If polymer is exposed to elevated temperature at some point it melts and fills the pores in the concrete, after filling whether the concrete will be durable?
In order to melt the polymer, the temperatures needed will likely be sufficiently high to cause partial dehydration of the binder and potentially lead to micro-cracking. So, you would also need to heat the polymer to such a temperature that its viscosity drops sufficiently that it fills the cracks.
There are some polymers with sufficiently low melting points, but these are not very common (http://polymerdatabase.com/polymer%20physics/Polymer%20Tm%20C.html)
But, you would need the melting point to be sufficiently that it remained solid during use.
You would then also need to consider behaviour in contact with water.