ORCID ID: S. Chhaiba (
For some time the cement industry has been seeking procedures to effectively lower the higher energy costs involved in cement manufacture. Timahdit oil shale and Jerada coal waste could potentially be used as alternative raw materials to produce clinker. This study explored the possibility of applying those materials to a greener use, based on the reactivity and burnability of raw mixes containing Moroccan oil shale and coal waste. The findings showed that, irrespective of particle size, oil shale mixes delivered higher reactivity than coal waste materials, although reactivity was highest in the oil shale clinker with a particle size <45 μm. The clinkers bearing oil shale with a particle size <90 μm or a blend of oil shale and coal waste with a size <45 μm contained higher proportions of alite (>70 %).
Cement, in many ways an essential material, is used world wideas a concrete component. Portland cement is made primarily from finely ground clinker, which itself is composed of calcium silicate and aluminate minerals formed when limestone and other materials are burnt at high temperatures. The cement industry is believed to account for over 8 % of worldwide CO2 emissions (
Oil shale is one of humanity’s most promising sources of energy and chemicals (
Recent research (
The present study aimed to determine the reactivity and burnability of portland cement raw mixes in which conventional raw materials were replaced with Timahdit oil shale and Jerada coal waste.
The materials used in this study were collected from the Timahdit shale bed, characterised by four lithological zones (T, Y, X and M), and the coal waste (CW) stockpiled at Jerada. The four layers of Timahdit oil shale were mixed to a homogeneous blend (BOS). BOS and CW were each sieved to three particle sizes:
BOS<45 μm,
90 μm>BOS>45 μm,
125 μm>BOS>90 μm,
CW<45 μm,
90 μm>CW>45 μm,
125 μm>CW>90 μm
The mean chemical composition for each fraction of BOS and CW is given in
Chemical analysis (wt %) of blended oil shale and coal waste by particle size
Oxide | CaO | Al2O3 | SiO2 | Fe2O3 | SO3 | MgO | K2O | MnO | P2O5 | LOI |
---|---|---|---|---|---|---|---|---|---|---|
19.37 | 8.52 | 23.94 | 2.56 | 3.59 | 2.1 | 1.06 | 0.01 | 0.25 | 37.67 | |
16.45 | 10.44 | 28.89 | 2.87 | 3.56 | 1.73 | 1.16 | 0.01 | 1.01 | 33.26 | |
15.41 | 9.78 | 27.05 | 2.69 | 3.32 | 1.62 | 1.07 | 0.01 | 0.94 | 37.48 | |
0.57 | 21.63 | 52.03 | 4.65 | 0.91 | 0.99 | 2.97 | 0.06 | 0.07 | 14.26 | |
0.45 | 22.6 | 51.29 | 4.87 | 0.85 | 1.08 | 3.13 | 0.09 | 0.09 | 13.85 | |
0.66 | 20.54 | 53.22 | 4.47 | 1.16 | 0.89 | 2.76 | 0.08 | 0.08 | 14.54 | |
47.21 | 2.75 | 5.26 | 0.91 | 0.05 | 0.98 | 0.46 | 0.01 | 0.01 | 40.98 |
The reactivity and burnability of raw mixes prepared with BOS, CW, limestone and laboratory grade Fe2O3 were compared to assess the viability of using oil shale and coal waste as raw materials in portland clinker. Their composition is given in
Raw mix composition
Proportion (wt%) | ||||
---|---|---|---|---|
Limestone | BOS | CW | Fe2O3 | |
66 | 33 | – | 1 | |
72.5 | 26.51 | – | 1 | |
70.2 | 28.8 | – | 1 | |
83.06 | 15.54 | – | 1.4 | |
85 | 14 | – | 1 | |
85 | 13.8 | – | 1.2 | |
64.71 | 20.44 | 13.71 | 1.14 |
Raw mix reactivity was determined in an air atmosphere with DSC/TG/DTG and burnability by measuring the free lime in the mixes after burning at 1400 °C for 30 min and applying Fundal’s (
% CaO(free) = 0.31 (LSF-100) + 2.18 (MS- 18) + 0.73 Q45 + 0.33 C125 + 0.34 Aq
Where LSF (=CaO/(2.8SiO2+1.18 Al2O3+0.65 Fe2O3) is the lime saturation factor, MS (=SiO2/(Al2O3+ Fe2O3) the silica modulus, Q45 the fraction by weight of quartz particles >45 μm, C125 the fraction by weight of limestone particles >125 μm and Aq the fraction of clay materials >63μm.
The raw mixes were prepared as follows.
After blending in a turbula for 60 min, the raw materials were pelletised, set into platinum crucibles, clinkerised at 1400 °C for 30 min in a laboratory furnace, cooled at ambient temperature, ground and then sieved to under 45 μm. The free lime content in the clinkers was found with the ethylene glycol method as specified in Spanish standard UNE 80243: 2014 (
The majority elements in the raw materials were identified on a Philips PW1404 X-ray fluorescence spectrometer (XRF) operating at 40 kV and 70 mA and fitted with a Sc–Mo tube, Super-Q software and four analyser crystals: PX-1, GE, LIF200, and LIF220. Differential scanning calorimetry (DSC) and thermogravimetric (DTA/TG) analysiswere simultaneously applied to 30 mg samples using a SDTQ600 analyser. The heating rate was 10 °C/min, the range 25 °C to 1450 °C and the carrier gas CO2-free dry air, flowing at 100 mL/min.
For the morphological study clinker samples were encapsulated in epoxy resin, cut and polished and the surfaces etched with a solution of HNO3 in absolute ethanol for observation under a Nikon Eclipse ME600 reflected light microscope. Infrared spectra were recorded for KBr pellets at frequencies of 4000 cm-1 to 400 cm-1 on a Thermo Scientific Nicolet 600 FTIR spectrometer featuring a spectral resolution of 4 cm-1.
Clinker samples were polished and carbon coated for analysis on a JEOL JSM 6400 scanning electron microscope (SEM) operating at 20 keV.
Clinker mineralogy was characterised on a Bruker D8 Advance X-ray diffractometer fitted with a high voltage, 3 kW generator and a (1.54-Å Cu Ka) copper anode X-ray tube operating at 40 kV and 30 mA, using the following settings: divergence slit, 6 mm, variable; step time, 0.5 s; step size, 0.019746º; scan time, 23’47”; 2θ angle range, 5° to 60°. Rietveld refinement was used to quantify the crystalline phases identified by XRD.
Raw mix reactivity is defined in terms of the rate at which the reaction reaches completion at a given temperature. The reactions taking place during clinkerisation include thermal decomposition of the raw mix components, limestone in particular, solid state reactions between the acid and basic components to yield C3A and C2S and smaller proportions of C4AF and the formation of large amounts of C3S in the presence of the eutectic liquid (
The DSC/TG curves for the raw mixes reproduced in
DSC/TG heat curves for BOS-containing raw mixes.
DSC/TG heat curves for CW-containing raw mixes.
DSC/TG heat curves for BOS+CW-containing raw mixes.
The very low intensity exothermal signals observed for the BOS mixes between 1130 °C and 1279 °C, the CW mixes between 1260 °C and 1295 °C and the blended raw mix at 1130 °C were all attributed to solid state reactions and the concomitant formation of C2S (belite), C12A7, C4AF (FF), C3A (
Chemical analysis of clinkers with particle size <45 μm, expressed in oxides (wt%)
Oxide | CaO | Al2O3 | SiO2 | Fe2O3 | SO3 | MgO | K2O | ZnO | P2O5 |
---|---|---|---|---|---|---|---|---|---|
62.43 | 7.1 | 18.69 | 4.25 | 1.58 | 2.15 | 0.72 | 0.03 | 0.58 | |
63.96 | 6.12 | 18.72 | 3.77 | 1.29 | 1.79 | 0.65 | 0.02 | 0.47 | |
62.41 | 7.95 | 20.38 | 4.14 | 0.06 | 0.97 | 1.02 | 0.01 | 0.05 | |
63.22 | 7.04 | 18.61 | 4.12 | 1.42 | 1.87 | 0.87 | 0.02 | 0.44 |
The diffractograms for the clinkers reproduced in
Quantitative analysis of the phases in clinkers (wt%)
BOS < 45μm | CW < 45μm | BOS + CW < 45 μm | 45 μm < BOS < 90μm | |||||
---|---|---|---|---|---|---|---|---|
Phase | XRD, Rietveld | XRF, Bogue | XRD, Rietveld | XRF, Bogue | XRD, Rietveld | XRF, Bogue | XRD, Rietveld | XRF, Bogue |
70 | 54 | 56,40 | 29 | 74 | 60 | 69 | 60 | |
10,23 | 13 | 28,17 | 37 | 10,3 | 12 | 14,7 | 8,5 | |
3,5 | 12 | 8,50 | 14 | 4,53 | 14,48 | 3.2 | 10 | |
11,72 | 13 | 6,52 | 13 | 11,52 | 12,21 | 12,5 | 12,5 |
XRD patterns for BOS clinkers burnt at 1400 °C.
XRD patterns for CW clinkers burnt at 1400 °C.
XRD patterns for BOS+CW clinkers burnt at 1400 °C.
The explanation lies in that with Bogue, C3A is calculated from the total Al2O3 as found with XRF, assuming that all the Al present is in the form of C3A and the ferritic phase. In other words, the Bogue calculation fails to consider any Al which may be in the silicon positions in alite and belite. Furthermore, when cooling takes place slowly, C3A crystallises more effectively from the molten mass and the resulting ferrite contains less Al in its structure, whilst when the cooling rate is faster, less microcrystalline C3A forms and the ferrite phase has a higher Al content. Low clinker crystallinity may also be the reason for these differences, for in that case Al would be in vitreous or poorly crystallised interstitial phases. As in Rietveld analysis the XRD values are normalised to the total crystalline phase content and vitreous or poorly crystallised phases (which fail to crystallise during cooling) are disregarded, the silicate phases would be over estimated. The present findings showed that whilst the alumina content was much lower than found with Bogue, the difference for the ferritic phase was much narrower, suggesting that the BOS clinkers cooled quickly.
The CW clinker with particles <45 μm had a lower proportion of C3S than the BOS-containing clinkers, whilst its belite content was higher. This clinker had a higher Rietveld-determined C3A than ferrite content, although both were lower than the values found with the Bogue formula.
The FTIR spectra for the clinkers reproduced in
FTIR spectra for BOS clinkers.
FTIR spectra for CW clinkers
FTIR spectra for BOS+CW clinkers.
The SEM micrographs of the clinkers reproduced in
SEM micrographs of clinkers and EDX findings for interstitial phase.
EDX analysis identified the majority and minority elements in the phases in the micrographed regions of the clinker. According to the findings in
EDX-based chemical analysis of BOS and CW clinker crystalline phases (wt%) with particle size < 45 μm
Alite | Belite | Interstitial phase | |||||||
---|---|---|---|---|---|---|---|---|---|
Oxide | ClkBOS | Clk BOS+CW | Clk CW | Clk BOS | Clk BOS+CW | ClkCW | Clk BOS | Clk BOS+CW | ClkCW |
16.5 | 21 | 15.7 | 20.2 | 22.63 | 19.9 | 6.8 | 3.6 | 2.1 | |
1 | 1.2 | 0.5 | 1.7 | 1.02 | 1.4 | 14.5 | 19.5 | 16.2 | |
0.6 | 0.8 | 0.5 | 0.6 | 1.26 | 1 | 13.6 | 17.6 | 15.8 | |
0 | 0 | 0 | 0.7 | 0.31 | 0 | 0 | 0.1 | 0 | |
1 | 1 | 0.1 | 0.6 | 0 | 0 | 3.6 | 2.4 | 1.6 | |
0 | 0 | 0 | 0.3 | 0.6 | 0.5 | 0 | 0.2 | 0.2 | |
66.3 | 76 | 62.9 | 57.7 | 73.12 | 54.7 | 48.8 | 56.5 | 40.6 |
The optical micrographs of resin-set, polished thin sections of clinker are shown in
OM micrographs of: (a) BOS clinker, raw mix particle size <45 μm; (b) CW clinker, raw mix particle size <45 μm; (c) BOS clinker, raw mix particle size 90 μm>BOS>45 μm; (d) BOS+CW clinker, raw mix particle size <45 μm.
Further to the micrographs of the BOS clinker with particle sizes 45 μm to 90 μm in
The lime saturation factor (LSF), silica modulus (MS) and alumina modulus (Al2O3/Fe2O3) for the clinkers studied are listed in
Lime saturation factor (LSF), silica modulus (MS),alumina modulus (Al2O3/Fe2O3) and free lime in clinkers and Fundal model calculation of chemical and particle size contribution to free lime
LSF | Al2O3/Fe2O3 | MS | Freelime (wt%) | Chemical contribution to free lime | Particle size contribution | |
---|---|---|---|---|---|---|
98 | 1.67 | 1.65 | 1.15 | -1.08 | 2.23 | |
103 | 1.62 | 1.9 | 2.78 | 1.27 | 1.51 | |
95 | 1.99 | 1.86 | 3.87 | -1.482 | 5.352 | |
90 | 1.92 | 1.7 | 2.68 | -3.58 | 6.26 | |
98.7 | 2.2 | 1.5 | 6.39 | -1.269 | 7.659 | |
95 | 1.99 | 1.75 | 19.53 | -1.79 | 21.32 | |
100 | 1.71 | 1.67 | 1.89 | -0.364 | 2.254 |
The free lime content also rose with particle size in the BOS clinkers. Further to the Fundal equation, after burning at 1400 °C for 30 min, the free lime content of a cement depends on the chemistry, mineralogy and particle size of the reagents (
The present findings on the suitability of Moroccan oil shale and coal waste as raw materials in portland cement manufacture support the following conclusions.
The main clinker phases in BOS, CW and BOS+CW are alite, belite, aluminate and aluminium ferrite.
Mixes containing oil shale deliver higher reactivity than coal waste materials, although reactivity is highest in the BOS raw mix with a particle size under 45 μm.
Raw mixes containing oil shale and a blend of oil shale and coal waste exhibit good burnability when the particle size is <90 μm, whereas burnability is poor in all the raw mixes containing coal waste, including the mix with particle size <45 μm.
Timahdit oil shale, alone or blended with Jerada coal waste, particularly where particle size is <90 μm in BOS and <45 μm in BOS+CW, is apt for use as a raw material in lieu of clay and as a partial substitute for limestone in cement manufacture. The resulting low (1400 °C) clinkerisation temperature would afford both economic (lower energy and civil engineering costs) and environmental benefits.
This paper has been carried out as part of a CSIC-Mohammed V University of Rabat, cooperation programm (i-COOPA20067). Facilities given by IETcc (CSIC) and funding from BIA 2013-47876-C2-1-P and BIA BIA2013-43293-R projects as well as the Regional Government of Madrid Community and European Social Fund (Geomaterials Programme2 S2013/MIT-2914) are gratefully acknowledged.