Low Temperature Synthesis of Belite Cement from White Sand and Lime

Low Temperature Synthesis of Belite Cement from White Sand and Lime

M. A. Tantawy

Chemistry Department, Faculty of Science, Minia University,

Minia, Egypt.

Abstract This paper studies the low temperature synthesis of -C2S from mixture of lime, BaCl2 and white sand with the ratio (Ca+Ba)/Si = 2. The Mixture was hydrothermally treated in stainless steel capsule at 180oC for 5 hours, calcined at 600-900oC for 3 hours and analyzed by FTIR, XRD, TGA/DTA and SEM techniques. Dicalcium silicate hydrate was formed by hydrothermal treatment of lime/white sand mixture at 180oC for 5 hours. -C2S was produced when hydrothermally treated lime/white sand mixture was calcined at 750oC for 3 hours. In lime/white sand mixture, -C2S does not stabilized and transformed to -C2S with raising

silicon atoms in the -C2S structure.8,9 The reactivity and hydraulic properties of stabilized -C2S depends on the

type and amount of the stabilizing ions.10 Stabilizing ions modify charge density localization of the electronic structure of -C2S and enhance its reactivity.11 Fast cooling may produce fine -C2S crystals affording transformation of -C2S to -C2S, even without need to stabilizer.12

temperature of calcination to 900oC. In contrast, -C2S was

-C2S

~ 700oC

'L-C2S

1160oC

'H-C2S

1450oC

-C2S

stabilized in lime/BaCl2/white sand mixture that was calcined at 900oC. This due to that Ba2+ ions stabilizes -C2S and retards its transformation to -C2S.

Keywords White sand, Lime, Hydrothermal treatment, Dicalcium silicate hydrate, Calcination, -dicalcium silicate.

1. INTRODUCTION

   Tricalcium silicate (alite, C3S) and dicalcium

   orthorhombic

   orthorhombic

   60 ~ 80oC

   -C2S

   monoclini c

   orthorhombic

   hexagonal

   silicate (belite, C2S) are the major components of Portland cement and determine most of the adhesive properties, strength and durability of Portland cement. Belite hydrates much slower than that alite. Whatever, alite and belite show about the same physical and mechanical properties after complete hydration.1 Production of Portland cement releases about 900-1000 kg of CO2 per tonne of clinker due to calcination of CaCO3 and fuel combustion.2,3 Production of Portland cement also consumes about 3000-4000 MJ of thermal energy per tonne of clinker for dry process and about 324-540 MJ of electrical energy per tonne of cement for grinding of raw materials and cement.4,5 Accordingly, the synthesis of low-energy reactive belite cement as an alternative to Portland cement is one of the most important challenges.

   Belite have five polymorphs (, , , L´ and H´) and their thermal transformation relationship is illustrated in figure (1).6 -C2S is stable at high temperature and transforms to '-C2S and finally to and -C2S at low temperature. -C2S is hydraulic whereas – C2S is a non-

   hydraulic and does not account for the setting and hardening of cement.7 -C S causes cracking of -C S

   Figure (1): Thermal transformation relationship of belite Polymorphs

   Dicalcium silicate hydrate (hillebrandite, Ca2(SiO3)(OH)2) was prepared by hydrothermal treatment of lime/quartz mixture with Ca/Si = 2.0 at 200-250°C for 5-

   10 hours.13 Hillebrandite was synthesized by mechanochemical treatment of amorphous precipitated lime/silica mixture at room temperature.14 Hillebrandite decomposes at about 500°C producing low-crystalline – C2S.15,16 The temperature at which -C2S begins to form decreases as the Ca/Si ratio and/or temperature of hydrothermal synthesis of hillebrandite becomes higher.17,18 Dicalcium silicate hydrate dissociates at 390- 490°C forming -C2S plus an intermediate phase that subsequently transforms to 'L phase at 920-960°C and yields -C2S on cooling.19 The aim of this work is preparation of belite cement from lime/white sand mixture hydrothermally treated and calcined at low temperature.

2. MATERIALS AND EXPERIMENTAL TECHNIQUES Freshly prepared lime was produced by calcination of limestone powder (purity > 99%) in an electrical muffle furnace at 950oC for 3 hours. Lime was cooled to room

   2 2 temperature in desiccator, milled and stored in tightly

   crystals forming dust (i.e dusting phenomenon) because – C2S crystals are larger than -C2S crystals. Dusting phenomenon can be prevented if -C2S is stabilized by inclusion of stabilizing ions (Fe3+, Al3+, Mg2+, Zn2+, Cr3+, Pb2+, B3+, Na+ and K+) which replace calcium and/or

   closed plastic bag to avoid carbonation. White sand was provided from Royal Cement Company, Minia, Egypt. Distilled water and analytical grade barium chloride were used without further purification. Mixtures of white sand, lime, BaCl2 (with the ratio Ca/Si or (Ca+Ba)/Si = 2) and

   distilled water (water/solid ratio of 5/1 by weight) placed in stainless steel capsule keeping the occupied volume equals

   0.67 of total volume capacity. The capsule was tightly closed to avoid sealing of water vapor, was shacked to obtain homogeneous suspension inside and was heated at 180oC for 5 hours in electric oven. The capsule was removed from oven and cooled to room temperature. The product of hydrothermal treatment was filtered, washed with distilled water, dried in microwave oven and calcined in an electric muffle furnace at 600-900oC for 3 hours. The calcined product was cooled to room temperature in desiccator, milled and stored in tightly closed plastic bottles. X-ray fluorescence XRF and X-ray diffraction XRD analyses were carried out by Philips x-ray diffractometer PW 1370, Co. with Ni filtered CuK radiation (1.5406 Ã…). The Fourier transform infrared FTIR analysis was measured by spectrometer Perkin Elmer FTIR System Spectrum X in the range 400-4000 cm-1 with spectral resolution of 1 cm-1. Scanning electron microscopy SEM was investigated by Jeol-Dsm 5400 LG apparatus. The thermogravimetric TGA and differential thermogravimetric analyses DTG were carried out with the aid of Shimadzu Corporation thermo analyzer with DTG- 60H detector with 10oC/min heating rate from room temperature up to 1000oC, under nitrogen atmosphere at 40 ml/min flow rate, the hold time at the appropriate temperature is zero.

   bands of silica that appear at 794 and 464 cm-1 were reduced significantly. Whereas, the absorption band of combined water appear at 3449 cm-1. Also, there is a new absorption band appear at 2924 cm-1. These observations indicate that the hydrothermal treatment initiates the reaction between lime and white sand leads to the formation of intermediate dicalcium silicate hydrates. The existence of unreacted lime and silica indicates that the hydrothermal treatment of the lime/silica mixture (Ca/Si=2/1) at 180oC for 5 hours does not drive the reaction to completion.

   TABLE I. THE CHEMICAL COMPOSITION OF LIMESTONE AND WHITE SAND DETERMINED BY XRF

   Oxide	Limestone	White sand
   SiO2	0.26	96.66
   Al2O3	0.16	2.87
   CaO	54.59	0.14
   Fe2O3	–	0.08
   MgO	0.29	0.02
   SO3	0.05	–
   Na2O	0.11	–
   K2O	0.03	–
   Cl-	–	–
   LOI	43.72	1.20
   TiO2	–	–
   Total	99.21	99.97

   Q

3. RESULTS AND DISCUSSION

Table (1) illustrates the chemical composition of limestone and white sand determined by XRF. The chemical composition results confirm that limestone and white sand are mainly composed of CaCO3 and SiO2 respectively in presence of small amount of impurities. Figure (2) illustrates the XRD patterns of lime and white sand. Lime mainly composes of calcium oxide (CaO) in addition to

Q

(b)

K K

Q

Q Q Q Q Q

. L

small amount of portlandite (Ca(OH)2) that may present due to partial hydration of lime. White sand mainly

(a) L

Relative Intensity, %

L

composes of quartz (SiO2) in addition to small amount of P P

kaolinite (Al2O3.2SiO2.2H2O). Figure (3) illustrates the

FTIR spectra of lime and white sand. The absorption band

3

that appear at 3640 cm-1 is due to OH- associated with portlandite (hydrated lime). The absorption band that appear at 875 and 1441 are due to the v2 and v3 of carbonate (CO 2-) present as a partial carbonation lime.20,21 The absorption band that appear at 452 cm-1 is due to Ca-O stretching vibration.22 The absorption bands of silica (Figure (3b)) appear at 1084, 795 and 464 cm-1 are corresponding to asymmetric stretching vibration of SiO Si, symmetric stretching vibration of SiOSi, and bending vibration of OSiO respectively.23

Figure (4) illustrates the FTIR spectra of hydrothermally treated lime/white sand and lime/BaCl /white sand mixtures. The absorption band of

10 20 30 40 50 60

2

Figure (2): XRD patterns of (a) lime and (b) white sand (K kaolinite, L lime, P portlandite, Q quartz)

Figures (5 and 6) illustrate TGA and DTA thermograms of hydrothermally treated lime/white sand and lime/BaCl2/white sand mixtures. Result of thermal analysis was illustrated in table (2). TGA/DTA thermograms illustrates the in situ sequence of thermal reactions and phase transformations that occur when hydrothermally treated lime/white sand and lime/BaCl2/white sand mixtures were calcined up to 900oC. Absorbed water was lost at 132oC.24 Residual portlandite

2 -1 Ca(OH)2 was dehydrated at 450oC.25 Dicalcium silicate

silica at 1084 cm was shifted to lower wavenumber value 1029 cm-1. This is an indication for lowering of the degree of polymerization of silicate structure due to reaction between white sand and lime under the hydrothermal conditions. At the same time, the intensity of absorption

crystallizes into -C2S at 659oC. -C2S transforms to '-C2S at 832oC.

Figure (7) illustrates the SEM micrographs of hydrothermally treated lime/white sand and

lime/BaCl2/white sand mixtures. Lime/white sand and lime/BaCl2/white sand mixtures have different morphologies. The former is characterized by appearance of large grains with rough surfaces whereas the later show the presence of small cube-like grains. The change in morphological properties is accompanying presence of Ba2+ ions that replace calcium and/or silicon atoms and modify the structure of calcium silicate hydrates.

were sifted to lower wavenumber values (516 and 951cm-1 respectively). This is an indication for transformation of – C2S to '-C2S.28 Figure (9) illustrates the FTIR spectra of hydrothermally treated lime/BaCl2/white sand mixture calcined at 600-900oC. Appearance of absorption band at 885 cm-1 even at 900oC indicates for existence of -C2S at higher temperatures. Ba2+ ions may stabilizes the -C2S and retards its transformation to -C2S or '-C2S phases. Except formation of some of -C2S as indicated from its characteristic absorption bands located at 520.

Relative Transmittance, %

100

(a)

TGA, %

95 (b)

90

85

80

0 100 200 300 400 500 600 700 800 900

4000

3600

3200

2800 2400 2000 1600

1200

800

400

Temperature, C

Figure (5): TGA thermograms of hydrothermally treated

1. lime/white sand and (b) lime/BaCl2/white sand mixtures

   Wavenumber, cm-1

   2

   Figure (3): FTIR spectra of (a) lime and (b) white sand

   0

-2 .

0

-4

132

-4

659

832

DTA, Uv

-8

Relative Transmittance, %

-12

(a)

-16 (b)

3449

(a)

1029

794

-20

450

0 100 200 300 400 500 600 700 800 900

Temperature, C

2924

464

Figure (6): DTA thermograms of hydrothermally treated

(a) lime/white sand and (b) lime/BaCl2/white sand mixtures TABLE II. RESULT OF THERMAL ANALYSIS

4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wavenumber, cm-1

Figure (4): FTIR spectra of hydrothermally treated

(a) lime/white sand and (b) lime/ BaCl2/white sand mixtures

Figure (8) illustrates the FTIR spectra of hydrothermally treated lime/white sand mixture calcined at 600-900oC. Calcination at 600oC was accompanied with the following observations; the absorption band of silica was shifted from 1029 cm-1 to lower wavenumber value (958 cm-1) at the same time, new absorption band appears at 876 cm-1. This is an indication for transformation of intermediate dicalcium silicates hydrate to -C2S.26 Raising the temperature of calcination to 750oC was accompanied with appearance of absorption band at 520 cm-1. This is an indication for transformation of -C2S to -C2S.27 At 900oC, the absorption bands located at 520 and 955 cm-1

Figure (10) illustrates XRD patterns of hydrothermally treated lime/white sand mixture calcined at 600-900oC as well as lime/BaCl2/white sand mixture calcined at 900oC for 3 hours. Dicalcium silicate hydrates do not appear in the XRD analysis (figure 9a) because of its amorphous structure. -C2S appears in lime/white sand mixture that was calcined at 600oC (figure 9b) and its amount increases with raising calcination temperature to 750oC (figure 9c). -C2S transforms to -C2S when lime/white sand mixture was calcined at 900oC (figure 9d).

This proves that -C2S was not stabilized in case of hydrothermally treated lime/white sand mixture that was calcined at higher temperature and when it was cooled it transforms to -C2S.8,9 In contrast, -C2S was stabilized in case of hydrothermally treated lime/BaCl2/white sand mixture that was calcined at 900oC (figure 9e). This proves that Ba2+ ions stabilizes -C2S and retards its transformation to -C2S. In other words, Ba2+ ions replace calcium and/or silicon atoms hence it stabilizes the structure of -C2S.10

L

Q Q

(e) Q L

(d) P

Q Q L

(c) P P Q L

Q

(a)

(b)

(b) P Q

Relative Intensity, %

P Q

(a)

P L Q L

Q

P P

P Q P Q P

Figure (7): SEM micrographs of hydrothermally treated

1. lime/white sand and (b) lime/BaCl2/white sand mixtures

   (d)

   Relative Transmittance, %

   (c)

2. (a)

   10 20 30 40 50 60

   2

   Figure (9): FTIR spectra of hydrothermally treated (a) lime/BaCl2/white sand mixture calcined at (b) 600, (c) 750 and (d) 900oC

   (d)

   Relative Transmittance, %

(b)

(a)

4000

3600

3200

2800

2400

2000

1600

1200

800

400

4000

3600

3200

2800

2400

2000

1600

1200

800

400

Wav enumber, cm-1 Wav enumber, cm-1

Figure (8): FTIR spectra of hydrothermally treated (a) lime/white sand mixture calcined at (b) 600, (c) 750 and (d) 900oC

CONCLUSIONS

A method for the low temperature synthesis of – C2S from lime/BaCl2/white sand mixture (Ca/Si=2 and 2% Ba) was described. Dicalcium silicate hydrate was formed by hydrothermal treatment of lime/white sand mixture at 180oC for 5 hours. -C2S was produced when hydrothermally treated lime/white sand mixture was calcined at 750oC for 3 hours. In lime/white sand mixture, -C2S does not stabilized and transformed to -C2S with raising temperature of calcination to 900oC. In contrast, – C2S was stabilized in lime/BaCl2/white sand mixture that was calcined at 900oC. This due to that Ba2+ ions stabilizes -C2S and retards its transformation to -C2S.

Figure (10): XRD patterns of hydrothermally treated (a) lime/white sand mixture calcined at (b) 600oC, (c) 750oC and (d) 900oC as well as lime/BaCl2/white sand mixture calcined at (e) 900oC

(P Portlandite, Q quartz, -C2S, L lime and -C2S)

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