A sepiolite mining by-product (SEP) has been studied as major component for lightweight aggregate (LWA) manufacture. Pellet bursting during firing was avoided by the addition of 2.5 wt% of thermoplastic waste (P) and 2.5 wt% P + 2.5 wt% carbon fiber residue (FC) in powder form. The mixtures were pelletized and then sintered at 1225˚C for 4 minutes in a rotary kiln. Highly porous white LWAs with good mechanical strength were produced. A mineralogical study revealed the formation of amorphous phase (>50%) and minor proportions of enstatite, protoenstatite and diopside. Quartz was the only inherited mineral, appearing in the form of isolated phenocrysts within a general porphyritic texture. The result of this study suggests the promising use of sepiolite (whether or not in residue form) for the manufacture of high quality LWAs.
Se ha investigado la fabricación de áridos ligeros (LWAs) a partir de subproductos de la extracción de sepiolita (SEP). Durante la cocción, el estallido de los pellets se evitó mediante la adición de 2.5 wt% de residuo de plástico (P) y 2.5 wt% de P + 2.5 wt% de residuos de fibra de carbono (FC), ambos en forma de polvo. Las mezclas fueron peletizadas y sinterizadas en horno rotatorio a 1225˚C durante 4 minutos. Se obtuvieron LWAs blancos, altamente porosos y con buena resistencia mecánica. Un estudio mineralógico reveló la formación de fase amorfa (>50%) y proporciones menores de enstatita, protoenstatita y diópsido. El único mineral heredado fue el cuarzo, apareciendo en forma de fenocristales aislados dentro de una textura porfídica. Los resultados obtenidos sugieren que la sepiolita (ya sea en forma de residuo o no) puede tener un uso prometedor en la fabricación de LWAs de alta calidad.
Sepiolite is a fiber micro-channel structured hydrated magnesium silicate, which due to its high adsorption and physicochemical potential is very valuable in many industrial sectors (
Some of the earliest evidence of the effective use of sepiolite as a substitute for kaolin in porcelain mixtures dates from the late 17th and early 18th centuries (
However, there has been no prior evidence of the use of sepiolite as a key component in the manufacture of lightweight aggregates (LWAs) until the publications of Moreno-Maroto et al. (
In the first two studies by Moreno-Maroto et al. (
Therefore, as sepiolite use in low percentages has already been proven in the works cited above (
Through the production of lightweight aggregates, this research aims to find a solution for the large amounts of material rejected in the extraction plant for this type of clay. The role of thermoplastic and carbon fiber residues as additives will also be assessed.
The sepiolite waste (SEP) was provided by the clay mining company Tolsa, S.A. (Vallecas plant, Spain), which usually trades this kind of clay as a pet litter absorbent. SEP was rejected at the plant as its aggregate size (about <1mm) was not suitable for the market. According to the information given by the manufacturer, this type of sepiolite by-product is the one generated in greater quantities in the plant and therefore tends to be very homogeneous over time. Despite not being dangerous, from an environmental point of view, the valorization of this waste could suppose not only a reduction of the negative impact on the landscape generated by the large piles of rejected material, but also the use of a material with excellent physical-chemical properties, as an alternative to the exploitation of natural raw materials. After oven-drying at 60˚C for 72 h to ensure that the material was perfectly dry (constant weight), SEP was milled to <200 µm (
The thermoplastic material (P) was a linear polyethylene-hexene copolymer, which was ground below 0.5 mm under controlled temperature conditions, in accordance with Moreno-Maroto et al. (
Below are listed the parameters measured in the characterization of the raw materials, as well as the method and/or instrument of measurement together with the corresponding references:
Relative density (ρR): AccuPycTM 1330 He pycnometer (
Particle size distribution: Coulter® LSTM 230 laser diffraction analyzer (
Specific surface area (SSA): Methylene blue spot test (
Chemical composition: Inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Thermo Electron 6500 ICAP). Previous fusion with lithium metaborate and dissolving in acidic medium (
Loss on ignition (LOI): muffle-firing (1100˚C for 24 h).
Chemical suitability for bloating: Criterion 1. Recalculation of percentages of Al2O3, SiO2 and flux oxides (∑Flux = K2O+Na2O+CaO+MgO+FeO+Fe2O3) and plotting in the Riley (
Carbon content (total (TC), organic (OC) and inorganic (IC) carbon): Shimadzu® TOC-VCSH analyzer.
Atterberg limits, classification and maximum toughness: Liquid limit (LL) by Casagrande cup method (
Optimal moisture content (WOP) for pelletizing: PL × 1.234 (
Thermal behavior: DSC-TGA; SDT Q600 TA INSTRUMENTS (platinum crucible, air atmosphere, 20ºC/min, maximum temperature: 1200ºC).
The final mixtures were formulated on the basis of the characteristics of the raw materials, in conjunction with previous testing conducted in a Nannetti® TOR-R 120-14 tubular rotary kiln (used to sinter the LWAs).
In
The manufacturing process of the LWAs was carried out according to the following steps:
Preparation of the mixtures shown in
Addition of the water corresponding to WOP (
Maceration for 72 h in a hermetically sealed bag.
Extrusion using a Nannetti® laboratory-scale pneumatic extruder.
Pelletizing by hand into spherical granules of approximately 10.3 mm.
Drying: 48 h at room temperature + 48 h at 105˚C. The diameter of the dry pellets was ~7.9 mm, which represents a shrinkage by oven-drying of about 23.2 %.
Tube rotation speed: 2.5 rpm.
Preheating: 400-600˚C; 20 seconds; entry zone of the kiln tube.
Sintering: 1225˚C; 4 min; middle section of the kiln tube.
Cooling: Fast quenching at room temperature (around 25˚C).
Particle size distribution | Plasticity parameters | Class. Moreno-Maroto and Alonso-Azcárate ( |
LOI1100 | LOI T firing | ||||||||||||
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Sample | Type | ρR (g/cm3) | SSA (m2/g) | Mean (μm) | > 63 µm (%)b | LL | PL | PI | PI/LL |
|
WOP (%) | Plasticity | Texture | 24 h muffle | preh. 20 sec | 24 h muffle |
SEP as received | Raw material | 2.44 | 175 | 38.0 | 13.3 | 171.1 | 74.8 | 96.1 | 0.56 | 28.9 | 92.3 | CH | Clay | 9.79 | NA | NA |
SEP milled | Raw material | 2.44 | 343.3 | 18.5 | 13.3 | 171.1 | 74.8 | 96.1 | 0.56 | 28.9 | 92.3 | CH | Clay | 9.79 | NA | NA |
P | Raw material | 0.94 | NM | 327.6b | 100.0 | NP | NP | NP | 0.00 | 0.0 | NA | NP | NA | 100 | NA | NA |
FC | Raw material | 1.65 | NM | 330.5 | 45.8 | NP | NP | NP | 0.00 | 0.0 | NA | NP | NA | 99.79 | NA | NA |
SEP-P | Mix: 97.5% SEP + 2.5% P | 2.40a | NM | 26.2a | 15.5a | 173.1 | 83.6 | 89.5 | 0.52 | 23.0 | 103.2 | CH | Clay | 12.61 | 1.92 | 12.57 |
SEP-PFC | Mix: 95% SEP + 2.5% P + 2.5% FC | 2.38a | NM | 29.3a | 16.3a | 163.9 | 82.1 | 81.8 | 0.50 | 20.7 | 101.3 | CH | Clay | 14.99 | 3.03 | 15.24 |
aEstimated from the values obtained in the raw materials.
bData obtained by sieving instead of by Laser diffraction.
The measured parameters relating to the physical and mechanical properties in LWAs, together with the methods used and their references, are as follows:
LOI during kiln firing and preheating: Weight difference between the fired and the unfired specimens in a batch (25 granules).
Bloating index (BI): Average percentage change in diameter experienced by the specimens in a batch because of firing (
Loose bulk density (ρB): In accordance with EN-1097-3 (
Particle density (ρA), skeleton density (ρS) and water absorption after 24 h of immersion (WA24): According to Annex C of the EN-1097-6 (
Relative density of the aggregate solid phase (ρsolid): three specimens milled below 53 µm (Retsch® RM 100 rotary agate mill) and ρsolid measurement using an AccuPycTM 1330 helium pycnometer.
Total porosity (
Void percentage (
Single aggregate crushing strength (
The study of the mineralogy and the glass formation was performed by XRD with a PANalytical® X´Pert Pro model diffractometer in accordance with Moreno-Maroto et al. (
The texture of the sintered LWAs was observed by thin-section polarized light microscopy (TSPLM): 30 µm thick slices (
According to the results of
Expanded LWA is formed by the development of a mineral matrix with a viscosity suitable for retaining the gases released from negligible amounts of certain gas-generating components (
According to its chemical characteristics, SEP would not be suitable for retaining gases because its oxide composition is located outside the Riley (
Carbon content | Fluxing oxides | |||||||||||||
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Sample | TC | IC | OC | SiO2 | Al2O3 | FeO+ Fe2O3 | Na2O | K2O | CaO | MgO | PxOx b | TiO2 | LOI1100 | SiO2/∑Flux |
SEP | 0.85 | 0.41 | 0.44 | 54.8 | 7.2 | 2.0 | 0.5 | 1.6 | 2.0 | 21.9 | 0.1 | 0.3 | 9.79 | 1.96 |
FC | 82.11 | 0.00 | 82.11a | NA | NA | NA | NA | NA | NA | NA | NA | NA | 99.79 | NA |
P | 90.37 | 0.00 | 90.37 | NA | NA | NA | NA | NA | NA | NA | NA | NA | 100 | NA |
a Apart from the organic compounds from the epoxy resin, this result is mainly represented by inorganic pure carbon fibers which are also oxidable at high temperatures in air atmosphere
b Sum of all oxides of phosphorous
Based on the LOI data in
According to the DSC-TGA graphs in
Mineralogy (%) | ||||||||||||||||
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Aggregate name | % P | % FC | t (min) | T (˚C) | Q | Pg | Fp | Mc | Sm | Sp | Cal | Dol | Pet | Et | Dp | G |
SEP (unfired)a | 0 | 0 | - | - | 11.1 | 4.3 | 7.9 | 3.9 | 37.8 | 30 | 2.6 | 2.4 | - | - | - | - |
SEP-P-1225 | 2.5 | 0 | 4 | 1225 | 2.1 | - | - | - | - | - | - | - | 14.2 | 29.1 | 3.6 | 51.1 |
SEP-PFC-1225 | 2.5 | 2.5 | 4 | 1225 | 2.6 | - | - | - | - | - | - | - | 10.9 | 31.4 | 4.3 | 50.8 |
a Although the mineralogy of SEP (unfired) has been measured in the mixture not containing FC or P, the same results are applicable to the mineral fraction of the mixtures containing these two additives.
On the other hand, and in accordance with Moreno-Maroto et al. (
Traditionally it has been thought that in order for LWA to expand, it is important to avoid massive loss of gases during the heating of the material (
Similarly, despite the faint results related to particle size distribution and chemical composition, the suitability of the studied materials for LWA production will be examined in the following sections according to experimental findings, which show the actual behavior of the materials beyond their theoretical feasibility.
Density, ρ (g/cm3) |
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Name | % P | % FC | t (min) | T (˚C) | diam. (mm) | BI (%) | LOIfiring (%) | ρB | ρA b | ρS c | ρsolid d | WA24 (%) |
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SEP-P-1225 | 2.5 | 0 | 4 | 1225 | 7.96 | 1.02 | 14.58 | 0.84 | 1.33 | 1.58 | 2.61 | 11.84 | 49.15 | 15.75 | 33.40 | 67.74 | 6.48 | 4.88 |
SEP-PFC-1225 | 2.5 | 2.5 | 4 | 1225 | 7.93 | 0.58 | 17.11 | 0.82 | 1.37 | 1.58 | 2.63 | 9.64 | 47.80 | 13.27 | 34.53 | 68.99 | 5.93 | 4.32 |
SEP-Pa | 2.5 | 0 | 48ha | 105a | 7.88 | 0 | 0 | 0.97 | NA | NA | NA | NA | NA | NA | NA | NA | 4.67 | NA |
SEP-PFCa | 2.5 | 2.5 | 48ha | 105a | 7.88 | 0 | 0 | 0.99 | NA | NA | NA | NA | NA | NA | NA | NA | 3.34 | NA |
a Unfired pellet, so that t and T refers to oven-dry conditions (48 hours at 105 ˚C) prior to start the actual firing stage in the rotary kiln.
b ρA is equivalent to the parameter called “oven dry density” (ρLrd) in the Annex C of the standard EN-1097-6 (
c ρS is equivalent to the parameter called “apparent density” (ρLa) in the Annex C of the standard EN-1097-6 (
d ρsolid is equivalent to the parameter called ρmatrix in other previous publications (9,10,35). The reason of this change is to avoid any confusion with the term “matrix” related to textural characteristics.
It is noteworthy that SEP-P-1225 and SEP-PFC-1225 exhibit quite similar properties in all aspects. As can be seen in
Apart from the density and bloating results explained above, these two LWAs have similar porosity:
Moderate WA24 results have been recorded (11.84 % and 9.64 % in
Despite this, the presence of “flaws” in LWAs is not as important as in other ceramic materials where the aesthetics of the pieces is paramount. Indeed, fractures or surface pores could have positive effects in LWAs, for example, by improving bonding with the cement paste when used in lightweight aggregate concrete (
Mineralogical composition is a key factor in aspects such as the appearance of a negative alkali-silica reactivity or the development of a good adhesion between the cement and the aggregate (
The mineralogy of SEP-P-1225 and SEP-PFC-1225 is similar (
The detection of diopside is also reported in other studies related to LWA manufacturing from wastes (
Since the neo-formed pyroxenes require lower crystallization temperatures than those used inside the kiln, it is likely that they crystallized during the final cooling phase and especially during the initial heating (which may be slower than the final cooling).
This process would be somewhat similar to what happens when lava cools suddenly, generating what is known as a strong
The use of FC as an additive has led to the formation of microspheres (
The production of lightweight aggregates using sepiolite plant rejects (SEP) as the main component has been assessed. The effect of plastic (P) and carbon fiber (FC) wastes when added in low proportions to SEP has also been studied.
The main conclusions that can be drawn are summarized below:
Although its composition and particle size is theoretically inadequate according to the Riley (
Despite the above, the high plasticity of sepiolite requires the addition of some gas-pressure mitigating additive (P and FC in the present investigation) to avoid the bursting of the pellets when placed in the kiln. However, other more conventional degreaser components should be tested in future investigations.
Apart from the decomposition of P and FC, the processes of clay mineral dehydroxilation and water release could be important in the formation of pores.
In terms of the changes that have occurred in mineralogy and texture, it is worth highlighting the important development of amorphous phase in the aggregates (>50 %) together with the neo-formation of pyroxenes (enstatite, protoenstatite and diopside). Quartz was the only inherited specie, appearing in the form of isolated phenocrysts within a generally porphyritic porous texture.
In conclusion, the use of sepiolite as the major component in LWA manufacturing can be an excellent alternative for harnessing this material in a different way, especially from those plant processed fractions that are “unmarketable”. Thanks to this new perspective these materials can be valorized, meaning an additional economic and environmental asset.
This research has been supported by the research project PEII-2014-025-P of the Junta de Comunidades de Castilla-La Mancha (JCCM) and the PhD grant number PRE-7911/2014 whose funds come from the Consejería de Educación, Cultura y Deportes of JCCM and the European Social Fund (DOCM 2014/10620 and DOCM 2016/12998 BDNS (Identif.): 323799). Special thanks to Tolsa, Innovarcilla and ICSA-Aernnova, the companies that have provided us the raw materials, without which this study would not have been possible.