaction and products. The stoichiometry of C3S hydration products vanes with lime concentration in the same way as synthetic C-S-H until C/S c.a. 1.5 corresponding to about [CaO] =20 mmol/1. Beyond this concentration, single phase C-S-H samples were obtained with 1.8<C/S<2. The discontinuity of the variation of stoichiometry with concentration is characteristic of an invariant point where two phases are in equilibrium: C-S-H with C/S≤1.5 we named s-C-S-H (corresponding to Taylor’s C-S-H(I)) and C-S-H with C/S≥1.8 we named γ-C-S-H. The structure of all samples derives from tobermorite structure as revealed by XRD, IR and 29Si MAS NMR spectroscopies, with a drierketten arrangement of silicate chains of which the length is obtained from the Q2/Q1 ratio. Chains are mainly pentamers when C/S≅ 1 and dimers when C/S≥1.5. A structural model is proposed for accounting for the evolution of stoichiometry, charge balance and chain length. In β-C-S-H, the C/S increase is mainly due to elimination of bridging tetrahedra and insertion of calcium ions balancing SiO− charges while in γ-C-S-H, it is due to insertion of calcium ions balancing OH− which may be localised in the structure in place of missing bridging silicates.