Pore solution

Air-Entrainment Agents. Materials that are used to improve the abiUty of concrete to resist damage from freezing are generally known as air-entrainment agents. These surfactant admixtures (see Surfactants) produce a foam which persists in the mixed concrete, and serves to entrain many small spherical air voids that measure from 10 to 250 p.m in diameter. The air voids alleviate internal stresses in the concrete that may occur when the pore solution freezes. In practice, up to 10% air by volume may be entrained in concrete placed in severe environments. 

Unfortunately, the protection provided by concrete can be overcome by contamination of the concrete by chloride. Chloride, when entering the concrete as a contaminant of the mix constituents, is to a large extent (about 90%) complexed within the cement matrix and only a small percentage is free in the pore solutions. The present codes of practice ban the use of chloride-bearing additives and restrict the amount of chloride present in concrete. For normally reinforced concrete made with ordinary Portland cement it should be not more than 0.4% chloride ion with respect to the cement content weight/weight. 

When mature concrete is contaminated by chloride, e.g. by contact with deicing salts, the cement chemistry is more complex, and less chloride is taken up by the cement hydrate minerals and a larger proportion is free in the pore solutions and can therefore pose a greater hazard. When embedded steel corrodes, the production of a more voluminous corrosion product pushes the concrete from the steel with resultant cracking and spalling of the concrete. 

Concrete is a composite material composed of cement paste with interspersed coarse and fine aggregates. Cement paste is a porous material with pore sizes ranging from nanometers to micrometers in size. The large pores are known as capillary pores and the smaller pores are gel pores (i.e., pores within the hydrated cement gel). These pores contain water and within the water are a wide variety of dissolved ions. The most common pore solution ions are OH", K+ and Na+ with minor amounts of S042" and Ca2+. The microstructure of the cement paste is a controlling factor for durable concrete under set environmental exposure conditions. 

Lithium has been found to prevent ASR expansion [37]. It is used either to mitigate further distress in ASR-affected structures by topical application of lithium solutions or as a means of using ASR aggregates in new structures when other methods of ASR mitigation are not feasible. As a critical amount of lithium is needed in the pore solution of cement paste to arrest the expansion [38], a method to spatially resolve and quantify the lithium is desirable. 

Diamond [38] has shown that considerable amounts of lithium become bound during the hydration process. A study of the influence of hydration on the observable lithium content in cement paste samples was undertaken. In parallel, a traditional approach to lithium content in the pore solution was conducted (physical extraction of the pore solution and subsequent chemical analysis). The relationship between pore solution extraction and bulk MR is shown to be linear (see Figure 3.4.15). This result indicates that MR will be able to image lithium held in the pore solution but not lithium bound to the cement paste matrix. MR and pore solution extraction results also confirm that large amounts of lithium become bound during hydration. 

MRI lithium measurements are playing an important role in the development and understanding of the role of lithium in treating and preventing ASR. The spatial resolution and speed make this measurement technique very attractive for monitoring the removal of lithium from the pore solution during ASR and also in investigating how well various surface treatments will slow ASR. 

Concentration Polarization As a reactant is consumed at the electrode by electrochemical reaction, there is a loss of potential due to the inability of the surrounding material to maintain the initial concentration of the bulk fluid. That is, a concentration gradient is formed. Several processes may contribute to concentration polarization slow diffusion in the gas phase in the electrode pores, solution/dissolution of reactants/products into/out of the electrolyte, or diffusion of reactants/products through the electrolyte to/from the electrochemical reaction site. At practical current densities, slow transport of reactants/products to/from the electrochemical reaction site is a major contributor to concentration polarization ... 

These processes may be affected by chemical admixtures, particularly the formation and properties of the protective layer. Also, admixtures remaining in the pore solution may further influence nucleation and growth of the hydration products, causing volume expansion, outward mechanical pressure on the protective gel layer and its subsequent disruption. 

Tishmack, J. K., Olek, J. Diamond, S. 2001. Characterization of pore solutions expressed from high-calcium fly ash-water pastes. Fuel, 80, 815-819. 

Now we might consider what is in fact the common clay mineralogy of sandstones. Shelton (1964), Bucke and Mankin (1971) find it to be most often dominated by kaolinite. This mineral although hydrous, is conspicuous by its lack of alkalis. Thus one could suspect that alkali activity in pore solutions of sandstones is, or was, frequently low, lower at any rate than adjacent mica-bearing shales. Laboratory studies by Hanshaw and Coplen (1973) and Khareka and Berry (1973) would give a plausible explanation for such a phenomenon. If solutions are forced hydrostatically across the argillaceous membrane, ionic species in solution are selectively... 

Chemical analyses of sediment pore solutions from deep sea cores... 

Keywords Porous materials, bulk water and pore solution, micro and macro pores, micro-... 

Bulk or free water and gas are in the macroscopic pores with a hydraulic diameter greater than 0.1 fim. The gel pores are filled with pore solution (gel water). Their diameter is much smaller (1 - 30 nm). During cooling below the freezing point of bulk water ice is formed in the larger pores with sufficient su-... 

Taking into account the aforementioned effects of ice formation in porous materials, a macroscopic quintuple model within the framework of the Theory of Porous Media (TPM) for the numerical simulation of initial and boundary value problems of freezing and thawing processes in saturated porous materials will be investigated. The porous solid is made up of a granular or structured porous matrix (a = S) and ice (a = I), where it will be assumed that both phases have the same motion. Due to the different freezing points of water in the macro and micro pores, the liquid will be distinguished into bulk water ( a = L) in the macro pores and gel water (a = P, pore solution) in the micro pores. With exception of the gas phase (a = G), all constituents will be considered as incompressible. 

Chloride, sulfate, and pH analyses of pore solutions from the three sites are shown in Figure 6. Figure 7 depicts organic matter, total sulfur and total iron for the same cores and Figure 8 shows data on forms of sulfur. 

Table 3.1 pH and oxygen sensitivities of graphite and lead electrodes [19] -approximate experimental values obtained in synthetic concrete pore solutions... 

Based on Equation 10.3, chemical mobility differs from water mobility by a factor of 1 + (pb/x)Xd. This factor is also known as the retardation factor. The larger the retardation factor, the smaller is the velocity of the chemical species in relationship to the velocity of water. Note, however, that the retardation factor contains a reactivity factor (Kd) and two soil physical parameters, bulk density (pb) and porosity (t). The two parameters affect retardation by producing a wide range of total porosity in soils as well as various pore sizes. Pore size regulates the nature of solute flow. For example, in very small pores, solute movement is controlled by diffusion, while in large pores, solute flow is controlled by mass flow. 

Pore solutions are present in practically all sediments which are not completely consolidated and often contain more dissolved solids than sea water. 

If the aqueous phase was sea water when the sediment was formed, its composition was in most cases changed by physical, chemical-mineralogical, and biological processes. Pore solutions provide the chemical components for diagenetic alterations. 

Evidence considered in Sections 5.3.1 and 8.3 indicates that the C-S-H gel of calcium silicate or cement pastes has a layer structure, and that, together with a pore solution, it forms a rigid gel in which the pores range in size from macroscopic to enlarged interlayer spaces of nanometre dimensions. One can therefore define a water content only in relation to a specified drying condition. Three such conditions will be considered. 

There are few data on the concentrations of CaO and SiO, in the pore solutions of C3S pastes (w/s < 1.0), as opposed to suspensions, and those that exist are conflicting as regards Csjoz (B56,011). The values of after long times (e.g. 27mmol 1 ) (Oil) appear to be significantly above the solubility ofCH this has been attributed to incorporation of SiO, in the CH (W22), but might also be due to the small crystal size of some of this phase. 

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