1. Suggest a mechanism for early hydration that explains the observed induction period characteristic of portland cement.
2. What experimental evidence that supports your hypothesis?
3. What role does mass transfer play in early age hydration?
4. Can we discern reaction orders from existing experimental datasets and what are reasonable rate experssions to be using for modeling of hydration?
5. What stoichiometries should we use for C3S hydration and can you suggest a reasonable reaction path mechanism as a starting point for modeling purposes?
6. What evidence exists for discerning transport properties of C-S-H?
These are good questions and should get the ball rolling.
It might be good to distinguish between “pure” systems and those that have been intentionally retarded. An ideal STARTING POINT is to see how much agreement there is on the unretarded alite system. If we can have clarity on that point, we have a solid basis for expanding the discussion to portland cement.
A question somewhat related to your 6th question is, “What is the origin of inner-product and outer-product C-S-H and, more generally, what chemical/physical factors determine the growth morphology of C-S-H?” I think this question will generate a lot of discussion based on recent experimental results from EPFL and also because fundamental modeling of microstructure development hinges on relating the local growth conditions to the growth morphology.
This is a good good collection of questions for discussion. Related to a number of them, but perhaps worth considering on its own as well is what we really mean by the induction period and its end. Does this definition or concept have meaning for individual components or is it purely a bulk property. If the former, we would need to understand the early time hydration of each reactive component and how the presence of other components affect the observed bulk cement property.
Agreeing with Jeff Schweitzer, if we focus on alite for a moment, can we really nail down the mechanism of the “induction” period? We have one theory that C3S superficially hydrolyzes and has an apparent solubility product that is extremely low as a result. Another theory is that a metastable hydrate barrier layer forms as a separate phase and kinetically prevents C3S from dissolving rapidly. Yet another theory, I think, is that the mechanism of C3S dissolution itself changes dramatically as the driving force for dissolution changes, resulting in a large decrease in its dissolution rate.
We often say that nucleation and growth of C-S-H are the rate-controlling steps during the induction period, and models of kinetics are often developed by assuming that the rate of nucleation of growth of C-S-H is equal to the rate of hydration for purposes of predicting calorimetry curves, etc. This is sufficient for such predictions (e.g. the BNG model) but we also need to understand what slows the rate of C3S dissolution to get a more complete picture of the chemistry.
Questions for Panel:
1. Suggest a mechanism for early hydration that explains the observed induction period characteristic of portland cement.
2. What experimental evidence that supports your hypothesis?
3. What role does mass transfer play in early age hydration?
4. Can we discern reaction orders from existing experimental datasets and what are reasonable rate experssions to be using for modeling of hydration?
5. What stoichiometries should we use for C3S hydration and can you suggest a reasonable reaction path mechanism as a starting point for modeling purposes?
6. What evidence exists for discerning transport properties of C-S-H?
These are good questions and should get the ball rolling.
It might be good to distinguish between “pure” systems and those that have been intentionally retarded. An ideal STARTING POINT is to see how much agreement there is on the unretarded alite system. If we can have clarity on that point, we have a solid basis for expanding the discussion to portland cement.
A question somewhat related to your 6th question is, “What is the origin of inner-product and outer-product C-S-H and, more generally, what chemical/physical factors determine the growth morphology of C-S-H?” I think this question will generate a lot of discussion based on recent experimental results from EPFL and also because fundamental modeling of microstructure development hinges on relating the local growth conditions to the growth morphology.
This is a good good collection of questions for discussion. Related to a number of them, but perhaps worth considering on its own as well is what we really mean by the induction period and its end. Does this definition or concept have meaning for individual components or is it purely a bulk property. If the former, we would need to understand the early time hydration of each reactive component and how the presence of other components affect the observed bulk cement property.
Agreeing with Jeff Schweitzer, if we focus on alite for a moment, can we really nail down the mechanism of the “induction” period? We have one theory that C3S superficially hydrolyzes and has an apparent solubility product that is extremely low as a result. Another theory is that a metastable hydrate barrier layer forms as a separate phase and kinetically prevents C3S from dissolving rapidly. Yet another theory, I think, is that the mechanism of C3S dissolution itself changes dramatically as the driving force for dissolution changes, resulting in a large decrease in its dissolution rate.
We often say that nucleation and growth of C-S-H are the rate-controlling steps during the induction period, and models of kinetics are often developed by assuming that the rate of nucleation of growth of C-S-H is equal to the rate of hydration for purposes of predicting calorimetry curves, etc. This is sufficient for such predictions (e.g. the BNG model) but we also need to understand what slows the rate of C3S dissolution to get a more complete picture of the chemistry.