Dicalcium silicate hydration

The hydration or more precisely the hydrolysis of C2S occurs analogously as in the case of C3S and can be schematically written as follows 2C2S+4H — C3S2H3+CH. Therefore the amount of calcium hydroxide formed per mole of silicate is three times lower. 

As it has been mentioned in Chap. 2, C2S occurs mainly as P polymorph. The rate of its hydration is substantially lower, however, there is a common opinion that the mechanism of reaction is the same as in case of C3S. The layer of hydrates on the C2S surface is not observed under the electron microscope, even at very high magnification. After 24 h the P-C2S surface is like the C3S surface after 5 min. l dration 

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The properties of cured Pordand cement are affected by these four constituents of the manufactured Pordand cement. Tricalcium silicate hydrates and hardens rapidly, giving rise to the initial set and eady strength. Increased concentrations of tricalcium silicate causes an increase in the early strength of Pordand cement concretes. Dicalcium silicate hydrates and hardens more slowly, giving the cured concrete its strength increases beyond one week. 

FURNACES, FUEL-FIRED] (Vol 12) a (A) Dicalcium silicate hydrate [15630-58-7]... 

INSECT CONTROL TECHNOLOGY] (Vol 14) d (D) Dicalcium silicate hydrate [54694-02-9]... 

Name l.l-nm tobermorite" l.l-nm tobermorite a-Dicalcium silicate hydrate... 

The products of dicalcium silicate hydration are identical or almost identical to those formed in the hydration of tricalcium silicate however, the amount of calcium hydroxide formed is distinctly lower. The hydration rate of dicalcium silicate is sigrrificantly lower than that of tricalcium silicate, even though it may be influenced to a certain degree by the selection of the dopant ion and the cooling rate. 8ome highly reactive forms of dicalcium silicate have been synthesized in recent years, but are of theoretical importance only. 

Figure 6.13 Radio-frequency pulse sequences for (a) the S / CP NMR experiment, (b) a spin-lock experiment for direct determination of T,p and (d) a spin-lock experiment combined with CP for determination of the 7 p relaxation time, (c) Plot of the observed, transferred magnetisation M,(t) as a function of the CP contact time (t = Tcp) in Si H CP/MAS NMR spectra (7.1 T, = 4.0 kHz) of a mineral sample of kaolinite (circles) and a synthetic sample of a-dicalcium silicate hydrate (diamonds). The experiments employed yB /2]c ss ...

Dicalcium silicate (2CaO SiO ) is very important in the final strength of the cement. This compound hydrates very slowly. The average dicalcium silicate content is 25% to 35%. 

The tobermorite obtained in the hydration of tricalcium silicate (Ca3Si02), / -dicalcium silicate (/ -Ca2Si04), portland cement, and concrete is a colloid, with a specific surface area of the order of 300 sq. meters per gram. To give an idea of how the elementary particles of tobermorite look, Figure 7 is an electron micrograph of a few particles (obtained by L. E. Copeland and Edith G. Schulz at the Portland Cement Association Research and Development Laboratories). These particles look like fibers, but if you watch them closely, you see that they are very thin sheets, rolled up as one would roll up a sheet of paper. At the lower end the sheets are partly unrolled. When one prepares tobermorite by the reaction of lime and silica, one usually obtains crumpled sheets, which are not rolled up. The electron microscopists tell us that the sheets are very thin, of the order of a single unit cell in thickness. 

Figure 7. Electron micrograph of tohermorite obtained from hydration of ff-dicalcium silicate...

The conclusion is that both the body structure and the surface structure of tobermorite are highly reproducible. Whether we use tricalcium silicate or fi-dicalcium silicate as starting solids, whether we use a water to solid ratio of 0.7 or 9.0, whether we use paste hydration or ball-mill hydration or a third type which I have not discussed (which gave the six other points on the curve), we wind up with a tobermorite having very nearly the same body structure and surface structure. 

The hydration of tricalcium silicate C3S and dicalcium silicate C2S (for abbreviations see below Table 5.3-6) are responsible for the further. solidification of Portland cement. This reaction only begins in earnest after ca. 4 hours. Initially long needles of calcium silicate hydrate are formed, which bond the cement particles together. Later, smaller needles of calcium silicate hydrate fill the gaps left. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate. 

Portland cement is typically composed of about 25% P-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)... 

Si NMR has also been applied to a hydration study of actual Portland cements, the broad spectra of which are composed of the spectra of the component tricalcium silicate and p-dicalcium silicate (Figure 4.40) (Barnes et al. 1985). Integration of the Si spectra of the discrete cement silicates and Portland cement hydrated under identical conditions allowed a comparison of the hydration characteristics (Figure 4.41) and... 

Figure 4.40. Changes in the Si spectra of the cement minerals tricalcium silicate (C3S) and 3-dicalcium silicate (C2S). and ordinary Portland cement (OPC) containing these minerals brought about by hydration at 20°C for one week. From Barnes et al. (1985), by permission of copyright owner.

The hydration reactions of tricalcium and dicalcium silicates can be illustrated as follows ... 

It should be noted that significantly more cement and hydration phases are known in literature [12]. For example, dicalcium silicate (2 CaO Si02 C2S), an important clinker phase which accounts for approx. 5-10 wt-% of the cement used to prepare the samples, was deliberately left out of the refinement. Although dicalcium silicate could be positively identified in the diffraction patterns of the unhydrated cement, its quantification in the Rietveld refinement was not reliably possible after the samples had been exposed to water and the phase was, therefore, excluded. 

Matkovic, B. and others, "Influence of BaSO on the Formation and Hydration Properties of Calcium Silicates, Parts I and II, Doped Dicalcium Silicates," Ceramic Bulletin, Vol. 60, Nos. 8 and 11,1981b, pp. 825-829 and 1164-1167. 

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