Advances in Cement Research, 1997, 9, No. 33, January, 17-23

X-ray mapping of interstitial phases in sulphate resisting cement clinker

E. BAcKSTROM* and S. HANSENt

Euroc Research AB; National Centre for HREM

Industrial sulphate-resisting cement clinker has been investigated by elemental mapping and X-ray microanalysis (EDS) in a scanning electron microscope; powder X-ray diffraction (XRD) and X-ray fluorescence analysis. The interstitial material mainly consists of ferrite CaN)! (nAlo 7!( 7)Feo'89(6)Mgo,o8( 2)MnOO5(2)Sio'!6( 4) TiO03( l)PO009( 3) 05 (EDS) , plus minor amounts of an orthorhombic aluminate (XRD) and potassium sulphate (EDS). In the elemental maps (Ca, Si, AI, Fe, K. S), recorded using the characteristic Ka X-rays; alite, belite, ferrite, potassium sulphate and pores are readily identified, while aluminates are only rarely distinguishable. Free lime, periclase, unreacted silica, and secondary alite should be easy to recognize, but were not observed. Chlorine or sodium were not detected in the clinker. More potassium sulphate was associated with belite, than with a lite, and the texture of the interstitial potassium sulphate suggests that at high temperature, molten potassium sulphate wets the belite crystals, but not the alite crystals.

Introduction In ordinary Portland cement clinker, the interstitial

material mainly consists of aluminate and ferrite (idealized formulas Ca3Al206 and CazAlFe05), as well as sulphates of calcium, sodium and potassium. The fine-grained interstitial phases are formed from melts surrounding the crystals of alite and belite (ideally Ca3SiOs and Ca2Si04) in clinker, upon cool-ing from the highest kiln temperatures. Sulphate-resisting cements are relatively rich in iron and the ferrite therefore predominates strongly over aluminate, among the interstitial oxides of the clinker.

Early properties, e.g. workability, of concrete made from these two types of cement, depend on the rapid hydration reactions between the aluminate and ferrite phases and the added sulphates. The reactivity of aluminate and ferrite in cement is not only dependent on the amount of sulphate present, but also on the

I exact nature of the sulphate compounds. The hydration behaviour of ferrite is less well known

* Euroc Research AB, Ideon Lund, Solvegatan 41, S-223 70 Lund, Sweden t National Centre for HREM, Inorganic Chemistry 2, Chemical Center, Lund University, P. O. Box 124, S-221 00 Lund, Sweden

(ACR 252) Paper received 2 January 1996; last revised 15 February 1996; accepted 13 March 1996.

compared to aluminate, although it is of prime importance in sulphate-resisting cements. Using syn-thetic solid solutions Ca2AlxFe2-xOs, it has been shown that the reactivitr increases with aluminium content in pure water,2, while in the presence of gypsum the iron-rich members are more reactive.4

X-ray microanalysis of clinker ferrites indicates a typical composItIon of Ca2AlFeo6(Mg, Mn)o2(Si, Ti)o.zOs in ordinary Portland cement clinker, while for sulphate-resisting cement clinker Ca2Alo7 Feoy(Mg, Mn)o.z(Si,Ti)o20s is suggested.s It has also been demonstrated by transmission electron microscopy studies,6,7 that the composition can vary drastically from the centre to the edge of a ferrite grain.

It is thus reasonable to assume that the hydration rate in cement is affected by, among other things, the actual composition of the clinker ferrite and by the sulphate phases present. In this paper, the interstitial oxide and sulphate phases, in production clinker for a sulphate-resisting cement are investigated, mainly by scanning electron microscopy techniques.

Experimental procedures Preparation of samples

Polished clinker. The material used in this study consisted of Swedish high-ferrite production clinker sampled during 1994. Clinker was cast in epoxy resin

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Backstrom and Hansen

and the cylinder with clinker was then sawed into two pieces, using 1,4-butanediol as the cooling medium. The fresh surface was then smoothed on a rotating disc of diamond grit, with ethanol as cooling medium. The surface was further polished in three steps: silicon carbide (600-800 mesh) and butanediol on glass plate (2 min); silicon carbide (1200 mesh) and butanediol on rotating disc (2 min); and finally diamond paste (1-2 ,um) and butanediol on rotating disc (30 min). The surface was washed with ethanol between polishing steps and after the last step. Polished areas were coated, by evaporation, with a thin layer of carbon and the rest of the cylinder was covered with conducting paint, in order to minimize electrical charging in the micro-scope. Samples were kept in sealed plastic bags before study.

Selective extraction of ferrite and aluminate. The clinker was ground to an average particle size of 5 ,um. Then 5 g of the sample was stirred with 25 g salicylic acid and 300 ml methanol for 24 hours. The solution was vacuum-filtered and the residue washed with acetone. For energy dispersive X-ray microanalysis, small amounts of extracted clinker were mounted on copper stubs covered with a conducting polymer. A thin layer of carbon was then evaporated on to the samples.

X-ray powder diffraction and fluorescens analysis The samples used in the powder diffraction experi-

ments consisted of the interstitial material prepared by selective extraction. X-ray diffraction spectra (Fig. 1) were recorded in reflection mode, using CaF2 as internal standard, on a Siemens D500 diffractometer

F141 500

33 34 28 ( degrees)

Fig. 1. Section of powder X-ray diffraction spectrum re-corded using an extracted interstitial sample. One calcium aluminate peak (A400 + (40) and the largest ferrite peak (Fl41) are marked 18

Table 1. X-ray fluorescence analysis of clinker for a sulphate resisting cement and Bogue calculation

Oxide Weight % Phase Weight % Volume %

CaO 662 C3S 577 61 Si02 23-4 C2S 23-6 24 Ab 0 3 3-6 C3 A Ig 2 Fe203 46 C4 AF lJ. 11 Na20a 00 979 100 K20 05 MgO 08 Ti02 02 Mn20] 02 P20 S 01 S03a Q:.l

997

a Other methods indicate normal contents of around 005% Na20 and 0-4% S03 in the clinker.

(radiation CuKa ), 28-step size 0'005, counting time per step 10 s). Elemental analyses on a production clinker (Table 1) and two leached interstitial materials (Table 2) were performed using a Philips PWl480 X-ray fluorescence spectrometer.

X-ray mapping and microanalysis A JSM-840A scanning electron microscope, inter-

faced with a Link ANlOOOO energy dipersive X-ray microanalysis system, was used in these studies. X-ray microanalysis (point analysis, accelerating voltage 20 kY, probe current 6 nA, effective counting time 100 s) was performed on extracted interstitial material (Table 2) and sometimes also on polished clinker samples.

Elemental maps (Figs 2 and 3) were collected on polished samples using the characteristic Ka X-ray radiation of the elements; Ca, Si, AI, Fe, K, S. Maps

Table 2. Atomic composition offerrite extracted jrom dif-ferent clinkers determined by X-ray microanalysis in a scanning electron microscope (no 1) or by X-ray fluores-cence spectrometry (no 2 and 3) Elementa No. Ib No. 2 NO.3

Ca 2'01(7) 201 201

Al 0'71(7) 093 092 Fe 0'89(6) 075 075 Mg 0'08(2) 013 013 Mn 0'05(2) 003 003 Si 0'16(4) 0'13 c 013' Ti 0,03(1) 002 002 P 0009(3) Q:Ql Q:Ql sum atoms 193 200 199 sum charges 587 601 598

0 5d 5" 5"

a Potassium and sulphur excluded b Average of six measurements with standard deviations referring to the last digit in parenthesis (analysed area :::::3 ,urn) C Assumed equal to magnesium due to presence of excess silica d Oxygen content calculated assuming Fe(1II). Mn(1I). Ti(IV) etc and then normalized to five oxygens

Advances in Cement Research, 1997, 9, No. 33

X-ray mapping of inter. titial phases in cement clinker

a b

c d

e

Fig. 2. A repre elltatil'e et 0/ elemental lIIap recorded by energy di persive analy is 0/ characteristic Ka X-r

Backstrom and Han en

a b

c d

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Fig. 3. Elemelltal IIWP of an allomalou ' clinker wilh large area of alile. pOla illln IIlphale and POIG illm aluminGle (arrowed in Ihe aluminium map): (a) alcium. (b) ilieon. (e) aluminillm. (d) irQn. (e) pOla ' ill/II. (f) !llp/llIr. The ize of Ihe map i 120 X 120 !lIn

Re ult and di cu ion Elemental analy e

The oxide composition of the cement clinker in the pre ent inve tigation, Table 1, i imilar to the oil-well

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ement clinker rudied in R f. 6, and the uncorrected Bogue alculation in the ame table indicate that ferrite can be expected to be the predominating inter titial pha e. The volume fraction f the main pha e have al 0 been e timated for compari on with

At/I'allces ill emem Re ear, h, 1997, 9. o. 33

experimental clinker images, although the volume and weight fractions are quite similar due to a small variation in density of the four phases.

The ferrite compositions derived from elemental analyses of extracted interstitial materials are com-pared in Table 2. The two samples analysed by x-ray fluorescence spectrometry contained excessive amounts of silica. Particles giving Si, as the only detectable element, were also found in the scanning electron microscope, but no crystalline silica phase was observed by powder X-ray diffraction. We assume that the silica is an amorphous residue of the silicates dissolved in the sample preparation procedure. If we assume the presence of an equal number of Si and Mg atoms, since Mn and Ti already balance each other, a reasonable ferrite stoichiometry is obtained.

The X-ray microanalysis, no. I in Table 2, is quite similar to the expected average composition of a fer-rite in sulphate resisting clinkers,s which is Ca 1.98 Alo72Feo90Mgo17Mno02Sio14 Tioo5 0 s. The two ana-lyses by X-ray fluorescence are intermediate between this formula and the more aluminium-rich composition typical for a ferrite in Portland cement clinker,s i.e. Cal98AllooFeo62Mgo17Mnoo2Sio14 TioosOs. The differ-ence in Al and Fe content observed by the two techniques, is probably due to the sample size. With X-ray fluorescence a relatively large specimen is analysed and it will then contain a small amount of aluminate in addition to the ferrite, which increases the AI/Fe ratio somewhat.

The analysis results confirm the conclusion6- s that the main substitution in clinker ferrite is of the type:

2MIII +-t MIl + M'v

with MIll (AI, Fe); MIl (Mg, Mn); and M'v (Si, Ti).

Phase analysis of clinker Powder X-ray diffraction. X-ray diffraction spectra

of selectively extracted interstitial phase, shows it to consist almost exclusively of ferrite, only one alumi-nate peak is readily detectable at 28 = 329 see Fig. l. A comparison with literature spectra of ferrite-aluminate mixtures,9 tentatively identifies this peak as the overlapping 4 0 0 and 04 0 reflections of ortho-rhombic aluminate (a ~ b = 109, c = 151 A). The integrated intensities of this aluminate peak and the stongest ferrite peak, i.e. I 4 I at 28 = 338, were estimated assuming the peaks to be mirror symmetric. Ferrite is also orthorhombic, with a = 55, b = 146, c = 53 A. Using the reference intensity ratios cited by TaylorS and the peak area ratio (ferrite/aluminate = 189) estimated from Fig. 1, we arrive at 9% aluminate and 91 % ferrite by weight. This agrees favourably with the values 11 % aluminate and 89% ferrite obtained from the Bogue calculation in Table 1, after deduction of calcium silicates. Both the Bogue calculation and the powder diffraction determination are subject to errors, but since they agree it is reasonable to assume

Advances in Cement Research, 1997, 9, No. 33

X-ray mapping of interstitial phases in cement clinker

an aluminate content of around 10 wt % in the oxide interstitial phase.

Elemental mapping. In typical elemental maps, like the ones in Fig. 2, the following phases are easy to recognize; alite, belite, ferrite and potassium sulphate. The silicates alite and belite form crystals, 5-50 .um in size, while ferrite and potassium sulphate form separate interstitial phases. The crystals forming the interstitial phase are usually micrometer-sized and cannot be resolved by the present technique; although pores in the interstitial phase are readily distinguished.

Calcium is present in decreasing concentrations in; alite, belite, ferrite ~ aluminate, potassium sulphate ~ pores. The light areas in Fig. 2(a) thus represent alite, grey areas are belite, while in the dark areas, forming the interstitial phase, it is usually possible to distin-guish between ferrite/aluminate-potassium sulphate on one hand and pores, which are black, on the other. Potassium sulphate probably appears similar to ferrite/aluminate because of its higher background radiation, compared to a resin-filled pore. The larger angular crystals consist of alite and the smaller rounded ones consists of belite, as expected. Free lime has not been observed, but should be easy to identify due to a calcium concentration higher than that of alite, and the absence of silicon. The content of free lime in the clinker under study is normally around I % by weight, when determined by titrimetric methods. Free lime is expected to form rounded grains, 1O-20.um in size, close to alite5 in Portland cement clinker.

Preliminary results indicate that the calcium maps are the most suitable for evaluation of the volume fractions of different phases in clinker, by image processing techniques. Alite, be lite and interstitial phases plus pores can be represented as three distinct grey-levels and their area fractions are then calculated by the software. The separation of interstitial phases and pores is more difficult, but ought to be possible.

Silicon in Fig. 2(b) is present in a greater content in belite (light), than in alite (grey). Between the silicate crystals, the silicon content is very low (dark areas) and no details can be identified.

If a clinker of the present composition is cooled at an insufficient rate, secondary growth of small alite crystals can occur. No clear example of this behaviour has been observed, which indicates that sufficiently rapid cooling has been applied. Free silica (unreacted sand) should give a silicon signal stronger than belite. This, in combination with the absence of other signals, would allow straightforward identification. No free silica has been observed in the clinker and indicates that the raw material has been properly ground.

Aluminium and iron maps are shown in Fig. 2( c) and (d). A comparison of the maps shows that Al and Fe are distributed in a similar way and occur between the silicate crystals as an oxide interstitial phase consisting mainly of ferrite. Small areas giving a

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Backstrom and Hansen

signal for Al (and Ca) but not Fe, indicative of aluminate, are only rarely observed. When they are observed, they usually are enriched in potassium, cf. the discussion of potassium maps below. Most alumi-nate crystals are probably smaller in size than the resolution of the technique used, which can be estimated from the maps to be around one micro-metre.

Potassium and sulphur maps are in Fig. 2(e) and (t). In most cases K and S appear together in areas with a low content of Ca, as can be seen in the lower parts of the figures. Point analyses in the scanning electron microscope, of larger areas of this type, indicate a 2: I molar ratio K:S and no Ca. This phase is probably the orthorhombic low-temperature form of K2S04 (arca-nite), and forms a sulphate interstitial material. In the present clinker, with molar ratios S03/K20 < 1 and K20/Na20 = 5-10, almost all S03 can be expected to be present as water-soluble sulphates, i.e. K2S04 and possibly some Ca2K2(S04)3 (calcium langbeinite).l0 The rest of the potassium is dissolved in aluminate and silicate phases. During the formation of clinker, sulphates and oxides form separate melts,5 and potassium sulphate also enters the gas phase,11 which further decomposes to K20, S02 and O2. Sulphur-containing areas without potassium have not been observed.

Small areas (1-3,um), rich in potassium, but without a clear sulphur signal are sometimes observed, e.g. in the central parts of Figs 2(e) and (f). These areas are situated in the interstitial material, usually associated with alite and containing calcium and aluminium, but little iron. Due to the small size of the grains, it is in most cases unclear if they should be interpreted as potassium-substituted Ca3A1206, or a potassium aluminate, or possibly a mixture of the two. An exception is in Fig. 3, where an exceptionally large clinker grain is shown. As can be seen in the calcium and silicon maps (Fig. 3(a) and (b)), large alite crystals are present, while belite is almost absent. The aluminium and iron maps (Fig. 3(c) and (d)) show that most of the interstitial material contains both metals and consists of ferrite. A micrometre-sized area, richer in aluminium than the ferrite, is arrowed in Fig. 3( c). It contains considerable amounts of potassium (Fig. 3(e)), but no sulphur (Fig. 3(t)) or calcium (Fig. 3(a)). This area thus consists of a potassium aluminate and it is surrounded by potassium sulphate. The most likely potassium aluminate in a clinker is KAI02. If KAI02 is actually a minor phase in the clinker, which needs further study to be confirmed, it can be expected to influence the early properties of the cement paste, since NaAI02 is k b . I 12 nown to e a potent settmg acce erator m concrete.

Occasionally, areas rich in potassium but without aluminium, calcium or sulphur are encountered. These are found on specimens that have been stored in plastic bags for some time and can be observed in the

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scanning electron microscope as small crystals grow-ing on top of the carbon film. These crystals are formed by reactive compounds (potassium aluminate?) in the clinker on exposure to the ambient atmosphere.

The potassium maps show that be lite is richer in potassium than alite. This difference is small but clearly visible in most map sets. Point analyses of belite reveal the presence of both K and S. This could either be a specimen preparation artefact, i.e. smearing out of potassium sulphate during polishing, or due to solid solution formation between belite and arcanite.

Magnesium and chlorine are critical elements in cement clinker. No areas with a strong magnesium signal, corresponding to periclase grains, have been observed in the maps. As mentioned above, the ferrite phase exhibits some magnesium doping. No chlorine concentrations have been identified in the clinker. Although this is not to be expected, since the clinker is known to contain less than 001 wt% Cl.

Sodium has not been detected above background level in the maps, but the average sodium content in the clinker is only around 005 wt%.

Distribution of potassium sulphate The elemental maps indicate that the oxide inter-

stitial phases are evenly distributed in the clinker. This is clearly not the case for interstitial potassium sulphate. In 27 sets of X-ray maps, potassium sulphate is observed in only 19 of them. Also in the map sets where potassium sulphate does occur, it appears to be unevenly distributed among the two silicates alite and belite. In 3/4 of the cases, more potassium sulphate is situated between belite crystals, than between alite crystals. Potassium sulphate associated with alite crystals forms well defined rounded areas in the interstitial material. In the case of belite, the sulphate often fills the gap between the silicate crystals and thus covers the belite. In the images, the potassium sulphate thus often appears in more or less worm-like shapes associated with belite grains, see Fig. 2( e).

This type of texture suggests several possible modes of formation. The scenario we find most plausible is that the observed texture reflects the situation when the clinker is formed. At the clinkering temperature, two unmixable melts exist together with solid silicates. During cooling of the clinker, the oxide melt crystal-lizes at a relatively high temperature, and forms an intimate intergrowth of micrometre-sized ferrite and aluminate crystals. The sulphate melt wets the belite crystals, but not the alite crystals. Due to these surface tension effects, most of the sulphate melt is present in belite-rich regions of the clinker. Little or no oxide phase can crystallize where the sulphate fills the space between belite crystals. On further cooling, the sul-phate melt finally solidifies at a relatively low temp-erature.

Another model involves ex-solution of potassium sulphate from a potassium sulphate-belite solid solu-

Advances in Cement Research, 1997, 9, No. 33

tion, formed at high temperatures. Although this model does not account for the uneven distribution of potassium sulphate in belite-rich areas.

Due to modern production techniques the average composition of a cement is well controlled. Never-theless variations in early reactivity are sometimes observed and these can seldom be related to variations in the average composition of the cement. It is consequently reasonable to assume, that the reactivity is affected by the local distribution of elements in the cement grains. A wide spectrum of solid state phenomena can be envisaged, that could affect the reactivity. It ranges from the texture and composition of the phases on the micrometre level to crystal defects present on the atomic level.

The clinker ferrite investigated here occurs in two, rather different, local environments. Ferrite with alite contains relatively small amounts of potassium sul-phate, while ferrite in be lite is associated with more potassium sulphate. In the case of cement grains, the presence of calcium sulphate must also be considered. Still, the local reactivity on the surface of a cement grain could be very different, when water is added, depending on which phases that are occurring ',,-gether. These implications from clinker microscopy, will be considered in future hydration studies.

In conclusion, X-ray mapping is a suitable method for the study of how the interstitial oxides and sulphates are distributed in cement clinker. This dis-tribution of phases could be of importance for the early reactions in cement paste. In order to character-ize the calcium and possibly potassium aluminate phases and to study the textural relationship between calcium aluminate and ferrite in the interstitial oxide phase of rapidly cooled clinker, a method with better spatial resolution is needed, i.e. transmission electron mIcroscopy.

Acknowledgements Financial support form the Euroc Fund, Euroc

Research AB and Cementa AB is acknowledged. The authors thank the laboratory of Euroc Research

Advances in Cement Research, 1997, 9, No. 33

X-ray mapping of interstitial phases in cement clinker

AB in Slite for performing X-ray fluorescence analy-sis, powder X-ray diffraction and polishing of speci-mens, as well as Eric Henderson and Erik Viggh for helpful comments.

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Portland cement hydration. Adv. Cem. Res.. 1988, 1, No.2, Apr., 67-74.

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3. DE KEYSER W. L. and TENOUTASSE N. The hydration of the ferrite phase of cements. Proceedings of the Fifth International Symposium on the Chemistry of Cement, Tokyo, 1968, vol. 2, 379-386. Cement Association of Japan, Tokyo, 1969.

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9. REGOURD M. and GUINIER A. The crystal chemistry of the constituents of Portland cement clinker. Sixth International Congress on the Chemistry of Cement, Moscow, 1974, 1. 63. Stroyizdat. Moscow. 1976.

10. POLLITT H. W. W. and BROWN A. W. The distribution of alkali in Portland cement clinker. Proceedings of the Fifth Interna-tional Symposium on the Chemistry of Cement, Tokyo, 1968, vol. I. 322-333. Cement Association of Japan, Tokyo. 1969.

II. KOSUGI T. Measurements of the vapor and dissociation pressures of potassium sulphate and carbonate at high temp-eratures. Bull. Chem. Soc. Jpn., 1972, 45, 15-19.

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Discussion contributions on this paper should reach the editor by 28 July 1997.

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