Evolution of Mineralogical Phases by ²⁷ Al and ²⁹ Si NMR in MK ‐ Ca ( OH ) ₂ System Cured at 60°C

Abstract

The evolution of the metastable phases in metakaolin/ Ca ( OH ) 2 systems cured at high temperatures, remains mostly unknown, newer techniques may now help to establish both the kinetic mechanism of the pozzolanic reaction and the thermodynamic stability of the main hydrated hexagonal phases: Stratlingite ( C 2 ASH 8 ) and tetra calcium aluminate hydrate ( C 4 AH 13 ). For this reason this work examines the kinetics of the pozzolanic reaction in the MK / Ca ( OH ) 2 system over 123 d at 60°C using nuclear magnetic resonance spectroscopy ( 27 Al and 29 Si NMR ). The results obtained by 27 Al and 29 Si NMR show that during the first 30 h, the metastable phases C 2 ASH 8 and C 4 AH 13 , coexist with the cubic phase ( C 3 ASH 6 ) obtained directly from the pozzolanic reaction. The gel C–S–H is clearly identified after 21 h of reaction, whereas at shorter times the C–S–H bands overlap those with the unreacted metakaolin ones. After 123 d of pozzolanic reaction, the first signs of the cubic phase are detected, a consequence of the conversion reaction of the metastable phases, and a phenomenon not previously identified.