Permeability Portland cement pastes

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...

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 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. 

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... 

The steep increase of the water permeability above approximately 25 % porosity (corresponding to a wjc ratio of 0.45 with a degree of hydration of 75 %, Figure 1.6) is the background of the specified values in the codes of practice for high quality concrete. For instance. Table 1.2 shows the relationship between water/cement ratio and degree of hydration in order to achieve segmentation of the macropores in a paste of Portland cement. 

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. 

In assessing the dmability of concrete made with limestone-modified Portland cement one has to bear in mind that— at a constant water/cement ratio— the porosity of the paste, and along with it its permeability, increases with increasing limestone content in cement Nevertheless, adequate durabihty may be preserved, as long as the amormt of added limestone is not excessive (Tezuka et al., 1992). 

Owing to a higher water requirement for hydration, the porosity of hardened aluminous cement pastes is lower than that of Portland cement at the same water/cement ratio. This has a favorable effect on permeability and along with it on the corrosion resistance and freeze-thaw resistance of the hardened material. However, the conversion to aluminous cement is associated with a distinct increase of both porosity and permeability, which must be considered when this cement is used in practice. 

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). 

Concrete based on calcium aluminate cement performs rather well if exposed to sulfate solutions, especially if made with a low water/ cement ratio and high cement content. However, cases of expansion and cracking have also occasionally been reported (Scrivener and Capmas, 1998). The reasons for the good sulfate resistance of this type of cement are not obvious. It is mostly attributed to a surface densification of the hardened material, resulting in a very low permeability of the formed surface layer, and/or to the absence of calcium hydroxide in the system (Scrivener and Capmas, 1998). Unlike Portland cement and related binders, magnesium sulfate solutions are less aggressive to calcium aluminate cement than alkali sulfate solutions. This is due mainly to the absence of the C-S-H phase in the hardened calcium aluminate cement pastes, which is particularly sensitive to the action of magnesium sulfate. 

The final effects of carbonation are strongly dependent on the quality of Portland cement and on the permeability of the hardened cement paste. Cements with a small amount of alkalis are less exposed to carbonation, and the same concerns cements blended with fly ash and blast-furnace slag. Carbonation occurs at the highest rates at relative humidity about 40-70%. Near 0% or 100% there is Utde or no carbonation. 

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). 

---