EFFECT OF PH ON THE HYDRATION OF TRICALCIUM SILICATE

The suspension hydration of coarse-grained (0.07 m2.g-1) tricalcium si licate (C3S) in relatively large amounts of water (1 g per 500 mL) was studied as a function of time (up to 48 h), temperature (10-degrees, 25-degrees, and 40-degrees-C), and pH (8, 10, 11, 11.5, 12). The pH of the solution was maintained with dilute HCI, and both solid and solut ion samples were withdrawn periodically for analysis. At earliest time s, the microstructure of the hydration products suggests that a fine-g rained, highly conformable hydrate forms on the C3S 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 parabo lic rate constants (k(ps)) calculated for this period increase with in creasing temperature and acidity of the solution. Similarly, activatio n energies calculated using the Arrhenius equation range from 2.0 kJ.m ol-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. Depe nding on the pOH of the solution, the initial calcium silicate hydrate (C-S-H) was observed either to persist throughout the 48-h experiment s (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) greater-than-or-equal -to 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) g reater-than-or-equal-to 10(-2.5)), the reticulated phase which is prec ipitating is identical to the Type I/II C-S-H observed in non-pH-contr olled experiments. Experiments confirm and strengthen the hypothesis t hat the system CaO-SiO2-H2O contains at least two distinct hydrated ph ases and that the activity of the hydroxide ion plays a dominant role in determining the nature and microstructure of the hydration products which form.