How to get information on reaction kinetics from experimental data

[Aoxuan_WANG]

Hi everyone! I am new to the forum and seek advise in modelling.
I would like to use the kinetics module in PHREEQC to constrain the production of a reaction product (because PHREEQC usually gives the result of the final equilibrium, whereas actual experiments usually have a defined time). I currently have experimentally obtained production amounts of the substance at various points in time, how do I apply this data to the kinetics module?

[dlparkhurst]

Read the manual on KINETICS and RATES, and look at some of the examples in the manual. There are examples in the forum as well.

[Aoxuan_WANG]

Thank you for your reply. I've looked at the manual and studied some examples, but it hasn't helped. There are examples for minerals that have explicit kinetic equations, but unfortunately the substance I am trying to model does not have explicit kinetic equations. So I don't have a clear goal for the next step. Do you have any impressive examples of this?

[Aoxuan_WANG]

In fact the mineral I want to model is Friedel's salt, a corrosion product of cement, but common mineral dissolution rate libraries or kinetic models don't cover it, which bothers me. Is there any possible solution to this problem?

[dlparkhurst]

You must use RATES and KINETICS if you want to model a kinetic reaction with PHREEQC. If there is no rate expression that you can find for Friedel's salt, and you still want to do some modeling, then you must derive a rate equation that is appropriate and fit your data to that rate equation.

I do not know what information you have to develop a rate equation. You could simply fit a function of time (a polynomial or exponential), but that may not have much transfer value. If you have concentration data, then you can try to use a functional form that takes advantage of that information.
 
I am not very knowledgeable of processes in concrete, but you may want to consider SOLID_SOLUTIONS or EXCHANGE to try to account for the variable composition of the solids. However, I would start with fixed compositions, and move on to variable compositions only if you need to. Kinetics for variable compositions is much more complicated.

[Aoxuan_WANG]

Thank you for your advise. This is indeed a relatively difficult problem to solve.