Hi Richard,

It all depends on what exactly do you need the measurements for. For example, if you are not interested in sub-surface cavity and porosity measurement then nitrogen absorption-desorption isotherms can give you the most accurate data for surface area and pore size distribution.

Since your sample is too big for any standard equipment I can suggest you use a similar process to cast smaller samples with different dimensions and acquire data as a function of apparent (geometric) area. The scaling from smaller samples to bigger one should then be possible.

I hope it helps.

There is a simple way to recognise the preliminary permeability with an initial surface absorption method of concrete cube sample with a surface of 100 *100 mm at least (http://www.zealinternational.com/page-pdf.php?id=390)