Permeability composite cement pastes

The gel pores form a part of CSH (calcium silicate hydrate), and may be classified as micro pores or meso pores. The principal difference between gel and capillary pores is that the former are too small to be filled by the hydration products and for capillary effects, it means that no menisci are formed. The gel pores occupy between 40% and 55 % of total pore volume, but they are not active in water permeability through cement paste and they do not influence the composite strength. Water in the gel pores is physically bonded. It is believed that gel pores are directly related to shrinkage and creep properties of the cement paste. 

Figure 6.10 shows how the permeability of cement paste increases with the tvic ratio. At first it increases slowly and beyond wIc = 0.5 it increases rapidly. The permeability may therefore be regarded as an indirect and partial measure of porosity. Considering the components of cement-based composites it may be assumed that the permeability is determined more by matrix properties than by aggregate ones. Particularly, the matrix/aggregate interface may have the largest content of pores and microcracks which affect the overall permeability (cf. Chapter 7). 

The porosity and pores structure will be of special importance for liquids transport in concrete. These both properties relate directly to w/c ratio and the degree of hydration, because porosity of concrete is affected mainly by cement paste the porosity of aggregate is generally very low. Continuous pores volume is increasing with w/c ratio and decreases with degree of cement hydration. There ate the permeability controlling factor, because the transport of liqitids in concrete composite occurs principally in continuous pores. 

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