The synthesis of belite clinker was studied using industrial wastes: paper sludge, cement kiln dust and rice husk ashes, as substitutes for natural raw materials. Wastes were characterized by XRF, XRD and TG analysis. Different formulations were prepared to produce clinker at 1300, 1350 and 1400 °C. The clinker obtained was characterized using optical microscopy, XRD and
Industrial activities contribute significantly to the generation of wastes; due to the lack of planning and ignorance of processes which they can be used, most of the wastes are destined for final disposal. For example, in the cement industry, a solid waste known as cement kiln dust (CKD) corresponds to 3-4% of the total cement produced, which by 2019 reached a world total production of 4.2 billion tons (
Recent research reports the use of CKD as a mineral addition to cement, finding satisfactory results (
Other authors (
The environmental impact associated with the extraction of raw materials, added to the important CO2 emissions related mainly to the decarbonation of limestone and the high energy consumption, supplied mostly with fossil fuels, makes research in this sector to be oriented towards the design and manufacture of alternative cements which required less energy for their production. An alternative is replacing conventional fuels and raw materials in order to reduce CO2 emissions (
In this regard, it has been investigated the possibility of producing cement with lower demands of calcium carbonate, for which low lime saturation factor (LSF) is handled, this is associated with the higher contents of belite phase in the clinker. It is estimated that the production of this type of belite cement requires 15 to 20% less energy compared to the production of alite cement, what is beneficial due to the cost of energy and the reduction of CO2 emissions (
Consequently, the environmental benefit of belite cements over OPC cements mainly are the increase in energy savings, the reduction in the burning temperature and the decrease in CO2 and NOX emission levels (
However, compared to the use of OPC cements, belite cements generally have less hydraulic activity during the first hours of hydration, so that research focuses on the use of additives that improve hydraulic activation, the use of “remelting reaction” and the “sol-gel method”. Nevertheless, these methods are not widely used on an industrial scale (
In the same way, there are studies in which some methodologies are implemented, by which raw materials required in the manufacture of cement are partially replaced (
In accordance with this, in the present research work, it is of interest the evaluation of the total substitution of raw materials required in the production of belite clinker by using three types of industrial waste: paper sludge (PS), cement kiln dust (CKD) and rice husk ash (RHA). The objective of this research is the reduction of primary resources, energy demand and CO2 emission in the clinker production, providing value of this type of by-products, that is currently not widely used. On the other hand, with the obtaining of the belite clinker, lower consumption of CaCO3 will be obtained and lower temperatures will be required, which results in lower energy consumption and a decrease in CO2 emissions.
For clinker preparation, the mixture of three wastes generated in different industries was used: (i) paper sludge (PS); (ii) cement kiln dust (CKD) and (iii) rice husk ashes (RHA). The PS comes from the tissue paper industry; in this process, limestone (CaCO3) and kaolin are added to fulfill the filler function, which are concentrated in the paper sludge. The CKD is generated in the production of cement, where part of the limestone has an incomplete reaction and the finest dust is carried by the air stream (
The paper sludge was previously calcined at 800 °C, the CKD and RHA were dried at 105 °C. The residues were milled and sieved (< 90 µm) individually.
Using the tool Solver, the mixing ratios of the residues were calculated, in order to prepare three formulations (
Mixtures composition (wt. %).
Formulation | PS | CKD | RHA | LSF | SM | AM |
---|---|---|---|---|---|---|
20.27 | 68.79 | 10.94 | 95 | 2.20 | 2.13 | |
26.17 | 63.90 | 9.92 | 100 | 2.10 | 2.32 | |
25.37 | 65.65 | 8.98 | 104 | 2.00 | 2.28 |
Each formulation was burned in a fixed bed electric oven (Carbolite model BLF 17/3). The heating ramp to obtain the clinker involved the following stages: (i) heating at 10 °C/min up to 800 °C; (ii) residence time for 30 minutes at 800 °C to promote complete decarbonation; (iii) heating at 10 °C/min to the maximum temperature: 1300, 1350 or 1400 °C; (iv) remain 45 minutes at the maximum temperature to promote complete clinkering and (v) forced convection cooling.
The by-products were characterized in terms of moisture content and ignition losses (EN12880:2001) (
The chemical composition of the residues, presented in
Chemical composition of residues determined by XRF.
(wt %) | CaO | SiO2 | Al2O3 | Fe2O3 | MgO | Na2O | K2O | P2O5 | TiO2 | Cl | SO3 | LOI |
---|---|---|---|---|---|---|---|---|---|---|---|---|
29.28 | 13.50 | 3.37 | 0.38 | 0.60 | 0.59 | 0.06 | 0.11 | 0.16 | 0.04 | 0.16 | 51.69 | |
59.48 | 9.76 | 5.18 | 3.73 | 0.69 | 0.24 | 0.88 | 0.35 | 0.29 | 0.22 | 0.87 | 18.12 | |
0.87 | 88.28 | 0.07 | 0.09 | 0.72 | 0.04 | 3.11 | 2.04 | --- | 0.54 | 0.86 | 3.15 |
LOI: loss on Ignition at 800 °C
The chemical and mineralogical composition were studied as described above for the residues. The content of free lime (f-CaO) in clinker samples was determined by means of the ethylene glycol method (
Clinker samples were milled and sieved (< 45 µm). Cement samples were prepared in proportion 95% wt of each clinker obtained and 5% wt of calcium sulfate dihydrate. A water/cement ratio of 0.55 was used. Heat flow was evaluated during cement hydration by conduction micro calorimetry, using a TAM Air microcalorimeter with an isothermal method and an air thermostat fixed at 25 °C. The samples were filled in glass ampoules and placed inside the kit for 40 minutes to achieve thermal stability. Then, the water injection was performed with a mixing device (admix), it was homogenized for two minutes. Finally, data were collected for a period of 120 hours.
The PS contains a humidity of 48.20%, while the CKD and RHA, due to their generation in thermal processes, have much lower contents; 0.65% and 1.65% respectively.
Result of the TG analysis of paper sludge (PS).
XRD patterns (a) paper sludge, (b) rice husk ash and (c) CKD.
The analysis of rice husk ash (RHA) shows that it is mainly composed of SiO2; moreover, it contains 3.11 wt% of K2O, which may come from traces of fertilizers applied in rice cultivation. In XRD pattern of RHA (
The XRD pattern of the CKD (
Because the PS have higher losses on ignition (LOI), the thermogravimetric behavior of this residue was studied. In
Chemical composition of each formulation XRF.
(wt%) | CaO | SiO2 | Al2O3 | Fe2O3 | MgO | Na2O | K2O | P2O5 | TiO2 | Cl | SO3 | LOI |
---|---|---|---|---|---|---|---|---|---|---|---|---|
55.10 | 17.82 | 4.88 | 2.39 | 0.71 | 0.43 | 1.46 | 0.45 | 0.28 | 0.72 | 0.77 | 14.79 | |
53.22 | 16.33 | 5.10 | 2.44 | 0.71 | 0.35 | 1.14 | 0.36 | 0.30 | 0.65 | 0.76 | 18.42 | |
54.14 | 15.65 | 5.08 | 2.44 | 0.70 | 0.33 | 1.19 | 0.39 | 0.31 | 0.59 | 0.76 | 18.21 |
The percentage of free lime
F1 | F2 | F3 | |||||||
---|---|---|---|---|---|---|---|---|---|
1300 | 1350 | 1400 | 1300 | 1350 | 1400 | 1300 | 1350 | 1400 | |
1.07 | 0.47 | 0.40 | 1.06 | 0.33 | 0.32 | 1.52 | 0.40 | 0.33 |
Through the Rietveld refinement it was identified and quantified the presence of the alite (C3S), belite (C2S), celite (C3A) and felite (C4AF) phases (
Mineralogical analysis of clinker (% wt.) through Rietveld.
Temperature (°C) | Formulation | C3S (Monoclinic) | C2S (Beta) | C3A (Cubic) | C4AF | Amorphous | Rwp |
---|---|---|---|---|---|---|---|
1350 | F1 | 8.7 | 50.9 | 4.3 | 3.9 | 32.2 | 9.0 |
1400 | F1 | 10.3 | 29.5 | 0.5 | 1.3 | 58.4 | 9.3 |
1350 | F2 | 9.5 | 46.7 | 8.0 | 3.3 | 32.5 | 8.4 |
1400 | F2 | 10.3 | 27.7 | 2.6 | 1.4 | 58.0 | 8.9 |
1350 | F3 | 9.3 | 43.5 | 14.4 | 2.9 | 29.9 | 8.9 |
1400 | F3 | 12.5 | 24.0 | 3.0 | 1.8 | 58.7 | 8.6 |
Clinker XRD. (a) Clinkering temperature 1350 °C, (b) Clinkering temperature 1400 °C. C3S (ICDD 01-086-0402), C2S (ICDD 01-077-0409), C3A (COD 96-100-0040), C4AF (COD 96-900-3337).
The formation of the mineralogical phases of clinker was confirmed by means of optical microscopy analysis (
Average size of alite and belite in the obtained clinker.
Average size | F1 |
F2 |
F3 |
|||
---|---|---|---|---|---|---|
1350 °C | 1400 °C | 1350 °C | 1400 °C | 1350 °C | 1400 °C | |
10.0 ±1.6 | 18.6 ± 2.3 | 8.8 ± 1.5 | 12.9 ±1.4 | 12.6 ±1.2 | 13.5 ± 3.1 | |
7.2 ± 2.3 | 7.8 ± 1.6 | 7.2 ± 1.4 | 6.9 ± 1.6 | 8.1 ± 1.1 | 9.2 ± 2.2 |
Clinker optical microscopy (a) Clinker F1 at 1350 °C, (b) Clinker F2 at 1350 °C, (c) Clinker F1 at 1400 °C and (d) Clinker F2 at 1400 °C.
On the other hand, the polished clinker sections were coated with a graphite layer of 8 nm, and analyzed in a scanning electron microscope (SEM) equipped with an x-ray spectrometer (EDS). Figure
Clinker electron microscopy (a) Clinker F1 at 1350 °C, (b) Clinker F2 at 1400 °C, (c) Clinker F3 at 1350 °C and (d) Clinker F3 at 1400 °C.
In
Chemical composition of the alite (C3S), belite (C2S) and interstitial phases (IP), by means of de SEM/EDS (wt%).
Element | Formulation F1 |
Formulation F3 |
||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
1350 °C |
1400 °C |
1350 °C |
1400 °C |
|||||||||
C3S | C2S | IP | C3S | C2S | IP | C3S | C2S | IP | C3S | C2S | IP | |
65.07 | 63.82 | 63.35 | 58.59 | 60.92 | 50.45 | 63.44 | 65.86 | 62.50 | 68.57 | 46.08 | 66.66 | |
16.01 | 15.77 | 3.81 | 16.86 | 13.81 | 7.84 | 14.98 | 17.62 | 12.81 | 16.43 | 16.61 | 14.63 | |
1.67 | 1.86 | 3.11 | 1.74 | 1.92 | 10.36 | 3.43 | 2.17 | 5.36 | 1.65 | 6.95 | 2.20 | |
1.63 | 1.14 | 17.04 | 1.67 | 0.00 | 12.20 | 2.23 | 1.49 | 3.74 | 0.00 | 2.88 | 2.09 | |
0.59 | 1.52 | 0.81 | 1.13 | 1.60 | 0.97 | 0.83 | 0.88 | 0.59 | 0.56 | 0.90 | 1.07 | |
1.05 | 0.24 | 0.55 | 1.06 | 0.33 | 2.89 | 1.15 | 0.00 | 0.48 | 0.00 | 1.59 | 0.57 | |
0.60 | 0.79 | 0.88 | 0.91 | 0.49 | 0.68 | 0.61 | 0.00 | 0.69 | 0.81 | 1.30 | 0.54 | |
0.56 | 0.94 | 0.47 | 0.74 | 0.82 | 0.98 | 0.57 | 0.00 | 0.00 | 0.56 | 1.38 | 0.54 | |
0.53 | 0.00 | 0.00 | 0.73 | 0.00 | 0.67 | 0.57 | 0.00 | 0.00 | 0.00 | 1.05 | 0.00 |
Microcalorimetry of cements prepared from clinker obtained at 1350 °C, (a) First peak; (b) Second peak.
Among the heat flow curves corresponding to the first peak, it is found that in the case of C2 cement, more heat is released in the first minutes of hydration. In addition, for C2, a second peak is not found, which means that the reactivity of C2 cement is greater and almost instantaneous. Despite having a C2 cement with an important content of the belite phase (46.7% by weight), the hydration reactions are carried out in the first minutes; this is associated with the high contents of amorphous phase in this cement (32.5% by weight), the content of celite phase (8% by weight) and the small size of belite (7.2 µm) that gives it a greater reactivity as has been suggested by some researchers (
On the other hand, the area under the heat flow curve was calculated in intervals of time between 0-0.99 hours and 1-25 hours, from which it is found that during the first hour of hydration, the heat flow of the C2 cement is 43.5% higher than in C1 cement, and 50% higher than in C3 cement. When comparing the results presented in
Heat flows in the hydration of prepared cements from each clinker formulation at 1350 ° C.
ID | Formulation | First peak Area t= 0 – 0.99 h | Second peak Area t= 1 – 25 h | Qaccum t=2 h (J/g) | Qaccum t=100 h (J/g) |
---|---|---|---|---|---|
C1 | F1 | 6.36 | 4.85 | 35.66 | 108.72 |
C2 | F2 | 11.26 | 0.23 | 53.43 | 149.79 |
C3 | F3 | 5.53 | 10.21 | 25.87 | 211.57 |
As observed in
Cumulated heat microcalorimetry (Qaccum) in prepared cements obtained clinker at 1350 ° C.
For the three cements, in the first 2 hours of hydration (induction period), the heat released is fundamentally controlled by the size of the alite and by the content of belite and amorphous material. It seems that for that initial period the content of C3A is not so important. After 100 hours of hydration, the highest heat released is associated to the sample that has the highest percentage in the sum of all phases (C3S + C2S + C3A + C4AF), which corresponds to formulation 3.
The analyzed by-products have chemical compatibility with the raw materials traditionally used in the production of industrial clinker. The set of these residues, was composed mainly of CaO, SiO2, Al2O3 y Fe2O3.
Belite clinker was obtained from the formulations in which 20-25% of paper sludge, 60-69% of CKD and 9-11% of rice husk ash were incorporated; consequently, 100% of the natural raw materials required in clinker production were replaced.
The increase in clinkering temperature favored the formation of alite phase; however, the belite phase predominates, which may be due to the stabilization of belite when traces of elements such as potassium and sulfur are present, because they hinder the reaction of belite with CaO, preventing the formation of alite. It was also found that at higher temperatures than those presented in this research (1450 ºC) the clinker was melted completely.
Even when there are important contents of the belite phase, the prepared cements, with obtained clinkers at 1350 °C, have great heat release at early ages, due to the amorphous content and the small size of the crystals.