DISTRIBUTION OF SULFATES IN PORTLAND CEMENT CLINKER

Transcription

1 DISTRIBUTION OF SULFATES IN PORTLAND CEMENT CLINKER Duncan Herfort Aalborg Portland, Denmark Abstract A method for predicting the distribution of SO 3 between the sulfate and non-sulfate phases is presented. The method uses the phase rule and the phase diagram for the K 2 SO 4 -CaSO 4 system to identify the range of temperature and composition across which the solid solution of sulfates in the silicate phases can increase as a function of increased sulfate content in the system. More specifically, it is predicted that the content of SO 3 in the silicates will increase as a direct function of increasing SO 3 /K 2 O ratios, and that above the SO 3 /K 2 O ratio necessary for anhydrite formation, the concentration of SO 3 in the silicates will remain constant. This model was tested on experimental data for both industrial and laboratory clinker and found to be essentially valid. The maximum possible content of SO 3 in belite was found to be approximately 2 wt% in rapidly cooled laboratory clinker, and 1.5 wt% in industrial clinker. 1. Introduction The role played by sulfates in the formation of Portland cement clinker has been widely studied. Pollitt and Brown (1) studied the distribution of alkalis and sulfates in a large number of works and laboratory clinkers over a wide range of alkali/sulfate ratios. Although they focused mainly on the distribution of alkalis, their results showed that not all the SO 3 is present in a water soluble form, particularly at high SO 3 /alkali molar ratios. They suggested that all of this non-water soluble SO 3 was present as anhydrite. Numerous studies have since shown this to be incorrect (e.g. 2-6), and that some sulfate is incorporated in the other phases, particularly in the belite phase were concentrations are 4 to 5 times higher than in alite (7). Concentrations in the belite phase can range up to about 2 wt% (7). It has been suggested by some workers that sulfate bound in the belite phase, or as relatively insoluble anhydrite, can lead to late ettringite formation analogous to delayed ettringite formation (DEF) in concrete produced at elevated 18

2 temperature (8,9). Work designed to test this hypothesis has shown this to be extremely unlikely (10-12). This paper begins by describing a general model based on the fundamental rules of phase equilibria for predicting the distribution of sulfates in Portland cement clinker. The accuracy of this model is then tested by applying it to data previously published by the present author. 2. General model for the distribution of sulfates Portland cement clinker without impurities and solely consisting of the four major oxides, CaO, SiO 2, Al 2 O 3, and Fe 2 O 3, can be defined as a 4 component system. After the four clinker phases, C 3 S, C 2 S, C 3 A and C 4 AF have formed on cooling below the invariant melting temperature of 1338ºC, the system can be described as being divariant. This is seen from the phase rule P + F = C + 2, where P is the number of phases, F is the number of degrees of freedom, and C is the number of components. At constant pressure the phase rule can be reduced to P + F = C + 1, and the above system is univariant, i.e. with just one degree of freedom. The degree of mutual solubility between the four clinker phases is therefore solely dependent on temperature. Changes in the chemical composition of the system at constant temperature will result in changes in the relative contents of the four clinker phases, but their not in their composition (i.e. the extent that they can form solid solutions with each other). Addition of SO 3 as an additional component at temperatures above approximately 1300ºC results in the formation of anhydrite as an additional phase. At lower temperatures most of the sulfate should occur as sulfospurrite, 2C 2 S.CS, and Kleins phase, C 4 A 3 S (13,14). However, although these phases are commonly formed on heating towards the burning zone in the cement kiln, they do not reform on cooling. Addition of K 2 O (as the major alkali in most Portland cements) as a further component gives the 6-component system, CaO, SiO 2, Al 2 O 3, Fe 2 O 3, SO 3, K 2 O. Applying the reduced phase rule, P + C = F + 1, to this system, a sub-solidus, univariant assemblage, in which mutual solubility of phases is solely dependent on temperature, must therefore consist of 6 phases. At SO 3 /K 2 O molar ratios higher that 3 (neglecting solid solution in the major clinker phases), the 6 phase assemblage will consist of anhydrite, Calangbeinite (C 3 KS 3 ), and the 4 major clinker phases. At ratios between 1 and 3 the assemblage will consist of arcanite, (KS ), and Ca-langbeinite, in addition to the 4 major clinker phases. At lower SO 3 /K 2 O molar ratios, only 5 phases are present, i.e. arcanite, 19

3 and the 4 major clinker phases. This assemblage is divariant, and the solubility of SO 3 and K 2 O in the major clinker phases will, in addition to temperature, also depend on the SO 3 /K 2 O molar ratio. Although this last condition applies to most Portland cement clinkers, the subsequently model developed will concentrate on systems in which the SO 3 /K 2 O molar ratio is greater that unity. In reality, the sub-solidus, univariant conditions for the above 6-component system will not apply because cooling of industrial clinker is too rapid for equilibrium conditions to prevail. Instead, the solubility of SO 3 in the clinker phases, and in particular the silicate phases which is the main subject of this paper, can be expected to depend on equilibrium conditions corresponding to a relatively high temperature, or range of temperatures, found between the cement kiln s burning zone and cooler (typically from 1450 to 1100 ºC). This is illustrated in the following by two hypothetical examples in which the equilibrium conditions are considered at 1200 and 1100ºC, respectively. Equilibrium conditions for the CaSO 4 -K 2 SO 4 binary system are shown in figure 1 (adapted from 15). The sulfate phases within this system can be considered more or less independently by treating it as a sub-system within the 6-component system for CaO, SiO 2, Al 2 O 3, Fe 2 O 3, SO 3, and K 2 O. At the temperature range shown (i.e. less than 1220ºC), the 4 major clinker phases, C 3 S, C 2 S, C 3 A and C 4 AF, will have already formed on cooling. 20

4 Temperature, ºC Figure 1. Binary diagram for K 2 SO 4 -CaSO 4, shown as molar ratios, from (15). 6 SO 3 content in cli nke r, w t% 4 2 lab clinker works clinker kiln coating K 2O content in clinker, wt% Figure 2. Range of SO 3 and K 2 O contents in the clinkers studied. 21

5 If we consider equilibrium at 1200ºC, assemblages with CaSO 4 /(CaSO 4 + K 2 SO 4 ) ratios of less than approximately 0.9, corresponding to the composition of the liquidus curve at this temperature, will consist of a sulfate melt and the 4 major clinker phases. This is a 5 phase assemblage and therefore divariant, which is obvious by the fact that SO 3 /K 2 O ratios can change in the sulfate melt as a function of the overall SO 3 /K 2 O molar ratio of the system. The divariant condition also allows the solubility of sulfates in the silicate phases to vary as a function of the SO 3 /K 2 O ratio, so that as CaSO 4 is added to a Portland cement clinker, both the SO 3 /K 2 O ratio in the sulfate melt, and the SO 3 concentration in the silicate phases will increase 1. At CaSO 4 /(CaSO 4 + K 2 SO 4 ) ratios greater than approximately 0.9 at 1200ºC, anhydrite becomes stable. The additional phase reduces the number of degrees of freedom to 1 (corresponding to temperature), and the composition of the sulfate melt will remain constant with further increases in CaSO 4. Importantly, the composition of all phases present will remain constant, including the concentration of sulfate in the silicates, and the only change to the assemblage will be an increase in the relative content of anhydrite. This is an important concept, because it shows that there is a well defined limit to the solubility of sulfates in the silicate phases, which at any given temperature corresponds to the SO 3 /K 2 O ratio at which anhydrite begins to form. The situation where equilibrium prevails at 1100ºC is analogous to the above, but where the maximum solubility of sulfate in the silicates is reached at a lower SO 3 /K 2 O ratio (corresponding to a CaSO 4 /(CaSO 4 + K 2 SO 4 ) ratio of approximately 0.75 above which anhydrite is stabilised). The implication of this is that slow cooling favouring equilibrium at lower temperatures results in a maximum concentration of sulfate in the silicates at a lower SO 3 /K 2 O ratio of the system. 3. Application of the model to experimental data for clinker In order to test the above model, Electron Probe Micro Analysis (EPMA) data for the belite phase is evaluated for clinker of wide ranging alkali and SO 3 contents. Experimental and production details for these clinkers are given elsewhere (4,14). For the purpose of this paper they can be divided into two groups, rapidly cooled laboratory clinker, and more slowly cooled works clinker and burning zone coating (comparable to 1 Assuming a SO 3 /K 2 O molar ratio > 1. At SO 3 /K 2 O molar ratio < 1, the concentration of K 2 O in the non-sulfate phases will decrease as shown by Pollit and Brown (1). 22

6 SO3 content in belite, wt% lab clinker works clinker kiln coating SO 3 content in clinker, wt% Figure 3. Relationship between SO 3 contents in belite (determined by EPMA) and SO 3 contents in the clinker. 2 SO3 content in belite, wt% 1 lab clinker works clinker kiln coating SO 3 /K 2 O ratio in clinker, wt% Figure 4. Relationship between SO 3 contents in belite and SO 3 /K 2 O ratios in the clinker. 23

7 clinker in terms of composition, and maximum burning, but more slowly cooled). The SO 3 and alkali contents (alkalis as equivalent K 2 O, K 2 O being the dominant alkali in all clinkers studied) are given in figure 2. The relationship between the concentration of SO 3 in belite and the total SO 3 content in the clinker is shown in figure 3, where no clear correlation is seen to exist. A much better correlation is found between the content of SO 3 in belite and the SO 3 /K 2 O ratio in the clinker shown in figure. 4. Interstitial sulfate phases are shown in the BSE micrograph shown in figure 5 with a lath shaped anhydrite crystal surrounded by Ca-langbeinite. 20µm anhydrite Ca-langbeinite alite C 3 A C 4 AF belite Figure 5. BSE image of works clinker containing anhydrite. Application of the phase equilibria model to the data plotted in figure 4 gives the trends shown in figure 6. Since the model predicts that the concentration of SO 3 in belite remains constant in the presence of anhydrite, these concentrations are assumed to level off at concentrations of just over 1.5% for the slowly cooled works clinker and burning zone coating, and just over 2% for the rapidly cooled laboratory clinker. The steeper slope of the slowly cooled clinker indicates that under divariant conditions, the solubility of SO 3 in belite increases with decreasing temperature. 24

8 General application of the model to data in the literature has not been attempted by the present author. In attempting to do this, care should be taken to ensure that sufficient SO 3 is present in the clinker to saturate the silicate phases. This should generally correspond to approximately 0.5 wt% SO 3 in the clinker, unless belite contents are significantly higher than 25%. anhydrite 2 SO3 content in belite, wt% 1 0 lab clinker works clinker kiln coating SO 3 /K 2 O ratio in clinker, wt% Figure 6. Relationship between SO 3 contents in belite and SO 3 /K 2 O ratios in the clinker, for works clinker and kiln coating, and rapidly cooled laboratory clinker. 25

9 4. Implications for concrete durability Concerns regarding high sulfate contents in clinker have largely centred on speculation that sulfates are released slowly from the belite phase and so-called hard burned anhydrite. The rate of dissolution of anhydrite has been dealt with by at least two authors (4,10,11), who have shown that anhydrite reacts at essentially the same rate as natural anhydrite, and is fully consumed in a matter of hours to a few days. Both works and lab clinker containing anhydrite discussed in the present paper are included in this study (4,11). The clinker has also been used to produce cements which were subsequently tested by monitoring the expansion of EN196 mortar prisms stored at 20 and 40ºC (10). No significant expansion was observed after two years. Indeed, using EPMA data for the concentrations of SO 3 in belite to predict the amount of ettringite that can potentially form by the late release of SO 3, this seems highly unlikely. For a typical Portland cement containing 25% belite, the maximum concentration of 1.5% SO 3 corresponds to less than 0.4% SO 3 by weight of the clinker, i.e. substantially less than the amounts of SO 3 released from the C-S-H phase as a result of heat curing. In fact, since half the belite will normally have reacted at 28 days, the sulfate tied up in this way will be considerably less. Furthermore, since it has also been shown that this SO 3 is invariably stoichiometrically matched by Al 2 O 3 (4,5,7), any sulfate released from the belite phase will form monosulfate, not ettringite. 5. Conclusion The fundamental rules for phase equilibria can be used to predict the relationship between sulfate contents in clinker and the concentrations in the silicate phases. It is predicted that the concentration of SO 3 in belite (and the other clinker phases) will increase with increasing SO 3 /K 2 O ratios in the clinker, reaching a maximum when the SO 3 /K 2 O ratio is high enough to form anhydrite at any given temperature. Experimental data confirm these predictions and indicate a maximum concentration of SO 3 in belite of approximately 1.5% in industrial Portland cement clinker, and 2.0% in rapidly cooled laboratory clinker. 26

10 References 1. Pollitt, H.W.W., Brown, A.W. The distribution of alkalis in Portland cement clinker Proc. 5th Int Symp Chem Cem, Tokyo 1968, vol. 1, Ghose, A, and Barnes, P. Electron Microprobe Analysis of Portland Cement Clinkers, Cement and Concrete Research. Vol. 8, No.6, (1978) Kristmann, M, Portland Cement Clinker Mineralogical and Chemical Investigations, Cement and Concrete Research, Vol. 8, No1, (1978), Herfort, D., Soerensen, J., Coulthard, E. Mineralogy of sulfate rich clinker and the potential for internal sulfate attack World Cem Res and Dev 28 (5) (1997) Miller, F.M., Tang, F.J. The distribution of sulfur in present-day clinkers of variable sulfur content Cem Concr Res 26 (12) (1996) Hall, C., Scrivener, K.L. Oilwell cement clinkers Advn Cem Bas Mater 7 (1) (1998) Taylor, H.F.W., Sulfates in Portland clinker and cement, These proceedings. 8. Mielenz, R.C., Marusin, S.L., Hime, W.G. and Jugovic, Z.T., "Investigation of prestressed concrete railway tie distress". Concrete International, Vol. 17, no. 12, (1995) Hime, W.G., "Clinker Sulfate: A Cause for Distress and a Need for Specification", Concrete for Environment Enhancement and Protection", Edited by R.K.Dhir and T.D.Dyer, (1996) , Published by E & FN Spon, London, UK. 10. Herfort,D, Rasmussen.S, Jøns,E & Osbæk,B. Mineralogy and Performance of Cement Based on High Sulfate Clinker, Cement, Concrete, and Aggregates, CCAGDP, Vol. 21, No. 1, June (1999), Michaud, V., Suderman, R., Sulfate solubility in high SO 3 clinkers Proc. 10th Int Congr Chem Cem, Gothenburg 1997, vol. 2, (11) 10 pp. 12. Klemm, W.A., Miller, F.M., Plausibility of delayed ettringite formation as a distress mechanism Considerations at ambient and elevated temperatures Proc 10th Int Congr Chem Cement, Gothenburg 1997, vol. 4 (59) 10 pp. 13. Pliego-Cueco,Y.B. and Glasser,F.P., Role of Sulphates in Cement Clinkering: The Calcium Silicosulphate Phase", Cement and Concrete Research, Vol. 8, No.4, (1978), Pliego-Cueco,Y.B. and Glasser,F.P., Role of Sulphates in Cement Clinkering: Subsolidus relations in the system CaO-Al 2 O 3 -SiO 2 -SO 3 ", Cement and Concrete Research, Vol. 9, No.1, (1979), Mikimov,S.M., Kri lova,n,i, and Bergman,A.G. Akad. Nauk Uzbeksk. SSR, Tashkent. Inst. Khim. Trudy, 2,99 (1949) 16. Herfort,D., Coulthard,E.L., Grundvig,S. & Zimmermann.H.D., Mineralogy of Coatings in a Precalciner Kiln, Proceedings of the twentieth International Conference on Cement Microscopy, Guadalajara, Mexico, (1998),