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 (16) with a Restch^® SK 100/C Spezialstahl arm mill.

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. (9). The carbon fiber residue (FC) was generated and supplied by the company ICSA-Aernnova (Toledo, Spain), specialized in the manufacture of aeronautical components based on carbon fiber composites. To facilitate the addition of the material in the mixtures, also FC was ground below 0.5 mm, again in line with the methodology indicated in a previous work of the authors (10). As explained above for SEP, in the cases of FC and P, the wastes generated are very homogeneous in terms of composition, structure and properties, being almost invariable over time.

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:

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 Table 1, it is observed that SEP presented a meaningful SSA due to its small grain size (Figure 1), especially after milling (343.3 m²/g), which resulted in a very high plasticity (Table 1 and Figure 2). Once oven-dried, it was observed that SEP was able to adsorb about 10 wt% of water from the ambient humidity. Previous trials in the rotary kiln showed that SEP was not adequate (without any additive) for LWA manufacture by itself, since the pellets burst in the kiln preheating zone very quickly. The reason is thought to be that the porosity of the SEP pellet is too low to allow a rapid release of water vapor outwards, leading to relatively high internal pressures. The addition of 2.5 wt% of P and 2.5 wt% P + 2.5 wt% FC (SEP-P and SEP-PFC in Table 1, respectively) prevented the pellet from bursting, so these two mixtures were finally selected for LWA manufacturing. The main properties of the mixtures associated with particle size distribution, ρ_R, chemical composition, SSA, carbon content and plasticity were also measured by applying the same protocols previously indicated for the raw materials. Additional 24-hour LOI tests were performed at the selected firing temperature (1225˚C).

The manufacturing process of the LWAs was carried out according to the following steps:

According to the results of Table 1 and Figure 2a, the sepiolite studied is highly plastic, with LL of 171.1 and PI/LL ratio of 0.56, yielding a T_max of 28.9 kJ/m³. When mixed with FC and P (which are nonplastic), the plasticity diminishes slightly due mainly to a small increase in PL. Consequently, T_max values are 20 and 23.7 kJ/m³ for SEP-P and SEP-PFC, respectively, figures that are in the same order as other ceramic clays studied (44). Thus, both the original sepiolite and the resulting mixtures are classified as clay-textured CH materials (31), because in addition to their significant plasticity, their sand content is low (13-16 %). Although their location is far from the Gippini (32) areas (Figure 2b), the toughness results point out that the materials could be suitable for LWA production.

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 (45). To achieve such conditions, not only is an adequate heating ramp required (usually sudden heating), but the material must also have appropriate chemical, mineralogical and particle size properties for the process of pore formation and bloating.

According to its chemical characteristics, SEP would not be suitable for retaining gases because its oxide composition is located outside the Riley (24) area (Figure 3) and the SiO₂/∑Flux ratio is less than 2 (Table 2) (25). Likewise, if the limits of Cougny (46) on particle size distribution are considered (Figure 1), again none of the raw materials under study would be theoretically adequate for LWA manufacturing.

Based on the LOI data in Table 2 (LOI ~10 %), SEP can produce an important volume of gas when heated. However, the percentages of OC and IC are relatively low (0.44 % and 0.41 %, respectively; Table 2). Consequently, their contribution to gas release is not expected to be significant. According to the XRD data (Table 3 and Figure 4), the carbonates are calcite and dolomite (2.6 % and 2.4 %, respectively), while other mineral species capable of generating gases in this sample are phyllosilicates, mainly smectite and sepiolite (37.8 % and 30 %, respectively).

According to the DSC-TGA graphs in Figure 5a, the most important loss of mass takes place at temperatures below 700˚C. Previous studies (1, 2, 47) show that this phenomenon is closely related to the release of hygroscopic, zeolitic and coordinated water. On the other hand, and based on the same references cited above, the dehydroxylation of sepiolite and montmorillonite explain the LOI at the temperatures ranging 700-850˚C. These reactions are usually accompanied by the release of gaseous water molecules, although according to Heller-Kallai et al. (48), H₂ could also be formed. As the percentages of calcite and dolomite are low (<3 %, Table 3), it is highly likely that their peaks cannot be detected in Figure 5a, because they have probably been overshadowed by those of the phyllosilicates.

On the other hand, and in accordance with Moreno-Maroto et al. (9-11), P and FC practically decompose at 100%, giving rise to gases that could favor the development of a porous structure (Table 2 and Figure 5b, c). Similarly, the exothermic decomposition suffered by their components is remarkable (Figure 5b, c), which is in agreement with the bibliography (49-55). This aspect may also be a key point in the development of a viscous matrix.

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 (56). However, recent research by Moreno-Maroto et al. (45) refutes this idea, showing that bloating occurs at a stage when more than 99% of the gas has been lost to the atmosphere, so that the pores of the aggregate develop from negligible amounts of gas. Therefore, the application of a thermal shock is not so much intended to prevent the incipient loss of gas, but rather to favor reducing conditions inside the aggregate, leading to the development of a viscosity in which pores can be formed from the available gas.

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.

Figure 6 shows an overview of the outer face and the core of the LWAs, whose main technological properties are presented in Table 4. Although SEP did not seem suitable for producing LWAs according to their particle size and composition (see Section 3.1), both SEP-P-1225 and SEP-PFC-1225 meet the requirements of the LWA standards. Thus, they comply with ρ_B <1.20 g/cm³ (0.84 and 0.82 g/cm³, respectively) and ρ_A<2.00 g/cm³ (1.33 and 1.37 g/cm³), as established in EN-13055-1 (57) (Table 4). The average diameter of the aggregates obtained is 7.96 mm (standard deviation, sd = 0.24) and 7.93 mm (sd = 0.31) in SEP-P-1225 and SEP-PFC-1225, respectively, which indicates that both types have even experienced slight bloating (BI of 1.02 and 0.58 %). This implies that real sintering tests are necessary before discarding any raw material for not possessing the best characteristics “on paper”.

It is noteworthy that SEP-P-1225 and SEP-PFC-1225 exhibit quite similar properties in all aspects. As can be seen in Figure 6, both aggregates are generally white. The addition of FC has fostered the formation of a black core (Figure 6b) with pores that contain unfired carbon fibers inside (Figure 7c and Figure 8b). Beyond this, the addition of carbon fiber has not entailed any supplementary benefit, as the similarity between the results of SEP-P-1225 and SEP-PFC-1225 reflects (Table 4). In any case, the fact that FC has not worked with the materials used in this investigation is not an indication that it is an unsuitable additive. Thus, for example, the work previously published by the authors mixing FC with ornamental rock sludge and lower amounts of sepiolite-rich by-products (10), showed excellent results when adding the carbon fiber. This shows that the suitability of such a material in the manufacture of LWAs will depend on the raw materials and the manufacturing conditions, so its effects should be studied on a case-by-case basis.

Apart from the density and bloating results explained above, these two LWAs have similar porosity: P_T , P_O and P_C are close to 50 %, 15 % and 35 %, respectively. These results are in the same order as some others reported in LWAs also sintered from mineral and carbon fiber wastes (10). A particularity of the SEP-LWAs is that their porosity is uniformly distributed throughout the aggregate inner section, so that, from a structural point of view, there is no clear differentiation between core and shell (Figure 7b and Figure 8a, b). This structure was formed because an adequate viscosity was generated very quickly, so that even part of the gases released in areas close to the surface were trapped, thus avoiding the formation of a non-porous thick shell.

Moderate WA₂₄ results have been recorded (11.84 % and 9.64 % in Table 4), which are significantly lower than others reported in commercial LWAs, for instance, in Arlita G3, whose WA₂₄ is 30.9 % (9). This occurs because the pore interconnection and the shell permeability are not very high (P_O is not either), as rapid firing at high temperatures has favored the formation of closed porosity and a shell which, although thin, is highly vitrified. Some of the water entry routes could be the deep cracks formed along the aggregate surface (Figure 6), which can even extend to the outermost core areas (Figure 7b). The development of these fractures may be related to the high plasticity of the sepiolite raw material and its mixtures (Figure 2), which are located very far from the “acceptable” and “optimal” extrusion zones for ceramics (Figure 2b). In fact, this resulted in the need to use a very high volume of water to shape the pellets (Table 1: W_OP around 100 %), which in turn would explain the large shrinkage that the green pellets experienced when oven-dried (23.2% shrinkage, as indicated in Section 2.4). Equally noteworthy is the high capacity of the dry granules to adsorb hygroscopic moisture from the environment (~10 %), as explained in Section 2.3. Regarding this aspect, a sudden release of this water when turned into gas could have encouraged the cracking.

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 (58). With respect to the latter, the single aggregate crushing strength (S) is about 6 MPa and the S/ρ_A of 4.3-4.9 N/m·g, which are values that exceed those obtained in Argex AR 4/10 - 550 (S = 1.27 MPa and an S/ρ_A = 1.5 N/m·g) (11). These results are undoubtedly encouraging if these LWAs are intended for the production of, for example, structural lightweight concrete. However, this can only be reliably demonstrated by an additional study in concrete samples, something that is beyond the scope of this work. The latter is also particularly important because the single aggregate crushing strength test is usually associated with a high dispersion (in this case, sd data are 2.65 and 2.26 for SEP-P-1225 and SEP-PFC-1225, respectively). This confirms that the final evaluation in terms of mechanical strength should be carried out with real concrete specimens, even though, as in this case, the data are promising.

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 (59, 60). Similarly, high proportions of amorphous phase could favor better pozzolanic activity, as well as a reduction in thermal conductivity, which is very interesting from an energy point of view (61, 62). The mineralogical composition of the raw material and the sintered LWAs is detailed in Table 3, based on the diffractograms of Figure 4.