Belite

It is doubtful whether the polymorphs can be reliably distinguished by DTA, since both the M, and the M3 forms are metastable at low temperatures and transform to Tj at about 700°C (M4,M5). This effect is normally swamped by the endothermic p to aY transition of the belite. Due to the effect of substituents, neither T3 nor Mj appears to form on heating, and endothermic transitions occur at 790-850 C, giving M, and at 950 C, giving M3. These could occur irrespective of the form initially present however, clinkers high in MgO, in which the alite is present entirely as M3, normally show no thermal effects attributable to the alite until more than lOOO C, when transition to R occurs. 

A substituted C3S has been found in nature and given the mineral name hatrurite (G5). The rock containing it is interesting because it contains other phases present in clinker and appears to have been formed under conditions somewhat similar to those existing in a cement kiln. 

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Belit, n., Belith, m. (Cement) belite. bellen, v.i. bark clamor, belolmen, v.t. reward, remunerate, beluften, v.t. (Aero.) ventilate, pressurize. Beluftung, /. airing, ventilation, belustigen, v.t. amuse, divert,... 

Alkali aluminate (5-calcium disilicate monosulfate), phase in Portland cement clinker, 5 472t Alkali aluminate (8-calcium disodium trialuminate), phase in Portland cement clinker, 5 472t Alkali belite, phase in Portland cement clinker, 5 472t Alkali blue toners, 14 318 Alkali borate glasses, 12 572, 573, 584 Alkali catalysed pad-dry-bake procedure, 9 485... 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Calculate the overall Ca and Si contents (expressed as CaO and Si02) of a Portland cement clinker that has 55% alite, 30% belite, 5% alumi-nate phase, and 10% ferrite (assume ideal compositions for the latter two). Would you expect this to behave as a fast or a slow setting cement ... 

The ferrite phase makes up 5 15% of normal Portland cement clinkers. It is tetracalcium aluminoferrile (CajAIFeOj) substantially modified in composition by variation in Al/Fe ratio and incorporation of foreign ions. The rale at which it reacts with water appears to be somewhat variable, perhaps due to differences in composition or other characteristics, but in general is high initially and intermediate between those of alite and belite at later ages. 

Type I belite grains are those that have crystallized from the liquid at temperatures above about I420°C and thus as a-C2S. The primary striations arise on cooling through the a to a transition, in which the symmetry decreases from hexagonal to orthorhombic, each set of striations thus representing a different orientation of the a structure. The transition to a -CjS that occurs on further cooling does not further increase the number of orientations, but that from o l to P, in which the symmetry falls to monoclinic, causes each orientation to split into two. This is the cause of the secondary sets of striations described above. 

Belite crystals have been observed that show only one set of striations. whose behaviour in transmitted light between crossed polars shows them to be twin lamellae. Such crystals are typically irregular in shape they have been called Type II belites, and are rare in modern clinkers. The striations arise from the aY to p transformation. Such crystals can form either by... 

XRD powder evidence shows that in the majority of clinkers the belite is predominantly or entirely of P-CjS structure (GI,YI), though some peaks are broadened (Gl) and the presence also of both a and a (presumably u l) forms has been reported (GI,Yl,RI,OI). Characterization of the polymorphic form is rendered difficult by the similarities between their powder patterns (Fig. 1.5) and by overlaps between the peaks and ones of other phases, especially alite, but has been aided by examination of fraetions in which the belite has been concentrated by chemical (Rl) or heavy liquid (Yl) separation. 

The striations in the common. Type I grains of belite are zones into which impurity ions rejected during the transition from a to a have been concentrated (Y1,01,C1). In some cases, they appear to have retained the a-CjS structure (Y 1,01,FI). In contrast, a study on synthetic belites showed that the material exsolved between the lamellae or at grain boundaries was often... 

Studies on synthetic belites suggest the possible occurrence of modulated structures, i.e. ones in which compositional or structural variations occur with a statistical periodicity that is not a rational multiple of that of the basic structure (H5). Such effects might occur in clinker belites. 

Table 1.2 includes an average composition for belites in ordinary production clinkers, based on the results of many studies by X-ray microanalysis (T2,K1.G3,B2,G4,S1,H3,H4) or chemical analysis of separated material (Yl). As with alite (Section 1.2.3), most of the results from different laboratories are in relatively close agreement. Belites in white cement clinkers have also been analysed (B3). Pure C2S, for comparison, contains 34.9% of Si02 and 65.1% of CaO. 

Clinker belites typically contain 4-6% of impurity oxides, of which the chief ones are usually AI2O3 and Fe203. Correlations have been reported to exist between the MgO contents of belite and clinker (Kl), the Fe20, contents of belite and clinker (Kl), and the total impurity content of the belite and the MgO content of the clinker (B4). Early reports mention a compound KC23S,2, but the K2O content of 3.5% corresponding to this formula is well above those found in clinker belites, and recent electron optical work indicates that the limit of K2O substitution is about 1.2% (C1). 

In general, clinker belite compositions indicate ratios of other atoms to oxygen above the theoretical value of 0.75. As Regourd et al. (R3) noted, the C2S structures are too densely packed for the presence of interstitial ions to appear likely, and it is more probable that vacancies occur in the oxygen sites, with possible concomitant shifts in the positions of other atoms. Table 1.3 includes atomic ratios, with a possible allocation of atoms to sites. 

Regourd et al. (R3) synthesized several belites of P-C2S type with compositions similar to those found in clinkers and determined their cell parameters. With increased substitution, the crystallinity tended to decrease, causing broadening of XRD powder peaks. Two of their results are given in Table 1.5. 

Table 2.3 lists some phases containing MgO that are in varying degrees relevant to cement chemistry. It is not a complete list of phases with essential MgO in the CaO-MgO-AljOj-SiOj system. As seen in Chapter 1, some MgO is also taken up by all four of the major clinker phases, typical contents being 0.5-2.0% for alite, 0.5% for belite, 1.4% for the aluminate phase, and 3.0% for the ferrite phase. Magnesium oxide (periclase), like calcium oxide, has the sodium chloride structure it is cubic, with a = 0.4213 nm, space group Fm3m, Z = 4, = 3581 kgm (S5) and refrac-... 

Reactions below about 1300°C, of which the most important are (a) the decomposition of calcite (calcining), (b) the decomposition of clay minerals, and (c) reaction of calcite or lime formed from it with quartz and clay mineral decomposition products to give belite, aluminate and ferrite. Liquid is formed only to a minor extent at this stage, but may have an important effect in promoting the reactions. At the end of this stage, the major phases present are belite, lime, aluminate and ferrite. The last two may not be identical with the corresponding phases in the final product. 

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