Tricalcium silicate phase

Rapid cooling ol the clinkers to obtain a high content of the tricalcium silicate phase... 

The crystal stmctures of the individual tricalcium silicate phases are similar, and differ mainly in the spatial orientations of the Si04 tetrahedra. 

The reasons for the fast setting of OPC+CAC blends are not fully understood. It is assumed that in blends high in Portland cement the rapid setting is due to the formation of ettringite in a reaction between the CA phase of the calcium aluminate cement, calcium sulfate present in the Portland cement, and calcium hydroxide formed in the hydration of the tricalcium silicate phase present in Portland clinker ... 

The amount of Ca(OH)2 formed in this reaction is less than that produced in the hydration of C3S. The dicalcium silicate phase hydrates much more slowly than the tricalcium silicate phase. 

Mineralizers are added to the raw feed to aeeelerate the kinetics of reactions by modifying the solid and liquid state sintering. The temperature of decomposition of calcium carbonate is lowered in the presence of mineralizers. In the synthesis of C3S and C2S, DTA has shown that some phosphates, earbonates, sulfates, and ehlorides deerease the deearbonation temperature and that of the formation of the silieates.[ °l Caleium fluoride acts both as a flux and a mineralizer in promoting the formation the tricalcium silicate phase. DTA thermograms have been applied to study the effect of mineralizers on the melting behaviors and crystallization temperatures of clinker. Early formation of liquid can be established by thermal techniques. 

Thermal analysis data on the hydration of dicalcium silicate are sparse because it is time consuming to follow the reaction of this phase which is very slow. The characteristic products obtained during its hydration are not much different from those formed in C3S hydration. Also, the major strength development that occurs in cement in the first 28 days (a period of practical significance) is mainly due to the tricalcium silicate phase. TG, DTG, and DTA investigations of C2S were carried out by Tamas.t The sensitivity of the instrument had to be increased substantially to detect the peaks due to the decomposition of calcium hydroxide and calcium carbonate, especially at earlier times. In Fig. 21, the DTA, DTG, and TG curves of C2S hydrated for 21 days and 200 days are given. A comparison of these peaks with those obtained from C3S pastes shows substantial differences in the intensity value of the peaks. The 200 day C2S sample shows a weight loss of 4%, whereas C3S hydrated for 21 days indicates a loss of 13%. 

Jeffery, J. W., The Tricalcium Silicate Phase,Symp. Chem. Cements, pp. 30-38, London (1952)... 

There has been continued interest in developing an organic-based accelerator. Ramachandran and coworkers studied the effect of o, m, and p-nitrobenzoic acids on the hydration of tricalcium silicate.The m and p nitrobenzoic acids acted as accelerators. The acceleratory effect was attributed to the complex formation between the organic compound and the C-S-H phase on the surface of the tricalcium silicate phase. 

Figure 16. Effect of SNF on the hydration of tricalcium silicate phase in a high alkali cement.

Portland cement clinker. In addition to the major dicalcium silicate and tricalcium silicate phases, the structure shows silicate and tricalcium silicate phases, smaller amounts of tricalcium aluminate (3Ca0-Al203), and millerite (4Ca0-Al203-Fe203). On reaction with water, the clinker forms a complex hydrated product. This product is the cementitious material. It is a noncrystalline calcium silicate gel resulting from the tricalcium silicate and dicalcium silicate. The microstructure of set cement is shown in Figure 11.25. 

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. 

Calculated X-ray powder diffraction patterns for tricalcium silicate and clinker phases... 

Type (b) Periclase, dicalcium silicate, merwinite or tricalcium silicate and melt are phase compositions developed for steel making refractories (bj and b2 types). 

The alumina modulus is the ratio of alumina to iron oxide. If the alumina modulus is 0.637 or less, no tricalcium silicate in formed in the melt phase. 

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. 

The early strength of concrete is the result of hydration of tricalcium silicate C jS) and tricalcium aluminate (C.,A) phases of Portland cement. When mixed with water C.-,S hardens rapidly and both C.,S and C..,A release heat. 

The setting reactions of the varions tricalcium silicate cements are very similar, as described in Chapter 8. They also resemble the setting of Portland cement. The initial setting involves the hydration of the alite (Ca SiO ) and belite (p-Ca SiO ) phases to form a poorly crystalline gel phase consisting of calcinm hydroxide in calcium silicate hydrate (approximate formula CajSi O ) [102], After the initial hardening, further condensation reactions occur which improve the strength and give rise to short silicate chains within the structure [103],... 

In the process formation two stages can be distinguish at the range of lower temperature to about 1300 °C, when the reactions proceed, as a rule, in the sohd state, in the presence of very low quantities of the liquid phase, and at higher temperature, at which it is already about 25 % of the melt. It is assumed that the dicalcium sihcate and calcium aluminate and ferrite are formed principally as the result of reactions in the sohd state. However, tricalcium silicate is formed by ciystalhzation from the liquid phase. 

The liquid phase, its chemical composition and physical properties, and principally viscosity and surface tension, are very important, because in industrial process tricalcium silicate is crystallizing from this phase. Calcium oxide and dicalcium silicate are dissolved in the liquid phase which became oversaturated towards C3S. The ciystallization of tricalcium silicate begins at about 1300 °C, but in prevaiUng mass proceeds at 1450 °C, i.e. at maximum chnkering temperature. 

Tricalcium silicate is the most important component of Portland cement clinker. Its share as a rule is overpassing 55% and reactivity with water has the decisive effect on paste hardening. At room temperature CjS is triclinic. The structures of different polymorphic phases of tricalcium silicate was determined by Jeffery [99], The basis was the rhombohedral pseudo-stracture with hexagonal unit cell [99] ... 

The tricalcium silicate hydration, and as a matter of fact, the tricalcirrm silicate hydrolysis is of special importance, because this phase is a dominant component of Portland cement clinker, constitnting more than 60 %. Moreover, the tricalcium silicate hydration is often considered as a good model of cement reaction with water. 

In 1936 Insley (about whom much more is said later) clearly showed that (1) alite is tricalcium silicate (CjS), (2) two different habits of dicalcium silicate (C S) comprise belite and felite, and (3) celite is tetracalcium aluminoferrite (C AF). Insley s descriptions and illus trations of clinker phases remain the basis for much of succeeding publications by others. 

Chromy, S., "High-Temperature Microscopic Investigation of Tricalcium Silicate and Dicalcium Silicate Phases in Portland Cement Clinker," Journal of the American Ceramic Society, Vol. 50, 1967, pp. 677-681. 

Tricalcium silicate is an essential constituent of all Portland clinkers. In its crystalline lattice it contains Ca cations in combination with 8104 and anions in a ratio of 3 1 1. Upon heating or cooling tricalcium silicate undergoes a series of reversible phase transitions ... 

Tricalcium silicate is thermodynamically stable only between about 1250°C and 2150° C. It melts incongmently above 2150°C, and becomes thermodynamically unstable with respect to dicalcium silicate and calcium oxide below 1250°C. Thus, to produce a clinker containing tiicalcium silicate, the raw mix has to be heated to a temperature that is higher than the lower limit of thermodynamic stability of this phase, and has to be cooled rapidly enough to prevent a noticeable decomposition of tricalcium silicate in the course of cooling. Such a conversion of C3S to C2S and C may take place especially at temperatures slightly below 1250°C, whereas at ambient or only moderately elevated temperatures tricalcium silicate remains preserved for an indefinite time. 

Upon contact with water, tricalcium silicate undergoes hydration, yielding an amorphous calcium silicate hydrate phase called the C-S-H phase (or just C-S-H) and calcium hydroxide as products of hydration. The rate of hydration will depend on the quality and quantity of dopants incorporated within the crystalline lattice, on the cooling rate in the production of the chnker, on the fineness of the cement, and on other factors. 

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. 

Immediately upon contact of the cement with water, in the pre-induction period, a rapid dissolution of ionic species in the liquid phase and the formation of hydrate phases get under way. Alkali sulfates present in the cement dissolve completely within seconds, contributing K, Na, and SO " ions. Calciiun sulfate dissolves imtil saturation, thus contributing Ca and ions. Small amoimts of tricalcium silicate (about 1-5%)... 

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