Development of high performance ceramic lightweight aggregates by recycling carbon fiber, plastic and mineral wastesinnovations in materials, methods and technological applications

Resumen

In accordance with the existing environmental issues and the basic principles of the Circular Economy on waste management, the suitability of five different wastes has been assessed for lightweight aggregate (LWA) production: a granite and marble sludge generated from ornamental rock sawing (COR), a factory sepiolite reject (SEP), household polyethylene-hexene thermoplastics (P), carbon fiber remnants from the production of aviation pieces (FC) and a heavy metal polluted material (MAZ), which was taken from the tailings of an abandoned mine. This Doctoral Thesis is made up of six main lines of research and a secondary one which can be summarized as follows: i) Analysis of the suitability of mixtures rich in granite and marble sludge for the production of LWAs: Although some LWA varieties were sintered from SEP and MAZ as major components, this investigation has been focused mainly on COR as it is a relatively common and widespread residue. Hence, different mixtures were prepared using COR as major component, while FC and P were examined as bloating additives and SEP as a binder, since the plasticity of COR itself was very low to be properly molded into spherical pellets. Once the wastes were milled (FC and P) or disaggregated (COR and SEP) to a very fine particle size, a preliminary study of their physicochemical properties and their thermal behavior was carried out in order to devise the appropriate batches. In accordance with the results obtained, a base mixture was prepared by blending 90 wt% COR and 10 wt% SEP (mixture called COS) to confer plasticity as well as 0, 2.5, 5 and 10 wt% of P or FC to check their suitability as pore-forming elements. The resulting mixtures were kneaded with their optimum water volume, extruded, shaped into pellets by hand, oven-dried and finally fired at 1100, 1125 and 1150 °C for 4, 8 and 16 minutes in a laboratory-scale rotary kiln. The main technological properties of the aggregates related to bloating, density, porosity, loss on ignition, water absorption and mechanical strength were determined. In addition, SEM microscopy was used to study the microstructure of some selected specimens. Of a total of 49 types of aggregate using the granite-marble sludge as major constituent, 42 were lightweight. However, those LWAs coming from COS mixture without any additive or when it was P did not exhibit either bloating or the typical LWA shell-core-macropore structure, but one consisting of micropores and microchannels. The increase in temperature and time of heating involved a greater sintering in these samples, which in turn was translated into higher shrinkage, density and compressive strength values, but less porosity and water absorption. In fact, the addition of plastic did not entail any improvement, but rather the opposite, as the resistance to crushing dropped significantly. From those batches, only the LWA sintered without P at the minimum time (4 min) and minimum temperature of firing (1100 °C) displayed adequate features to assess its water suction capability. The results pointed out that this LWA could be suitable in hydroponics and/or water filtration systems, even better than the commercial LWA called Arlita G3. For its part, the addition of FC did promote bloating and the formation of a highly porous shell-core structure. It was remarkable the presence of unburnt carbon fibers embedded in the mineral matrix, which helped to enhance the mechanical strength. Although all the FC varieties met the regulatory requirements on LWA density, the specimens fired at 1150 °C are noteworthy, as they were particularly lightweight, yielding a particle density of 1.1-1.2 g/cm3, which is far below the maximum standardized limit of 2.0 g/cm3 applicable to LWAs. ii) Analysis of the impact of firing conditions on the mineralogy and texture of the samples: In order to assess the influence of the heating temperature (1100, 1125 or 1150 °C), the dwell time (4, 8 or 16 min) and the additive (FC, P or none) on the mineralogy and the texture, twelve of those aggregates were selected and subjected to a thorough study by means of microscopy and thermal techniques, as well as by X-ray diffraction, analyzing the polycrystalline powder diagrams according to the Rietveld method. Only small proportions of quartz, plagioclase and alkali feldspar withstood the sintering process from the original mineralogy, while glass increased from 36 to 70 % as temperature and dwell time were risen. Some augite (6.5 %) was neo-formed at about 980-1025 °C, far below the sintering temperatures. The addition of P barely affected the mineralogy, while FC promoted the development of glass and a highly porous fiber-microsphere-holding texture in which the phenocrysts observed in other specimens were almost absent. It was remarkable that the glass formation was connected with lower solid-phase density, less water absorption and greater closed porosity in the aggregates. iii) Analysis of the suitability of mixtures rich in sepiolite rejects for LWA production: A less comprehensive study with the sepiolite-rich sample, SEP, and the mining waste, MAZ, as major components was also performed. Regarding the former, some tentative tests showed that when SEP is fired without any gas-pressure-mitigating additive, the pellets burst inside the kiln very quickly. This undesirable effect was prevented by the addition of 2.5 wt% of P. In order to find out if carbon fiber entailed any impact, another mixture was prepared by adding 2.5 wt% P + 2.5 wt% FC into SEP. The resulting pellets were heated at 1225 °C for 4 minutes. White color LWAs of high porosity, low density and significant mechanical strength were obtained. A meaningful volume of glass was evolved in all the aggregates (>50 %), just as less proportions of enstatite, protoenstatite and diopside as neo-formed species. Quartz was the only inherited mineral, appearing in the form of isolated phenocrysts inside an overall porous porphyritic texture. The addition of FC did not mean any remarkable improvement in this case. iv) Manufacturing of lightweight aggregates from a heavy metal polluted mining waste: As MAZ was a sandy material, it was sieved <63 µm to collect the fraction in which the heavy metals adsorbed to phyllosilicates and organic matter are usually concentrated. Just like COR, MAZ plasticity was poor, so it was blended with 10 wt% SEP to gain consistency (MAS mixture). Likewise, two other mixtures were prepared: the first one by adding 2.5 wt% of thermoplastic and the second one containing 2.5 wt% of carbon fiber so as to check the effect of these two additives as bloating agents in MAS. The pellets were fired at 1175 °C for 4 minutes. Dark color LWAs of high porosity, low density and good mechanical strength were obtained. The addition of P and FC enabled the development of a porous core with well distributed pores surrounded by a very thin shell, which contrasted with the thick cortex formed when these components were not used. While the addition of FC did not provide real advantages in the technological properties, P promoted greater bloating and lower density. A high volume of glass was evolved in all the aggregates (≥ 60 %) and augite was the main neo-formed mineral. v) Study of the immobilization and changes in the fractionation of chemical elements associated with sintering: The impact of LWA sintering on the fractionation and immobilization of thirty-three metallic elements, including a good handful of heavy metals and rare earth elements, has been assessed thoroughly by means of the BCR method. Four fractions were considered: F1 (weakly adsorbed), F2 (Fe–Mn (hydr)oxide-bound), F3 (sulfide- organic matter-bound) and FR (immobile, e.g., aluminosilicate-bound). Seven LWA varieties were selected: one sintered from the mixture COS (90 % granite-marble sludge + 10 % rejected sepiolite), five from COS-2.5FC (COS + 2.5 % of carbon fiber waste) and one from MAS (90 % heavy-metal-rich mine tailing + 10 % sepiolite). Both the unfired mixtures and the SEP sample were also subjected to the BCR protocol in order to evaluate the effect of firing on the mobility of the elements. The major components detected were Fe and Mg, this last one coming mostly from the sepiolite. Besides, MAS presented concentrations of Zn, Pb, Ni and As exceeding the legal limits established in USA (US EPA, 1993; USDA, 2000), as well as the ones ruling in Spain (Real Decreto 1310/90), which are based on the Council Directive 86/278/EEC of the European Union (Council Directive, 1986). The study was mainly focused on the mixture COS-2.5FC and its selected LWAs, which were sintered at different temperatures (1100, 1125 and 1150 °C) and dwell times (4, 8 and 16 min). It has been observed that although most of the elements follow the expected redistribution (a decrease in their concentrations of F1, F2 and F3 in favor of FR), this pattern is not followed in many cases, and in fact, complex trends depending on heating intensity have been observed (e.g., in Fe, Co, Ni, Cu and As). Accordingly, the elements have been grouped into different classes based on the fractionation behavior observed. As a general rule, with some exceptions, LWA sintering has caused the desired effect by increasing the immobilization of the elements to a greater or lesser extent. Only arsenic has exhibited a clear increase of its bioavailability at the expense of a pronounced reduction of FR. An environmental evaluation based on regulatory thresholds and the Risk Assessment Code (RAC) applicable to the BCR test, has indicated that the aggregates manufactured with the granite-marble sludge as main component do not entail any negative environmental concern, while that prepared from the mining-tailing would not be appropriate for agricultural uses in its current state due to its high concentration of As and the high leachability recorded for this metalloid. vi) Application of selected aggregates in the manufacture of lightweight concrete: Since LWAs are usually intended for lightweight concrete production, three of the varieties containing carbon fiber (the most interesting according to their technological characteristics) have been selected for this purpose. Prismatic concrete specimens were prepared in triplicate using these LWAs as coarse fraction. Additional specimens containing a normal-weight aggregate, a commercial lightweight aggregate and no coarse aggregate (mortar) were prepared for comparison. Water/cement ratios of 0.45 and 0.55 were studied. A comprehensive analysis of the properties related to workability, density, porosity, mechanical strength, elastic modulus and thermal transfer was conducted. Interfacial transition zones were examined through SEM-microscopy. Compressive strengths between 35 and 55 MPa and low values of density and thermal conductivity have been obtained with the LWAs developed in this investigation. Furthermore, the concrete samples manufactured from them have displayed the highest ratios relating the mechanical strength over the density and the thermal conductivity, meaning more balanced features than the other specimens, even exceeding the values of the normal-weight concrete. vii) Secondary research line: development of a new methodology to determine more accurately the plasticity of raw materials and their optimum moisture content for extrusion and molding: This Doctoral Thesis has also addressed the development of a new method to measure plasticity. This method has been of great importance in measuring not only the consistency of the samples but also their optimal water content for extrusion and pelletizing. Furthermore, this new approach has enabled us to develop new soil classification systems and a new definition of clay, whose potential application goes beyond even the manufacture of LWAs, covering fields such as ceramics, agriculture or geotechnical engineering. On balance, it has been proven that the wastes embraced in this research are suitable for LWA production, highlighting some varieties (e.g., some of those containing carbon fiber) whose characteristics in terms of strength/density ratio exceed those of commercial LWAs, which is very important in structural lightweight concrete production. Other specimens have shown excellent features for agriculture, while from an environmental standpoint, beyond the profit linked to the recycling process, the use of highly-polluted materials to manufacture LWAs could be promising not only to obtain high-quality materials intended for concrete, agriculture or civil engineering, but particularly to immobilize heavy metals from contaminated sites.