Porosity of geopolymer concrete is highly dependent on the concentration of the alkali content in concrete.

If alkali content is more , the it produces more porous geopolymer gel,Hence this tends to increase in the size of pores ,which further results in increase in water absoption.

In comparision with geopolymer concrete, tendency to absorb water is high in case of opc

Int J Mol Sci. 2012; 13(4): 4388–4395.

Published online 2012 Apr 10. doi:  10.3390/ijms13044388

PMCID: PMC3344220

Fly Ash Porous Material using Geopolymerization Process for High Temperature Exposure

Mohd Mustafa Al Bakri Abdullah,1,* Liyana Jamaludin,1 Kamarudin Hussin,1 Mohamed Bnhussain,2 Che Mohd Ruzaidi Ghazali,1 and Mohd Izzat Ahmad1

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refer to  this article

Here's a PhD thesis from a few years' ago which might be of interest to you. (there's a chapter on measuring porosity in geopolymers from page 48)

https://www.ideals.illinois.edu/bitstream/handle/2142/44764/Brayden_Glad.pdf?sequence=1

Our recent work has shown that the total porosity of the NaOH activated slag mortars decreases with increasing the alkali dosage. The results have good agreement with the strength data. The lower total porosity at high alkali dosage is simply due to the enhanced degree of slag activation.

https://www.researchgate.net/publication/315541408_Effect_of_alkali_dosage_on_alkali-silica_reaction_in_sodium_hydroxide_activated_slag_mortars

Geopolymer can exhibit both higher or lower total porosity than OPC system, depending on types of alkali activators, more details can be found in the following book:

Caijun, Della Roy, and Pavel Krivenko. Alkali-activated cements and concretes. CRC press, 2006.

Article Effect of alkali dosage on alkali-silica reaction in sodium ...