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Effect of pH on the Hydration of Tricalcium Silicate
Author(s) -
Ramachandran A. R.,
Grutzeck Michael W.
Publication year - 1993
Publication title -
journal of the american ceramic society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.9
H-Index - 196
eISSN - 1551-2916
pISSN - 0002-7820
DOI - 10.1111/j.1151-2916.1993.tb03691.x
Subject(s) - silicate , chemical engineering , mineralogy , chemistry , materials science , organic chemistry , engineering
The suspension hydration of coarse‐grained (0.07 m 2. g −1 ) tricalcium silicate (C 3 S) in relatively large amounts of water (1 g per 500 mL) was studied as a function of time (up to 48 h), temperature (10°, 25°, and 40°C), and pH (8, 10, 11, 11.5, 12). The pH of the solution was maintained with dilute HCl, and both solid and solution samples were withdrawn periodically for analysis. At earliest times, the microstructure of the hydration products suggests that a fine‐grained, highly conformable hydrate forms on the C 3 S grains as early as 2 min. Once formed, the growing hydrate layer imparts anywhere from 0.5 to 6 h of diffusion control to the dissolution reaction. The parabolic rate constants ( k ps ) calculated for this period increase with increasing temperature and acidity of the solution. Similarly, activation energies calculated using the Arrhenius equation range from 2.0 kJ·mol −1 at pH 10 to 20.6 kJ·mol −1 at pH 12. Values less than 21 kJ·mol −1 are generally indicative of diffusion control in aqueous systems. Depending on the pOH of the solution, the initial calcium silicate hydrate (C─S─H) was observed either to persist throughout the 48‐h experiments ( a OH <10 −2.5 ) or to act as a nucleation site for a second, less soluble, highly reticulated C─S─H hydrate ( a OH ≥ 10 −2.5 ). The chemical composition of the solution phase and the characteristics of the evolving microstructure over the course of the reaction lead to the conclusion that under high‐pH conditions ( a OH ≥ 10 −2.5 ), the reticulated phase which is precipitating is identical to the Type I/II C─S─H observed in non‐pH‐controlled experiments. Experiments confirm and strengthen the hypothesis that the system CaO–SiO 2 –H 2 O contains at least two distinct hydrated phases and that the activity of the hydroxide ion plays a dominant role in determining the nature and microstructure of the hydration products which form.