PERMEABILITY OF CEMENT PASTE

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

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])...

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. 

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 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. 

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. 

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],... 

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). 

It has also been observed that Portland cement pastes with added sihca fiime contain an increased number of hollow shell pores, also called Hadley grams, in the size range 1-15 //m (Kjellsen and Atlassi, 1999). They are formed primarily at early ages by dissolution of the cement grains, and in pastes without sihca fitme usually become filled with flesh hydrates as the hydration progresses. It appears that in pastes with added silica fiime the number of empty shells is significantly increased. These hollow shell pores appear to be connected to the continuous capillary pore system by much smaller gel pores, and then-presence contributes to a reduced permeability of the paste. 

Brouwers, 1998). In the presence of these additives the dissolved calcium hydroxide tends to react with the residual non-reacted fly ash, yielding additional amounts of C-S-H, and thus reducing the permeability of the paste. The optimum amounts of these additives to be interblended with Portland cement were found to be around 35 mass% of fly ash and about 8 mass% of silica fume (van Eijk and Brouwers, 1998). However, cement combined with 70 mass% of granulated blast furnace slag behaves not too differently from a cement that contains just Portland clinker alone (Faucon et al., 1996). 

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. 

The sensitivity of concrete structures to sulphate attack is strongly related to the exposure conditions. Structures in an environment of high sulphate content in the air or in w ater, for example sewage tunnels, are particularly vulnerable. After sulphate ions penetrate the pore system of cement paste, complex reactions start with C3 A leading principally to two kinds of processes gypsum corrosion and sulphoaluminate corrosion (Mindess etal. 2003). The products of sulphate reactions with cement expand and can cause cracking and destruction. The permeability of the material s structure and the quality of cement decide upon the rate of these processes. Special Portland cements as well as high alumina cements may be used for elements exposed to sulphates (cf. Section 4.1.1). 

Fig. 8-5 shows the relation between the transport properties of cement paste (expressed as coefficient of water permeability) as a function of w/c ratio and degree of hydration (Powers et al., 1954). It is clear that at w/c ratios <0.45, the paste is practically impermeable to water whereas at w/c >0.55 there is a dramatic increase in permeability. This break point is the reason why a w/c ratio of <0.45 is specified in the codes of practice for high quality concrete (Ben-tur et al., 1997). 

Scrivener, K. L. 1988. The Use of Backscattered Electron Microscopy and Image Analysis to Study the Porosity of Cement Paste . In Pore Structure and Permeability of Cementitious Materials, Proceedings of Materials Research Society Symposium 139 129-140. 

In the absence of cracks and large channels in the concrete, the permeability is a function of the paste water-cement ratio. 

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. 

Cement batch number Surface area of cement (cm g - ) (air permeability) Total alkali content of cement SO4 content of cement Air content of paste (% by vol.)... 

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. 

Partial replacement of Portland cement by a mineral addition can greatly decrease the permeability to water provided the age is such that sulhcienl reaction of the addition has occurred. For a paste with w/s 0.47 and 30% replacement of cement by pfa, cured at 20°C for 1 year, Marsh et al. (M87) found a permeability of 10 m s the corresponding paste of pure... 

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. 

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. 

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