Portland cement paste constituent phases

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. 

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. 

In type M, just as in type K cement, the matrix of the hardened paste is produced by the hydration of the existing Portland cement. Ettringite, the phase responsible for expansion, is formed in a reaction of calcium monoaluminate (CA), the main constituent of high-alitmina cemerrt, with calcium sulfate and calcium hydroxide, formed in the hydration of Portland cement and also added to the system as a constituent of the expansive additive ... 

As to the phase transformations, in blast furnace slag cement pastes increasing amounts of melilite [a sohd solution of gehlenite (C2AS) and akermanite (C2MS2)] are formed as the amount of slag in the original cement increases, at the expense of yff-dicalcium silicate, which is formed in Portland cement pastes under similar conditions. In pastes made from cements that contain distinct amoimts of fly ash, yff-dicalcium silicate is also the main final decomposition product. In parallel, quartz, which is a common constituent of fly... 

In contact with MgSO the reaction of the srrlfate ions with the constituents of the cement paste is essentially iderrtical. Parallel to it, however, a decalcification of the C-S-H phase occurs, resirlting in a distinct strength loss of the material. Bracite [Mg(OH)2] is precipitated in the surface zone, in addition to gypsum. Because of these additiortal reactions, high-iron Portland cement does not exhibit any improved resistance to MgSO attack, contrary to the action of other soluble sirlfates. 

The two most important constituents of Portland cement are alite, a form of tricalcium silicate, and belite, a form of dicalcium silicate. In their hydration both calcium silicates yield—in addition to calcium hydroxide—a nearly amorphous calcium silicate hydrate phase (C-S-H phase), and this hydration product is mainly responsible for the strength and other physico-mechanical properties of the hardened cement paste. 

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