The synergy of cement and metakaolin tends to reduce the pore size to about a tenth of the diameter within the first days. This is valid to a replacement until the 20% level and about 27% water in which most of the Portlandite formed will have reacted to form additional CSH* or CSAH** phases.
Through the formation of these phases the pores will be filled by additional binding material. Due to the lower pores diameter the water uptake is reduced.
The total pore volume depends on the w/b ratio***. With very high porosity the advantages of metakaolin replacement will decrease.
The reduced overall pore size given by metakaolin will lead to more strength, higher density and more acid resistance
* CSH - calcium silicate hydrate
** CSAH - calcium silicate aluminum hydrate
*** w/b ratio - water to binder ratio