Calcium Aluminoferrite Research Articles

Use of Selected Minerals as Secondary Raw Materials in Cement Industry Part II: Effect on Clinker Structure

The use of selected secondary materials for improving the reactivity of cement raw mix, is a challenging subject. It is known that certain foreign elements exert a considerable effect on the cement raw mix burnability and clinker structure, even at low concentration. This work is part of a project on the use of selected minerals as a source of elements having a positive effect on the burnability of cement raw mix. The minerals, iron rich wolframite, stibnite and a wolframite- stibnite assemblage, were selected in order to introduce W, Sb and S in the cement raw mix. One reference and twelve modified mixtures, prepared by mixing the reference sample with 0.5, 1.0, 1.5 and 2.0 % w/w of the above minerals, were studied. Their effect on the burnability is discussed in Part I. The subject of this paper is their effect on the structure of the produced clinker. The XRD studies, performed in samples that were burned at 1450 o C, showed that the diffraction patterns correspond to a structure of a typical clinker, obtained at the above temperatures. In addition, samples sintered at lower temperature (1200-1400 o C) were examined in order to investigate the effect of the additives on the intermediate products. It is concluded that the added compounds affect mainly the structure of calcium aluminate and calcium aluminoferrite phases, due to their dissolution in the melt formed during the sintering. Scanning Electron Microscopy was used in order to examine the structure of the clinker and the distribution of the foreign elements in its main phases. It is shown that W and Sb are mainly concentrated in the solidified melt. In addition, bigger and more roundish alite crystals are found in these samples. These modifications are attributed to the effect of the added elements on the properties of the melt.

Characterization of Liquid Exsolved by Remelting Reaction of Belite

We examined the chemistry and structure of the liquid that was exsolved by the remelting reaction of belite. Electron probe microanalysis revealed that the liquid concentrated the impurity components (e.g., Na2O, K2O, Al2O3, and Fe2O3) of the parent belite. Micro‐Raman spectroscopy showed the similarity of the spectra of the exsolved liquid and amorphous calcium aluminoferrite solid solution. The transition from the crystalline to the amorphous state of the solid solution was induced by increasing the excitation laser power. Accordingly, the liquid at ambient temperature was most likely in a glassy state. The chemical composition of the liquid, deduced from the above experiments, was ∼81 mass% Ca2AlFeO5 and ∼19 mass% Ca2SiO4. The alkalies, which have never been detectable in crystalline calcium aluminoferrite, were probably necessary for the vitrification of the liquid.

Hydration-resistant lime refractories from Egyptian limestone and ilmenite raw materials

Four refractory lime batches were processed from the pure limestone economically available at Beni Khaled, El Menia, Egypt up to firing at 1550°C using a two-stage firing process. The ferri-ilmenite ore existing at Abu Ghalaga, Eastern Desert was added as a dopant material in amounts of 0.5, 1.0, 2.0, 3.0%. Densification parameters and hydration resistance of the fired grains were investigated. The most dense hydration resistant grains were selected to assess their refractory quality by determining load-bearing capacity and thermal shock resistance. These results were interpreted in the light of phase composition and microstructure of the fired grains. It is concluded that dense and hydration resistant lime grains can be processed by doping the pure limestone powder with 2.0–3.0 wt.% of ferri-ilmenite before firing up to 1550°C. Such level of ilmenite content has contributed in the densification of lime particles in the solid state and also by limited amount of the developed liquid phase. Hence, direct-bonded lime network is formed with partial interruption by a platey calcium–alumino–ferrite–titanate phase, which crystallized on cooling from the liquid phase at the grain boundaries of the lime–lime network. This improves the bulk density of fired grains to about 3.2–3.3 g/cm 3 and its rate of hydration to 4.15–3.80 g/h without significant deterioration of its load-bearing capacity and thermal shock resistance.

A Study of the Celite Part (Part VII)

The author performed systematic research on calcium almino-ferrite phase in portland cement by means of optical microscope and powder X-ray diffraction, and presented 7 reports to the Journal. They are Part I, Vol. 45, No. 533, pp. 279-299 (1937), Part II, Vol. 45, No. 534, pp. 361-373 (1937), Part III, Vol. 45, No. 535, pp. 433-446 (1937), Part IV, Vol. 45, No. 537, pp. 614-631 (1937), Part V, Vol. 45, No. 540, pp. 880-896 (1937), Part VI, Vol. 46, No. 541, pp. 12-20 (1938), and Part VII, Vol. 46, No. 542, pp. 66-76 (1938).The ferrite phase in portland cement is known to be crystallized from molten liquid containing CaO, Al2O3, Fe2O3, and SiO2 together with calcium alminate phase. It was concluded, in the last report of the series, that the composition of calcium almino-ferrite was 6.2CaO:2.2Al2O2:Fe2O3 when Al2O3/Fe2O3 ratio of starting mix exceeded 1.4 in the ternary system CaO-Al2O3-Fe2O3, and that the addition of SiO2 had no effect on the ferrite composition.The calcium alumino-ferrite was called celite, similarly to alite and belite, and it was supposed to be a compound with the composition of 4CaO:Al2O3:Fe2O3. The author found it possible to prepare ferrite phase with any composition in the solid solution series between 2CaO:Fe2O3 and 6.2CaO:2.2Al2O2:Fe2O3, or Ca2(AlxFe1-x)2O5 where 0<x<0.7, for the first time. And all text books in cement chemistry show the phase equilibrium diagram as Fig. 1.In addition to the micro-structure observation with optical microscope, crystalo-chemical consideration based on X-ray diffraction determination was applied, and the research out put has contributed not only to ceramic science and but also practical cement technology to a great extent till now, in ceramic field including cements, refractories, and so on.The contribution of the author to ceramics field spread in wide range and extra-ordinal. One of those is the introduction of powder X-ray diffraction technique to ceramic research. It was really hard for him to assemble X-ray apparatus for himself and find his way to analyze experimental results. He started to evacuate the tube in the morning, and the X-ray experiment was continued through night. But, today, it is one of the most popular and important tool for ceramists, we depend strongly on him today and in future.Explanatory article can be seen at Vol. 41, pp. 251-259 (1933), showing illustration of X-ray tube at that time as Fig. 2. And the earliest research report, Vol. 42, pp. 313-322 (1934), is on calcium aluminate, as an out put from fundamental research on castable refractory.

Quantitative Aspects of 27Al MAS NMR of Calcium Aluminoferrites

27Al MAS NMR spectra of synthetic calcium aluminoferrites, Ca2AlxFe2−xO5 with x = 0.93, 1, 1.33, reveal only a few percent of the expected intensity for the 27Al central transition, indicating that the calcium aluminoferrite phase in Portland cements can barely be observed by 27Al MAS NMR. This result supports the use of 27Al MAS NMR for quantitative analysis of the tricalcium aluminate phase in Portland cements.

PREPARATION OF IRON RICH CEMENTS USING RED MUD

Possibility of producing calcium sulfoaluminoferrite (SAF) (C4(A,F)3S)-calcium aluminoferrite (C4AF) based cements using lime + red mud + bauxite + gypsum has been investigated. The effects of composition, firing time and firing temperature on the properties of cements produced has been studied. The characteristics of the cements produced have been found to be strongly dependent on the raw mix composition and firing temperature but not so much on firing time. Some of these cements possess strengths comparable to and at times even more than ordinary Portland cement (OPC). Since the red mud used contains significant amount of titania, effect of titania on pure sulfoaluminate phase has also been studied. © 1997 Elsevier Science Ltd

Hydration products of calcium aluminoferrite in the presence of gypsum

Hydration products of C 6A 2F, C 4AF and C 6AF 2 in the presence of gypsum were studied by DTA, XRD, EDAX and IR techniques. An iron-rich phase, containing little calcium, was found in their hydrates. DTA proved to be an effective method to detect this phase if it was enriched. Good linear relationships were obtained between optical densities and Fe/(Fe+Al) ratios in AFm phases when infraed absorption bands between 1200-960 cm −1 were fitted by Gaussian-line-shape curves. The correlations suggested that the intensity of the absorption of ν 3-SO 4 2− could quantify the amount of Fe(III) in AFm phases. A possible explanation of this result was tentatively proposed as well.

Effect of cooling rate and Na 2O content on the character of the interstitial materials in portland cement clinker

Both the chemical and phase compositions of the crystalline interstitial phases in clinker vary significantly depending on the cooling rate and Na 2O content of clinker. In slowly cooled clinker the calcium aluminoferrite phase is definitely lower in amount and Al 2O 3/Fe 2O 3 ratio than in rapidly cooled clinker. The coexistence of Na 2O encourages this change. With slow cooling, SiO 2 and MgO in the ferrite phase decrease in content with an increase in the ratio of SiO 2 to MgO due to the precipitation of periclase. In the calcium aluminate phase MgO decreases as well; but SiO 2, in the coexistence with Na 2O, remains high even on slow cooling because of the coupled ion exchange Ca 2++Al 3+→Na ++Si 4+, which causes the crystal system to change from cubic to orthorhombic.

Effect of burning conditions and minor components on the color of portland cement clinker

The substitution of Me 2+ (Mg,Zn) for Al 3+ and Fe 3+ in the calcium alumino-ferrite phase, which introduces extrinsic oxygen vacancies in the structure, is essential to the change in color from yellowish brown to dark grey. However, an increase of the ratio Me 4+ (Si, Ti)/Me 2+ in the ferrite phase, which, for example, is encouraged by slow cooling in nitrogen, reduces the number of extrinsic oxygen vacancies and prevents the above color change. With C 6A 2F solid solutions the color change is accompanied by a definite increase in electric conductivity, indicating the significance of electrons released by the oxidation of FeO in addition to the occurrence of oxygen vacancies.

The application of thermal analysis and conduction calorimetry on the formation and hydration of calcium aluminoferrite

DTA was used to study the formation of calcium aluminoferrite from CaO, Al2O3 and Fe2O3 (proportion=2∶ x∶ 1−x),x=0-3/4) and the effects of C4A3 $$\mathop S\limits_{\text{\_}}^{\text{\_}} $$ C11 A7·CacF2, β-C2S and MgO on that process. The fusion of CF and the eutectic of CaO-Fe2O3, CA-C12A7, CA-C3A, C12A7-CA-C3A and CA-C2AxF(1−x) will be favourable for the formation. The presence of β-C2S and MgO will decrease its formation temperature by an extra eutectic while C4A3 $$\mathop S\limits_{\text{\_}}^{\text{\_}} $$ . and C11A7·CaF2 will not. The early heat evolution during the hydration of C2AxF(1−x) were measured by the conduction calorimeter made by ourselves and the products after 1 day hydration were examined by TG and DTA. The results revealed that the peak of heat evolution rise, the amount of the hydrating product iron hydrate gel decreases, CAH10 and C3(A, F)H6 increase with the increase of the value x. The more MgO (up to 5%) soluted in C6AF2, the higher the hydration heat for this solid solution is, when TiO2 soluted in C6AF2 is 2%, the hydration heat for the system reaches the highest.

Surface composition of anhydrous tricalcium aluminate and calcium aluminoferrite

Electron spectroscopy and optoacoustic spectroscopy (OAS) have been used to study the surfaces of synthetic tricalcium aluminate, Ca3Al2O6 and calcium aluminoferrite, Ca2AIFeO5. The surfaces of these compounds have compositions which differ markedly from those of the bulk. The surface of tricalcium aluminate is depleted in calcium and enriched in aluminium and also carries relatively stable hydroxyl groups, which can be detected by OAS. Calcium aluminoferrite has a surface enriched in aluminium and depleted in calcium and iron. Again, some hydroxyl groups are present on the surface. These differences in surface composition are discussed with particular reference to the early stages of reaction with water.

Mössbauer analysis of iron phases in brown coal ash and fireside deposits

The iron phases present in an electrostatic precipitator ash, an uncooled ash deposit and a cooled superheater ash deposit from Hazelwood Power Station, Australia, burning Morwell brown coal has been examined using Mössbauer spectroscopy. The principal iron phase in the precipitator ash and the uncooled ash deposit from a hot gas offtake was calcium aluminoferrite (Ca2Fe2 − xAlxO5). Minor amounts of hematite (α-Fe2O3) and magnetite (Fe3O4) were also detected in the precipitator ash. The cooled superheater ash deposit contained a (Mg, Fe, Al) oxide spinel as the primary iron phase; small quantities of hematite were also detected in this deposit close to the heat exchanger interface. The formation of these iron phases has been rationalized on the basis of the average composition of coal delivered to the power station and supplementary ash chemistry data obtained from other techniques. The evidence suggests that the calcium aluminoferrite in the precipitator ash is derived from inorganic constituents (distributed throughout the coal organic matrix) and the hematite and magnetite are of mineral origin (discrete particles).

Hydration products of calcium aluminoferrite

Hydration products of Ca 2AlFeO 5, steam cured at 72 C, have been identified using X-ray diffraction and Mössbauer spectroscopy. Iron-bearing phases are hydrogarnet Ca 3Fe yAl 2−y(OH) 12 with y = 0.22, and non-crystalline ferrihydrite Fe(OH) 3.nH 2O. These two are readily distinguished in Mössbauer spectra at 4.2 K where they give, respectively, a single unsplit absorption line and a magnetically split hyperfine pattern with H hf = 501 kOe. Addition of 5 wt% lime to the calcium aluminoferrite increases the proportion of iron in the hydrogarnet to y = 0.32 while leaving the ferrihydrite unchanged, whereas 5 wt% gypsum eliminates iron from the hydrogarnet entirely and produces an even more disordered non-crystalline ferric hydrate.

Some possibilities of Raman microprobe in cement chemistry

Laser microprobe “MOLE” has been used to obtain Raman spectra of individual crystals of alite and belite from two industrial clinkers. Important differences are observed. Problems arising in the study of C 3A, calcium alumino-ferrites and hydration are discussed.

The composition of the tricalcium aluminate and ferrite phases in Portland cement determined by the use of an electron-probe microanalyser

Synopsis Each of the four main phases of Portland cement contains various elements in solid solution, the extent and nature of which may affect the hydraulic properties. A direct determination of the composition of these phases has only become possible by the use of the electron-probe microanalyser. The paper describes the analysis of the tricalcium aluminate and calcium alumino-ferrite phases. It was difficult to obtain reliable and accurate analyses of these in Portland cement clinkers and results were confirmed by the investigation of laboratory preparations where the tricalcium aluminate and calcium alumino-ferrite phases were predominant. The tricalcium aluminate phase was found to be approximately: (Ca2·78Na0·16K0·02) (Al1·75Fe0·16Mg0·07Si0·l0)O6 The calcium alumino-ferrite phase in a normal Portland cement clinker was found to be: Ca5·97Al3·23Fe2·13Si0·14Mg0·15Ti0·23Mn0·09O15 and that in a sulphate-resisting cement clinker to be: Ca6·16Al2·24Fe3·22Si0·14Mg0·07Ti0·12Mn0·03O15 The properties of a Portland cement are largely governed by the relative amounts of the four major phases present. The normal ‘Bogue’ method of compound calculation from the chemical analysis assumes that no solid solution occurs in these phases and that the cement clinker is at equilibrium. This method of compound calculation is compared, for a series of cement clinkers, with a calculation where solid solution is taken into account, with a calculation where the ferrite composition as given by X-ray diffraction is used, and also with the direct determination of the phases by X-ray diffraction and optical microscopy. It seems likely that the normal method of compound calculation underestimates the C3A content of sulphate-resisting cement clinkers.

Studies on the Hydration of Calcium Aluminoferrite Phase and its Glass in Portland Cement Clinker

Interstitial phases in portland cement clinker, consisting of calcium aluminate, calcium aluminoferrite and its glasses, are so complicated that have not yet been well investigated, especially on the hydration of these minerals.The authors attempted to clarify the reaction mechanism in the initial stage of the hydration of these minerals (in ten hours) by a adiabatic calorimeter which had been already reported in a previous paper.The results obtained are as follows;(1) Hydration in the water without gypsum and alite.In general, calcium aluminoferrite (crystalline) showed the increase in the rate of heat liberation with increasing Al2O3 content. The rate of heat liberation of quenched materials was smaller than that of crystallized materials. It was suggested that in the hydration of portland cement, the finest hydration products precipitate near the cement grain and the rate of hydration depends on the diffusion of water through the products layer on surface of the grain. From the above reason, considering that the nature of the film varies with time, parabollic equation seemed to represent the hydraton process well. In the other hand, as the hydration rate obeyed logarithmic equation well, the occurrence of the flaw path or the loosed structure zone in the hydration product layer, which were caused by the transformation of the hydration product, was assumed in the same hydration prosess. Moreover the approximte activation energy of diffusion through the hydration product layer was calculated.(2) The amount of Fe2O3 which 3CaO⋅Al2O3⋅6H2O can take in solid solution became smaller with the increase in hydration temperature.(3) Hydration in the water with gypsum and alite.In this case, the rate of heat liberation of quenched materials was greater than that of crystallized materials. It was suggested that the rate determining step is the liquid-solid interaction, because the relationship between heat liberation and time was expressed as straight line. When gypsum was depleted, hydration proceeded the same as in the water without gypsum and alite, and then logarithmic equation was applied. In addition, activation energy in solid-liquid interaction was calculated for C4AF (E=11.0kcal/mole).

Action of Water on Calcium Aluminoferrites.

The action of water on seven preparations of the calcium aluminoferrite solid solution series, ranging in composition from 6CaO·2Al2O3·Fe2O3 to 2CaO Fe203, was studied (a) by leaching the powdered preparations with water, (b) by stirring in a large volume of water at different temperatures from 10 to 100 °C, and (c) by paste hydration at 1, 25, and 45 °C. The initial reaction occurring immediately on contact between the aluminoferrite and water is one of incongruent solution. The Al2O3, most of the CaO, and a trace of Fe2O3 pass into solution, and a residue of Fe2O3, possibly hydrated and amorphous, remains. If the suspension is sufficiently concentrated to give a supersaturated solution, a hydrated precipitate will form. At room temperature the major precipitated phase is 2CaO·Al2O3 8H2O, or a limited solid solution of the same. At 70 °C a solid solution of 3CaO·Al2O3 6H2O, in which about one-tenth of the Al2O3 is replaced by Fe2O3, is precipitated. The rate of reaction increases with temperature and with the proportion of alumina in the aluminoferrite. Hydration in paste form progresses in a different manner. The hydrogarnet phase produced varies in composition with the parent aluminoferrite, but is always somewhat poorer in Fe2;O3 The hexagonal plate phase, 4CaO·(Al2O3,Fe2O3)·nH2O is produced from the aluminoferrites high in F(e2O3, especially at the lower temperatures. Another platy phase, 2CaO·Al2O3·8H2O (or its limited solid solution) is formed from preparations highest in Al2O3, also at lower temperatures. Intermediate members give chiefly the hydrogarnet phase even at 25 °C.

Solid solutions of calcium aluminoferrites

1. Solid solutions of calcium aluminoferrites are formed with the participation not of the cubic form of ScaO· 3Al2O3. but ofthombic (a′) form (which is structurally closer to 2C&0- Fe2O2 and to 4CaO·AI2O2· Fe2O2) the other components being dicalcium ferrite and a certain amount of free calcium oxide. 2. The compositions of the aluminoferrite containing the maximum amount of aluminate in solid solutions corresponds to 17.5% by weight of ferric oxide. The formula of such an aluminoferrite can be written 8CaO· 3AI2O3· Fe2O3. 3. 4CaO· Al2O3· Fe2O3 does not form solid solutions containing 3C&0- Al2O3. 4. Aluminoferrites present in mixtures containing calcium aluminates, calcium silicates, or excess free calcium oxide are of variable composition. 5. The aluminoferrites of industrial clinkers are of variable composition. 6. Calculation of the mineralogical composition of clinker must take into account the formation of an aluminoferrite of the composition indicated by the results of microscopic investigation. This procedure increases the accuracy with which the contents of other clinker minerals can be calculated.