Portland cement 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. 

This section considers a number of extremely important structure types in which A1 combines with one or more other metals to form a mixed oxide phase. The most significant of these from both a theoretical and an industrial viewpoint are spinel (MgAl204) and related compounds, Na- -alumina (NaAlnOi ) and related phases, and tricalcium aluminate (Ca3Al20g) which is a major constituent of Portland cement. Each of these compounds raises points of fundamental importance in solid-state chemistry and each possesses properties of crucial significance to... 

Alite is the most important constituent of all normal Portland cement clinkers, of which it constitutes 50-70%. It is tricalcium silicate (CajSiOj) modified in composition and crystal structure by incorporation of foreign ions, especially Mg ", AP and Fe. It reacts relatively quickly with water, and in normal Portland cements is the most important of the constituent phases for strength development at ages up to 28 days, it is by far the most important. 

Portland cement clinkers contain small amounts of alkalis and sulphates derived from the raw materials and fuel. Both alkalis and SO3 can be present in the major clinker phases, but tend to combine preferentially with each other to form alkali or potassium calcium sulphates, and it is necessary to consider these components together. In addition, silicate and aluminate phases containing sulphate can form either as intermediates or in undesirable deposits in eement making, and a calcium aluminate sulphate is a major constituent of some expansive and other speeial cements. 

Numerous important mixed oxide phases are formed by the combination of aluminum oxide and one or more other metal oxides. The foremost of these from an industrial viewpoint are spinels, jS-aluminas, and tricalcium aluminate, an important constituent of Portland cement. 

Brownmillerite phases form an important constituent of cement powder. Portland cement, one of the most widely used types, is made by heating mixed limestone and clay in a kiln. This results in multiple reactions together with partial melting and sintering of the mixture, producing a dense mass called clinker, which is ground to produce cement powder. The resulting complex material contains four major... 

A chemical analysis alone cannot describe the form, particle size, or mineralogy of the feed. SiO from a chemical analysis does not necessarily mean quartz, nor does Fe Oj necessarily imply hematite. Analysis by X-ray diffraction (XRD) quite accurately records most of the detectable mineralogical varieties and with calibrated standards allows an estimation of abundance. But XRD cannot elucidate the particle form or size, and virtually misses the occurrence of amorphous materials such as glass or poorly crystalline materials such as limonite, FeO(OH), a major constituent in many iron sources for portland cement. Phases below the detection limit by XRD can easily be seen in the microscope. However, chemical and XRD analyses of each of the raw materials individually, in sieved fractions, and in their blended combination in the feed, are immensely helpful, indeed strongly recommended, for routine... 

Daimon, M. and others, "Crystal Structure Analysis of Major Constituent Phases in Ordinary Portland Cement," 9th International Congress on the Chemistry of Cement, New Dehli, 1992, pp. 17-22. 

In ordinary Portland clinker (corresponding to ASTM type I Portland cement) tricalcimn silicate is the most abundant phase, present in amormts between about 50% and 70%. Dicalcium silicate usually constitutes 15-30% of the clinker. Typical amounts of tricalcium aluminate are 5-10%, and of the ferrite phase 5-15%. In special Portland clinkers the individual clinker pliases may be present in increased or reduced amounts, or may be absent entirely. Free lime, free MgO and alkali sulfates are minor constituents of Portland clinker that are present only in small amounts or may be absent entirely. Calcium sulfate is only rarely a constituent of Portland clinker, but is intergroimd in limited amounts with it, to obtain Portland cement. In Table 2.1 different types of Portland cement are compared. 

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 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. 

Ca2Al(0H)g.2H20].0H. In C4AHjc, an additional layer of H2O molecules is present between the principal layers. Tetracalcium aluminate hydrate is not among the hydrate phases formed in the hydration of calcium aluminates constituting calcium aluminate cement however, it may be formed in the hydration of tricalcium aluminate, a constituent of Portland cement. 

In mixes with Portland cement as the main constituent, the setting is due to a rapid formation of ettringite and the hydration of calcium aluminate cement. The hydration of the calcium silicates has little influence on the setting process, but contributes to the subsequent strength development (Gu et al., 1994). In addition to the phases formed in the hydration of pure Portland and calcium aluminate cements, stratlingite (gehlenite hydrate, C2SAHg) may also be formed in the hydration process. 

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... 

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