This study analyzed the behavior of two laboratory-synthesized calcium silicates, C 3 S and C 2 S, after hydration in 8-M NaOH and in water as a control. Two-and 28-d mechanical strength values were determined and the products were characterized with XRD, TEM, and 29 Si and 23 Na MAS NMR. The results showed that hydrating C 3 S in a highly alkaline medium had no significant effect on the mechanical development of the material, whereas in C 2 S hydration, that medium hastened hydration substantially, impacting setting and hardening times. This finding has technological implications, given the low early-age reactivity of dicalcium silicate under normal hydration conditions.
This study analyses the behavior of calcium silicates C 3 S and C 2 S hydrated in two alkaline media, Na 2 CO 3 and Na 2 SO 4 . The silicates were synthesized with laboratory reagents and hydrated in water, to which solid-state alkaline activators with 4 wt% Na 2 CO 3 or 4 wt% Na 2 SO 4 were added. Two-and 28-day mechanical strength values were determined and the reaction products were characterized with XRD, SEM/EDX, and 29 Si and 23 Na MAS NMR. The findings showed that the presence of Na 2 CO 3 hastened hydration kinetics and stimulated early-age mechanical strength development in both silicates. The most significant effect of sodium sulfate, however, was observed in the 28-day material in both silicates, in which it raised strength by stimulating the precipitation of C-S-H gels with a high percentage of Q 2 units.akali activation, calcium silicate, calcium silicate hydrate, cements, gels
This study explored the behavior of laboratory-synthesized tricalcium aluminate (C 3 A) in alkaline media. In all the trials, the C 3 A was hydrated with water containing a certain percentage of alkali activators (4% Na 2 CO 3 , 4% Na 2 SO 4, or 8% Na 2 SO 4 by weight of C 3 A). Two-and 28-day mechanical strength values were determined and the products were characterized with XRD, DTA, and NMR. The compressive strength of the C 3 A pastes rose in the presence of the alkaline activators used in this study. The alkalis induced the formation of calcium monocarboaluminate (C 4 AČ H 11 ) as the main hydration product in all cases. In addition, when Na 2 SO 4 was added, a sodium calcium sulfoaluminate, phase U (C 4 ASˇNH 16 ), formed. The presence of this mineral, whose formation is associated with high pH values, furnished indirect information on the alkalinity in the medium.
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