Cement paste consists of hydrates of various cement compounds as C3S, C2S, C3A, C4AF and Ca (OH)2 and the gross volume available for these products of hydration is the sum of absolute volume of dry cement and the volume of the mix water. Thus the cement paste consists of gel of hydrates of cement compounds, Ca(OH)2 and gel pores either are filled with water or remain empty.
It has been found that gel has 28% pores or voids, but these pores are so small that hardly any water can pass through them under normal condition. The permeability of gel pores is found to be about 7 x 10-16 m/s, which is about 1/100th of the cement paste as a whole. Thus there is no contribution of gel pores to the permeability of cement paste.
The permeability of hydrated cement paste as a whole is greater due to the presence of larger capillary pores in the paste, which depend on the w/c ratio in the paste. Generally the w/c ratio is the main contributing factor to 5 permeability. At low w/c ratio, not only the extent of capillary cavities is less, but the 5 diameter of pores also is small.
The pores or cavities formed at low w/c ratio get filled up in c few days by the products of hydration of cement. Only very large cavities formed due to high water/cement ratio (usually more than 0.7) will not be filled up by the products of hydration, which are responsible for the high permeability of the paste. Thus the permeability of cement paste is a function of its porosity.
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The capillary porosity is governed by the w/c ratio and by the degree of hydration and the permeability of cement paste is also mainly dependent on these two parameters. A relation between the permeability and w/c ratio for mature cement paste (93% hydrated cement) has been shown in Fig. 17.2. From the study of Fig. 17.2, it will be seen that for a given degree of hydration permeability is lower for paste with lower w/c ratio, specially below w/c ratio 0.6 at which the capillaries become filled with hydrated, products of cement.
From the Fig. 17.2, it will also be seen that as w/c ratio increases from 0.4 to 0.7 the coefficient of permeability increases more than 100 times. For a given w/c ratio, the permeability decreases with age as cement continues to hydrates and fills some of the original water pores as shown in Fig. 17.3.
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The reduction in permeability of cement paste is faster at lower w/c ratio. ACI standard 301-89 suggests that for water tight structural concrete w/c ratio not more than 0.48 for exposure to fresh water and not more than 0.44 for exposure to see water should be used. The value of max. permeability is often recommended as 1.5 x 10-11 m/s.
The influence of filling or segmentation of capillaries on permeability shows that the permeability is not a simple function of porosity. It has been observed that two bodies may have similar porosities, but different permeabilities. In fact one large passage connecting capillary pores will result in a large permeability, while the porosity will remain virtually unchanged.
Reduction in permeability of concrete paste with progress of hydration is shown in Table 17.5 below:
From the above Table 17.6, it will be seen that cement paste even with high w/c ratio of 0.7 is quite impervious as that of Granite whose coefficient of permeability is 5.35 x 10-10 m/s. This value of coefficient of permeability is so small, that there is no possibility of any water to seep through it. But in actual practice, it is observed that mortars and concrete exhibit much higher values of permeability than shown in above Table 17.6. Definitely this is not due to the permeability of aggregate used in mortar or concrete. The aggregate is as impermeable as that of cement paste as can be seen from above table.
In actual structures the higher permeability of mortar or concrete is due to the following reasons:
1. The development of fine or micro cracks due to thermal stresses and long term dry shrinkage.
2. Due to the development of large micro cracks in the transition zone.
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3. Due to the development of cracks and volume change on account of various minor reasons.
4. Existence of entrapped air due to insufficient compaction.