Portland cement paste phases

Fig. 7.3 QXDA results for the fractions of the clinker phases reacted in Portland cement pastes. Filled circles Copeland and Kantro (C39), w/c = 0.65. Diamonds Bezjak et al. (B98), sample C2. Vertical lines Osbaeck and Jons (08), range for 7 samples. Open circles Dalziel and Gutteridge (DI2). Open squares Patel et al. (P28), samples cured at 100% RH. Filled squares Tang and Gartner (T34), clinker interground with gypsum.

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... 

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. ... 

C-S-H having Si/Ca 0.62, Al/Ca 0.09, with a hydrotalcite-type phase having Al/Mg 0.38. As with Portland cement pastes, the material described in Fig. 9.1 as C-S-H may really be an intimate mixture of the latter with AFm and. to a minor extent, hydrotalcite-type structures. 

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. 

J.J. Thomas, J.J. Chen, H.M. Jennings D.A. Neumann (2003). Chem. Mater., 15, 3813-3817. Ca-OH Bonding in the C-S-H gel phase of tricalcium silicate and white Portland cement pastes measured by inelastic neutron scattering. 

Basic properties of Portland cement pastes are attributed to the C-S-H gel. Therefore this phase is a field of interest and the subject of numerous investigations [32]. However, in spite of this, the structme and chemical composition of C-S-H cause several discussions. It is the effect of colloidal constitution of this phase and variable, not well defined composition, depending on liquid phase composition, primarily of calcium ions concentration. Moreover, the morphology of this phase transforms as a function of hydration or maturing time of the samples. 

The higher differences, in phase composition are observed in the pastes autoclaved in the saturated water vapour at temperature range 120-200 °C. The phase composition of the calcium sihcate hydrates is primarily the function of C/S in the starting mixture. In Portland cement paste without additions the C-S-H (I) is transformed quickly to the C-S-H (II) and the latter already at 25 °C into a-C2SH [194, 199]. The proper addition of the ground quartz gives the 1.1 nm tobermorite. This phase contains aluminium in solid solution, which causes the transition to xonotlite, at temperatures exceeding 160 °C difficult... 

Fig. 6.38 Composition of the liquid phase in Portland cement paste subjected to thermal treatment at 90 °C and then cured in water at 20 °C, plotted as afunetion of time. (According to [151])...

The role of the ferrite phase, generally identified as brownmillerite, should be mentioned too. In the case of sulphate attack this phase can be the source of almninate ions [237] moreover the ferrite ions can form the analogue of ettringite or to substitute the aluminate ions in all calcium aluminate phases [222]. The latter case is undoubtedly the most common one in the Portland cement paste. However, the reaction of sulphate ions with ferrites is slower. There is a view that the F/Al ratio in the hydrated phases is lower than in brownmillerite hence, some amount of iron(in) hydroxide is always present [222] (see also Sect. 4.1.1.). This hydroxide occurs in the gel-like form and therefore the diffusion of ions through the gel layer is slowed down. Therefore, the corrosion process is hindered. The other phases containing the Fe ions can be produced too, it is discussed in Chap. 3. 

The examination of chemical composition of slag glass surface at the early age of hydration has shown that it is modified immediately after the contact with the Uquid phase [6]. As the result of incongment dissolution of slag grains on their surface the l er of C-S-H is formed, with, however, lower C/S ratio than in the Portland cement paste. When the hydration of slag is activated by alkalis, this phase contains the Na+ orK+ ions. 

The C-S-H formed has lower C/S ratio than in the Portland cement paste. Take-moto and Uchikawa [23] studied the chemical composition of C-S-H phase binding C3S crystals with fly ash particles. They found a significant decrease of C/S ratio from approximately 2 in the vicinity of C3S to the very low value near the fly ash grain surface (Fig. 8.5). 

Uchikawa [18] determined the composition of C-S-H phase in the Portland cement paste and in the pastes with 40% fly ash and slag addition after 4 years of hydration. The results are given in Table 8.1. 

Calcium hydroxide is formed— besides C-S-H—as the second product of C3S hydration. This happens because the C/S molar ratio within the C-S-H phase is always distinctly lower than that of the original C3S. In hydrated tricalcium silicate or Portland cement pastes calcium hydroxide is present in the form of crystals up to about 10-30 pm large, and in this crystalline form is called portlandite. 

The main constituent of a mature Portland cement paste is the C-S-H phase. Its mean Ca0/Si02 molar ratio may vary, depending on the composition of the cement, the water/cement ratio, and the hydration temperature. On the micrometer scale this ratio may vary between about 1.2 and 2.3. The C-S-H phase formed in the hydration of Portland cement also contains limited but variable quantities of foreign ions. 

The second most important phase of the hydrated Portland cement paste is calcium hydroxide (poitlandite), present in the form of relatively large crystals embedded within the C-S-H phase. The AFm phase is usually intimately intermixed with the C-S-H phase. The amount of AFt declines after reaching a maximum, and this phase is usually absent in mature pastes. As long as it is present, it is distributed within the C-S-H matrix in a microcrystalline form. Some non-hydrated clinker residua may be present even in mature pastes, especially at lower water/cement ratios. 

At equal starting water/cement ratios and degrees of hydration the porosity of pastes made from sulfobelite cements is distinctly lower than that of ordinary Portland cement pastes. This is due to the higher amount of water bound within the crystalline lattice of the solid phases formed in the hydration of the former cement, especially in that of the AFt phase. 

Compared with ordinary Portland cement pastes made with the same water/cement ratio, the porosity of matnre alite-fluoroaluminate cement pastes is distinctly lower, mainly because of the high combined water content of the resulting ettringite phase. 

Just like ordinary Portland cement pastes, alite-fluoroaluminate cement pastes undergo carbonation in a reaction with the CO2 in air. However, the rate of this process is slower than in similar ordinary Portland cement pastes, mainly because of lower porosity (Kim et al., 1992). X-ray diffraction studies have revealed a gradual disappearance of peaks belonging to the AFt and AFm phases, and the formation of gypsum, calcite, and vaterite (Knofel and Wang, 1994). The strength of the material is not adversely affected by carbonation (KnOfel and Wang, 1992). 

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