Cement paste permeability

N. Banthia and S. Mindess. Water permeability of cement paste. Cement Concrete Res, 19(5) 727-736, September 1989. 

The graphs given in Fig. 1.38 show the logarithmic relationship between the water-cement ratio and the permeability coefficient of hardened cement paste. Thus concrete with a paste water-cement ratio of 0.4 will be almost impermeable. Water-reducing agents can be used to reduce the water- cement ratio, so ensuring that the permeability is kept to a minimum. 

The type of data produced by H O and N adsorption is relevant to gel pores having radii up to about 50 A,2but larger pores and capillaries exist in the hardened cement paste and probably are more significant in determining the porosity or permeability of the hardened paste in concrete to gases and liquids. 

Whatever the fine structure and reactions associated with hydrating cement paste, the permeability of the hardened matrix will depend on the sizes of interconnected capillary openings remaining after hydration. 

The permeability of concrete and the rates at which ions and gases diffuse in it are of major importance for durability. We shall consider only the behaviour of cement paste. 

Pastes inpregnated with PMMA or sulphur are still sufficiently permeable to water that expansion occurs on long exposure (F46). In polymer-impregnated (S108) and MDF (R64) cement pastes, there is evidence of interaction between Ca ions and carboxylate and possibly other groups of the polymer. In MDF pastes made with calcium aluminate cement, the polymer (PVA) was found to inhibit the normal hydration reactions of the cement, but to react with Ca and AH to give an ionically cross-linked polymer and calcium acetate. TEM showed the material to be essentially a dispersion of grains of clinker or hydration products in a continuous polymer matrix. 

Cardenas HE, Struble, LJ. (2006). Electrokinetic nanoparticle treatment of hardened cement paste for reduction of permeability. Journal of Materials in Civil Engineering Jul/Aug 546-560. 

Figure 1.6 Influence of capillary porosity on strength and permeability of cement paste (a). Capilla7 porosity derives from a combination of water/cement ratio and degree of hydration (fa) (Powers [7] from [3])...

The decrease in capillary porosity increases the mechanical strength of cement paste and reduces the permeability of the hydrated cement paste (Figure 1.6). A distinction should be made between capillary pores of larger dimensions (e. g. >50 nm), or macropores, and pores of smaller dimensions, or micropores [3]. The reduction in porosity resulting of both the macro- and the micro-pores plays an essential role in increasing mechanical strength. 

Crete surface to the bulk of the concrete. Permeability is high (Figure 1.6) and transport processes like, e. g., capillary suction of (chloride-containing) water can take place rapidly. With decreasing porosity the capillary pore system loses its connectivity, thus transport processes are controlled by the small gel pores. As a result, water and chlorides will penetrate only a short distance into concrete. This influence of structure (geometry) on transport properties can be described with the percolation theory [8] below a critical porosity, p, the percolation threshold, the capillary pore system is not interconnected (only finite clusters are present) above p the capillary pore system is continuous (infinite clusters). The percolation theory has been used to design numerical experiments and apphed to transport processes in cement paste and mortars [9]. 

Silica fume. Silica fume (SF) is a waste product of manufacturing ferro-sihcon alloys. It consists of an extremely fine powder of amorphous silica. Average particle diameter is about 100 times smaller than that of Portland cement and the specific surface area is enormous 13000-30000 m /kg compared to 300-400 m /kg for common Portland cements. Silica fume shows an elevated pozzolanic activity and is also a very effective filler. For these reasons, addition of silica fume to Portland cement may lead to a very low porosity of the cement paste, increasing the strength and lowering the permeability. It is usually added in the proportion of 5 to 10 % and it is combined with the use of a superplasticizer in order to maintain adequate workability of the fresh concrete. 

Cement paste characteristics, for example, strength and permeability significantly depended on its nanostructure features in particular nanoporosity. In recent years, electron microscopy has been demonstrated to be a very valuable method for the determination of microstructure. Numerous studies on the influence of nano-SiO on the microstructure of plain cement mortar have been carried out. The results showed that nano-SiO particles formed a very dense and compact texture in the hydrate products and decreased the size of big crystals such as CH. In this chapter in order to study the microstructure of RHA mortar, with and without nano-SiO, a SEM was used. The microstrueture of the RHA mortar with 3% replacement of nano-SiOj and without nano-SiO at a euring age of seven days are presented in Fig. 5.5 and Fig. 5.6, respeetively. Results showed that the nano-SiOj particles improved the dense and compact microstructure of RHA and generated a more homogenous distribution of hydrated products. 

Fig. 5.60 Relation between the total porosity (determined with helium method) and permeability of water saturated pastes (according to [133]) empty circles—Portland cement pastes, black circles—pastes from cements with mineral additions, cured at 20-80 C, one year measurements...

Fig. 5.61 Relation between the maximum pores radius and permeability of hardened cement paste, (according to [134])...

There were attempts to relate the permeability of concrete to the properties of interfacial transition zone. However, the unambiguous results were not obtained. According to Roy [142], the constraction of interfacial transition zone surface does not play important role in concrete permeability, while Valenta [143] has quite opposite opinion. This problem will be discussed in Chap. 6 where the construction and properties of interfacial transition zone will be presented [144], In the light of the studies by Richet and Oliver [145] it is evident that the porosity of inteifacial transition zone in traditional concretes (w/c = 0.5 or more) has a significant influence on the permeability of concrete this permeability is a hundred times higher than in the case of cement paste and rises with the size of aggregate (Fig. 5.68). However, the effect of the transition zone on the diffusion of ions is not so evident, because the locally increased water content in this zone, decrease the w/c ratio in cement matrix outside it, which consequently limits the diffusion, thus a total effect can be negligible [145],... 

Fig. 6.24 Permeability coefficient of hardened cement paste vs w/e, for entirely hydrated eement clinker. (According to [64])...

On the other side the distinguishing of acid corrosion has the justification Portland cement paste is not stable in water solution with pH lower than 10, or even 10.5 [68]. Therefore the concrete in such an enviromnent is readily corroded. However, concrete from calcium aluminate cement is stable with no special protection, at pH of water solution reduced to 4 [69]. As it was aforementioned, beside of pH value there are the other factors involved diffusion rate (permeability) and, as in all the chemical reactions, the solubility of products. 

It is assumed that cement paste plays a role of semi-permeable membrane which is impermeable for the silicate anions formed as a result of reaction. Some authors postulate that this semi-permeable membrane is composed of sodium and potassium sUicates gel formed during reaction. A concentration gradient appears and linked with him the osmotic pressure. Recently, the difference in discussions of swelling mechanism, attributed to the sorption of water or to the osmotic pressure, became insignificant. There is a view that sodium and potassium ions can diffuse through the semi-permeable membrane more easily than the calcium ions. 

The corrosion process consists in migration of chloride ions into the concrete with simultaneous OH ions diffusion in an opposite direction, from the leached cement hydrates. Therefore the porosity, and consequently the permeability of concrete is of fundamental importance. Cement matrix is a part of concrete which is subjected to the destructive action as a first one. The attack of chloride ions on cement paste will be then discussed first. However, some information concerning the effect of aggregate on the chloride ions diftusion is needed for comprehensive presentation of phenomena occurring in concrete. 

It is obvious that the same mechanisms can operate in opposite directions it means that the introduction of ions from the surrounding liquid medium inside the paste can be possible. The mechanisms mentioned above are controlled by pores stracture and permeability of cement paste. 

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. 

The content of concrete component readily soluble is the other effloreseenee controlling factor, beside permeability. This refers generally to the cement paste, from which the sodium, potassium and calcium ions ate released to the pore solution in concrete. 

At low fly ash additions— that is, below 15%—the extent of carbonation in mature fly ash concrete tends to be equal to or lower than that in similar concrete mixes with no ash, in spite of the lower calcium hydroxide content of the formed hydrated cement paste (Buttler et al., 1983 Hobbs, 1988 Goni et al, 1997). This is due mainly to the reduced permeability of the paste to CO2. However, at higher fly ash contents the resistance to carbonation is significantly reduced (Goni et al, 1997). 

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