Comparison between Permeability of Aggregate and Cement Paste of Concrete

Do you know, what are the usual porosity of cement paste and natural aggregate? The usual capillary porosity of hardened cement paste in concrete lies between (30-40) percent.

In natural aggregate this value is usually 3 percent and in rare cases exceeds 10 percent. In concrete production our usual interest is to make it less permeable and we will try to keep permeability of aggregate as low as possible.

Thus cement paste have large porosity. Does it mean cement paste have more permeability? We are providing a table below; try to understand the comparison between permeability of cement paste and rocks. Here rocks represent aggregate that we are discussing.

Consider, granite have co-efficient of permeability 5.36X10-9 cm/s which have similar permeability of cement paste having water cement ratio 0.70 (the paste is sufficient matured).

Now consider an aggregate particles having 3% porosity but have permeability of a poor cement paste of mature concrete. Form this table it is clear aggregate of above range have permeability that varies with that of cement paste having water/cement ratio of (0.38 to 0.71).

Here a term is used ‘matured’ cement paste, when do they become mature? Of different water-cement ratio maturity comes after different periods as below:

Water-cement ratio	Maturity period (days)
0.4	3
0.5	14
0.6	180
0.7	365

 

The curing condition is moist curing. Dear reader in our next post we will discuss about permeability contribution of aggregate in relation to their porosity.