Mössbauer , XRD , and Complex Thermal Analysis of the Hydration of Cement with Fly Ash

Hydration of cement with and without �y ash is studied with Mössbauer spectroscopy, �RD, and thermal analysis. Iron in cement is present as Fe-ions and occupies two octahedral positions, with close isomer shis and quadrupole splittings. Iron in �y ash is present as Fe and Fe, and the Mössbauer spectra display three doublets—two for Fe in octahedral coordination and one for Fe. A third doublet was registered in the hydrating plain cement pastes aer the 5th day, due to Fe in tetrahedral coordination in the structure of the newly formed monosulphate aluminate. In cement pastes with �y ash, the doublet of tetrahedral iron is formed earlier because the quantity of ettringite and portlandite is low and more monosulphate crystallizes. No Fe(OH)3 phase forms during hydration of C4AF. e �y ash displays pozzolanic properties, which lead to lowering of the portlandite quantity in the cement mixtures and increasing of the high temperature products.


Introduction
Tricalcium aluminate (C 3 A) and tetracalcium alumoferrite (C 4 AF) are main clinker phases in cement.e hydration of C 3 A proceeds quickly, and during reaction with gypsum, ettringite is formed and at later stages, monosulphate occurs accompanied by monocarbonate and aluminates with different composition.e hydration of C 4 AF proceeds slower, but �nally, there are formed hydrate products with composition similar to the products of the C 3 A hydration.In the presence of lime and gypsum, AFt phase is the main product of reaction, but the progress of hydration of C 4 AF is slowed down due to the formation of AFt layer at the surface of C 4 AF-grains.In a subsequent step, the AFt phase converts to AFm, prior to complete consumption of calcium sulphate [1].
e distinction between iron-free and iron-containing analogues is difficult to be found by use of the widely used methods for the study of cement because the structural differences between these phases are negligible [2].Mössbauer spectroscopy is a suitable method for the study of iron distribution in cement as well as its behaviour during the hydration process, because this approach allows to study directly the character of the bonding, the phase state, and the electronic environments of the iron ions [3,4].
Eissa et al. [5,6], A�� et al. [7], and Pascual et al. [8] have proved that iron in C 4 AF is bivalent (Fe 2+ ) and trivalent (Fe 3+ ) with the latter being in tetrahedral (T) and octahedral (O) coordination.e authors have found that iron atoms are weakly bonded in strongly deformed crystal lattice, while Ekimov et al. [9] derived formulae for the distribution of the atoms in both states.e studies of Vertes et al. [10] have shown the presence of �ne magnetic structure in nonhydrated brownmillerite, and Tamás and Vértes [11] have established that during hydration, this phase disappears and the whole content of iron in the solid solution being in octahedral coordination.
e studies of Pobell and Wittmann [12][13][14] on the hydration of Portland cement showed that the intensity of the doublets with which the Mössbauer spectrum is described changes, and aer 10 years, the doublet of iron in nonhydrated cement disappeared completely [13].
Harchand et al. [15][16][17] and Eissa et al. [18,19] have registered three doublets in the Mössbauer spectra of slag Portland cement-two doublets characterizing iron in octahedral and tetrahedral state and a third one-of iron in the hardened monosulphate.e authors have proved that during hydration the relative content of iron in octahedral state lowers, while that in tetrahedral state it increases.A correlation has been found between the ratio of the iron quantity in tetrahedral and octahedral state (Fe 3+ (T)/Fe 3+ (O)) and the strength of the cement paste.
e conclusions of Harchand et al. [15][16][17] are that during hydration of sulphate-resistant Portland cement and slag Portland cement, no Fe(OH) 3 was formed.Mössbauer spectrum of sulphate-resistant Portland cement is with two doublets-the �rst one is of the ferrite phase, and the second one, whose intensity increases during hydration, is of iron included in the monosulphate (AFm phases).
Hassaan et al. [20,21] have registered two doublets in the Mössbauer spectrum of hydrating cement and have found that with time, the quantity of tetrahedral iron increases at the expense of octahedral iron.In the case of high aluminate cement, he has registered three doublets-of Fe 3+ (O), Fe 3+ (T1), and Fe 3+ (T2), respectively.e intensity of the last two changed with time, and this was accepted as a measure for the degree of hydration of the cement paste.
Dwivedi et al. [22], Rai et al. [23], and Singh et al. [24] have proved using the Mössbauer effect the delaying effect on the cement hydration of black gram pulse and superplasticizer, while Barathan et al. [25] have studied the hydration of Portland cement with addition of silica fume and have registered the transformation AFt → AFm.
e purpose of the present paper is to investigate the hydration of cement with and without addition of �y ash from TEPS by use of Mössbauer effects and other methods.

Initial Materials.
Two types of Portland cement are used-from the Bulgarian factories Zlatna Panega (PC1) and Holcim (PC2) and �y ash (FA) from TEPS Bobov Dol (Bulgaria) 100% classi�ed under 63 m, with speci�c surface of the particles    m  /g.e chemical compositions of the initial materials are given in Table 1 and the mineral contents in Table 2.

Samples and Methods
. e prepared samples are of plain cement and cement with addition of 10 wt.% of FA from the total quantity of the initial mixture with water-to-solid (W/S) ratio of 0.5.e samples were, then, hermetically closed in polymer containers up to the 24th hour of hydration, aer being removed and kept in water at temperature of 20 ∘ C up to a particular age for investigation.For the samples studied in the �rst 24 hours, there was applied an interruption of the hydration process by drying and subsequent soaking in acetone and ether.en, these samples were dried and grounded in the form of dust before analysis.
e speci�c surface of the ash particles was determined following the method of low temperature adsorption (  4 K)-a variant of the BET method.
e starting materials and the hydrating cement pastes were analyzed for determination of the quantity of Fe + and e Mössbauer spectroscopic investigation was performed with a spectrometer operating in a symmetric triangle and constant acceleration mode.e acceleration calibration is in relation to natural Fe-foil, and the Mössbauer source is 5 Co/Rh ( 5 Co is intruded in rhodium foil) at room temperature.e obtained data are processed by the least squares method with the "MOSFINT" program.e approximation model is a sum of doublets (tested for variants of 2, 3, or 4 doublets).e program allows approximation of the experimental data with the so-called thin approximation and with integral approximation [26,27] and incorporation of de�nite bonds and constraints on the approximated parameters, for example, equal widths, boundaries of the variations, and so forth, [28,29].
e powder XRD analysis was performed on a DRON �M1 diffractometer (Ni-�ltered Cu radiation, 40 k�/25 mA).e registration of the diffraction lines was performed by a scintillation counter.e complex thermal analysis of the samples PC1 and PC1FA was done with apparatus type 3427 MOM by heating from 20 to 1000 ∘ C with 10 ∘ C /min steps in air and Al 2 O 3 as inert material; the mass of the samples was 800 mg each.

Experimental Results and Discussion
Complex ermal Analysis.e thermal effects, registered on the 48th day of hydration of the samples of cement pastes PC1 and PC1FA (Figure 1) are connected with decomposition of the hydrate products and liberation of the hydrate water as follows [30,31]: Ca-hydrosilicates, Cahydroaluminates, and ettringite with temperature of dehydration up to about 435-450 ∘ C, further denoted as PI products, portlandite, Pt, with temperature of the endothermal peak at about 505-510 ∘ C, and products denoted PII including calcite and Ca-hydrosilicates with temperature of dehydration higher than 620-630 ∘ C (Table 3).Effect at 180-200 ∘ C was also registered for the cement paste with �y ash addition, which probably is due to the thermal decomposition of Ca-monosulfoaluminate or less probably due to hydrocarbon aluminate.e total content of the hydration products in the plain cement paste is higher from the respective one in the cement paste with �y ash addition, but recalculating it per 1 gram cement in the cement paste, it is seen that it is higher for the paste with addition of �y ash, most probably due to the pozzolanic reaction of the ash.e differences in the content of hydration products are more pronounced in the products with low temperature of dehydration (including ettringite in this group) as well as of portlandite in the cement paste with �y ash.As a result of the pozzolanic reaction of the ash with portlandite in the second paste, there was registered an increase of the quantity of the hydration products with higher temperature of dehydration.
Powder XRD Analysis.e powder XRD patterns of the cement pastes PC1 and PC1FA are presented on Figures 2 and  3, respectively.e phase analysis showed that aer 1 day of hydration, the main peaks of portlandite are with increased intensity for the cement paste with addition of �y ash.us, it follows that the �y ash stimulates the hydration up to the 24th hour.With ageing of the hardening process, the same intensities lower in magnitude as a result of the pozzolanic reaction between the �y ash and portlandite.
Mössbauer Spectroscopy.e Mössbauer spectra of the initial cements (Figure 4) were processed with a model of two doublets.e obtained Mössbauer parameters (Table 4) show that in both cement samples the Fe atoms are in threevalence states and are distributed in two positions, O1 and O2, in octahedral coordination with close isomer shis,  1 and  2 , and quadrupole splittings, Δ 1 and Δ 2 , and relative intensity, respectively, for the �rst and second position of  1 = 45-46% and  2 = 54-55%.A doublet, which characterizes Fe 2+ , was not recorded.e Mössbauer spectrum of the �y ash was processed with a model based on three doublets, with the �rst two of which corresponding to Fe 3+ in tetrahedral coordination, positions T1 and T2, respectively, while the third one corresponds to Fe 2+ .
Figures 5 and 6 represent Mössbauer spectra of cements PC1 and PC2 at different ages of hydration, and Tables 5 and  6 contain the respective Mössbauer parameters.Up to the 5th day of hydration, there is registered a redistribution of the Fe 3+ ions between the two octahedral positions, and the quantity of iron increases in position O2, which is connected with the hydration of C 4 AF.e ratio of the quantities of iron ions in positions O1 and O2 in the initial cements PC1 and PC2 is, respectively, 0.85 and 0.82, and aer 1 day of hydration, it is already 0.72 and 0.54, respectively.It is seen that the process of redistribution of the Fe 3+ ions between the two octahedral positions is more intense for cement PC2, which is connected with twice greater quantity of C 3 A than in cement PC1.Due to this reason, the speed of consumption of gypsum in it is higher and, thus, facilitates the hydration of C 4 AF [1,32].Between the 5th day, and 14th day, a doublet of three-valence iron in tetrahedral coordination (T) is formed as a result of lowering of its content in the octahedral position O2.is doublet in the cement pastes is due to the formation of iron-containing mono-sulphate-aluminate, which appears in the later stages of hydration [2,33,34].e intensity of the third doublet  3 increases up to the 48th day of hydration.
e Mössbauer parameters of the third doublet differ from these of Fe(OH) 3 (  2 mm/s, Δ  72 mm/s) and its gel (  4 mm/s, Δ  72 mm/s) [17], which means that during hydration of the tetracalcium alumoferrite no Fe(OH) 3 phase is formed or it is in a negligible quantity.
Figures 7 and 8 represent the Mössbauer spectra of hydrating cement pastes with addition of �y ash, respectively, PC1FA and PC2FA, and Tables 7 and 8 contain their Mössbauer parameters.e doublet of the three-valence iron in tetrahedral coordination is formed between the 1st day and the 5th day of hydration, earlier, and compared with the pure cement pastes, and its intensity  3 increases up to the 48th    day of hydration.e reason for this is that in the presence of �y ash, a smaller amount of ettringite and of portlandite is formed aer the 1st day, and the hydration of C 4 AF is hampered [35].e �nal result is that greater amount of monosulphate is formed because the ratio SO 4 /Al 2 O 3 in the system is lowered thus favouring AFm formation [36,37].

Conclusions
(1) In the initial cements, iron is present as Fe 2 O 3 and FeO phases.e three-valence iron is distributed in two octahedral positions with close isomer shis and quadrupole splittings.Due to its low concentration in the cement paste, iron in the FeO phase was registered only chemically.
(2) Iron in the �y ash composition is present in bi-and three-valence form, which in the Mössbauer spectra is registered by three doublets-two for the threevalence iron in octahedral coordination and one for iron in bivalence form.
(3) A third doublet was registered in the Mössbauer spectra of the hydrating plain cement pastes aer the 5th day, which is due to the presence of three-valence iron in tetrahedral coordination in the structure of the newly formed monosulphate aluminate.
(4) In the cement pastes with addition of �y ash, the doublet of tetrahedral iron was formed between the 1st day and 5th day of hydration because of the fact that in these cement pastes the quantity of ettringite and portlandite is low, and signi�cant quantity of monosulphate is formed.With the increased time of hydration, the relative quantity of tetrahedral iron is also increased at the expense of octahedral iron.
(5) No Fe(OH) 3 phase was formed during the hydration of C 4 AF.
(6) �e �y ash from "Bobov Dol" TEPS (Bulgaria) displays pozzolanic properties, which lead to lowering of the portlandite quantity in the cement pastes with �y ash and parallel increase of the quantity of the products with high temperature of dehydration.

F 1 :
ermal effects registered on the 48th day of hydration of samples of cement pastes PC1 and PC1FA.T 3: Weight loss during heating of the cement pastes PC1 and PC1FA on the 48th day of hydration.

F 2 :F 3 :
Powder XRD patterns of cement paste PC1 at different ages of hydration.Powder XRD patterns of the cement paste PC1FA at different ages of hydration.T 4: Mössbauer parameters of the initial materials.

F 4 :
Mössbauer spectra of the initial cements and �y ash.T 5: Mössbauer parameters of the of cement PC1 at different ages of hydration.Time of hydration  1 (mm/s)

F 5 :
Mössbauer spectra of cement PC1 at different ages of hydration.T 7: Mössbauer parameters of the of cement PC1FA at different ages of hydration.

F 6 :F 7 :
Mössbauer spectra of cement PC2 at different ages of hydration.Mössbauer spectra of cement paste with addition of �y ash PC1FA at different ages of hydration.

F 8 :
Mössbauer spectra of cement paste with addition of �y ash PC2FA at different ages of hydration.

T 1 :
Chemical composition of initial materials.
a e results are for dry (105 ∘ C) samples.Fe3+.e total content of iron, Fe  O 3 (t), was determined by optical emission spectroscopy with a source of inductively coupled plasma (ICPAES) aer alkaline melting with LiBO  dissolution with diluted nitric acid.e content of FeO was determined by titrating with KMnO 4 aer dissolution with H  SO 4 in the presence of Na  CO 3 , which is a reagent suppressing the oxidation of Fe + .e content of Fe  O 3 was determined as a difference from the total amount-Fe  O 3 (t) and the content of FeO multiplied by the coefficient for stoichiometric transformation (  ).
T 6: Mössbauer parameters of the of cement PC2 at different ages of hydration.