Water non-evaporable

It should be pointed out that deterioration under freeze-thaw conditions can also be caused by a mechanism other than the direct freezing of the non-evaporable water. The capillaries contain dissolved salts, such as hydroxides, sulfates and carbonates. As part of the water is frozen, the concentration of salts in the remaining water increases and water will flow by osmotic pressure from the gel pores to the capillary pores, setting up an additional disruptive pressure. 

Fig. 5.10 Degree of hydration of a tricalcium silicate paste in the presence of calcium chloride, measured by non-evaporable water content (Odier).

Fig. 7.43 Non-evaporable water content of concretes with (— (Johnston [140]).

Studies of the kinetics of the C3S hydration in the absence and presence of accelerators show that the extent or degree of hydration of the silicate phase in the presence of calcium chloride is considerably increased, right up to at least 28 days, whether measured by the quantity of lime produced [6] (Fig. 5.8), X-ray analysis [15] (Fig. 5.9), or the amount of non-evaporable water [16] (Fig. 5.10). Figure 5.8 also shows that a small amount of TEA retards the hydration of the C S phase for a considerable time, and the trend... 

Fig. 5.13 shows the general form of the curve relating the fraction of C3S consumed (a) to time in a paste of w/s 0.5 at about 25°C and with moist curing. Such curves have been determined using QXDA for unreacted C3S (e.g. Refs K20,OI0), though the precision is low for values of a below about 0.1. At low values of a, other methods are available, such as conduction calorimetry (e.g. Ref. P22), aqueous phase analyses (e.g. Ref. B63) or determinations of CH content or of non-evaporable water. At very early ages, it may be necessary to allow for the fact that the property determined depends on the nature of the hydration products e.g. precipitation of C-S-H begins before that of CH. 

Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of non-evaporable water, referred to the ignited weight. Copeland et al. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression ... 

The loss below the CH step is due to decomposition of C-S-H and the hydrated aluminate phases. Although the TG curves of pure AFm phases are markedly stepped in this region (Fig. 6.2), those of cement pastes normally show only slight indications of steps. Weak peaks can, however, sometimes be seen on DTG curves. The absence of steps is probably due to a combination of low crystallinity, the presence of other phases and the presence of AFm phases of different compositions in mixture or solid solution or both. For typical experimental conditions with a 50 mg sample, heating rate of 10 deg C min and Nj flow rate of 15 ml min , the volatiles retained at about 150°C, after correction for COj, correspond to the non-evaporable water, and those retained at about 100"C to the bound or 11% RH water, but this last temperature, in particular, is very dependent on experimental conditions (T5). 

For pastes of typical ordinary Portland cements cured for 3-12 months, the CH content found by thermal methods or QXDA is typically 15-25%, referred to the ignited weight (P29,R14,M37,H37,T17,D12). Pressler et al. (P29) found that for pastes of various ages of ordinary (US Type I) Portland cements, it was linearly related to the content of non-evaporable water, but that for cements high in belite (US Type IV), it tended to a maximum while the latter continued to increase. This is readily explained, since belite yields only a little CH on hydration. The author has noticed similar behaviour even with modern cements high in alite, and that the CH content can possibly even decrease slightly after 28-91 days (T5). il... 

Copeland et al. (C39,C25) treated CjS pastes, and C-S-H prepared in other ways, with various sources of AP, Fe or S04 ions. The XRD peaks of the phases providing the ions disappeared, and changes occurred in the micromorphology of the gels and in the non-evaporable water contents and fractions of the CaO extractable by an organic solvent. It was concluded that the ions were taken up by the C-S-H and that up to about one silicon atom in six could be replaced by aluminium, iron or sulphur, and that aluminium and iron could also replace calcium. These results suggested that the principal hydration product in Portland cement pastes was a substituted C-S-H, a conclusion that later appeared to be supported by the results of the... 

Water contents (pereentages on the ignited weight) Bound water 31.2% Non-evaporable water 21.9%... 

The and Na" are present in the cement partly as sulphates and partly in the major clinker phases (Section 3.5.6). When the phases containing them react, the accompanying anions enter products of low solubility and equivalent quantities of OH are produced. The K, Na and OH" ions are partitioned between the pore solution and the hydration products. Early estimates of the fractions remaining in solution were too high because non-evaporable water was used as a measure of bound water and the quantity of pore solution thereby overestimated. 

The phase composition of the paste by weight and by volume, and the non-evaporable water content, were calculated using procedures... 

Structure of the material by a model based largely on evidence from total and non-evaporable water contents and water vapour sorption isotherms. Powers later modified the model in minor respects (P34). 

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. 

The specific volume of the non-evaporahle water, is obtained by taking the volume of hydration product less those of the cement from which it was formed and the gel water, and dividing by the mass of non-evaporable water. This gives [(w/c )//) — V -mJ/m , or 0.74 X 10" m kg" . 

Fig. 8.3 shows the mass and volume relationships for a fully hydrated, saturated paste of w/c ratio 0.5, calculated using the above expressions and values. Following Powers and Brownyard, the hydrated cement is treated from a purely volumetric standpoint as a composite of reacted cement, non-evaporable water and gel water. The specific volume of the non-evaporable water was assumed to be 0.73 x I0 -gel water to be 1.00 x 10 m kg several reasons for example, the pore solution is in reality not pure water, but an alkali hydroxide solution with a specific volume (for 0.3 M KOH) of... 

For any specified drying condition, the calculated water contents arc lower and the porosities higher than those of pure Portland cement pastes, and this appears to be true in varying degrees of composite cements in general. Experimental observations support this conclusion. Non-evapor-able water contents of 2-year-old pastes of w/s ratio 0.5 typically decrease with slag content from around 23% for pure Portland cements to 10 13" for cements with 90% of slag (C42). For the paste to which Table 9.4 refers, the observed non-evaporable water content was 17.7% (H49). Porosities and their relations to physical properties are discussed in Section 9.7. 

As with the slag cement discussed in Section 9.2.7, the calculated water contents for different humidity states are lower, and the porosities higher than for comparable Portland cement pastes. The water contents are lower because replacement of CH by C-S-H or hydrated aluminate phases causes relatively little change in the HjO/Ca ratio, so that the water content tends to fall towards the value which would be given by the Portland cement constituent alone. Diamond and Lopez-Flores (D39) found that, for two 90-day-old pastes similar to that under discussion (30% pfa, w/s = 0.5), the non-evaporable water contents were l2.5 /o and 13.0%, while that of a Portland cement paste was l5.4 /o. The porosities are discussed in Section 9.7. 

Traetieberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with CH probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the CH content passed through maxima usually within the first day before beginning to decrease in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for CH content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements. 

Table 9.9 Contents of calcium hydroxide and non-evaporable water in some pastes of Portland cement with and without microsilica (percentages on the ignited weight) (H53)... 

Ramachandran and Feldman (R57) determined helium porosities and densities for cement pastes containing various quantities of CaCl2. They concluded that for a given degree of hydration, the effect of the latter was to increase the absolute density and thus also the porosity. At non-evaporable water contents of 12-16%, 1% or 2% additions of CaCl2 slightly increased the compressive strength, but 3.5% additions decreased it. 

The specific volume of compressed water depends on the mass of non-evaporable water Wn and on the mass of gel water Wg. Gel water is estimated by equation 4 [7], and w may be obtained by the simple difference Wd-Wg. 

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