Cement paste specific surface area

Water-cement ratio (by wt) Nature of mix Air content (%) Paste content (%) Voids in concrete (%) Specific surface area (mm) Void spacing factor (mm)... 

Table 3.4 The effect of various air-entraining agents at different concentrations on the specific surface area and computed spacing factor of air bubbies in cement paste...

An increase in the water-cement ratio of cement pastes leads to greater air entrainment and a decrease in the specific surface area of bubbles. However, the spacing factor is relatively unchanged, as shown in Table 3.5 [14]. 

Table 3.5 The effect of water-cement ratio of cement pastes on the air content, specific surface area and computed spacing factor...

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... 

Costa and Massazza (C44) concluded from a study of natural pozzolanas of varied types that reactivity in mixtures with CH at w/s = 2 and 40 C depends during the first 28 days on the specific surface area and at later ages on the contents of Si02 and AI2O3 in the active constituents. A comparative study of five natural pozzolanas and three low-CaO pfas in pastes with cement showed that the CH contents of the pozzolanic cements were considerably lower than those of the pfa cements at 3-60 days, but virtually the same at 90 days, the pozzolanas thus appearing to react more rapidly than the pfas at early ages but more slowly later. Determinations of the unreacted mineral admixture in pastes with CH showed that at 90 days 23-30% of the natural pozzolana had reacted, compared with 11-15% for the pfas. The similarity in CH contents suggests, however, that these values may not apply to mixtures with 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. 

B and b— are the experimental constants. This formula relates to the c lower than the value corresponding to the transformation of paste finm the dilatancy showing fluid to the pseudoplastic one. The constant b depends on the shear rate, type of material and shape of grains but not on the specific surface area and the grain size distribution. The constant B depends on the shear rate and specific surface area of cement. 

Fig. 5.12 Curves obtained by Bombled with rheograph [4, 5]. Paste with w/c=0.25 of cements with different specific surface area 7 201 mV kg, 2 248.5 mVkg, 3 307 mV kg, 4 403 m /kg, 5 511 mVkg...

The specific surface area of cement pastes was investigated applying usually H2O and N2 as adsorbates. The values obtained with HjO ate high— about 200 mVg. They ate proportional to the non-evapoiable water content in the paste, and irrespectively also of the porosity [39]. In the case of N2 application, a significantly lower values ate obtained, about 50 m /g [35] and they are not proportional to the non-evaporable water content. Use of the other adsorbates gives the results compatible with those for... 

There are some paste properties which influence the behaviour of fresh concrete. The workability of concrete mixture is the function of the rheological properties of paste which depend on the fine fraction content in cement. The yield stress value, being the measure of concrete mixture consistency at final stage of mixing, is increasing with the rise of cement specific surface area (Fig. 5.12). The initial setting time is significantly shorter in the case of this cement property. 

The C-S-H phase is an amorphous or nearly amorphous material of the general formula Ca0. Si02.H20, where both x and y may vary over a wide range. On the nanometer scale the C-S-H phase is stracturally related to the crystalline phases 1.4 nm tobermorite and jeimite. In cement pastes hmited amounts of foreign ions may be incorporated into the C-S-H phase. On the micrometer scale the C-S-H phase appears either as a dense amorphous mass or as a microciystalline material with an acicular or platelet-like morphology. The material contains pores with radii between about 1 and 10" nm, and exhibits a specific surface area exceeding 100 m /g. 

At comparable consistencies of the starting mix, alkali-activated slag cement pastes exhibit lower porosity than comparable Portland cement pastes, owing to the lower initial water/solid ratio. The proportion of pores with r<10 nm is usually higher in hardened AAS pastes (Shi et al, 1992) however, the actual pore size distribution also depends on the activator used. In a comparative study, mixes produced with sodimn silicate exhibited the finest and those made with NaOH the coarsest pore stmcture (Shi, 1996, 1997). The specific surface area of AAS cement pastes is higher (by about 35-55%) than that of comparable ordinary Portland cement pastes (Tailing and Brandstetr, 1993). 

FIGURE 2.98 Spin lattice relaxation time, T, as a function of the specific surface area of a set of cement pastes. (Drawn from the data shown by Gran and Hansen, Cement Concrete Res., 27,1319,1997.)... 

NMR quantitative analysis of water within a cementitious sample can be done in situ. The technique is nondestructive and noninvasive. It is possible to measure water in different pore (as discussed here) the specific surface area of cement pastes (Barberon et al. 2003 Halperin et al. 1994), the specific surface area of C-S-H including and excluding gel pores (Muller et al. 2013a) and the density and water fraction of C-S-H hydrates (Muller et al. 2013a,b), all in never-dried materials. 

Hardened cement paste mainly consists of a porous, semi-colloid phase of calcium sihcate hydrates physically incorporating crystalline calcium aluminate sulphate hydrates, the so-called cement gel. The cement gel has a very large active internal surface area that can adsorb water molecules it has been found through measurements that the specific surface S of hardened cement paste is of the magnitude of 200000 m /kg of the material. The specific surface is a measure of the internal, free surface in m per kg of the material. 

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