W/C ratio

The mix design was as follows strength 30 MPa, cement OPC 280 kg. fly ash 120kg, water 140kg, stone, 19mm 1110kg, stand 245 kg, crushed dust 585kg,W/C ratio 0.5. 

Fig. 6.10 Corrosion vs time in 3% NaCI as a function of water-cement (w/c) ratio and calcium nitrite content (NACE International, reprinted with permission).

When high dosages of superplasticizer are used to effect low W/C ratio and high workability. 

The moisture content of samples, in equilibrium, in the ESEM is varied at constant temperature (10°C), while lowering the pressure in the ESEM chamber from 9 torr to 2 torr. At the same time, relative humidity varies from 100% to 20%. The created chamber climate induces evaporation of the unbound (free) water in the cement paste (CP) samples without or with an embedded aggregate (to model simple concrete). It has been observed that curing conditions, sample age, water/cement (w/c) ratio, the presence of an aggregate, as well as the value of the RH, gives rise to different drying behaviour of the... 

Angular agg regate-reduction in w/c ratio with admixture Admixture 1.1 8.63 > [77]... 

Superplasticizer type Super- p/ostidzer AEA (ml kg of cement) W/c ratio by weight Type 1 cement (kg m 3) FACT Entrained air (%) Slump (mm) Density fkg m 3) f c at 28 days f c at 63 days (MPa) Stress applied Stress-strain rotio Test creep strain (in. jin x /0p"4)t... 

The suppression of superconductivity in (NH3)K3C60 has been attributed to the crystal symmetry lowering that lifts the luorbital degeneracy and decreases the (U/W)c ratio for the transition to the AFM Mott insulating state. This is aided by the increased interfullerene separation (10.57 A) in the ab plane (substantially larger than the value of 10.05 A found in K3C60), which reduces the bandwidth, Wy and increases further (U/W). 

A sample with a w/c ratio of 0.4 by weight (which is at the threshold water content required for complete hydration of OPQ was cured for 28 days in a dry desiccator with a relative humidity of <5%. All surfaces of the sample except the top face were sealed with Parafilm, to promote evaporation of water through this surface only. This was done to disrupt the hydration process in this part of the sample and hence introduce some inhomogeneity into the cured sample. Following the cure period, the Parafilm was removed and the sample was dried in an oven for 48 h at 105°C in order to remove all evaporable water. Water penetration studies were then carried out as described below. 

The penetration of brine into a range of OPC samples (with w/c ratios of 0.3, 0.4 and 0.5) was studied. The samples were cured for 28 days in a wet environment (relative humidity >95%) to ensure a homogeneous cure, and dried at 105°C for 48 h to remove all evaporable water. After drying, the samples were immediately placed into a saturated solution of NaCl for 4 days, after which the samples were fully saturated. The samples were then removed from the solution and imaged. The resulting 1-D profiles are presented in Fig. 19, where an increase in the signal is observed as the w/c ratio is increased, due to the increased pore sizes and hence increased uptake. 

In a second set of experiments, the rate of penetration of a brine solution into a cement sample was measured as a function of soaking time. The sample, which had a w/c ratio of 0.4, was heated in an oven at 105°C for 4 days to remove all evaporable water, and then sealed on all sides except the end face which was dipped in a saturated salt solution. The resulting 1-D profiles illustrated in Fig. 20 show the progression of the 23Na front into the sample with prolonged soaking times. 

Fig. 19. Profiles of the 23Na concentration in OPC samples with w/c ratios of 0.3, 0.4 and 0.5 soaked in brine, showing enhanced uptake for larger w/c values. (Reproduced from Ref. 98 with permission from Elsevier.)...

Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25°C and w/c ratios of 0.45-0.65. XRD powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsum and other calcium sulphate phases are no longer detectable after, at most, 24 h, and tbe clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. 

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 content of chemically bound water is approximately that retained on equilibration at 11% RH of a sample not previously dried below saturation. For fully hydrated pastes of typical cements, it is about 32%, referred to thel ignited weight (F13,T35). There are no systematic data relating this quantityi to cement composition. The total content of water essential for completei hydration in a saturated paste is defined as that present in such a pastel having the minimum w/c ratio at which complete hydration is possiblel... 

Opinions have differed as to the possibility of determining the hydrated) aluminate phases by thermal or X-ray methods. The determination of ettringite was discussed in Section 6.2.2. Bensted (B99), who used DTAJ found that for ordinary Portland cements the ettringite content increase with time during the first 2 h to maximum values of 2.2-2.8%, and that th quantity of ettringite formed at any given time increased with the w/c ratio ... 

The essential input data are (a) the bulk chemical composition of the cement, (b) the quantitative phase composition of the cement and the chemical compositions of its individual phases, (c) the fraction of each phase that has reacted, (d) the w/c ratio, (e) the COj content of the paste and an estimate of how it is distributed among phases, and (0 the composition of each hydrated phase for the specified drying condition. If (b) is unknown, it may be estimated as described in Section 4.4, and if (c) is unknown, it may be estimated from the age as described by Parrott and Killoh (P30), or, more simply though less precisely, by using empirical equations (D12,T37). If the phase composition by volume and porosities are to be calculated, densities of phases are also required. 

S o.40i 2 10,4- Table 7.4 gives other data assumed. By introducing also the w/c ratio and the densities of the phases, the volume percentages of phases for the specified drying condition may now be calculated, and, by difference, the porosity. Using data for the TG curves of the phases containing H,0 or COj or both, one may also calculate a simulated TG curve for the paste. The results of such a calculation are given in Fig. 7.4, the observed curve in which relates to the 14-month-old paste of Table 7.3. 

The first study using a wet cell, made at high w/c ratios, showed tubular growths radiating from the cement grains, which were considered to have formed by a silicate garden mechanism (D14). Later work showed that they were rich in calcium, aluminium and sulphur, and that they did not form if CjS was substituted for cement (BlOl). They have not been observed in the more recent studies made at normal w/c ratios, and do not appear to be a significant feature of normal cement hydration. 

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).

Taylor (T37) described a method for predicting the concentrations al any desired age after I day from the w/c ratio and the contents of total Na,0. total K2O, water-soluble Na20 and water-soluble K2O in the cement. It was assumed that the amount of each alkali cation taken up by the products is proportional to its concentration in the solution and to the quantity of products (C-S-H and AFm phase) taking it up. This led to the equation... 

Major influences on the kinetic curve of a cement include the phase composition of the clinker, the particle size distribution of the cement and the RH and temperature regimes during curing. Other influences include the w/c ratio, the content and distribution of admixtures, including gypsum, the reactivities of individual clinker phases and probably others, such as the microstructures of the clinker and of the cement particles. 

The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20°C and 100% RH, Patel el al. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, 1.00 and 0.51. If the RH was lowered to 80%, the corresponding values were 0.77, 0.19, 0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of RH on the hydration rate (da/d/) of each of the phases could be represented by a factor (RH — 0.55)/0.45. ... 

In contrast to these results, Locher (L39), working with pure CjS, found that hydration was more rapid at a low w/c ratio. 

Tattersall (T43) found that pastes of w/c ratio 0.28-0.32 and age 4.5 min followed the Bingham model at low rates of shear, but that at higher rates the structure broke down irreversibly. Several other investigators have obtained similar results, but negative hysteresis has also been observed (e.g. Ref. R30), probably due to the use of hysteresis cycles of long duration, in which the structural breakdown due to shear is outweighed by the effects of... 

To an extent that increases with the w/c ratio, fresh cement pastes exhibit the phenomenon of bleeding, i.e. settlement of the solid particles. The interparticle attractions are sufficiently strong that particles of all sizes settle at the same rate, typically about 2pms . Settlement also tends to increase the w/c ratio at the top and to decrease it at the bottom of the sample. It decreases with increased fineness or increased early hydration rate of the cement. In a concrete, it can produce layers of water beneath aggregate particles or reinforcing bars. 

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. 

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