Cement paste pores

Fig. 2.3. Upper. Pattern obtained by cross-section of a hardened cement paste. Pores and pieces of matter of many different sizes can be seen, and no characteristic length scale is apparent. Lower. A model capturing the essential property of the real material, viz., it is constructed according to the same logic of repetition by dilation, whatever the observational scale...

Moukwa M. and P.-C. Altcin (1988). The effect of drying on cement pastes pore structure as determined by mercury porosimetry . Cement and Concrete Research 18 745-752. 

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. 

The T) and T2 dependence is described by Eqs. (3.4.3) and (3.4.4) [34] where Qi and q2 are spin-lattice and spin-spin surface relaxivity constants, and S/ Vis the surface-to-volume ratio of the pore. These equations provide the basis of a methodology for crack detection in cement paste specimens [13]. 

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. 

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

A typical distribution of pore radii in the hardened cement paste of concrete was shown in Fig. 1.40 which indicated that the majority of pores lie in the region of 0.05 and 1.0 pm diameter and it is through these pores that water passes by applied pressure or capillary rise, as shown in Fig. 4.5(a). 

Fig. 5.23 Pore volume distribution of cement pastes from nitrogen adsorption. Curve 1 = cement with no admixture curve 2 = cement paste and 2% CaCl2 (Gouda).

This shrinkage mechanism occurs only in pores within a fixed range of sizes. In pores larger than 50 nm the tensile force in the water is too small to cause appreciable shrinkage and in pores smaller than 2.5 nm a meniscus cannot form [122]. The amount of cement-paste shrinkage caused by surface tension depends primarily on the water-cement ratio, but it is also affected by cement type and fineness and by other ingredients (such as admixtures, and supplementary cementing materials) which affect pore size distribution... 

Supercritical C02 treatment affects the microstructure of the cement paste. In the first stage of the sc C02 treatment, free water in the cement pores is extracted. As a consequence of this dehydration process, channels of about 50-pm diameter develop. Dissolved calcium in the free water reacts with the C02 and crystallizes with the C02 as calcite along the channel walls. In the second stage, the structural water of the hydrated cement phases is extracted. The carbonation of the portlandite to form more calcite takes place. Water, bound to the CSH surrounding the partially hydrated cement clinker particles, is partially replaced by a carbonate formation. The short fibers of the CSH-cement framework, which are responsible for the physical properties of the cement, are not affected (Hartmann et al., 1999). 

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. 

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. 

Unless a wet cell or cryo stage is used, the fine microstructure is much altered by dehydration in the instrument (J10,S41). However, localized drying occurs in any paste even before it is placed in a high vacuum, as soon as the RH falls below saturation. The water is lost initially from the wider pores, which are probably represented disproportionately on fracture surfaces. The state of the cement paste in a practical concrete may thus vary on both a macro and a micro scale between dry and saturated. 

Several studies on the quaternary systems of CaO-SiOj-HiO with NajO or K,0 have been reported (K19,S52,M52). Alkali greatly lowers the concentrations of CaO in the solution and raises those of Si02. The solid phase compositions are difficult to study. Determinations based on changes in concentration on adding CH to alkali silicate solutions are subject to considerable experimental errors, while direct analyses of the solid are difficult to interpret because the alkali cations are easily removed by washing. Suzuki ei al. (S52) considered that they were adsorbed. Macpheeef /. (M52) reported TEM analyses of the C-S-H in washed preparations obtained by reaction ofCjS (lOg) in water or NaOH solutions (250 ml). The C-S-H obtained with water had a mean Ca/Si ratio of 1.77 that obtained with 0.8 M NaOH had a mean Ca/Si ratio of 1.5 and a mean NujO/SiOj ratio of 0.5. These results do not appear to be directly relevant to cement pastes. The pore solutions of the latter may be 0.8 M or even higher in alkali... 

Fig. 7.2 Backscattered electron image of a mature Portland cement paste, aged 2 months. Successively darker areas are of unreacted cement grains (bright), sometimes with visible rims of hydration products, CafOH), other ( undesignated ) regions of hydration products, and pores (black). Scrivener and Pratt (S28).

Fig. 7.8 Concentrations in the pore solution (scaled as indicated in the cases of OH and SiOj) of a Portland cement paste of w/c ratio 0.5. After Lawrence (L. 2).

The content of non-evaporable water, relative to that in a fully hydrated paste of the same cement, was used as a measure of the degree of hydration. Portland cement paste takes up additional water during wet curing, so that its total water content in a saturated, surface dry condition exceeds the initial w/c ratio. Evidence from water vapour sorption isotherms indicated that the properties of the hydration product that were treated by the model were substantially independent of w/c and degree of hydration, and only slightly dependent on the characteristics of the individual cement. The hydration product was thus considered to have a fixed content of non-evaporable water and a fixed volume fraction, around 0.28, of gel pores. 

Brunauer and co-workers (B55,BI08) considered that the gel particles of the Powers-Brownyard model consisted of either two or three layers of C S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer water, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200m g for cement paste. Sorption isotherms using N2 give lower values of the specific surface area this was attributed to failure of this sorbate to enter all the pore spaces. 

In principle, isotherms at low partial pressures of the sorbate may be used to determine specific surface areas by the Brunauer-Emmett-Teller (BET) method (G64). In this method, it is assumed that molecules of the sorbate are adsorbed on surfaces that can include the walls of pores, provided that the distance between molecules on opposing walls is large compared with molecular dimensions. From a plot derived from the isotherm, and given the effective cross-sectional area of the sorbate molecule, the specific surface area of the sorbent and the net heat of adsorption are obtained. Using water as sorbate, specific surface areas of about 200 m per g of D-dried paste have typically been obtained for mature cement pastes of normal w/c ratios... 

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