Alite structure

Portiand cement clinker structures (18,19) vary considerably with composition, particle size of raw materials, and burning conditions, resulting in variations of clinker porosity, crystallite sizes and forms, and aggregations of crystallites. Alite sizes range up to about 80 p.m or even larger, most being 15—40 )J.m. 

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. 

Figure 5.8 The crystal structure of olivine, (a) The structure projected onto (100) showing serrated chains of octahedra running parallel to the c axis (b) oxygen coordination polyhedra projected about the Ml and M2 positions. Metal—oxygen distances in each coordination site are indicated. Cell parameters and interatomic distances are for fay-alite (from Smyth Bish, 1988).

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. 

A number of experimentally determined XRD powder patterns have been reported for CjS and alite polymorphs (PI), but because of uncertainties in the indexing and sometimes in identification of the polymorph, these are usefully supplemented by patterns calculated from the crystal structures (Appendix). The patterns of the polymorphs are closely alike, and in clinkers, interpretation is complicated by the fact that many peaks overlap ones of other phases. The pattern depends not only on the polymorph or... 

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. 

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-... 

There should be little free lime. What there is should occur as rounded grains, typically 10-20 pm in size, and associated with alite and interstitial material. Lime appears cream in sections etched with HF vapour. Its presence may be confirmed by a microchemical test using White s reagent (5 g of phenol in 5 ml of nitrobenzene + 2 drops of water) long, birefringent needles of calcium phenate are formed. The test also responds to CH. Alkali sulphates occur in the clinker pore structure they are etched black with HF vapour, and inhibit the etching of silicate phases with which they are in contact. 

Applications of IR and Raman spectroscopy to the study of clinkers and unhydrated cements have been reviewed (B39,B40). The laser Raman microprobe, with which regions of micrometre dimensions on a polished surface may be examined, has been used to investigate structure and crystallinity, especially of the alite and belite (Cl9). Spectroscopic methods for studying the surface structures and compositions of cements are considered in Section 5.6.2. 

Stade el al. (S70) examined C-S-H preparations containing Al by Al magic angle NMR. At a Ca/Si ratio of 0.95, the aluminium was tetrahedrally coordinated this is also true of aluminous tobermorite (K34), which is of broadly similar composition and structure. At higher Ca/Si ratios, octahed- ral aluminium was also present. The fraction of the aluminium in octahedral coordination increased with Ca/Si ratio, and at Ca/Si =1.51 reached unity. The aluminium in pastes made from alite or from C3S with a little CjA was / also largely in octahedral coordination (S71). Electron spin resonance and Mossbauer examinations of C-S-H preparations containing Fe showed / that the iron was in octahedral coordination (S72). 

Olefin selectivities also decrease with increasing bed residence time and chain size on Ru catalysts (4,14). For example, propylene selectivity decreases with increasing bed residence time without a corresponding increase in propane selectivity, leading to a net decrease in the fraction of the converted CO that appears as C3 molecules (Fig. 7b). Readsorbed olefins initiate chains that continue to grow and ultimately desorb as larger olefins or paraffins, ( alitative trends are similar on all supports and on both Ru and Co catalysts. The selectivity details depend on the support physical structure, on the density of exposed surface metal atoms, and on the intrinsic readsorption properties (j8r) of Co and Ru surfaces. 

The vertical, single shaft kilns were used for burning limestone to produce quicklime 2000 years ago. Romans used such vertical kilns to burn pozzolanic lime. The term pozzolan applies to the incoherent pyroclastic-si-alitic rocks occurring in the neighborhood of Pozzouli-Italy, and it has subsequently been extended to include a wide range of both natural and inorganic materials differing in nature, composition, and structure. 

The detailed structures of alite and belite are known. Both consist of isolated SiO, telrahedra, the so-called Q species. Following hydration to form Si-OH groups, there is a condensation reaction that results in pairs of SiO tetrahedra becoming joined as dimers, the so-called Q species [57]. Condensation reactions continue and these cause short-chain silicate species to form, with Q end groups and mid-chain units (ie, SiO tetrahedra joined at two comers to other SiO tetrahedra [62]). In addition, some of the mid-chain units contain aluminium rather than silicon, and so are CF(1 Al) species. 

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],... 

C. Nital is perhaps the most common etchant and stain for silicates and improves with age. Nital is 1.5 mL of nitric acid (HNO ) in 100 mL of ethyl, methyl, isopropyl, or amyl alcohol. The author routinely uses asolution of 1 mL of HNOj and 99 mL of anhydrous isopropyl alcohol. The solution quickly reacts in 6 to 10 seconds with alite and belite. At a 0.05% dilution the reaction time is 20 to 40 seconds. Ono (1995) relates alite reactivity to color produced with 0.2% nital. Depending on the relative reactivity of silicates, alite normally turns blue to green, belite is brown to blue—both silicates showing details of internal structure. Nital superimposed on a 20-second potassium hydroxide etch turns C3A light brown and colors the silicates. 

L. Dimethyl ammonium citrate (DAC) solution is prepared by dissolving 192.6 g of citric acid in 1 L of warm water. The solution is cooled and brought to 2 L by adding 891 mL of aqueous dimethyl ammonium solution (33 percent). A 5- to 10-second application of DAC on a polished surface structurally etches alite strongly and belite slightly. An optional preparatory etch with water for five seconds will aid in the identification of aluminate. 

Photograph 3-2 Blue coloration on alite with an otherwise uniform tan color on alite crystals. Possible explanations include differences in crystallographic orientation, chemical composition, structural state (for example, monoclinic versus triclinic), or perhaps combinations of these. (S A6612)... 

Alite crystal chemistry was discussed in a paper by Ono (1974) in which he described changes in the atomic structure of alite in response to such variables as solid solution, exsolution, thermal vibration, states of disorder, inversion, and partial decomposition. 

Extremely dense structure, large alite Kiln wall (Fundal, 1980)... 

Disappearance of ferrite phase, iron transformed to metallic state, clinker color changes to white, alite decomposition structures vanish Extreme reducing conditions (Woermann, 1960)... 

Relatively wide and more numerous belite borders on alite which shows lamellar structure, the latter well-developed upon withdrawal from kiln at temperatures lower than 1200°C Clinker burned under reducing conditions, removed from kiln at1200 C, aircooled. Similar texture when removed at 1150°C and quenched in water (Sylla, 1981)... 

Photograph 9-6 Nital-etched, broken surface of clinker, revealing belite lamellar structure. Note small belite inclusion in alite crystal (left center). (S A6732)... 

Photograph 9-25 Alite crystal with attached alkali sulfates. Lamellar structure in belite revealed in upper right corner. 30-min maleic acid etch on crushed, sieved clinker. (S A6748)... 

Burki and Braun (1988) asserted that the clinker structure from laboratory tests is principally determined by raw meal properties and to a lesser extent by the heating rate. Rapid heating was said to increase the alite formation rate, accelerated with a homogeneous raw meal made of a chalky, clayey limestone (cement rock). A coarse microstructure (large voids and large crystals) was produced from coarse meal made largely of monomineralic particles. No influence of the final size of the alite and belite was observed after variations in heating rates. 

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