1.2 Session Topic Modeling

Moderator: Jacques Marchand

4 Responses to “1.2 Session Topic Modeling”

  1. Now that I’m more involved in modeling, I can see that there are many important questions that we need to grapple with:

    1. What form of the rate equations should we be using. What experimental evidence is there in support of some of the recent models that have been suggested? This question is particularly directed at the reaction order to be used, i.e. [Ca++}^n, what is n, etc.

    2. What do published diffusion coeficients really mean, are they effective diffusivities or skeletal diffusivities for the materials, i.e. D for C-S-H (I) and (II).

    3. Should we be considering various alternative modeling platforms other than automaton? Are there other options and might they be of some use?

    4. Are there multiple ways to explain induction?

    5. What role might ionic transport play, i.e. we know that surface chages play a role in aggomeration and design of admixtures for rhology control, might then the same surface charges effect local ionic transport and hence hydration?

  2. A concept that I will discuss in my talk is that of a “reaction volume”, which is a limited volume within which reaction product can form during the early nucleation+growth period. The reaction volume seems to extend outward from the surface of the cement or C3S particles by a fixed distance. The evidence for this is indirect, but pretty compelling. Increasing the w/c does not increase the amount of early hydration as one would expect if all of the water-filled space was available to N+G. On the other hand, increasing the surface area of the powder greatly increases the amount of early N+G hydration. So some questions I would like to raise for discussion at the workshop are:

    1) Why does a reaction volume exist, i.e., what is the mechanism for limiting hydration product to form in this region?
    2) Is there an alternative explanation for these observations?

  3. Is the “reaction volume” another way of saying that nucleation and growth of C-S-H only occurs on C3S surfaces? If you assume that no nucleation can occur in the bulk solution, but only on C3S surfaces, then that would seem to explain both the lack of influence of w/c ratio, but the significant influence of C3S surface area. If so, then the mechanism would be heterogeneous nucleation of C-S-H on C3S surfaces or growth on existing C-S-H surfaces.

    This latter mechanism is the only one that has ever produced results consistent with published experiments when simulating the full chemistry and microstructure evolution with HydratiCA, and it is also the mechanism assumed by the Dijon model.

  4. I would like to clarify my comment at the workshop on the importance of temperature in the kinetics models. We know from calorimetry that the early age reaction is very exothermic and that the rate constant has a strong Arrhenius relationship. It also appears that the structure and water content of the C-S-H gel is temperature dependent. Therefore, the model needs to include a feedback loop for the effect of temperature on the rate of the reaction. This also implies that the thermal boundary conditions – adiabatic , isothermal etc – for the concrete object need to be specified. Computer models that simulatesimultaneously chemistry, temperature, stress (electromagnetics ect) are called multi-physics. COMSOL is a commercial version. We have been using it for a problem that involves induction heating. It is very powerful, but also expensive.

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