Capillary porosity

Fig. 5.16 Mercury intrusion porosimetry curves of C3S pastes showing differences in capillary porosity distribution.

Characteristics considered Infil- tration Aggre- gation >0.20 mm Silt and clay Clay Volume- weight Total porosity Non- capillary- porosity Organic matter Moisture- equiva- lent Sus- pension Dis- persion pH value... 

Young, J. F., Capillary Porosity in Hydrated Tricalcium Silicate Pastes, ... 

Fig. 8.5 Relations between porosities (volume percentages) and water/ccmcnt ratio for mature Portland cement pastes. The experimental data are for pastes at least 8 months old, and the calculated curves relate to a typical cement aged 18 months. Open symbols total water porosities. Filled or half-filled symbols mercury porosities. Curve A total water porosity. Curve B free water porosity. Curve C capillary porosity. References to data O (P20) O (S77) A (F33) V (M68) (S78) (F34) 9 (019) (M68) (D3I) 3 (H4I). In the last two cases, porosities by volume were estimated from data referred in the original sources to masses of dried paste, assuming the tatter to have contained 0.23 kg of water per kg of cement having a specific volume of 3.17 x 10 m kg h...

Parrott and co-workers (P30,P32,P35,P33) described a more sophisticated method for modelling the hydration process. The fraction of the total water porosity that was below 4nm was calculated by multiplying the volume fraction of C-S- H by an appropriate factor, which depended on whether the C-S-H was formed from alite or belite, the temperature and the amount of space available. The constants assumed were based on experimental data obtained using a procedure based on methanol sorption (Section 8.3.4). The effect of drying was allowed for (P35) by introducing a factor of 0.7 - -1.2(RH — 0.5) for 0.5 < RH < 1, or of 0.7 for RH 0.5. These refinements allow some deviation from the Powers-Brownyard postulate of a fixed volume ratio of gel porosity to product. Typical results for the volume fractions of pores larger than 4 nm in mature pastes of a cement with an alite content of 56% were approximately 0.26, 0.16 and 0.07 for w/c ratios of 0.65, 0.50 and 0.35, respectively (P32). For the two higher w/c ratios, these results are near the capillary porosities of Powers and Brownyard, but for w/c 0.35 the latter value is zero. 

Equation 8.7 is similar to that of Powers, but unreacted cement is considered equivalent to hydration product. Some of these equations break down at zero or high porosities, but for a wide range of intermediate porosities, assuming suitable values of the constants, any of them can give a reasonable fit to a given set of data. Rossler and Odler (R31) concluded that equation 8.10 was the most satisfactory. A relation identical to that of this equation, using capillary porosity, is implicit in a figure represented earlier by Verbeck and Helmuth (V5). 

Figure 3 Distribution of pore sizes of an hydrated Portland cement paste 4 months old (initial mass ratio water/cement of 0.4) obtained by mercury intrusion. The pore family related to the calcium silicate hydrate (some nm) and that of capillary porosity (a few hundreds ofnm) can be easily distinguished. The finest porosity is completely saturated with interstitial liquid while the capillary porosity is partially.

When concrete is considered instead of cement paste, the w/c ratio and the degree of hydration remain the main factors that determine the capillary porosity. Nevertheless, concrete is more complex because of the presence of the aggregates and the transition zone between aggregate and the cement matrix, where the structure of cement paste tends to be more porous [2, 3]. 

In determining the resistance to degradation of concrete and its role in protecting the embedded steel, not only should the total capillary porosity (i. e. the percentage of volume occupied by capillaries) be considered but also the size and intercormec-tion of capillary pores. Figure 1.6 shows the relation between the transport properties of cement paste (expressed as coefficient of water permeabihty) and the compressive strength as a function of the w/c ratio and degree of hydration [7]. 

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. 

This ratio depends therefore on the capillary porosity. The strength can be determined from the following empirical formula ... 

Fig. 5.48 Reduetion of capillary porosity vs time of hydration, (aceording to [112])...

Because of a good correlation between the capillary porosity, i.e. between the large pores and permeability, a relation will also occur between the permeability and strength of concrete. This relation was found experimentally by Torrent and Jomet [137] (Fig. 5.62). 

Fig. 5.67 Reduction of capillary porosity in the paste as a result of C-S-H phase formation, at addition of slag or fly ash (schematically) a capillary pores in cement paste after 15 day of hydration. b modification of pore structure as a result of C-S-H phase formation after longer time, (according to [140])...

Plasticizers and superplasticizers improve radically the pore stmcture (effect of w/c) and concrete becomes less permeable to air and water [159], Collepardi and Massida [159] found a capillary porosity and pore size lowering with decreasing permeability of concretes in which water reducers were used. The resistance to the sulphate attack was also improved [159]. 

In Table 6.4 the effective chloride ions diffusion coefficients in the pastes from various cements, determined by Page et al., are presented [191,211]. The NaCl molar solution in saturated Ca(OH)2 solution, as well as the saturated Ca(OH)2 solution were applied in two chambers of measuring stand. As can be concluded from these data, the chloride ions diffusion coefficient is ten times lower in slag cement paste and three times lower at 30 % fly ash addition. As it can be concluded, the resistance of slag and pozzolanic cements to the attack of aggressive water solutions is significantly higher. This relates undoubtedly to the much lower capillary porosity of these cement pastes. 

The diffusion is increasing with the w/c ratio of the paste, that means with the increasing capillary porosity [192, 226]. There is a close correlation between the w/c ratio (porosity) and the rate of diffusion in the paste, as it is shown in Fig. 6.51... 

Special cements with silica fume, giving condensed, compact pastes (DSP) with high content of ultra-fine particles, have very good freeze-thaw resistance [80], It is caused by extremely low capillary porosity and hence very low content of freezable water in these pastes. The same rerrrark can be related to the reactive powder concrete (RPC), for example the Ductal type composite [335], This question will be discussed in Chap. 10. 

However, in cementitious material, research related to the diffusion coefficient, the permeation coefficient, and quantification of the electric mobility has just made a start, and models corresponding to various materials and environmental conditions have not yet been constructed as of this stage. In the past, apparent diffusion coefficients were used including aggregates as shown on the left side of Fig. 4, but in practice, substances such as pore solution and ions did not move within the aggregates or the hydrates themselves, but they can move in capillary porosity and transition zones. Thus, an effective diffusion coefficient D, calculation equation is proposed as... 

The opposite occurs at the venule end of the capillary with fluid flowing in from the tissues into the blood. However, if this balance between the two opposing forces is upset under certain abnormal physiological conditions, fluid accumulates in the interstitial space resulting in a condition known as edema. In this diseased state, a normally negative interstitial fluid pressure becomes positive. This may happen due to cumulative or singular effect of several factors like high blood pressure, increased capillary porosity and/or low plasma protein content. 

Capillary porosity of approximately 15% volume, depending on amount of water. 

All kinds of chemical agents that attack the concrete-like materials are dangerous if allowed to penetrate into the material s structure and if there is enough moisture for chemical reactions. Impermeability of the matrix stops the penetration of chemicals and migration of water to such an extent that the reaction becomes slow with respect to the lifetime of the structure and the material has improved durability. As mentioned elsewhere, high performance materials with a low w/c ratio, low capillary porosity and high density are durable in conditions in which ordinary materials may exhibit sensitivity to destructive agents. 

According to Powers et al. (1959) the capillary porosity in cement paste is characterized by a percolation transition (from connected to disconnected) at a volume fraction of about 20% porosity, but this opinion is not confirmed by the tests shown by Ye (2005), who proved that the same porosity may correspond to the permeability different by one order of magnitnde, as shown in Figure 11.37 (curves for wic = 0.40 and tvic = 0.60 indicate the same porosity equal appr. 30.9%). 

Porosity of the OPC paste is determined by the ratio of water to cement a w/c ratio of 0.4 allows complete reaction of the OPC clinker with water and the gel pores with a diameter of several nm remain (Powers, 1954). Total porosity and capillary porosity for OPC cement paste (Fig. 8-3) increases... 

Figure 8-3. Total porosity and capillary porosity of OPC cement paste for different water/cement ratios...

Permeability is the abiUty of the refractory material to transmit the gas or liquid. The permeable porosity is a part of the capillary porosity. The permeability of the material may aid in understanding the corrosion resistance, the resistance to infiltration of metal, slag, or electrolyte, although there is no direct consequence. 

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