The aim of this study is the preparation of β-belite by a solid-state reaction using powdered limestone, amorphous silica and liquid alkali silicates. The raw materials were blended, the mixtures were agglomerated and then burnt. The resulting samples were characterized by X-ray diffraction analysis and scanning electron microscopy. Free lime content in the β-belite samples was also determined. The effects of CaO/SiO2 ratio (1.6–2.1), burning temperature (800–1400 °C), utilization of different raw materials (silica fume, synthetic silica, potassium silicate, sodium silicate, potassium hydroxide) and burning time (0.5–16 h) on free lime content and mineralogical composition were investigated. The purest β-belite samples were prepared from a mixture of powdered limestone, silica fume and liquid potassium silicate with a ratio CaO/SiO2 = 2 by burning at temperatures between 1100 and 1300 °C for more than 2 h. Decreasing of the CaO/SiO2 ratio led to rankinite formation and lower a burning temperature led to the formation of wollastonite.
Cement production is associated with high energy consumption and CO2 emissions, therefore the cement industry is facing challenges to reduce them. One of the routes is the manufacturing of clinkers based on dicalcium silicate (C2S–belite). Belite formation generates reduced amounts of CO2 and also energy consumption is lower in comparison with tricalcium silicate (C3S – alite), the main component of Portland cement (
Dicalcium silicate exists in several polymorphic forms at ordinary pressures: α, α´H, α´L, β and γ (
Although β-belite hydratation is much slower than that of alite, the later strength of the belite-rich and alite-rich cement pastes can be similar (
Several methods of the preparation of belite cement or pure belite have been studied closely in laboratory scale. The most common process, corresponding to industrial production, involves the blending of powdered raw materials eventually followed by agglomeration, and final burning of the mixture (
The present work is focused on the preparation of β-belite by a solid-state reaction, using raw materials produced in high volumes in industrial scale: powdered limestone, amorphous silica and liquid alkali silicates (water glasses). These alkali silicates were used here for their ability to bind effectively powdered raw materials, and as a natural source of reactive SiO2 and of alkali ions, the latter having been reported as β-belite structure stabilizers. Systematic investigation was carried out to determine the influence of C/S ratio, reaction temperature and the sort of silica and alkali ions used on phase composition and free lime content.
The raw materials used for dicalcium silicate preparation were partly powdered, such as synthetic silica (VP4; AV EKO-COLOR, s.r.o., Czech Republic), silica fume (České lupkové závody, a.s., Czech Republic) and limestone (Omyacarb 5VA; Omya), partly liquid, such as potassium silicate (Z. Ch. Rudniki S.A., Poland) and sodium silicate (Vodní sklo, a.s., Czech Republic).
The chemical compositions of powdered raw materials, determined by X-ray fluorescence, are presented in
Chemical composition (wt. %) of powdered raw materials
LOI |
SiO2 | Al2O3 | Fe2O3 | CaO | MgO | K2O | Na2O | TiO2 | P2O5 | ZrO2 | Cl | SO3 | |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Limestone | 43.7 | 0.50 | 0.27 | - | 55.1 | 0.41 | - | - | - | - | - | - | - |
Silica fume | 0.68 | 95.6 | 0.23 | 0.30 | 1.34 | - | - | - | - | 0.44 | 1.14 | - | 0.11 |
Synthetic silica | 4.23 | 92.3 | 0.22 | 0.06 | 0.16 | 2.14 | 0.01 | 0.47 | 0.06 | - | - | 0.02 | 0.38 |
LOI = Loss on ignition
Physical properties of powdered raw materials
BET (m2/g) | Specific gravity (kg/m3) | Bulk density (kg/m3) | |
---|---|---|---|
Limestone | 6.77 | 2658 | 787 |
Silica fume | 15.32 | 2204 | 303 |
Synthetic silica | 142.4 | 2111 | 174 |
Chemical composition (wt. %) of liquid alkali silicates
H2O | SiO2 | Al2O3 | K2O | Na2O | |
---|---|---|---|---|---|
Potassium silicate | 69.02 | 21.29 | 0.03 | 8.18 | 0.70 |
Sodium silicate | 64.59 | 22.59 | 0.07 | 0.26 | 12.75 |
XRD patterns of powdered raw materials.
Particle size distributions of powdered raw materials.
Pore size distributions of powdered raw materials.
Analytical grade potassium hydroxide (Lachner) and distilled water were used to prepare a 25% KOH solution.
The chemical compositions of powdered raw materials were determined by X-ray fluorescence (BRUKER S8 Tiger).
A BRUKER D8 Advanced X-Ray diffraction system (XRD) equipped with BRUKER SSD 160 detector and operating with Cu-Kα radiation at 40 kV and 25 mA was used for analysis of raw materials and prepared dicalcium silicates. XRD scanning was taken at the 2θ = 0.02 step over an angular range from 5° to 70° with 1 s counting time.
A Mastersizer 2000 laser diffraction particle size analyser (MALVERN Instruments) was used to determine size distribution of powdered raw materials. Agglomerates were disrupted by ultrasound treatment.
Pore size distributions of powdered raw materials were determined using AutoPore 9510 mercury intrusion porosimeter (Micromeritics), which operates with pressures from 0.01 MPa to 414 MPa.
A gas sorption analyser Autosorb iQ from Quantachrome was used for the determination of specific surface area by the Brunauer-Emmett-Teller method (BET).
Free lime content in dicalcium silicate samples was determined by a glycerine-alcohol test.
The morphology of silica fume and synthetic silica was studied by a scanning electron microscope Mira 3 from (TESCAN).
An inductively coupled plasma optical emission spectrometer OPTIMA 8000 (Perkin Elmer) was used to determine the content of micro-elements and K/Na ratio in liquid alkali silicates. Total content of alkali metals (Na, K) and content of SiO2 in alkali silicates were determined by conventional acid-base titration methods; the reason for their application being higher accuracy at higher concentrations compared with other methods.
Specific gravity was determined by the pycnometric method.
Ten mixtures M1–M10 were prepared by hand stirring in a vessel. Compositions of these mixtures, which are given in
Compositions of raw materials mixtures (wt. %)
Raw material | M1 | M2 | M3 | M4 | M5 | M6 | M7 | M8 | M9 | M10 |
---|---|---|---|---|---|---|---|---|---|---|
Limestone | 58.73 | 59.73 | 60.65 | 61.49 | 62.27 | 62.99 | 61.54 | 62.53 | 69.49 | 68.37 |
Silica fume | 17.27 | 16.27 | 15.35 | 14.51 | 13.73 | 13.01 | - | 13.48 | 21.36 | 21.02 |
Potassium silicate | 24.00 | 24.00 | 24.00 | 24.00 | 24.00 | 24.00 | 24.00 | - | - | - |
Synthetic silica | - | - | - | - | - | - | 14.46 | - | - | - |
Sodium silicate | - | - | - | - | - | - | - | 24.00 | - | - |
H2O | - | - | - | - | - | - | - | - | 9.14 | - |
KOH - 25% solution | - | - | - | - | - | - | - | - | - | 10.61 |
CaO/SiO2 (mol/mol) | 1.60 | 1.70 | 1.80 | 1.90 | 2.00 | 2.10 | 2.00 | 2.00 | 2.00 | 2.00 |
Conditions of β-belite preparation
Sample | Mixture | Agglomerates | Burning temperature (°C) | Burning time (h) |
---|---|---|---|---|
B1 | M1 | Tablets | 1 100 | 8 |
B2 | M2 | |||
B3 | M3 | |||
B4 | M4 | |||
B5 | M5 | |||
B6 | M6 | |||
B7 | M5 | Tablets | 800 | 8 |
B8 | 950 | |||
B9 | 1 250 | |||
B10 | 1 300 | |||
B11 | 1 400 | |||
B12 | M7 | Tablets | 1 100 | 8 |
B13 | M8 | Plastic material | ||
B14 | M9 | Tablets | ||
B15 | M10 | Tablets | ||
B16 | M5 | Tablets | 1 100 | 0.5 |
B17 | 1 | |||
B18 | 2 | |||
B19 | 4 | |||
B20 | 16 |
Chemical composition (wt. %) of β-belite samples
Sample | SiO2 | Al2O3 | Fe2O3 | CaO | MgO | K2O | Na2O | P2O5 | ZrO2 | TiO2 | CaO/SiO2 (mol/mol) |
---|---|---|---|---|---|---|---|---|---|---|---|
B1 | 38.08 | 0.34 | 0.09 | 56.85 | 0.42 | 3.42 | 0.29 | 0.13 | 0.34 | - | 1.60 |
B2 | 36.70 | 0.34 | 0.09 | 58.22 | 0.43 | 3.45 | 0.30 | 0.13 | 0.32 | - | 1.70 |
B3 | 35.41 | 0.34 | 0.08 | 59.50 | 0.44 | 3.47 | 0.30 | 0.12 | 0.31 | - | 1.80 |
B4 | 34.22 | 0.34 | 0.08 | 60.68 | 0.45 | 3.50 | 0.30 | 0.11 | 0.29 | - | 1.90 |
B5, B7–B11, B16–B20 | 33.10 | 0.35 | 0.07 | 61.79 | 0.46 | 3.52 | 0.30 | 0.11 | 0.28 | - | 2.00 |
B6 | 32.06 | 0.35 | 0.07 | 62.83 | 0.47 | 3.54 | 0.30 | 0.10 | 0.26 | - | 2.10 |
B12 | 32.99 | 0.35 | 0.02 | 61.56 | 1.00 | 3.56 | 0.42 | - | - | 0.01 | 2.00 |
B13 | 32.53 | 0.34 | 0.07 | 60.74 | 0.45 | 0.11 | 5.37 | 0.10 | 0.27 | - | 2.00 |
B14 | 34.34 | 0.38 | 0.11 | 64.10 | 0.47 | - | - | 0.16 | 0.40 | - | 2.00 |
B15 | 33.13 | 0.37 | 0.10 | 61.85 | 0.46 | 3.52 | - | 0.15 | 0.39 | - | 2.00 |
All the samples B1–B6 with C/S ratio from 1.6 to 2.1 were white.
XRD patterns of samples B1–B6 with different CaO/SiO2 ratio.
Free lime assessments (
Effect of CaO/SiO2 ratio on the free lime content.
The samples B5, B7, B8 were white in colour, while samples B9, B10 were light green. The sample B11 was amorphous green glass, obviously because of the applied temperature 1400 °C, which was higher than the melting point of mixture M5. The development of phase composition with burning temperature is shown in
XRD patterns of samples B5 and B7–B11 with different burning temperatures.
The effect of burning temperature on the free lime content is shown graphically in
Effect of burning temperature on free lime content.
Typical morphologies of the samples prepared at different temperatures are shown in
Morphology of sample B8 treated at 950 °C.
Morphology of sample B5 treated at 1 100.
Morphology of sample B11 treated at 1 400 °C.
The samples B7 and B8 are not homogenous. Spherical particles of silica fume are clearly visible in
The samples B5 and B12–B15 were prepared from various mixtures of raw materials, but the C/S ratio and the calcination procedure were kept the same. XRD patterns of these samples and the results of free lime determination, compared in
XRD patterns of samples B5 and B12–B15 prepared from different raw materials (LS - limestone, SF - silica fume, SS - synthetic silica, KS - potassium silicate, NaS - sodium silicate, KOH - potassium hydroxide).
Effect of different raw materials utilisation on the free lime content.
The sample B5, prepared from a mixture of limestone, silica fume and potassium silicate had the lowest content of free lime. The replacement of potassium silicate with sodium silicate (B13) or potassium hydroxide (B15) led to a slight increase in free lime content. A significantly higher increase was caused by the replacement of silica fume with synthetic silica (B12). The highest lime content in the sample B14 demonstrates a significant influence of alkaline ions on the solid-state reaction rate. X-ray diffraction patterns are consistent with the results of free lime determination. All samples contained β-belite. The samples B15, B12 and B14 contained lime, the amounts of which increased respectively. The sample B13 probably contained sodium calcium silicate (ICDD 731726) and the sample B14 cristobalite (ICDD 391425). The diffraction pattern of the sample B12 explains why it contains more free lime than the sample B5, although synthetic silica has significantly higher specific surface area than silica fume (
The samples B5 and B16–B20 were prepared from the same mixture M5 (C/S = 2) by burning at 1 100 °C for various times (0.5–16 h). All of these samples were white. The influence of burning time on mineralogical composition and free lime content are shown in
XRD patterns of samples B5 and B16–B20 with different burning time.
Effect of burning time on free lime content.
β-belite with low content of free lime (below 5 %) was prepared by burning a mixture of powdered limestone, silica fume and liquid potassium silicate (water glass) with a ratio C/S = 2 and K2O content about 3,5 % at temperatures from 1 100 to 1 300 °C for more than 2 h. Increasing C/S ratio above 2 led to the an increase in free lime content. Rankinite was formed at a C/S ratio under 2; its content increased (while β-belite content decreases) with decreasing C/S ratio. A decrease in the burning temperature to 950 °C resulted in an increase in free lime content and formation of wollastonite – an intermediate of β-belite formation. Liquid potassium silicate in the reaction mixture was replaced with liquid sodium silicate or potassium hydroxide, without substantial deterioration in β-belite purity. The replacement of silica fume with synthetic silica with a low bulk density did not prove as expected because of low miscibility with other raw materials.
The described procedure for β-belite preparation is very simple and uses raw materials produced in large volumes. Therefore, it indicates high potential for industrial application, especially if impurity content is reduced by optimizing the amount of alkali metal in the reaction mixture.
The author thanks Karol Bayer (Faculty of Restoration, University of Pardubice) for providing SEM pictures.
The publication is a result of the project Development of the UniCRE Centre (project code LO1606) which was financially supported by the Ministry of Education, Youth and Sports of the Czech Republic under the National Sustainability Programme I.