The Effect of Titanium Dioxide on the Structure and ...

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The Effect of Titanium

The Effect of Titanium Dioxide on the Structure and Reactivity of Ferrite T Duvallet1 , T L Robl1 , F P Glasser2 1 – University of Kentucky, USA 2 – University of Aberdeen, UK

Minor constituents can have a major impact on the structure and reactivity of Portland cement and Calcium Sulfoaluminate cement. An important example of this is the effect of titanium dioxide on the reactivity of the ferrite phase, the end member of which is brownmillerite. The effect of TiO2 on ferrite reactivity was investigated by forming the pure end member from reagent grade materials and adding in TiO2 at the dosages 0%, 0.5% 1.6% 2.6% and 3.6% by weight. The powders were mixed and pressed into pellets, preheated to 800 °C and then fired at 1350 °C for 30 minutes. The brownmillerite was milled for 1 hour and characterized by X-ray diffraction, energydispersive X-ray spectroscopy, isothermal calorimeter and thermogravimetric analysis. The addition of TiO2 was expressed in the principle XRD peak with values of TiO2 greater than 1.6% shifting it to higher d-spacing. For TiO2 greater than 1.6% the hydration and subsequent set time was found to be retarded, from only a few minutes for the materials with no TiO2 to approximately 5 to 6 hours. The addition of very low levels of TiO2 (0.5%) appeared to slightly increase the set time of the brownmillerite over the pure end member. The rate of strength development of mortar cubes also varied significantly with the higher TiO2 samples (i.e. 2.6 and 3.6%) having lower one-day strengths but much higher (by up to twice) at 7 and 28 days. It was demonstrated that the behavior of brownmillerite as a cementitious material was greatly affected by relatively small dosages of TiO2 indicating the importance of even low levels of minor components in the Portland cement and Calcium Sulfoaluminate cement.

Tristana Duvallet, University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky, 40511, USA.Ms. Duvallet is PhD Candidate in Materials Science and Engineering at the University of Kentucky,and received her Diploma of Engineer in Material Sciences from ESIREM, Dijon, France in 2008. She is currently investigating calcium aluminate cement systems. Dr. Thomas L. Robl, University of Kentucky, Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky, 40511, USA. Dr. Robl is an Associate Director of the Center for Applied Energy Research of the University of Kentucky. He is currently conducting research into the application of coal combustion and other industrial by products into low energy cement systems. Professor Fred P. Glasser, Chemistry Department, University of Aberdeen, Aberdeen, UK. Dr. Glasser holds the position Professor Emeritus at Aberdeen University. He is currently active in investigations of CSA and other low energy cement systems. Professor Glasser is preeminent among the experts in cement chemistry.

Keywords: Brownmillerite, Ferrite, Reactivity, Titanium dioxide

T Duvallet, T L Robl, F P Glasser

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INTRODUCTION The use of industrial by-products in the fabrication of Portland cement and Calcium Sulfoaluminate cement contributes to their sustainability. Using waste materials reduces cost and can also reduce CO2-emissions by lowering the amounts of limestone used in the raw mix. Examples of by-products can include red mud from bauxite processing, slag and coal combustion fly ash. [1, 2, 3, 4, 5] However minor elements as impurities in the by-products, and materials as well, can affect the chemical and physical properties of the cements and their performance. Several studies have been completed on the subject of incorporating impurities (alkali oxides, fluoride or transition metal oxide) into the cements [6]. This study focuses on the effect of titanium dioxide on the reactivity of the ferrite the end member of which is also called brownmillerite, nominally C4AF. This is an important phase in Portland cement, as well as in calcium sulfoferroaluminate cement (CSFA). Other research on this topic include that of Miller [7] who found that titanium has insignificant effect on the manufacturing of the cement when titanium dioxide (TiO2) is introduced at a low concentration levels. Kakali [8] found that the addition of titanium dioxide of 2% by weight in the raw materials of ordinary Portland cement does not show any significant increase in the rate of hydration. In fact, the addition of TiO2 slightly retards the hydration process during the first 2 days. But after 28 days its effect on the hydration rate becomes negligible. Teoreanu [9] observed that the mechanical properties of cement, such as compressive strength, where TiO2 was incorporated were better than without TiO2. Titanium dioxide (around 1%) can also reduce the melting temperature of clinker by about 50-100°C [10, 12]. Teoreanu [9], Knofel [11, 12], Marinho [13] and Hornain [14], found that after cooling to ambient temperature, titanium dioxide is preferentially partitioned into the ferrite phase. Marinho and Glasser [13] studied the effect of titanium dioxide in a concentration range from 1 to 10% on the crystal structure of the ferrite phase by XRD and electron diffraction. They regarded the structure as distorted perovskite and concluded that Fe3+ was substituted by Ti4+, charge being balanced by incorporation of additional oxygen atoms. From 1 to 9 % of Ti contents, the C4AF structure is disordered with respect to the random distribution of oxygen. But at higher Ti content, more than 9%, a different stacking arrangement appears, the formula of which is given as Ca5(Fe4-xAlx)TiO13. The hydration of ferrite phase with addition of gypsum was studied by Collepardi [15], who determined that the hydration of the ferrite phase is retarded by gypsum which was attributed to ettringite coating unreacted C4AF grains. They also observed the conversion of ettringite to monosulfate (AFm) following depletion of gypsum. Fukuhara [16] also observed the retardation of C4AF hydration by the presence of gypsum. The most realistic hydration reaction was the following:

Emanuelson [17] and Meller [18] confirmed the previous results. Ferrite hydration without gypsum produces first AFm phases, C2(A,F)H8 and C4(A,F)H19, followed by hydrogarnet. With addition of gypsum, AFt is the first phase formed followed by AFm phases due to the conversion of ettringite to AFm due to lack of SO42- ions and then followed by the hydrogarnet phase.


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Our research is focused on the effect on TiO2 at low levels, i.e. 0% to 4% on the properties of the ferrite phase. Hydration analyses, calorimetric analyses, TGA, EDS and XRD analyses and mortar strength testing were performed to determine the influence of the TiO2 on the C4AF.

EXPERIMENTS The raw materials used are reagent grade hydrated lime Ca(OH)2, aluminum hydroxide Al(OH)3, iron oxide Fe2O3 and titanium dioxide TiO2. In order to study the effect of the titanium dioxide on C4AF, different amounts of TiO2 were introduced in the raw mix of C4AF before firing. The formula used to make the solid solutions is: C4A1-xF1-xT4x [13] (which may also be written as Ca4Al2-2xFe2-2xTi4xO10+2x), with x equal to 0, 0.1, 0.3, 0.5 and 0.7 (Table 1). This stoichiometry results in samples that have 0% (BT-0), 0.52%, (BT-0.5), 1.57% (BT-1.6), 2.61% (BT-2.6), and 3.65% (BT-3.6) of TiO2 by weight. The raw mix was formed into pellets to provide more intimate contact among the oxide and improve reactivity. The pellets were then fired in an electric tube furnace, preheated at 800°C for 30 minutes and then fired at 1350°C for 30 minutes. The fired pellets were quickly removed from the furnace at the maximum temperature and rapidly air quenched. Other attempts have been made using firing temperatures below 1350°C, but were unfavourable as too much free lime persisted in the final product.

Table 1 Weight percentage of compounds into each sample BT-0 C4AF

BT-0.5 C4A0.99F0.99T0.04

BT-1.5 BT-3.6 BT-2.6 C4A0.97F0.97T0.12 C4A0.95F0.95T0.20 C4A0.93F0.93T0.28

Ca(OH)2

48.42

48.42

48.41

48.41

48.40

Al(OH)3

25.49

25.23

24.72

24.20

23.69

Fe2O3 TiO2

26.09 0

25.83 0.52

25.30 1.57

24.78 2.61

24.26 3.65

The samples were crushed and ground in a ball mill for one hour. Each sample of ferrite was then analysed for particle size, surface area and particle density as presented in Table 2. The skeletal density was measured by helium pycnometer AutoPycnometer 1320 (Micromeritics). Surface area was measured with the Blaine apparatus. The particle size distribution was determined by a particle size analyser Mastersizer 2000 (Malvern). Calorimetric analyses were performed on a TAM Air calorimeter at 23°C for 300 hours. Hydration tests were conducted over a period of 28 days for BT-0, BT-0.5 and BT-3.6 only. The mix was composed of 10 g of total cement and 4.5 g of distilled water (water/cement ratio of 0.45). The mix was blended in a plastic vessel. The vessel containing a wet towel to keep a relative humidity of 100% was tightly closed. The hydration was stopped at different


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times: 3-hour, 1-day, 4-day, 7-day and 28-day, by washing with acetone twice and drying for 1 hour in an oven at 50°C. The samples were stored in desiccators prior to TGA and XRD analyses.

Table 2 Density, surface area and particle size for each BT

BT-0 BT-0.5 BT-1.5 BT-2.6 BT-3.6

SKELETAL DENSITY BY HELIUM PYCNOMETER (g.cm-3)

SURFACE AREA BLAINE (m2.kg-1)

PARTICLE SIZE BY LASER ANALYSIS d(0.5) (µm)

3.852 3.907 3.878 3.904 3.858

255.77 273.82 201.89 187.68 191.66

10.733 10.481 8.739 10.853 10.176

The TGA instrument used is a TA Instruments SDT Q600 and the analyses were performed from 50 to 1000°C with a ramp of 20°C/min in a continuous flow of nitrogen gas. XRD analyses were performed on each sample of the various batches after firing to verify the obtained phases. The XRD equipment is a Philips X'Pert diffractometer operating at 45 kV and 40 mA with Cu K- radiation. The samples were ground by hand in a ceramic mortar and pestle, dry mounted in aluminum holders, and scanned from 8 to 60° - 2 . The samples with TiO2 addition were subjected to EDS analysis to verify that the TiO2 was fully integrated into the structure and that brownmillerite was the only phase. The composition determined from this analysis was close to the calculated compositional stoichiometry of the TiO2 contaminated brownmillerite within the limits of accuracy of the method as shown in Table 3.

Table 3 Atomic percentages of titanium in BT-0.5, BT-1.6, BT-2.6 and BT-3.6 by EDS At. % of Ti

BT-0.5 BT-1.5 BT-2.6 BT-3.6

Ideally

0.22

0.67

1.10

1.55

EDS

0.25

0.50

1.16

1.62

Mortar strength testing followed a modification of ASTM C109. The mix consisted of 500 g brownmillerite, 1375 g sand and 225 g DI water (water to cement ratio of 0.45). The very rapid set of BT-0 and BT-0.5 did not allow flow tests to be conducted. Also because of the rapid set and limitations on the materials available, smaller batches were employed, making one or two cubes at a time rather than the conventional 6. The cubes were tested in duplicate after 1, 7 and 28 days for compressive strength.


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RESULTS AND DISCUSSIONS Effect of Ti on the Brownmillerite Structure All the samples were found to be generally similar by XRD. They all showed that the ferrite phase C4AF to be the major component present. There are several “brownmillerites” listed in the ICDD files, 01-070-2765, with Mg and Si impurities, Ca2Al0.95Fe0.95Mg0.05Si0.05O5; was very close to that of BT-0, the end member material of this study. The Al-Fe brownmillerite without any mentioned impurities, 01-071-0667 was further to the left. The addition of TiO2 resulted in broadening and flattening of the X-ray diffraction peaks and also shifting them to lower 2 theta. Figure 1 presents the X-ray data from 33 to 35° 2 showing the range of this effect on the 141 reflection, for BT-0 and BT-3.6. It is found that the X-ray patterns for BT-0 and BT-0.5 are similar as are BT-2.6 and BT-3.6, with BT-1.6 intermediate between the two. The 141 peak also shifted to lower values with the shifts to larger d-spacing was the most pronounced for the BT-2.6 and BT-3.6 samples. The lowest level of TiO2 addition, BT-0.5, had little effect. The addition of TiO2 seems to cause a transformation of the lattice structure of the brownmillerite.

Figure 1 X-ray diffraction for 141 peak for BT-0 and BT-3.6

Effect of Ti on Brownmillerite Reactivity Isothermal calorimeter was used to track the exothermic reaction of the brownmillerite to hydrogarnet hydration reaction over a period of 300 hours. The most significant differences among the samples were in the first few hours. The samples BT-0 and BT-0.5 present similar curves (Figures 2 and 3), except that BT-0.5 has a slightly faster hydration reaction than BT-0 by about 15 minutes. BT-1.6 shows the initiation of hydration peak at around 30 minutes. BT-2.6 and BT-3.6 have delayed hydration which initiates at about 5 hours.


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Figure 2 Hydration reaction followed by isothermal calorimeter - Power/Cement Material

Figure 3 Hydration reaction followed by isothermal calorimeter - Energy/Cement Material The peak of the first derivative of the isothermal energy curve is related very closely to set time. The end member BT-0 sets very quickly, within a few minutes of hydration. The addition of 0.5% of TiO2 seems to accelerate the hydration reaction by a few minutes, while more than 1.5% of TiO2 retards the hydration reaction by about 5 hours. The delay in hydrogarnet formation was confirmed by both XRD and TGA. After 3 hours of hydration, BT-0 and BT-0.5 show both peaks of C4AF and hydrogarnet and BT-3.6 shows only peaks for C4AF. This is also evident in the TGA traces (Figure 4) where the weight loss from the hydrogarnet at 300 °C is missing from BT-3.6 at approximately 4 hours, but present in BT-0 and BT-0.5. After 1 day and until 28 day, all samples show X-ray reflections of C4AF and hydrogarnet, but the diffraction peaks for the hydrogarnet are less sharp for BT-3.6 at day 1 (Figure 5). Also, at 28 days BT-3.6 shows traces of calcium monocarboaluminate (3CaO.Al2O3.CaCO3.11H2O or C4AĆH11) (Figure 6).

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Figure 4 TGA results from hydration of BT-0, BT-0.5 and BT-3.6

Figure 5 XRD results for BT-0, BT-0.5 and BT-3.6 after 1-day hydration at room temperature


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Figure 6 XRD results for BT-0, BT-0.5 and BT-3.6 after 28-day hydration at room temperature

The mortar strength data is presented in Figure 7 for 1, 7 and 28 days. The low TiO2 samples, BT-0 and BT-0.5 were found to produce the highest mortar strengths at day 1, 7.6 and 7.8 MPa respectively versus 2.3, 5.5 and 4.9 MPa for BT-1.6, BT- 2.6 and BT-3.6. However by day 28 the high TiO2 samples had reversed this trend, with BT-2.6 and BT-3.6 having almost doubled the strength of the pure end member.

Figure 7: Compressive Strength of each BT after 1, 7 and 28 days of curing


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DISCUSSION AND CONCLUSION The effect of even small quantities of TiO2 on the reactivity of brownmillerite was found to be measurable. At the lowest levels, 0.52%, little change was noted in the x-ray pattern but the sample actually hydrated faster, and produced ultimately higher strengths that the Ti-free material. Additions of TiO2 at higher levels, 2.6% and 3.6%, produced notable shifts in the Xray of the ferrite, and the 141 peak was shifted to higher d-spacing. Both of these materials behaved in a similar fashion and displayed with a delayed set time, but had the highest long term mortar strengths. The intermediate sample, with 1.6% TiO2 was somewhat enigmatic having the lowest long term strength. The reactivity reported here is an intrinsic reactivity: the sample is wet and sufficient solid dissolves to increase the pH to its saturation value, close to 12. Thus the reaction is selfinitiating and self-accelerating as the pH increases. On the other hand, when ferrite occurs in a polyphase cement, pH change in the course of hydration is controlled in part by other phases and, moreover, the nature of the phases formed is affected by sulfate: the measured reaction is extrinsic. For present purposes, to demonstrate the role of Ti in reactivity, intrinsic reaction is the least complex to interpret. But for application to commercial or complex polyphase cements, it is also important to measure the extrinsic reaction. This is, however, more difficult to interpret as it is conditional, in the sense that numerical values depend on the exact proportions of the reactive components. In summary, the structure and behavior of brownmillerite as a cementitious material was greatly affected by relatively small dosages of TiO2, indicating the importance of even low levels of minor components in the Portland cement and in Calcium Sulfoaluminate cement.

ACKNOWLEDGEMENTS The authors wish to thank Dr. Y. Zhou and Dr. K. Henke for their help during this project.

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