On the global hydration kinetics of tricalcium silicate cement

We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power-laws in time, $x/(1-x)\sim (t/t_x)^\psi$. For early times $t < t_x$ we find an `accelerated' hydration ($\psi = 5/2$) and for later times $t > t_x$ a `deaccelerated' behavior ($\psi = 1/2$). The crossover time is estimated as $t_x \approx 16 hours$. We interpret these results in terms of a global second order rate equation indicating that (a) hydrates catalyse the hydration process for $t<t_x$, (b) they inhibit further hydration for $t > t_x$ and (c) the value of the associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6} liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually being furnished as a diffusion limited precipitation that the exponents $\psi = 5/2$ and $\psi = 1/2$ directly indicate a preferentially `plate' like hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.Comment: RevTeX macros, 6 pages, 4 postscript figure

Influence of the electrical field applied in chloride migration tests on the properties of concrete

Electrically accelerated short-term chloride migration tests have become very popular laboratory techniques, performed in order to obta in quantitative information on the ingress speed of chlorides in concrete. The common model assumption adopted for each chloride migration technique is that the applied electrifical field does not alter the concrete, i.e. the permeability of concrete will not change during the test. However, due to the electrical field, not only the chloride ions migrate but al so all other ions which are present in the external electrolytes and in the pore solution of concrete. The migration rates of all these ions depend (among other factors) on their mobility, which in the case of chlorides is much lower than for the hydroxyl ions. This can potentially lead to a decrease of the pH of the pore solution and therefo re to the dissolution of portlandite or to the reduction of the chloride binding capacity of the cement hydration products. This study addresses tbe influence of the electrical fi eld on the properties of concrete. Ageing of the samples by the electrical fi eld was done by performing the migration test with tap water used as the catholyte (instead of the NaCI solution) for 24 hours. Subsequently, the RCM test was performed on pre-aged samples as well as on the reference samples in order to compare their chloride transport properties. The results show that, although the system of concrete/external electrolytes is dynamic (the pH of electrolytes and the current are continuously changing during the test), the investigated chloride transport properties in concrete remain stable